No. 36
.t8l
)
tra= 2
d in
Tetrahedron Letters No. 36, PP 3393 - 3496·
©Pergamon Press Ltd. 1978. P.rinted in Great Britain.
THE 1-BICYCLO[
I. I. I
SlOD
op.on
le
Erlangen-Nlirnberg, 8520 Erlangen
University, Pittsburgh, Pa. 15213, USA
Recognition of the equivalence of certain transition metal fragments and groups
comprised of first row atoms and hydrogen 1 casts new light onto old problerr.s.
For example, Fe(C0) and CH+ (or BH) are isolobal; 2 both contribute two vacant
3
orbitals and a pair of electrons to structures in which they participate. 1
Hence, analogous geometrical arrangements are anticipated: I 3 vs II 4 ' 5 and
III 2 vs IV 5 .
Fe(C0) 3
-, ,. '
,, ' ..
~
'I
st
Universit~t
Federal Republic of Germany and the Department of Chemistry, Carnegie-Mellon
to a
i-F
)PENTYL CATION AS A CH+• TRIMETHYLENEMETHANE C0!1PLEX
Institut flir Organische Chemie,
h we
ectro-
-----~g;
Jayaraman Chandrasekhar, Paul von Rague Schleyer,* and H. Bern~ard Schlegel
(d) (I • 3) ;
in' the
,mistry
0040-4039/78/0901-3393~2.00/0
1
,r
\
I
""
\
',
R
r,=----;--:
I
\
P:'
'
I
t.'~- -- --- -_\,'
I
R
m
I
R
Since several trimethylenemethane fe(C0)
complexes (e.g. V) have been
3
characterized, 6 we may well expect CH+ to form stable complex VI with trimethylenemethane.7 The compatibility of orbitals between the CH+ and the C H
4 5
fragments is quite like those describing the electronic structures of II 4 and
8
5
IV. The six interstitial electrons in II, IV, and VIa fill strongly stabilized
~and~
orbitals, and the CH binding indicated by dotted lines in II, IV, and
VIa results. VIa may be recognized as a "nonclassical" form of the 1-bicyclo[l.l.l)pentyl cation (VIb).
1965.
3
, f.eiCOJ
,,
,
505.
I
,
,,'
':',' '
I
6
1
' '
~
+
~b
This cation has been studied by Wiberg as a putative solvolysis intermediate. 9
3393
No. 36
3394
For a highly strained bridgehead system, 1-bicyclo[
I .I .I
)pentyl chloride reacts
remarkably rapidly, three times faster (not many powers of ten slower) 10 than tbutyl chloride. However, the products, 3-methylenecyclobutyl derivatives formed
via VII, are exclusively ring opened. This suggests that relief of ring strain
concerted with ionization might be responsible for the enhanced rate, rather
than any special stability of VI. In addition, a recent study showed that the
spiropentyl cation VIII formed VII without the intervention of VI. 7
.d
H"
+~---1_
v~
w
We have studied the nature of the 1-bicyclo[ I .I .I )pentyl cation (VI) theoretically,
using both MIND0/3 semi-empirical 11 and RHF/ST0-3G ab initio methods. 12
At both these levels, VI is a local mlnlmum on the
c 5 H7 +
potential energy
surface. With MIND0/3 the f v structure VI was obtained by Davidon-Fletcher3
Powell optimization without-symmetry constraints. At ST0-3G, evaluation of the
'
pertinent force constants using analytically
calculated gra d.lents 13 showed the
f 3 v structure rigorously to be an energy minimum. However, the MIND0/3 barrier
for conversion of VI to VII along the symmetry allowed fs reaction coordinate
was only 1.8 kcal/mole. Such a low value is consistent with the observation of
ring opened products during solvolysis, but suggests that this opening is not
concerted with ionization.
The ST0-3G optimized structure of VI (Table) supports its description as a CH+
trimethylenemethane complex (VIa). In comparison with the ST0-3G structure of
14
the parent hydrocarbon, bicyclo[ I .I .I )pentane (IX, Tablel,
the c c bonds in
1 2
VI are lengthened, and the c c bonds are shortened significantly. Noteworthy lS
2 3
the flattening which takes place in the cation. The c c distance in VIII is
1 3
calculated to be 1.890A, the shortest nonbonded C-C separation known; that these
bridgehead carbons are strongly repulsive is indicated by their strongly
c 1 c 3 shortens to
1.640A, but the overlap population (-0.092) indicates that the antibonding
negative overlap population (-0.279) (Table). In cation VI,
character of the interaction is largely eliminated. This confirms a speculation
of Wiberg that reduction of the. nonbonded repulsion between bridgehead atoms
upon ionization might contribute to the observed rate enhancement. 8
The non-involvement of VI in the rearrangement of the spiropentyl cation VIII to
VII can be attributed to orbital symmetry prohibitions. Following Pearson, 15 the
bonds broken and formed during the conversion of VIII to VI can be classified ln
terms of the fs symmetry maintained during the reaction. The bonds broken
No. 36
3395
eacts
(indicated in VIII) transform as a' + a' while the bonds formed transform as a'
han t-
+a''
(see VIb). Thus the process is symmetry forbidden.
