+
Preliminary Draft of Chapter for “Green Chemistry” Book – 12/16/96
h
BNL-63763
Transition Metal Catalyzed Reactions of Carbohydrates:
A Nonoxidative Approach to Oxygenated Organics
Mark Andrews, Chemistry Department
Brookhaven
National Laboratory, Upton, NY 11973-5000
TABLE OF CONTENTS
I - Background and Introduction ........................................................................ 2
Oxygenated
Organics:
Problems with Hydrocarbon
Oxygenated
Organics:
Advantages
Oxygenated
Organics from Biomass:
Carbohydrate
Oxidation Approaches
.....2
Biomass Approaches
.....3
of Non-Oxidative
Methods and Research Opportunities
....4
Catalysis Research Needs ................................ .............................5
II - Progress to Date
Prior Literature
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . **o* . . . . ..*..
*o**oo..
*..
o* . . . . . . . . . . . . ..o
. . . ..o*
. . ..*oo
. . ..o...
o ‘7
................................ ................................................................ ...... 7
Aldose Decarbonylation
................................ ......................................................... 8
of Sugars ................................ ....................................... 9
Catalytic Hydrocracking
Pt and Pd Diolate and Alditolate Complexes ................................ ....................... 10
Catalytic Diol Deoxydehydration
................................ .......................................... 14
III - Opportunities for the Future ....................................................................... 15
Complexation
of Cyclic and Disaccharide
Sugars/
Development
of Catalytic Polyol Disproportionation
Development
of Catalytic Carbohydrate
Development
of Aqueous Organometallic
N-
Summary
N-
Bibliography
Other Metals ....................... 15
................................ ........... 18
Ionic Hydrogenolysis
Carbohydrate
........................... 19
Chemistry
..................22
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
*..
*...
*..
o.*..
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ●..0.
1
24
I - BACKGROUND
Oxygenated
Organics:
A prime
technologies
Problems with Hydrocarbon
target
for
new,
is the production
commodity,
andspecialty
ofoxygenated
repercussions.
recalcitrant
of petroleum
problems,
The following commercial
billion lb/yr nylon precursor, is currently
begins with the hydrogenation
catalytic air-oxidation
this ketone-alcohol
produce
partially recyclable.
example is illustrative.
produced2’3
of carcinogenic
mixture must be further
adipic
benzene
/ cyclohexanol
starting
oxidized
potentially
explosive conditions,
economics
Highlight
but
the
environment
1. While solutions
public
nitric acid to
which
are only
hazardous
waste streams.
almost
always
involves
specially designed reactor
problem.
affect not only process
perception,
are
summarized
in
may come from current
research efforts, other problems are inherent in the approach
strategies
by distillation,
two oxidations,
to some of these problems
we believe that alternative “green chemistry”
followed by
mixture in about 75%
oxides,
issues, which adversely
and
process which
a reduction,
which require expensive,
problematic
Adipic acid, a two
with corrosive
materials
systems to account for this latent safety/environmental
The fundamental
environmental
material
acid, but nitrogen
of organic
are currently
to cyclohexane,
Thus, the overall process involves
the air oxidation
in the fuel,
their utilization
by a multi-step
and a distillation, and generates several environmentally
Furthermore,
synthesis
or natural gas fossil feedstocks.
many with unfavorable
After recycle of unreacted
not only the desired
key compounds
cheap raw materials,
to give a cyclohexanone
yield at 5% conversion.
organic
Most of these products
and oxygen are presently
poses long-standing,
benign,
organics,
chemical markets.1
Approaches
Oxidation
environmentally
derived from the partial air oxidation
While hydrocarbons
AND INTRODUCTION
itself.
For this reason,
merit serious exploration.
DISCLAIMER
This report was prepared as an account of work sponsored
by an agency of the United States Government.
Neither
the United States Government nor any agency thereof, nor
any of their employees, make any warranty, express or
implied, or assumes any legal liability or responsibility for
the accuracy,
completeness,
or usefulness
of any
information, apparatus, product, or process disclosed, or
represents that its use would not infringe privately owned
Reference herein to any specific commercial
rights.
product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by
the United States Government or any agency thereof. The
views and opinions of authors expressed herein do not
necessarily state or reflect those of the United States
Government or any agency thereof.
DISCLAIMER
Portions
of this document
in electronic
produced
document.
image
from
may be illegible
products.
Images
the best available
are
original
Highlight 1. Fundamental
Technological
Issues in Hydrocarbon Oxidation
Issues
The chemical inertness of hydrocarbons leads to high reaction activation
energies, often resulting in poor selectivities due to greater
reactivity of primary oxidation products.
Poor air-oxidation selectivity wastes valuable feedstock.
Other oxidants, while frequently more selective, are much more
expensive or, if produced from oxygen (e.g., R02H), generate
stoichiometric co-products when used.
Oxidations with air or peroxides pose explosion hazards.
