Fundamental electronic proper9es of Carbenes Carbenes: neutral compounds featuring a divalent carbon atom with only six electrons on its valence shell -­‐ Four electronic configura4ons -­‐ Ground-­‐spin mul4plicity: dictates the reac9vity of the carbenes Singlet carbenes: ambiphilic character, in principle Triplet carbenes: generally regarded as diradicals -­‐ Ground-­‐spin mul4plicity is determined by the rela4ve energy of the σ and pπ orbitals -­‐ The nature of the Ccarbene subs9tuents will be a determining factor Role of the carbene subs9tuents Induc4ve Effects: -  σ-­‐electron withdrawing subs9tuents favors the singlet state -  σ-­‐electron dona9ng subs9tuents favors the triplet state CX2 carbene (C2v symmetry) Perturba9on orbital diagrams -­‐ Mesomeric effects typically play a more significant role Four-­‐electrons three-­‐centered π-­‐system -­‐ Stabiliza9on of the Ccarbene empty pπ
By dona9on from the 2 nitrogens. -­‐ Carbene lone pair stabilized by σ-­‐electron-­‐ withdrawing subs9tuents X = π-­‐electron-­‐dona9ng subs9tuent N-­‐Heterocyclic Carbene (NHC) Chemistry First isola9on of stable N-­‐heterocyclic carbene Arduengo 1991
JACS 1991, 113, 361. Cl
Ad N
N Ad
base
Ad N
N Ad
R
Coordina4on to transi4on metals N
M
N
R
strong donation σ
H
N
M
N
R
low π acceptor
Excep4onal stability of M-­‐NHC complexes Early Examples Mes N C N Mes
Cl
Ru
Cl
Ph
PCy3
Olefin Metathesis
Grubbs, Nolan, Herrmann (1999)
N
C
Pd
N
Pd(OAc)2
N
O
Cl
O
N
N
TfO
Suzuki Coupling
Nolan, Glorius (2003)
Cyclic Amino Carbenes Cyclic di-­‐amino carbenes R
R
R
N
N
N
C
N
N
N
R
200 < 13C NMR Ccarbene (ppm) < 250 C
C
R
R
Leading reviews: Acc. Chem. Res. 1999, 32, 913; Chem. Rev. 2000, 100, 39. « Abnormal » Carbenes/mesoionic Carbenes Generated via deprotona9on at the C4 posi9on R
R"
R
R"
N
C N
R
R'
N
C N
R'
R
Stronger σ-­‐donor than « normal » carbenes Review: Coord. Chem. Rev. 2013, 257, 755. Mesoionic form Example Ph
H
X
Dipp
N
Ph
N
Dipp
KN(SiMe3)2
- HN(SiMe3)2
- KX
Dipp
Ph
N
Ph
C N
Dipp
X = Cl, Br
Molecular structure Bertrand, Science 2009, 326, 556 Group 1 and 2 metal complexes supported by NHCs Dipp
Dipp
Dipp
N
N
C
+
MN(SiMe3)2 + M'{N(SiMe3)2}2
N
Dipp
N
C
N
Dipp
M
+ M'{N(SiMe3)2}3
C
N
Dipp
M = Li, Na, K
M' = Mg, Ca, Sr, Ba
Molecular structure Hill, Inorg. Chem. 2011, 50, 5234. Structurally Characterized NHC-­‐Mg(II) Species R'
N
C
N
R'
MgR2
R = alkyl
R'
N
C
N
R'
Ad
N
C
N
Ad
MgR2
MgEt2
First example of a NHC-­‐Mg(II) species Arduengo, J. Organomet. Chem. 1993, 462, 13. N
N C
Mes
Mes
Mg
Mg
Mes
Mes
C N
N
Dimers may form with less sterically hindered NHCs Applica9on of NHC-­‐bearing Mg(II) species in Grignard Allylic subs9tu9on Key effect of the NHC moiety (ac9ng as a Lewis base) to enhance the nucleophilicity of the R-­‐Mg group Tetrahedron LeE. 2010, 51, 5704. Postulated NHC Mg(II) species Imidazolium salts NHC-­‐borane Species First stable NHC borane species: Chem. Ber. 1993, 126, 2041. First X-ray characterized by Robinson in 2007
JACS 2007, 129, 12412
No hydrobora9on reac9vity via a 1,2 hydride migra9on Migra9on disfavored by the weak π accep9ng character of the NHC Interes9ng structural features Mes
N
C
BH3
N
Mes
Intermolecular interac9ons Chem. Commun. 2003, 1722 Close electrosta9c contacts reflec9ng the electrophilic nature of the Ccarbene Acta Cryst. C 2005, 61, o291 Common synthesis of NHC borane species: Fundamental reac9vity and applica9ons Review on NHC-­‐borane species: Angew. Chem. Int. Ed. 2011, 50, 10294. Stable species May be purified via chromatography Amino-­‐boranes: Solids, safe to handle CAAC-­‐borane adducts Unusual reac9vity with CAAC-­‐type carbenes Inser9on of the Ccarbene into a B-­‐H bond Bertrand, Angew. Chem. Int. Ed. 2010, 49, 9444. Bertrand, Angew. Chem. Int. Ed. 2013, 52, 7590. Neutral Tricoordinate Organoboron isoelectronic With Amines Boron(I) central atom: isoelectronic with N(III) in amines Bertrand, Science 2011, 333, 610