Sigma-bond metathesis

Textbook H: Chapter 6.5.1 – 6.5.2

Textbook A: Chapter 3.3.7

[M]-CR3 + H-E [M]-E + H-CR3[M]

CR3

H

E

Sigma-bond metathesis

It avoids the TD barriers of the C-H activation /substitution step.

It is found for early TM with d0 configuration.

Lu CH3

+ CH4

- CH4

LuCH3

H

CH

H

H

Lu

H3C

H

C

H H

H

++-

-

Cp*2Lu-CH3 + H-13CH3 Cp*2Lu-13CH3 + H-CH3Cp2*Lu

CH3

H

13CH3

Cp*2Lu-CH3 + H-CH3 Cp*2Lu-CH3 + H-CH3

Cp*2Lu-CH3 + H-H Cp*2Lu-H + H-CH3

Cp*2Lu-CH3 + H-CH3 Cp*2Lu-H + CH3-CH3does not give

Cp*2Lu-H + H-H Cp*2Lu-H + H-H

Cp*2Lu-H + H-CH3 Cp*2Lu-H + H-CH3does not give

+ H-CH3H

La

Cp Cp

HLa

Cp Cp

H

CH3

H

La

Cp Cp

H3C H

0 kcal/mol

73.2 kcal/mol

15.5 kcal/mol

+ H-SiH3H

La

Cp Cp

HLa

Cp Cp

H

SiH3

H

La

Cp Cp

H3Si H

0 kcal/mol

-3.6 kcal/mol

2.8 kcal/mol

Why position cannot be CThe reaction is best described as a nucleophilic substitution of H at either C or Si in the coordination sphere of Ln.

The energy barrier for C at the position is lowered with electronegative substituents (F), known to stabilize a hypervalent species, but not vinyl or phenyl.

The transition state is a pentacoordinated anionic CH5

- or SiH5- which is energetically

highly unfavorable for C and much more favorable for Si.

-H elimination

-hydride elimination as the reverse of -bond metathesis

4

Cp

ThCp

tBu

tBu

Cp

ThCp

tBu

Cp

ThCp

tBu

CH2

-agosticinteraction

H

Cp

ThCp

tBu

CH2

H

-bondmetathesis

starting

-bondmetathesis

ending

50 oC, 60 h-H elimination

bond metathesis with high-valent, late TMs Midterm 2005: Recently, Hartwig et al. (J. Am. Chem. Soc. 2005, 127, 14263-

14278) published mechanistic studies on the functionalization of arenes by diboron reagents catalyzed by iridium complexes:

5

For each step shown in the catalytic cycle indicate the mechanism (type of reaction). Where you can envision more than one possibility, write down all of them (at least two) discussing arguments that support your proposal or that are against it (at least one of each). If you consider that some intermediates are not shown, draw those intermediates. Indicate formal oxidation state and electron count for each iridium complex.

Electrophilic activation of C-H bonds

Shilov, 1972

Alkane activation step

Pd2+, Pt2+ and/or Pt4+, Hg2+, Tl3+.

LnMx+2X2 + R-H LnMx + R-X + H-X

LnMx+2X2 + R-H LnMx(R)(X) LnMx + R-X- HX

CH4 + D2OPtCl4

2-

CH3D + HDOD2O / CH3COOD

CH4 + PtCl62- + H2O (Cl-)

PtCl42-

H2O120 oC

CH3OH (CH3Cl) + PtCl42- + 2 HCl

(II)Pt

H2O

Cl OH2

Cl

+ CH4

- HCl

(II)Pt

H2O

Cl OH2

CH3

+ Pt(IV)Cl62-

- Pt(II)Cl42-

PtH2O

Cl OH2

Cl

CH3

Cl

H2O-CH3OH

-HClPt(II) + R-H

Pt(II)H

R

Pt(IV)

H

R

Pt(II)-R + H+

1,2-AdditionM=N bonds

M=C bonds

X

Zr

XR

NHtBuX

Zr

X

NtBu- RH + R'H

X

Zr

XR'