●
●
●
●
Soc ietal Issues
Hydrocarbons are a non-renewable, fossil resource.
Crude oil production is fraught with political and environmental concerns.
Transportation of natural gas and petroleum products poses safety and
environmental hazards.
Poor air-oxidation selectivity generates significant toxic waste streams.
●
●
●
●
Oxygenated Organics: Advantages of Non-Oxidative Biomass Approaches
One such alternative
biomass.
Biomass
approach is the production of oxygenated
carbohydrates
and “pre-oxygenated”.
strategies
oxygenated
The objective
oxygen
already
Advantages
societal
issues
now generally
present
are ever-growing
decision-making
change
the overall
economics
dominate
the
equation.
associated
with biomass
oxidation.
or redistribution
rather than its problematic
in Highlight
approach
are particularly
significant,
to governmental
of a process,
there
production,
environmental
such as monocrop
2.
The
as these
and industrial
costs can radically
such that feedstock
are potential
3
regulations
of the
addition.
are summarized
In particular,
While
shift in
approach
contributors
considerations.
to hydrocarbon
the partial removal
in these feedstocks,
of a biomass
This leads to a complete
organics compared
becomes
of this non-oxidative
advantages
from
are not only our most abundant organic feedstock,
but they are renewable
for producing
organics
prices
environmental
plantings
no longer
concerns
and the use of
.
+
fertilizers,
herbicides,
and pesticides,
these issues
should
be more tractable
than
those associated with fossil feedstocks.
Highlight 2. Advantages of Biomass Route to Oxygenated Organics
Technological
●
●
●
Advantaae~
Rich functionality in biomass fosters low reaction activation energies,
permitting mild reaction conditions and hence lower capital and
operating costs.
Hydrogen-bonding characteristics of carbohydrates should be very
helpful in promoting highly selective reactions.
Conversion processes will primarily involve reductive reactions, which
are typically more selective than are oxidative reactions.
I Adv~
●
●
●
Biomass is an abundant, widespread, sustainable resource.
Production and transportation of biomass are relatively safe and
environmentally friendly.
Biomass process solvents and waste streams are likely to be relatively
biocompatible.
Oxygenated Organics from Biomass: Methods and Research Opportunities
There
are
two
major
(Highlight 3), fermentation
traditional
approaches
and high-temperature
of these will be significant contributors
can effectively
sufTer from limitations
on the range of products
homogeneous
increasingly
energy input.
Recently,
transition-metal
utilize
biomass
pyrolysis.4
to future oxygenated
While both approaches
substantial
to
“We believe that both
organics technologies.
raw, multi-component
that can be obtained
we have proposed
catalysis,5-7
a technology
Highlight 3. Biomass Conversion Strategies
●
Fermentation
High-temperature pyrolysis
Homogeneous transition-metal
—
1---*
4
catalysis
biomass, they
and require
a new strategy
significant in the latter part of this century. 2
●
conversion
that
based on
has become
.
.
Our approach
recognizes
transition-metal
catalysts
functionalities
carbohydrates.
converting
that the high selectivity
are
(polyhydroxyl,
carbonyl,
As such, homogeneous
specific carbohydrates,
fermentation,
into valuable
homogeneous
match
and
oxygenated
the
acetal)
catalysis
obtainable
to
of homogeneous
rich
present
compared
to pyrolysis
from sugar-rich
organics.
however, the design of appropriate
of information
organometallic
homogeneous
conversions
feasible using transition-metal
straight
on the interactions
Highlight
biomass
means of
advantages
of a
and lower energy
fermentation.
At present,
systems is strongly inhibited
of native
We believe
catalysts.
in
fragile
crops and biomass
The potential
catalytic conversion
unique basic research opportunity.
types of carbohydrate
and
and
should be an effective
catalysis approach are its greater product versatility
requirements
by a lack
an excellent
and reactivity
carbohydrates
that this situation
presents
4 gives illustrate ve examples
that we have begun
to explore
with
a
of the
that might be
catalysis:
Highlight 4. Potential Metal-Catalyzed Carbohydrate Conversions
●
●
●
●
Hydrocracking to glycerol, ethylene glycol, and methanol.
Deoxydehydration to allylic alcohols, e.g., xylitol to pentadienol.
Disproportionation to unsaturated aldehydes or acids, e.g., glycerol to
acrolei n.
Dehydroxylation to cx,~-diols and derived compounds, e.g. glucose to
adipic acid, erythritol to tetrahydrofuran.
Carbohydrate Catalysis Research Needs
Highlight 5 gives the fundamental
effective attainment of transition-metal
objectives
catalyzed
While our primary focus has been on exploring
of organotransition
advantageous
metal complexes
identified
transformations
and understanding
with carbohydrates,
to model these interactions
as being essential to
of carbohydrates.
the interactions
we have also found it
by studying the simpler case of metal–diol
5
.
.
Highlight 5. Fundamental Metal–Carbohydrate Research Objectives
Synthesis and characterization
of organometallic carbohydrate and
diolate complexes as models for catalytic intermediates.
