Electronic Supplementary Information forligands carbenes ...
Carbenes and Analogues Carbenes are 6-electron carbon centres, typically -bonded to two other...
-
Upload
gilbert-gilmore -
Category
Documents
-
view
215 -
download
0
Transcript of Carbenes and Analogues Carbenes are 6-electron carbon centres, typically -bonded to two other...
Carbenes and Analogues
Carbenes are 6-electron carbon centres, typically -bonded to two other groups, and with two lone electrons.
These can be spin-paired (singlet) or spin-non-paired (triplet):
triplet singlet
C C
R
R
R
R
Heavier Group 14 Analogues
In the singlet state these compounds can be Lewis bases or Lewis acids.
When R is a “small” group these species oligomerize.
If R is large or is there is a site for intermolecular coordination to the E center (increases the coordination number up to three or four) then these species can be stabilized.
ERR
Silylene
Silylene is the silicon analogue of the carbene, and is similarly unstable.
They cannot be isolated by simple reduction of the dihalide:
Me2SiCl2 + 2 K 2 KCl + (Me2Si)n + Me(Me2Si)mMen = 5, 6, 7; m < 100
But photolysis or thermolysis of these species afford silylenes:
c-(Me2Si)6 Me2Si: + c-(Me2Si)5
Interestingly, thermolysis either in air (300oC) or under nitrogen (1330oC) produces SiC, a popular industial abrasive.
Silylenes are also highly reactive and thus transient species
Thermal Routes to Silylenes
Me2C
Me2CSiMe2 Me2C CMe2 + :SiMe2
Elimination of a silylene from hexamethylsilirane occurs at 80oC.
It is an equilibrium reaction:
The evolved silylene adds into the three-membered silirane ring to alleviate ring strain:
+ :SiMe2
Me2C
Me2CSiMe2
Me2C
Me2C SiMe2
SiMe2
Formation of Si=Si - DisilenesBefore coming back to silylenes we need to realize that one reaction path might be dimerization and formation of compounds containing Si=Si.
Such species are not widespread – Why?
Closer to the top of the group, the s and p orbital energies are closer together.
Si=Si is weaker than two Si-Si bonds and the double bond is not usually kinetically stable (C=C is also weaker than two C-C, but kinetics helps)
As a result silylenes tend to aggregate into rings and clusters.
6 R2Si: (R2Si)6
bulky ligands might protect the double bond
Formation of Si=Si
The first isolable disilene (Si=Si formation) was generated in 1981 by the elimination of silane to make the disilene:
Mes2Si(SiMe3)2 (h) Me6Si2 + Mes2Si=SiMes2
Employs bulky mesityl groups
Structure is trans bent:
Most are planar with Si-Si about 2.15A
E ER
R
RR
18 deg
Double Bonds Between E
Small out of plane angle
Si-Si distance 0.2 Å shorter than that of a single bond
No tendency to dissociate to monomers in hydrocarbon
To date approx 30 stable disilenes have been isolated and characterized (Si=Si 2.14-2.25 Å, approximately planar core geometries)
Formation of Si=SiPhotolysis:
J. Am. Chem. Soc. 1993, 115, 10428.Key to formation is protection of the Si=Si with bulky groups
Reduction:
=
Thermal Routes to Silylenes
Thermolysis of a Si=Si bond gives a silylene.
Note that only due to the tremendous steric hinderance is the silylene around long enough to form a Lewis base adduct
J. Am. Chem. Soc. 1997, 119(6), 1456.
Theoretical Ph2SiCNPh
Although the compound was not structurally characterized, calculations show the silicon to be sp3, as would make sense with a silylene-Lewis base adduct.
Reactivity of Silylenes
Silylenes insert into bonds to silicon.
(Note that SiMe2 is really hexamethylsilirane heated up to ~80oC to generate dimethylsilylene in situ)
R3 SiH + :SiMe2 R3Si-SiMe2HR3 SiOR + :SiMe2 R3Si-SiMe2OR
Me3SiC CSiMe3 + :SiMes2
Me3SiC
SiMes2
CSiMe3
1
Interestingly, 1 is so stabilized by the mesityl groups (steric) that this compound can be precipitated from methanol.
Germylenes, Stanylenes, Plumbylenes
As we go down the periodic table, the stability of the carbene derivatives goes up.
With bulky R groups, these can be stable monomers:
EX2 + 2LiHCMeSi
Me Me
Me
Si
MeMe
ECH
Me
SiMe
Me
MeSi
MeMe
CH
Me
Si
MeMe
MeSi
MeMe
EX2 + 2 LiR 2 LiX + :ER2 (R = CH(SiMe3)2)
ERR
Germylenes, Stanylenes, Plumbylenes
Most common way to make transient germylenes (photo- or thermoloysis):
Other examples of stable monomeric species:
GeR2
Ph
Ph Ph
Ph
Ph
Ph
Ph
Ph
GeR2+ R = Me, Et, Ph etc
MMes
Mes
Mes
Mes
M = Ge, Sn, Pb(via metathesis) M
F3C
F3C
CF3
CF3
F3C
F3C
M = Sn, Pb(via metathesis)
Germylenes, Stannylenes, Plumbylenes
The two-coordinate species are considered to be sp2 hybridized with a vacant p orbital. They tend to be colored due to the n to p transitions
Three coordinate species viewed as sp3 (Td) and four-coordinate as dsp3 (tbp)
SnOHMe
OMe
OMe
MeO N
N
Sn
H
Me3Si
SiMe3
H
SnH
Trip
Trip
Ar
SnH
Ar
Sn
H
chloride analogue also known to dimerize
NtBu
tBu
Sn
NN
Sn
N
2
Formation of Ge=Ge and Sn=SnIf the steric bulk isn’t great enough, these compounds will dimerize - Do not have planar geometry
E ER
R
RR
E ER
R
RR
The lone pairs are in the sp2 orbital on “E”, and it is energetically more favorable to overlap with the empty p orbital rather than promote the electron pair into the p orbital for a more classic double bond.