Jrmed
Besides II, IV, and IV, a number of carbocations can be equated to isolobal
transition metal equivalents, but this is not always successful. 16 further
he
reports will be presented subsequently.
Acknowledgements
We thank Professor John A. Pople for his encouragement. At Erlangen, the fonds
der Chemischen Industrie and at Pittsburgh, the National Science Foundation
provided financial support. This work was facilitated by a NATO Grant.
tically,
Table. Calculated Structures and Energies of VI and IXa
the
MIND0/3b
RHF/ST0-:3Gc
MIND0/3b
RHf/ST0-3Gc
1.497 (0.998)
1.507 (0.671)
1.549 (0.952)
1. 55 2 (0.677)
1. 59 3 (0. 86 7)
1. 59 7 (0.581)
1.549 (0.952)
1. 55 2 (0.677)
1. 58 3 (0.331)
1.640 (-0.092) 1. 86 2 (0.028)
1.890 (-0.279)
1. 093
1.090 (0.548)
1.107 (0.907)
1.088 (0.752)
(0.698)
1.112 (0.952)
1. 087
the
ier
r
te
of
r
Jt
r
r
c1c2
c2c3
c1c3
C3H4
(0.873)
ln
lY
lS
:hese
lon
1.106 (0.945)
1. 088
<C1C2H56
151.9
150.8
143.1
142.5
<H5C2H6
108.5
115. 7
104.3
111.1
Energy
253.4d
r
C2H5
-190.75749e
(-192.72309)e,f
I
to
the
d in
58.8d
(0.770)
-191.61541e
(-193.60941)e,f
a Bond lengths in~' angles in degrees. b Bond indexes in parentheses.
c Mulliken overlap populations in parentheses. d 6H~ in kcal/mole. e Total
energy in Hartrees.
geometry.
f
Energy using the 4-31G basis with ST0-3G
opti~ized
3396
No. 36
References
1.
K. Wade, Adv.
2.
11. Elian and R.
11. 11.
L.
Inorg.
3.
F. Emerson, L. Watts, and R.
R. E. Williams, Inorg.
5.
6.
Hoffmann, J. Am.
Chern.
~Q,
Soc.,
AlrnenningeJ,, A.
(1959); D.
C. Andrewc; and G.
C. Andrews, G.
Magyar and C.
J.
8.
J.
11.
B,
210
8~,
(1975); M.
355
Hoffmann,
Pettit, J. Am.
Chern.,
(19Ci9); A.
7.
10.
(1975).
1R61
J.
P.
Lillya, J.
Gajewski and 11. J.
Davidson, J.
114S
Soc.,
~7,
132
P.
Org.
(197
).
(1%5).
Stohrer and
413
(1975).
Inorg.
(1968);
\'iahl, Acta Chern.
Organomctal.
Duce, ibid.,
Organometal.
Chang, J.
12,
(1972).
Haaland, and K.
Davidson, and D. A.
Elian,
ibid.,
Chern.
(1971); W.
H. Hogeveen and P. \'J. Kwant, Ace. Cherr.. Res.,§,
M. R. Cl1urchill and K. Gold, Chern. Cornmun., 693
D.
9.
1
Chen, D. M.P. 11ingos, and R.
4.
R.
~§,
Radiochem.,
Hoffmann, Inorg. Chern.,
H~·
Chern.,
Chel'l.,
~7,
~~'
99
755
~~.
Chem.,
95
Chern.,§,
Scand.,
~~'
393
(197S); E.
401
1145
(l'll?);
S.
(1976).
(1978).
B. Collins and P. v.R. Schleyer, lnorg. Chem., ~~' 152 (1977).
K. B. Wiberg and V. Z. Williams,,Jr., J. Am. Chern. Soc.,§~, 33'13 (1967).
R. c. Bingham and P. v.R. Schleyer, J. Am. Chem. Soc.,~~' 3189 (1971).
R. c. Bingham, 11. J. S. Dewar, and D. II. Lo, J. An,. Chem. Soc., ~7, l?S~
( 19 7 5).
12. The Gaussian 70 series of prosrams were used:
R.
Ditchfield, 11.
D.
Newton, and J. A.
Chemistry Program Exchange,
\'1.
J.
Hehre, VI. A.
Pople, Program )Jo.
Indiana University, Bloomington,
Chern.
Ind.
B.
Schlegel, S. \•lolfe, and f.
14. 11.
D.
Newton in "Applications of Electronic Structure Theory", Ec. H.
Schaefer III, Plenum Press, New York, 19'17, Chapt.
15.
R.
G.
Pearson,
Phys.,
~~'
H.
13.
Bernardi, J.
Lathan,
236, Quantum
5, p.
T.
(197cJ).
E'.
223.
"Symmetry Rules for Cherr:ical Reactions", Wtley-Interscience,
New York, 1975.
15.
3632
Clark and P. v.R.
Schleyer, Nouveau J.
Chern.,
submitted.
(Received in UK 26 June 1978; accepted for publication 7 July 1978)