. Understanding factors affecting complexation stabilities and selectivities.
● Exploring
reactivities of carbohydrate/diolate
complexes.
● Determining
solvent effects on complexation and reaction chemistries.
● Discovering
catalytic cycles to effect carbohydrate/diol redox chemistry.
●
chemistry.s’g
Solvent
issues
are also a significant
concerns here include
simple volubility
solvent
via
coordination
hydrogen-bonding
carbohydrate
donor
interactions,
problems,
atoms
interference
or hydroxyl
and product
redox transformations
secondary
isolation
groups,
focal point.
with catalysis from
effects
problems.
and catalytic intermediates
+Hz
/
r
~1
HO
+H2 / –H20
HO
I
R
LmM’H
+H2
R
2 RCH20H
RH2C-CH2R
For sugars, R = CH20H, etc.
6
of solvent
Some
of the
that we envision
and will describe later include the following:
LmM’H
The
.
II - PROGRESS TO DATE
Prior Literature
One of the first reported homogeneously
catalyzed
was by Kruse of ICI in 1976, the RuHCl(PPhs
and ketose sugars to the corresponding
reactions
)3 mediated
hydrogenation
~0
CHZOH
c
o
0“:,~’owfip:t:
H
OHH—
HO
H
OH
H
H
OH
HO
H
H
*
*
RUHCL3
OH
transfer
example
Prior to our studies,
hydrogenation
of
an
CGHA(P*P~e)2)Pt(CsHsOs),19
glycerol.
is capable
derived
OH
from the simplest
is observed
was observed:
carbonyl
of catalytic
of sugars, 13-1* and one
complex,
possible
in the formation
in an asymmetric
present in
reports
carbohydrate
of glycerol poses minimal regioselectivity
study, only 1,2-glycerol complexation
H
$
the “latent”
related
reactions
organometallic
1,3-) compared to up to 15 possibilities
OH
aldehydo-glucose
of hydrogenating
there were also several
While diastereoselectivity
complexation
of open-chain
and disproportionation
isolated
H
GLUCITOL
Note that, despite the small proportion
group.
H
CHZOH
GLUCOSE
solution, 12 the metal catalyst
OH
HO
H2
A
Kruse(ICI)
of aldose
alditols in amide solvents: I“’ll
H,
H
of carbohydrates
carbohydrate,
of this complex,
considerations
C fj alditol.
(1,2-
(only 1,2- vs
In the reported
.
Aldose Decarbonylation
Our first work in metal-mediated
approach
to carbohydrate
“descent
carbohydrate
aldose sugars (e.g., glucose, R = H), by Wilkinson’s
H.
H
catalyst, Rh(PPhJsCl:
O
c+
of
21’22
$H20H
CH20H
OH
+
involved a novel
chemistry, 20 the decarbonylation
of series”
7H20H
chemistry
+
tH20H
CH20H
o
HO
~=
u
OH
NO
Solvent
=
Q
OH
While the reaction
oxygenated
solvents,
0.01%)
the study confirmed
organometallic
metal reagent
(<
of open-chain,
aldehyde
convenient
and economical
of alditols
(sugar
disaccharides
to the production
1 that amide
are an effective medium
of native carbohydrates.
to selectively
of
for
Again, the
react with the tiny fraction
form of the sugar present
without
are clean and predictable,
resort
to
offering a
synthesis
alcohols)
of authentic reference samples of certain types
,
for comparison with natural produ~ts. For example,
can be prepared
(as illustrated
a multi-step
(NMP),
groups. 23 The reactions
protecting
glycosylpentitols
ability
suited
Kruse’s early observation
transformations
has the notable
temporary
constitute
nor particularly
such as lV-methyl-2-pyrrolidinone
conducting
(NMP)
Andrews,Klaeren& Goul(
is not catalytic
organics,
RhCl(CO)L2
~H3
in a simple
above
synthetic
for lactose),
challenge
8
single step from readily available
a process
with generation
that would otherwise
of concomitant
waste
.
*
With
streams.
products,
ketose
including
furandimethanol
fi-uctose.6
resistant
sugars,
when
the Kruse
FDM is a difunctional
polymers,
increasingly
hydrogenation
aromatic
monomer
and is also a precursor
important
decarbonylation
and dehydration
of furans. 24 We have also observed
a variety
(FDM)
we observed
system
formation
of
is applied
to
used to manufacture
to tetrahydrofurandimethanol,
flamean
diol.25
Catalytic Hydrocracking of Sugars
Our second major study also involved
carbonyl
group present
in monosaccharide
aldose and ketose carbon–carbon
by H2Ru(PPhs)A
hydrogenation,
chemistry
sugars.
