"Organometallics" Study Meeting #2

Contents (ref:'11. 4. 14. (Thu.)
Shogo HASHIZUME (M2)

p.341-384)

8-2. Chemistry about Group 14 Elements
8-3. Chemistry about Group 15-17 Elements
8-4. Ate Complex and Hypercoordinate Compounds
8-5. Reaction of Carbon-Metal Bonds

8-2. Chemistry about Group 14 Elements
Si, Ge, Sn, Pb : More electronically positive than carbon (C)But ionicity of M-C bonds are much lower thanthat of other organometallics such as alkyllithium.

M C+ -8-2-1. Substitution at Group 14 Elements
A- They has chirality (of course, if A~D are all different).
- Nucleophilic substitution (SN2-M) is much faster than carbon.
- That's because of stable 5-coordinate intermediate.
- It means stereochemical inversion does not always occur.

MD

B
C

(M = group 14 elements)

Tendency of stereochemical inversion or retention
inversionleaving group : highly polarlized
nucleophile :softretention
low polarlizedhard

8-2-2. Ef f ects of Substituentseffect:Trimethylsilyl (TMS) group shows high electron donating effect when it is at acarbon adjacent to -electron system (such as benzylic or allylic).
RRR M

It's due to hyperconjugation between M-C bond's orbital and -orbital.

C CHyperconjugation also accounts for rapid SN1 reaction.
R3M
R3M
A

R3M

SN1B

AA

B

B

X
X
vertical stabilization-effect works against lone pair of oxygen/nitrogen at -position to decrease oxidation potential.
R3Si
O- e-R3Si
O

C-Si bond cleaveage

effect:- Silyl group combined directly to -electron system shows relatively strong electron
withdrawing conjugative effects.
- This electron withdrawing nature is measured in hyperfine coupling constant of radical
anion of mono-substituted benzene.
orbital (pseudo- Origin of this effect is considered to be("inverse" hyperconjugation)(Traditionally, it's considered to be d- conjugation of Si.)orbital) of Si-C bond.

RMCR(p-d) conjugation

MCR"inverse" hyperconjugation

8-2-3. Unstable Spieces of Group 14 Elements
Group 14 elements can form chemical spieces like carbon.
2-coordinate chemical spieces- Silyrene, germyrene, and stannyrene are generally
unstable like carbene.

examples:TMSt-BuTMS NN- Silyrenes or germyrenes are all singlet unlike carbenes.

MM- Silyrene shows similar reactivity with singlet carbene,
but not react with C-H bonds or C-C bonds.

TMS NNTMS

t-Bu- They form Lewis pair with ethers or amines.

(M = C, Si, Ge)

Cations(M = Ge, Sn)Ph

3-coordinate silyl cation is too unstable and highly electrophilic.
AnionsSiPhPhOClO3covalent bond
3-coordinate silyl anion is stabilized by aromatic substituent like phenyl group.
Stabilization of silyl anion by phenyl group is considered to be due to inductive effect.

Radicals
Radicalic reductions by silyl hydride or stannyl hydride are well-known.
Pyrmid inversion of silyl radical is slow because of unplanar structure.

SiDouble bonds
H2M=MH2 are not planar and take trans-bent structure.
These double bonds are considered to be donating-bonding between two 2-coordinate spieces.

MM
donating-bondingM

M

trans-bent structure8-3. Chemistry about Group 15-17 Elements
8-3-1. Group 15 Elements (P, As, Sb, Bi)3-coordinate compounds
trialkylphosphine : R3P
High ionization potential, low basicity (compared with amine)
This tendency is rationalized by theoretical calculation of hibridized orbital of EH3 (E = N or P).
P-H bond has high p-nature and lone pair has high s-nature.
ER3 compounds : for stabilization of transition metal complex as -donating/ -accepting ligands
The higher p-nature of lone pair, the stronger ligand. (R3P>R3As>R3Sb>R3Bi)
Onium salts and ylides
R2R1HC+ R3P

X

R2R1HC PR3

base

R2R1C PR3

R2R1C PR3ylideylene

X

Donation of ylide structure is higher.

8-3-2. Group 16 Elements (S, Se, T e, Po)
Thiol, sulfide, etc.
- Features of REH (E = O, S, Se, Te)

ROH

RSH

RSeH

RTeH

Aciditylowhigh

Nucleophilicityof RE-

lowhighNature of RE-hardsoft- Stabilization of adjacent carboanion

Lip

Due to p - * conjugation*R2
R1Se
Ph- Weaker stabilization of adjacent carbocation than oxygen
RE

CR

EC
Because interaction of 2p -3pand 2p -4p are weak.

(E = O, S, Se, Te)- Neighboring group participation
Hydrolysis speed : EtSCH2CH2Cl >>> EtOCH2CH2Cl
ClOHSEt SEt

Et SThis stabilization is non-vertical (different from that of Si's -cation stabilization).

8-3-3. High-Period Group 17 Elements
High-period halogens (especially iodine) form stable high-coordinate compounds.
AcO OAc
I OAc
OXI
X

(Dess-Martin reagent)O
organoperiodinane

organoiodinane(X = F, Cl, Br, etc.)

8-3-4. Organof luorine Chemistry
Different features from other halogens- Unreactive for halogen-lithium exchange (due to small atomic radius = unstablity of highvalent spieces).
- Most electronegative element.
- "van der Waals radius" similar to that of H offers intriguing property for bioactivity.
Electronic effects
via -bondstrong electron-withdrawing inductive effect (-I effect)
via lone pair of p-orbital
electron-donating resonance effect (+R effect) and -inductive effect (+I effect)
T hese orthogonal ef f ects determine the reactivity of f loride compounds.
repulsion
+

CF-FF
C C

C C-I effect+R effect

FFF-I effect
CF3E++

C

CF3+E+
E
meta-selectiveE
para-selective
Fluorination reaction- Nucloephilic fluorination: KF, Et2NSF3, etc.

