08.05.16
Literature seminar

Suzuki Yuta (M2)

Metal / σ-bond interactions– toward an understanding of C-H activation –☯ Contents
12

σ-complex (η2-R-H bonds).........................................................................................................................
2bonds)1.1

Formation of σ-complex ..................................................................................................................... 2

1.2

Structure and Dynamics of σ-Complex (MHm(η2-E-H)n) (n=m≠2(n=m≠0)..............................................
0)

1.2.1

E = H ............................................................................................................................................ 3

1.2.2

E = Si............................................................................................................................................
5Si

1.2.3

E = C.............................................................................................................................................
5C

σ-CAM (σ(σ-bond Complex Assisted Metathesis) ....................................................................................... 6
2.1

NonNon-classical metathesis mechanism ............................................................................................... 6

2.2

Catalytic cycle via σ-CAM ................................................................................................................. 7

2.2.1

Borylation .................................................................................................................................... 7

2.2.2

Hydrogenation, HydroSilylation................................................................................................
7
HydroSilylation

2.3
3σ-CAM for the EE-E bonds (E≄(E≄H)........................................................................................................
H)
8

Various Types of Mechanism ~ or more simple mechanism? ~..............................................................
8~
3.1

C-H bond activation mechanisms ..................................................................................................... 8☯ Main source
Robin N. Perutz and Sylviane Sabo-Etienne Angew. Chem. Int. Ed. 2007, 46, 2578

1

1σ-complex (η2-R-H bonds)
1.1

Formation of σ-complex☯ Intermediate of Oxidative addition☯ Equilibrium changes by what M, L, Eare used
☯ σ(H-E): electron donor☯ Three-center two-electron bondCharacterized by X-ray diffraction and NMR:
1.2

E = H, Si, B, C (etc.?)

Structure and Dynamics of σ-Complex (MHm(ηη2-E-H)n) (n=m≠≠0)“Classical”
Classical” interaction of σ-complex

Metathesis

Anotherbehavior21.2.1 E = H
This type of complex is the most studied family.
i2☯ First observation of η2-H2 complex (X(X-ray diffraction, Neutron diffraction):diffraction) W(CO)3(P Pr3)2(η -H2)

(Kubas. G. J. et. al JACS 1984, 106, 451)
☯ First observation of the equilibrium between (η(η2-H2) and (η(η1-H)(Xiao-Liang Luo et. al, JACS 1990, 112, 6912)(η2-H2) ligand in maltihydride complex is not so stable→ Forcible introduction of hydrogen into complex using strong acid
HBF4･OEt2 (pKa=- 0.4)
CH2Cl2, 193K

8 coordinated: crowded

7 coordinated: not crowded
1H NMR analysis

Keq = [2b2b]/[2a2b 2a]:2a Determined by the 1H NMR integration∆H = 1.1 ± 0.2 kcal mol-1→ 2b is slightly less stable than 2a∆S = 2.4 ± 0.8 cal mol-1 K-1
250 K

200 K

→ Result of the rotation of the η2-H2 ligand in 2b(in CH2Cl2)“Shallow minima” B

T1(msec): longitudinal relaxation time (縦緩和時間)(T1 value mainly depends on temperature and atom density
around the target atom)( Re-H )Phenomena A→ Due to the exchange of η2-H2 and (H)2 ligands of 2b( Re-(H)2 )APhenomena B→ Due to the exchange between 2a and 2b

Existence of equilibrium between hydrogen ligands
ligands☯ How is T1 observed?: JACS 1988, 110, 4126)

3

Search for detail exchange mechanism…
mechanism…
☯ Gibbs free energy of activation‡

2b → 2bDegenerate∆G1 (213K ) = 9.9 ± 0.2 kcal mol-1

2b → 2a

Oxidative addition∆G2 (213K ) = 11.6 ± 0.4 kcal mol-1‡If 2b → 2b include 2b → 2a → 2b process:

‡‡

‡

‡

∆G1 ≥ ∆G2But this time:∆G1 < ∆G 22b → 2b is not via OA/RE route.