via a combination
H
resulting in the formation
$
OH
Hz (300 psi)
100 “C, 24 h
NMP
-----
2H
-----
+
OH
and carbonyl group
CH20H
HO
+
CH’20H
-----
-----
J-
------
------
of fmctose
are unreacted
H
H
OH
H
OH
t
Glycerol (15%)
CH’20H
--
0
ORUH ~
HO 2
+
species present
hydrocracked
of lower polyols: 5J6
CH20H
HO
The hydrocracking
that
\HOH
RUH2L4 (2%)
CHZOH
-----
reaction
Hexitols(64%)
H
OH
This work demonstrated
of a retro-aldol
0
H
upon the latent
single bonds could be catalytically
CH20H
HO
dependent
/ RUH2
is very selective for glycerol; the only other significant
fructose
and the two hexitols
9
derived from simple
.
hydrogenation
accelerates
of the fructose carbonyl group. Addition of a basic co-catalyst
the reaction and increases the selectivity for hydrocracking
hydrogenation
selectivities
formation
(3 l% yield ofglycerol).
are then somewhat
previously
selectivity
propanediol).
is potentially
heterogeneous
(no formation
and enhanced
Key findings of these studies
occurs under much milder conditions (100 ‘C/
patented
over simple
the mass balance and cracking
reduced, due to sugar degradation
of Cz, Cd and C5 fragments.
transformation
greater
Unfortunately
(KOH)
are that the
300 psi Hz) than with
catalysts (2OO ‘C / 2000 psi Hz )26-34 and with
of partially
deoxygenated
products,
An improved version of this chemistry would be desirable
a good precursor to a variety of oxygenated
e.g.,
1,.2-
as glycerol
organics.
Pt and Pd Diolate and Alditolate Complexes
Though it is clearly possible to do catalytic chemistry involving
carbonyl
groups,
polyhydroxylic
polysaccharides
masked
the
compounds
characteristic
rather
Building
functionality
is the diol unit.
such as starch
as an acetal,
monosaccharides.
most
and cellulose
than
This
on the brief
present
grouping
literature
in
is found
where the carbonyl
as the equilibrating
carbohydrate
group
hemiacetal
reports
these
even
in
is fully
found
in
of (Lz)Pt(diolate)
complexes, 19’35 we have now made an extensive study of bis(phosphine)
platinum(H)
diolate and sugar alcohol complexes. 8’9 The key to this entire study was provided by
the development
bis(phosphine)
of a new method for synthesizing
diolate complexes,
reaction of a
platinum carbonate, L2Pt(COs),36 with the diol or alciitol:
10
This reaction
is nearly
determinationof
thermoneutral,
relative complexation
on the diol and ancillary
different
a similar
phosphines
ligand.
For different
reactions
ofthesubstituents
1,2-diols
(dppp), these cover arangeof
with ethane-l,2-diol
,therelative
allow
with Lz =
almost
complexation
100.
constants
For
cover
The total range, from (1,2 -bis(dicyclophexylphosphino)ethane)-
range.
Pt(pinacolate)
at
the
low
Pt(phenylethane-l,2-diolate)at
cases, electron-donating
binding
exchange
constants as afunction
phosphine
l,3-bis(diphenylphosphino)propane
hence various
constants.
end
to
(cis-1,2-bis(diphenylphosphino)ethene)-
the high end, is estimated
tobe
groups on both the diol and phosphine
Complexationof
l,3-diols
over 10s.
In all
lead to lower diol
is about afactorof100–1000
lower
than that of 1,2-diols.
With
alditols,
complexation
regioselectivities
favoring
to
isomeric
occur
via
are observed,
coordination
complexes
an
a, ~-diol
are
possible,
linkage.
which vary strongly
In
even
after
practice,
with alditol
11
significant
stereochemistry,
to internal threo diol units (e.g., 2,3-galactitol,
over erythro and terminal diol units (3,4-galactitol,
requiring
3,4-mannitol)
2,3- and 1,2-mannitol):
.
—
CH20H
H+C)H~
::
H
~
83%
17%
2.3-Isomer
~p
@El
/0
/p(\
o
[
/p\
1
=
Ph
cH@H~11%
H
89V0~
470 ~
OH
HO
H
HO
H
H
t
Pyridine
+
time
interactions
scale
HOH2C
EZl
R2
J
‘Ph
1,2-Isomer
2,3-Isomer
17%~
~
18%
3,4-Isomer
82%~
H
‘: $
HOH2C
Andrews& VOSS
~
on the
NMR
time
scale.
~h
:
~
5%
is fast on the
Hydrogen-bonding
to these regioselectivities,as
Hs.
, .O..
3%
Pyridine
structure of (dppp)Pt(3,4-mannitolate
~h
~
“r
not kinetic, as isomerization
contributors
the x-ray diffraction determined
@zml
HO
t_cD,a2_J
but slow
are important
HOH2C
1%~
71~0
These ratios are thermodynamic,
laboratory
RI
~
‘H
CH20H
1670
. ..
“+OH
]
~
\
cH@H
7% ~
86-
\/
~
H+OH
Ph
Ph
OH
+
HoH2y
1,2-Isomer 14% ~
illustrated
by
):9
?