CH2ClN- Electrophilic fluorination: FClO3, CF3OF,, etc.
N 2BF

4F

Perfluoroalkylation reaction
RfI-+

Nucleophilic substitution with Nu is unavailable.
(Radicalic cleavage, halogen-lithium exchange to generate Rf -)8-4. Ate Complex and Hypercoordinate Compounds"Ate complex" = anionic complex salt having hypercoordinate central atom
"Hyper coordinate compounds" = central atom has more electron over Octet Rule.
8-4-1. Bonding of 5-Coordinate Compounds
X

X
X

M

X

X

X
X
tirigonal bipyramid(TBP) structureX

MXX

rectangular pyramid(RP) structure

Muetterties Rule : More electronegative ligands take apical position andmore electropositive ligands take equatrial position in TBP structure.

8-4-2. Bonding of 6-Coordinate Compounds
XX M XX
X
X
octahedronstructure

8-4-3. Isomerization of Ate Complex
Isomerization of 5-coordinate compounds
Two hypothetical mechanism is probable:
pseudorotation mechanism (Berry)turnstile rotation mechanism (Ugi)- pseudorotation mechanism
X1
X3

X1
X5

X3

M

X5X4M

X5
X4

X2

X2

X1
M

X5X4X2M

X2
X1

X4Isomerization of 3-coordinate compounds- vertex inversion
X1

P

X2
X3

X1

X2
P X3

- edge inversion
PX1

X2
X3

P
X3

X2

X2
X3

X1

X2
X1

The former is enagetically advantageous.

X3
X3

M
X4

- turnstile rotation mechanism

X5

X3

P
X1

Isomerization of 6-coordinate compounds- twist mechanism (Bailor) : few examples
X1
X4
X3

X3
X5
X6

M

X4
X6

X2

X5
X1

M
X2

- multisteps mechanism : many examples
X1
X4
X3

X1
X5
X6

M

- X6

X2

X4
X3

M

X1
X5
pseudorotation

X2X2M

X1X5+ X6X3X4X2

M

X5X6X38-4-4. Reactivity of High-Coordinate Compounds
Roughly devided into two patterns:- Organo-substituent of e-rich central atom works as nucleophile.
- Organo-substituent works as electrophile and e-rich central atom works as LG.
Example of the former caseHO
SiF4+RH

IF
Si F
O FOH
R

R(silicate)

Other examples : Peterson, W ittig, Corey-Chaykovsky, etc.

Example of the latter case
PhF

Me

BF4+ LiCuMe2

8-5. Reaction of Carbon-Metal Bond8-5-1. Reaction Mechanism
SE2 reaction (1 step): stereochemical retention or inversion
HOMOCM

LUMO

HOMOCEEM

LUMO

retention : allowedinversion : partially allowed

Mechanism via electron transfers (multi steps) : stereochemical invertion or racemization
M R + X2electr on
tr ansf er

M R X2

M RX

X

M RX

X

in solvent cageor racemization if these intermediates are diffused (not in solvent cage)

R X+M X
inversion

8-5-2. Alkyl-Metal Bonds
Organometallics of Group 1, 2Li

Me+ (MeO)2SO2OBOM
Hretention

OBOMH
Ni-Pr2(MeO)2CO

Ni-Pr2
Me O

Me O

O
CO2Meretention

Ni-Pr2OMe O

Li
TMEDAOCO2Meinversion

ClCO2MeLi

bromination
reagent

Br

Br+(retetion)(inversion)

25 : 75
97 : 3

with Br2 :with BrCH2CH2Br :Organometallics of Group 12, 13, 14

B + EtCO2D200 C
D3H

H
OMe MeMeR2B
Et*putative mechanism
REt
R B O
retention
RO
DH+ MeONa

R2B
Et

I2

H

MeI

inversion

R H

retention

HEt

RR
Hg +H OAc

HOAc

Hg
RR

low Lewis acidity of Hg

8-5-3. Allyl-Metal Bonds
Ph
Me3Si

Ph
H

t-BuCl
TiCl4H

H

H
E

inversion

H

E

SiMe3
H
Met-BuPh

Phor Me
H

H

SiMe3

Strong interaction between alkene's -orbital and C-Si
bond's -bond offers vertical position of Si group.

8-5-4. Alkenyl-Metal Bonds
Alkenyl silane

DPh

SiMe3H

D

H

H

MeH

H

Ph
H

H

Br2

SiMe3

Me3Si

Nu

MeH

H
retention

SiMe3
MeMeBrBr

H

H
inversion

HBr

120 rotation

Br
H

Br2H

2H

Br

H

H

D

SiMe3NuH
Br

anti-addition

n-Hex

SiMe3BrPhH
Ph

DHSiF5n-Hex

F5Si2

H

H
n-Hex
SiF560 rotation

Ph

H

H

Br
retention

2Alkenyl borane
Br2
R

H

H

B(OH)2H

NaOH

R

Br

MeONa

Br

H
B(OH)2

R

H

H

B(OH)3

R

Br

H

H

inversion

I2

R
H

H
B(OH)3I

H

H

B R'
HO OH

Br2

R
H

H R'
B OH
BrOH

r ear rangement

H

R

Br

H

H

I
retention
R

R'

R'

H

H
inversion

H
B(OH)2R

H

H

R'
retention

MeONa
R

R

heat