E E RRR
R
Double Bonds Between E
out of plane angle 41° Sn-Sn distance 2.768(1) Å, which is slightly shorter than that of a single bonddissociates to (SnR2) monomers in hydrocarbon
Back to Disilenes: Bonding
Si=Si bond is much weaker than C=C bond consequently the HOMO LUMO gap is much lower leading to absorption in the visible
Bonding in Disilaethenes - Isomerization
Isomerization of disilenes is much easier than for alkenes because the Si=Si bond is weaker
Initially produced photochemically Thermally isomerized at room temp! (Note strain relief)
Reactivity Disilenes1,2 - Additions
The reactivity of disilenes has some things in common with alkenes including 1,2 addition processes, but they tend to be more reactive!
This would not go without a catalyst for an alkene
Reactivity of Disilenes: 2+2 cycloadditions
Unlike simple alkenes, disilaethenes readily
undergo a variety of 2+2 cycloadditions
Formation of Ge=Ge and Sn=Sn
An electron transfer method (reduction) can be used for formation of Ge=Ge (and Sn=Sn) bonds. (This also occurs for Si)
Ar2ECl2 + LiNap
Ar2E EAr2
Ar2E Ar2E EAr2h+ naphthalene
Another method is to exchange ligands with a stable germylene. If the new ligand is less bulky, formation of the dimer occurs:
((Me3Si)2N)2Ge: + LiR R2Ge=GeR2 + LiN(SiMe3)2
Reactivity of Disilylenes
Although the relatively weak Si=Si bond can be easily opened to form rings and disubstitutions, disilylenes never react like silylenes (insertion into bonds:
R2Si=SiR2 + X2 R2XSi-SiXR2
R2Si=SiR2 + HX R2XSi-SiHR2
The closest reactivity to insertion is the reaction with oxygen, which goes by a coordination pathway:
R2Si SiR2 + O2
R2Si SiR2
O
O
R2Si SiR2
O OR2Si
O
O
SiR2
Double Bonds to Carbon
From a synthetic standpoint, it is interesting to have double bond to carbon that could be exploited for C-C bond formation.
The first work was done by gas phase thermolysis to produce a highly reactive species.
Me2Si
H2C
CH2
CH2 H2C CH2 H2C SiMe2+
Me2Si
H2C
SiMe2
CH2
Because of the difference in electronegativity, it is common to consider a silaethene in equilibrium with a charge separated species
H2C SiMe2 H2C SiMe2
Stable Silaethenes
Stable silaethenes can be made by exploiting the electron-withdrawing power of the carbon substituent, and by increasing steric bulk, to prevent dimerization:
O
R(Me3Si)3Sih
OSiMe3
(Me3Si)2Si R
Si
Si
Me3Si
R
Me3Si
OSiMe3
R
Me3SiOSiMe3
Me3Si
This head-to-head dimerization occurs when R = Me, but not when R = CEt3, adamantyl
Stable Silaethenes
The silaethene Me2SiC(SiMe3)(SiMetBu2) is made from an intermolecular metathesis:
Me2Si
LiCl
SiMetBu2
SiMe3
Me2Si
SiMe3
SiMetBu2
+LiCl
Si-C = 1.702 ATwist of 1.6 deg.
Stable Silaethenes
This is stabilized by methyl exchange in a four-centered methyl exchange:
SiMetBu2
SiMe3Me2Si
SiMetBu2
SiMe2Me2SiMe
SiMetBu2
Me3Si SiMe2
Of course, this exchange can happen with the SiMetBu2 moiety as well, but is less stable:
SiMetBu2
SiMe3Me2Si
SiMetBu2
Me3Si SiMe2
SitBu2
Me3Si SiMe3
Stable Silaethenes
A similar route to silaallene:
SiMes*(Ad)FiPr
MeO
Pri
Br
iPr
OMe
iPr2 BuLi
-LiBr, LiF CSi
Ad
*Mes
Pri
Pri
OMe
iPr
OMe
iPr
GermaethenesGe=C bonds can also be stabilized by lithium halide elimination:
Me2Ge
LiX
SiMe3SiMe3
Me2Ge
SiMe3
SiMe3
+ LX
The germaethene is more stable than the silaethene due to the difference in electronegativity, but is more labile due to the weaker double bond.
This gives it a broader range of reactivity.
Element- Element Double BondsTrans-bent configurationThe angle increase and the H-E-H angle decreases with increasing atomic numberThe ratio of the double to single bond length also increases with increasing atomic number.
Types of R4E2 Structures
All Sn and Pb along with some Ge and Si dissociate into type F- the inert pair effect
Energetic differences between C and D are small esp. for Si
Kinetic and thermodynamic stabilization through substituent design
Structural types of R4E2
Transition Metal-E Double BondsAnalogues of carbenes – variation of substituents and metal offers wide range of species
Description of bonding
Transition Metal-E Double Bonds
• short tungsten-germylene bond of 2.4590(16) Å vs. 2.667(3) Å• planar geometry around the germanium atom of the germylene ligand suggesting sp2 hybridization
Organometallics 1999, 18, 4468-4470
Transition Metal-E Double Bonds
Description of bonding
Several complexes with K structure are known – supports zwitterionic formalism (structure H)
The First Stable CyclotrisileneThe silicon analogue of cyclopropene:
3 in 65% yield together with tris(tertbutyldimethylsilyl)chlorosilane as the sole byproduct
Iwamoto, J. Am. Chem. Soc. 1999, 121, 886