OH
,,,,\\i
)
‘\?’
OH
/ptJ
<
/!
‘h
Ph
\
>J
] .0
Ho”
Detailed
studies
show that the hydrogen-bond
platinum
diolate oxygens is comparable
The intramolecular
are thus sufficiently
hydrogen-bonding
acceptor
(HBA)
to that of the strongest
interactions
strong that they are retained
12
strength
neutral
in the Pt alditolate
of the
13BA’s.~7
complexes
to a large extent even in neat
HBA solvents
such as pyridine,
leading
to only minor changes
in complexation
isomer ratios as a fi.mction of solvent (cf. comparisons in above Figure).
While
thermally,
the (L2)Pt
diolate
they undergo
ketones
can bereduced
starting diolate ii-omthe
reaction.
fragments
are run in the presence
oraldehydes
are remarkably
oxidative-cleavage
of hydrogen
and catalytic
tothe corresponding
When the
HzRu(PPhs)A,
alcohol.
Synthesis
be integrated
Thus it is possible to selectively
the
of the
into the overall
then corresponds to hydrocracking
single bond.
stable
of the diol C–C!
and a reactive (L2 )PtO species.g
platinum carbonate canalso
The net transformation
diol carbon–carbon
complexes
facile photochemical
bond to give two carbonyl
photolyses
and alditolate
of the starting
convert mannitol
into glycerol: 6
OH
$
HOit I
While this reaction
carbohydrate
carbohydrate
is clearly not of practical utility, the results demonstrate
complexation
substrates,
palladium
K. S. Koenig)
and alditols
selectivity,
facilitated
carbohydrate
were disappointing,
bind about
chemistry.
a factor
of 100 times poorer
it is very sensitive to decomposition
oxidative-cleavage
Initial results (S. K. Mandal
however, as competition
been isolated,
symmetry
by the rich functionality
of
active metal than Pt(II), we have
(L2 )Pt(II) center.
Hoffiann
that
can be converted into reaction selectivity.
corresponding
thermal
OH
NMP
Since Pd(II) is a much more catalytically
investigated
OH
0.1 HzRu(PPhs)4
‘“OH
HO
E
~2
1.0 (dppe)Pt(COs)
OH
HO
OH
Hz,hv
Furthermore,
studies show that diols
to (L2 )Pd(II)
13
than to the
while (dppp)Pd(3,4-mannitolate)
has
by water and shows no sign of
of the central C–C bond, possibly
constraints. 8
and
due to Woodward
-
Catalytic Diol Deoxydehydration
Since Pt(II) and Pd(II) diolate complexes
we looked elsewhere
activity,
transformations.
for possible
exhibited
systems
Based on several stoichiometric
and Gable39 in conjunction
designed and implemented
with their studies
no signs of thermal redox
to effect catalytic
reactions reported by Herrmann38
of alkene
oxidation,
we have now
the catalytic cycle shown to the left below:7
HO
OH
)--’
Ph
-2~0
Cp*Re03
*
~hF
90“c
+
+
PPh~
O=PPh3
ttt
50,
t“
II
4.2 “~.O.I h
.,. r
6
R,
mdrews & Cook
For the simple diol phenyl- l,2-ethanediol,
well in solvents such as benzene
sacrificial
equal
to
reductant
that
Cp*ReO(phenylethanediolate),40
by
4
The initial
Gable
for
suggesting
A
i- 7
‘/—\
8
Time (h)
12
using triphenylphosphine
rate of the reaction
alkene
that extrusion
extrusion
16
A primary
14
as a
is essentially
from
is the rate-limiting
In donor solvents, such as tetrahydrofuran
the catalyst dies after a few turnovers.
,“
1
the reaction to give styrene proceeds very
or chlorobenzene
(above right).
observed
in the catalytic cycle.
carbohydrate
pure
step
(T13F) and NMP,
cause of this has been identified
(over-reduction
occurring
of the catalyst to a Re(III) species)
have been discovered
(use of an acid co-catalyst
reducing
agent).
protected
sugar l,2:5,6-diisopropylidenemannitol
The reaction is stereospecific
Alditols are also deoxydehydrated,
erythritol
product butadiene
as well.
to give a thiocarbonate,
give
phosphine
the alkene,
substrates,
to the corresponding
sulfide,
This technique
of the
trans alkene.
,4-diol,
but the fully
is at least as good as most
the typical approach
being reaction
which is then heated with a phosphine to
and carbon
the reaction shows promise
sacrificial
allyl alcohol and the C4 alditol
and cis -2-buten-l
current methods for converting diols to alkenes,
with thiophosgene
or a weaker
this from
as shown by the conversion
glycerol yielding
giving not only 3-buten- 1,2-diol
deoxygenated
and ways to prevent
dioxide.41
For carbohydrate
of selectively
converting
specific
groups into another reactive but readily differentiated
functional
group, an alkene,
or more specifically,
the
need
an allyl alcohol.
for inefficient,
carbohydrate
syntheses.
environmentally
generating
Future
implementations
Improvements
could significantly
temporary
benign could potentially be developed
the sacrificial reductant.
to biomass-derived
waste
This methodology
hydroxyl
protecting
that
would
reduce
groups
be even
in
more
based on carbon monoxide as
of this sort might then lead to viable routes
commodity oxygenated
organics as well fine chemicals.