NHtBuX = Cp: Bergman

X = NHtBu: Wolczanski

1988

Mo NO - C6H6Mo NO

Mo NO

- CMe4

Mo NOL

L

Mo NOR

HH

RHL

Mo NOR

RH

MoNOL

H

Legzdins, 2003

-H elimination -hydride elimination as the reverse of 1,2-addition

8

LnMHC

H

R

-H eliminationLnM

CHR

H

Li

tBu

Ta

Cl

CltBu

tBu

tBu

Zn

tBu

2Ta

tBu

tBu

tButBu

tBuHH

HH

Ta

tButBu

tBu

tBu

TaCl53 -H elimination

not isolated

Migratory insertions

U

M XM U X

U = CO, C2H4, C2R2, NO, CR2, CNR, RCN, O2, CO2

X = H, alkyl, aryl, OR, NR2

Migratory insertions

Two possible mechanisms:

Labeling studies show that Me migrates.

10

Me

M COM C

Me

O

C

M

Me O

Me migratesCO migrates

MnOC

OC 13CO

CO

C

CO

O Me

if CO movesMn

OC

OC CO

CO

Me

CO

25%no label

+ MnOC

OC 13CO

CO

Me

CO

75%

Me is cis tothe label only

MnOC

OC CO

CO

Me

CO

25%no label

+ MnOC

OC 13CO

CO

Me

CO

50%cis

MnMe

OC 13CO

CO

CO

CO

25%trans

+

if Me moves

cis : trans2 : 1

U

M XM U X

U = CO, C2H4, C2R2, NO, CR2, CNR, RCN, O2, CO2

X = H, alkyl, aryl, OR, NR2

experimental results

If CO moves

11

MnOC

OC 13CO

CO

C

CO

O Me

MnOC

OC CO

CO

Me

CO

25%no label

+

MnOC

OC 13CO

CO

Me

CO

75%

MnOC

OC

CO

C

CO

O Me

- 13CO

25%

MnOC

13CO

CO

C

CO

O Me

- CO

+

75%

CO moves

CO moves

If Me moves

12

MnOC

OC 13CO

CO

C

CO

O Me

MnOC

OC Me

CO

CO

CO

25%no label

+

MnOC

Me 13CO

CO

CO

CO50%cis

MnMe

OC 13CO

CO

CO

CO

25%trans

+

MnOC

OC

CO

C

CO

O Me

- 13CO

25%

MnOC

13CO

CO

C

CO

O Me

- CO

+

75%

Me moves

Me moves

Migratory insertion examples

Insertion of CS2

(OC)5Mn R + CS2 (OC)5MnS

RS

Insertion of SO2

(OC)5Mn R + SO2 (OC)5Mn S R

O

O

S-bound

Cp2TiMe2 + SO2 S Me

O

O-boundOCp2Ti2

Reactions of alkyls with electrophiles

LnMCH2 CH2

H -H elimination

LnM

H

H2C

CH2LnM H H2C CH2+

-H elimination: retro-migratory insertion

-Olefin insertion

• -olefins can insert from two positions:

1,2-insertionR

1-position

2-position

ZrMe

ZrMe

ZrMe

­R

RR

Zr

RMe

ZrMe

ZrMe

ZrMe

­R

Zr

MeRR

R

2,1-insertion

• 1,2-addition is the major mode of insertion; 2,1-insertion usually leads to chain termination in polymerization reactions.

elimination: retro-migratory insertion

Modes of blocking -H elimination No -hydrogens The alkyl is oriented so that the beta position cannot access the metal

center (steric bulk or rigidity). The alkyl would give an unstable alkene as the product.

15

CMe3

M

neopentyl

CMe2Ph

M

neophyl

SiMe3

M

"silyl-neopentyl"Ph

M

benzyl

M R

M

alkynyl

norbornyl

LnMCH2 CH2

H -H elimination

LnM

H

H2C

CH2LnM H H2C CH2+