Ill – OPPORTUNITIES FOR THE FUTURE
Complexation of Cyclic and Disaccharide Sugars/ Other Metals
A key extension
preparation
of our previous
of compounds
(L2)PtII
alditol complexation
derived from cyclic and disaccharide
more typical of primary biomass
sugars such as glucose,
well as being better models for the biomass polymers
also more complex and will provide a demanding
15
studies is the
sugars,
fructose,
which are
and sucrose, as
starch and cellulose.
test of complexation
They are
stereo- and
.
regio-selectivity
since the a- and ~ -anomers
forms are in equilibrium
via the open-chain
of both pyranose
7H20H
~-Pyranose
HO
H
H$
~
CH20H
/
HO 1
a-Pyranose
OH
H+OH
OH
0>0
HOT
aldehydoGlucose
HO
fl-Furanose
complexation
model
studies
A somewhat
we have examined
methyl- cx-mamopyranoside.
is four times
isomer is essentially
Based
unreactive,
probable
that
(Z. H. Shriver), internal cis/trans competition
suggesting
not counting
observed
trends,
sugars.
of the cis
For underivatized
the four from the open-chain
the higher
acidity
CG
form.
of the anomeric
involving this site. It is also
to the metal will significantly
16
should be
twelve possible isomeric a, ~-diolate
hydroxyl protons should favor complexation
that complexation
complexation
that very high selectivities
in furanose
there are theoretically
that can be formed,
1,2-diol,
in
than for cis -cyclohexane - 1,2-diol, while the trans
for cis vs trans coordination
on our previously
hemiacetal
show
smaller ratio (ea. 2:1) is observed for the one
For cyclopentane-
stronger
sugars such as glucose,
complexes
with cis - and trans-cycloalkanediols
of the cis isomer is favored over the trans isomer by a factor of seven
sugar prototype
achievable
OH
OH
a-Furanose
‘H
for cyclohexane- 1,2-diol.9
isomer
0>0
b
e
Our prior
ring
form of the sugar:
yH20H
HO
and furanose
alter the equilibrium
ratios of the sugar isomers present,
the pyranose
form by preferential
cyclic dihydroxyethyl
side-chain
The common
for example,
increasing
the furanose form over
binding to a cis-furanose
diol unit or to the exo -
in a C (j furanose sugar.
disaccharide
sucrose will actually be a much simpler case since
its carbonyl group is masked as a full acetal and only the two trans- (x,&diol units in
the glucopyranose
the
fragment
fructofuranose
experimentally
ring
would be expected
presumably
observed
polysaccharides
cellulose
a 2,3-trans-diol
in
ct,~-Diol
(M.
A.
OH
o
O>OH
products
The
Andrews).
are
common
Units in Sucrose, Starch and Cellulose
~
HO
HOH2C
ratio
Two
unreactive.
unit.
HOH2C
O.O
being
the trans-diol unit in
and starch have only one unique U,&diol unit to complex,
glucopyranose
~
a 3:1
to coordinate,
~
I
2 trans-Pyranose
1 m.wzs-Furanose
~os”)
n
CH20H
w
1 ?rans-Pyranose / Repeat Unit
OH
Another
reactivity
characteristics
particularly
periodic
essential
those
greatly
to participate
polarization
with
the carbohydrate
the complexation
other
metals
besides
and
Ptll,
and those from other parts of the
coordination
chemistry
of early,
metals should be quite different from that of late metals with
to date.
reducing
is determining
that are more redox active
oxophilic transition
expected
objective
of carbohydrates
In particular,
table.
have studied
future
Lone pairs
on the coordinated
in n-backbonding
their hydrogen-bond
of the metal–oxygen
diolate
oxygens would be
to the electron-deficient
acceptor
ability.
This, and the decreased
bond, should significantly
17
metal center,
alter the coordination
selectivities.
Results from both studies with more biomass relevant
and with other metals will be helpful in predicting
catalytic reaction selectivities
carbohydrates
and understanding
the kinds of
that may be achievable with native sugar substrates.
Development of Catalytic Polyol Disproportionation
We have intriguing
may be possible
external
to effect catalytic
reducing
phenylethanediol
agent
a disproportionation
initiators
is the product
and 1,2-phenylethanediol
is not consumed.
Similarly,
as substrate
conversion
that it
without
addition
of an
reaction.
Thus
1,2-
itself,
of only
amounts.
hence
we conclude
using ct-methylstyrene
proceed via diol disproportionation,
diol product(s).
to l-hexene
in the
of alkene
development
though we have not yet identified
disproportionation
stoichimetrically
reaction
of
seems
the
following
potentially
type
attainable
the oxidized
of
catalytic
and
is
-“
certainly
+2“20
L =+%x”
02
OH
Proposed
18
about
polyol
achievable with a suitable catalyst:
‘07-0” -
From
from the diol must
While there is obviously still much that we don’t understand
reactions,
as
show that the a-methylstyrene
of 1,2-hexanediol
that the formation
One
the reaction
of styrene as an initiator proceeds without oxidation of the styrene.
these experiments,
these
styrene
Cross-over experiments
auto catalytic behavior.
the initiator
diol deoxydehydration
suggest
and a suitable “initiator”, also present in catalytic
of the most effective
presence
via
results (G. K. Cook) which
is converted to styrene in about 60% yield in the presence
catalytic Cp*Re03
exhibits
preliminary
This chemistry
could
be used to produce
carboxylic acids from biomass feedstocks,
esters or from the hydrocracking
valuable
unsaturated
aldehydes
and
the glycerol coming from either fatty acid
of fructose or other carbohydrates.
Development of Catalytic Carbohydrate Ionic Hydrogenolysis
Our colleagues
hydrogenation
chemistry
number of organic
acetals,45
net removal
National
that accomplishes
substrates,
and alcohols,4G
(C&0JW(CO)3H
alcohol,
at Brookhaven
ranging
utilizing
of oxygen,
loss of water,
presumably
hydrogenation
of a
from alkenes42 and alkynes43 to ketones, 44
hydride
For alcohols,
transfer
and
a strong
acid,
this reaction accomplishes
by a mechanism
and hydride
generate the hydrocarbon
the stoichiometric
a metal
+ triflic acid (HOTf).
have developed an ionic
Laboratory
e.g.
the
involving
protonation
of the
to the resulting
carbenium
ion to
product:
1.2 CF3S03H
OH
+
Cp(CO)3WH
~
22 ‘C, 5 rninuws
+
[i,,]
-*P,::)3W,0TD
Bullock & .%ng
The qualitative
ions.
reaction
rates reflect the stabilities
Thus, deoxygenation
room temperature.
temperature,
of tertiary
Deoxygenation
alcohols
of secondary
of the intermediate
carbenium
(above) occurs within minutes
alcohols requires
at
hours at room
while most primary alcohols are inert under these conditions.
Based on these results and other organometallic
of these reactions are very plausible,
as illustrated
19
literature,
for a diol:
catalytic versions
OH
@
LnM
H2
Y
,H
R
<
‘H
OH
Proton
Transfer
Oxidative
Addition
H20
Y
LnM@
OH
4
\
R
\
R
Proposed
There are a large number of cationic dihydrides / molecular
dihydrogen
now known,47-49 many of which have sufficient acidity to protonate
as diols.
The resulting intermediate
by lone-pair
Subsequent
charged internal carbon, leading
alcohol that should be stable towards further reduction.
this reactivity
substrates
hydride attack
to a primary
With polyols as substrates,
would lead to cx,co-diols, which are valuable
compounds
in their own
right or they may be converted to other products such as tetrahydrofuran
C4 alditol
readily
erythritol)
available
synthesis
such
carbenium ion in this case would be stabilized
donation from the adjacent hydroxyl group.
should occur at the more positively
complexes
or adipic acid (from CG alditols,
from the hydrogenation
of glucose
e.g., sorbitol
and/or
and mannitol,
fructose).
of adipic acid would clearly be a much more environmentally
(from the
Such a
benign route
to this product than the current route described in the introduction.
A number
of problems
develop this chemistry.
will need to be addressed
by experimentation
to
The first is to show that diols and polyols will undergo this
20
type of ionic hydrogenation.,
phenylethane-
Preliminary
experiments
1,2-diol is reduced by (C5H5)W(CO)3H
(N. M. Brunkan)
+ HOTf in dichloromethane
solution
at room temperature
to give the expected 2-phenylethanol
product
(via the intermediate
complex [Cp(CO)s(HOCH2CH
epoxide,
reduced
a more
direct route to the proposed
to 2-phenylethanol
such as 1,2-hexanediol,
the reaction
stopped
n-propanol,
the rate
2Ph)]+[OTfJ-).
epoxide
diol stage.
intermediate,
Propylene
inhibiting
step.
leveling
The
second
problem
that will dissolve sugars.
of magnitude
experiments
indicate
more
acidic
that the problem
than
The problem
pose a problem.
binds too strongly
coordinating
summary,
here is to avoid
A good
has a pKa = –6, ca.
alcohols.
Here
test
of the diol or polyol.
of the metal dihydride,
could also
apparently
because the triflate
counterion
to the vacant metal site required for effective oxidative-addition
A potential
counterion
solution
disappointing,
of parameter
to this problem
is to use a much more weakly
such as BAr’A– (Ar’ = 3,5-bis(trifluoromethyl)phenyl)).50
while the preliminary
here are somewhat
wide range
ionic
Thus, initial attempts (Song and Bullock) to react metal triflates
with Hz have not been very successful,
of hydrogen.
protonated
to
diol is
the
of the solvent.
will be esterification
The final step of the catalytic cycle, reformation
that
oxide was reduced
will be to conduct
of the acidity of the acid by protonation
is
Simple diols,
further indicating that dissociation of water from the protonated
orders
associated
Styrene
NMR spectra suggesting
choice might be acetic acid, whose conjugate acid [CH3C(OH)2]+
four
as the primary
more quickly under the same conditions.
at the protonated
hydrogenation in solvents
excessive
pronated
were not reduced, however,
show that
In
results for ionic hydrogenation of diols discussed
they represent very limited explorations
space available,
e.g., with respect
of the
to the metal and its
ligands which together are known to have a strong effect on the hydricity
of the metal hydride (a range of 10s in rates of reaction with trityl cation).5 I
21
Development of Aqueous Organometallic Carbohydrate Chemistry
Given the high water volubility
low cost, the development
is a desirable
water-soluble
chemistry
benign
carbohydrate
propylene
transformations
has traditionally
associated
to an aqueous
of a carbohydrate
competition
problems
medium.
hydroxyl
done in organic
but whether
with water
this
While
solvents,
For catalytic
problems
cycles based on
however,
complexation
the same functionality.
diol functionality
is sufficient
for binding
55 For
of sugars, such
we see no inherent
functionality,
could arise, as water contains
over water,
applications.
has been commercialized.
hydrocracking,
chelate effect operative with a carbohydrate
advantage
been
catalysis systems
that rely on the carbonyl functionality
mediated
coordination
synthesis
and
52-54and one aqueous process utilizing
hydroformylation,
as our fructose retro-aldol
with conversion
biocompatibility
homogeneous
organic
complexes are now well-known
them as catalysts,
and water’s
of aqueous carbohydrate
goal for industrially
most organometallic
of sugars
The
offers some entropic
for the carbohydrate
successfully
compete
to the metal
center
determined.
For those systems for which water proves detrimental,
remains
to
to be
there are still a
number of solvents, such as 2V-methyl-2-pyrrolidinone
(NMP) and dimethylsulfoxide
(DMSO), which are well-suited to metal-carbohydrate
chemistry
reasonably
attractive under today’s health and environmental
Some hydroxyl reactions,
not depend on coordination.
carbonylation
substrate,
system
however,
Recently an aqueous, homogeneously
hydroxymethylfurfural
and applied
hydrogenolysis,
conditions,
to a carbohydrate-derived
5 -formylfuan-
the catalyst is also capable of
the alcohol group in HMF, giving 5-methylfurfural:
22
do
catalyzed alcohol
(HMF) to give the new compound
2-acetic acid. 56 Under different reaction
deoxygenating
standards.
such as ionic alcohol
has been reported
and are considered
.
H+OH
LH20H
Pdll J P(C~H4S03- )3
+Co
H+/H20
Fructose
A
‘“’CGCH”
‘3CT5’CH
;heldon
This
latter
carbonylative
hydrogenolysis
c02
reduction
as amethod
chemistry
ofdehydroxylating
may be an alternative
to ionic
carbohydrates.
IV - SUMMARY
There
United
is a critical
States
chemical
industry
to zero-waste
produce.
Carbohydrates
processes
might
biomass
catalytic
as legislative
and cradle-to-grave
replace
however,
with
concerted
catalyzed
reactions
chemistry
technology.
responsibility
economical,4
research
Oxygenated
pressures
resource,
which
the
the selective
effectively
in the
push the
they
for some
conversion
synthesis
of
of a
if appropriate
organics, found in avariety
are particularly
efforts,
processes
for the products
While
could compete
systems can be found.
such as nylon and polyester,
that
and economic
fossil feedstocks.
is still not generally
conversion
friendly
represent a plentiful, renewable
or fine chemical,
ofproducts
believe
industry
economically
to~uels,
commodity
need for new environmentally
attractive
homogeneous
targets.57
transition
We
metal
could play a significant role in bringing about this future green
23
.
,-
IV - BIBLIOGRAPHY
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Acknowledgments.
major contributions,
The author would like to thank his co-workers for their
experimental
Chapter:
Nicole
Brunkan,
Klooster,
Kristina
Koenig,
research
was carried
AC02-76CHOO016
Division of Chemical
Gerald
Santosh
and intellectual,
Cook, George
Mandal,
out at Brookhaven
with
the U.S.
to the studies discussed in this
Klaeren,
Wim
Zachary Shriver, and Eric Voss.
This
National
Department
Gould,
Stephen
Laboratory
of Energy
and supported
Sciences, OffIce of Basic Energy Research.
27
under contract DEby its