☯ Activation enthalpy and activation entropy of 2b → 2a‡‡∆H 2 = 1.0 ± 0.4 kcal mol-1,cf.) trans-IrX(CO)(PPh)2∆S 2 = −42 ± 2 cal mol-1 K-1(1)trans-IrXH2(CO)(PPh)2 (X = halogen)

H2‡∆S‡ = −20to − 24 cal mol-1 K-1∆H = 10 − 12 kcal mol-1,(2)(William H. Saunders, Jr. et. al JACS 1985, 107, 8049)
In spite of intramolecular mechanism of (1),(1)∆S‡ of (1) is smaller than that of (2) which is intermolecular mechanism.

Direct 2b → 2b exchange??
☯ …But in this paper, no more direct observation could not achieved.
Their final prediction for 2b → 2b exchange mechanism

*2b*

*
3

*2b3

☯ Compound 3 cannot be detected at least NMR because of this middle instability.
☯ This is also the first example of having described non-classic σ metathesis mechanism
41.2.2 E = Si
(Robin N. Perutz et. al Organomet. 2002, 21, 5347)☯ Double σ-bond complex☯ A is stabilized by H-Si interaction(bottom figure)☯ Stable even in 376 K

Bulky, but cis
exceptionally stable

H-Si secondary interaction

1.2.3 E = C
Direct observation of σ-C-H complex is rare so far.
(ex.1) One of the two example (so far) ofobserving σ-C-H complex by X-ray diffraction(ex.2) NMR study(Graham E.B. et. al JACS 2005, 127, 4134)(Karsten M. et. al JACS 2003, 125, 15734)

21

3

B: using 2,2,4,4-d4-pentane(500MHz, 163K)☯ Relative intension
C1 / C2 / C3 = 6 / 6.07 / 2.90
It is so difficult to observe σ-C-H bonds unless using finely tuned complex, for example, using
low-valent, cooridnatively unsaturated, tris-aryl oxide uranium(III) complex (ex.1).
52

σ-CAM (σ-bond Complex Assisted Metathesis)
2.1

Non-classical metathesis mechanism

Some of the examples shown in the section 1 are about non-classical behavior of σ-complex. In order
to explain these phenomena, another type of metathesis mechanism was invented.

Classical mechanism(σ-BM)

Non-classical mechanism

σ-Complex

Oxidative addition,

as intermediate

Reductive elimination

OA/RE○○

σ-BM××σ -CAM○×62.2

Catalytic cycle via σ-CAM

Following examples are estimated by experimental evidences and DFT calculation.
2.2.1 Borylation(Michael B. H. et. al JACS 2005, 127, 2538)☯ From DFT calculation, the barriers
for pathways involving a OA/RE
sequence were higher than σ-CAM
pathways.
16’
16’
CH3
M
H

High TS(RO)2B

unfavorable
B(OR)2 = Bpin

18’
18’

21’
21’

☯ Step ① was not calculated by DFT,
however, reaction of the 16-electron①flagment 6’ with borane produces
stable 18-electron complex 21’21’.

20’20’(M = CpRh(Bpin))

2.2.2 Hydrogenation, HydroSilylation

7

2.3

σ-CAM for the E-E bonds (E≄≄H)☯ Ultimate version of σ-CAM.
☯ agostic C-C complex is known, but
σ-C-C complexes are not known

3

Various Types of Mechanism ~ or more simple mechanism? ~3.1

C-H bond activation mechanisms(Michael B. H. et. al JACS 2007, 129, 12068)

Recently, several groups have suggested alternative mechanisms that appear to be in between
OA/RE and σ-BM. Michael et al. tried to unify this chaotic situation.
MAσMAσBMmetal-assisted σ-bond metathesis
OATS
oxidatively added transition state
σ-CAM
σ-bond complex assisted metathesis
OHM
oxidative hydrogen migration
OA/RE
oxidative addition/
σ BMOATS/σσ-CAM
MAσσBMOHM TS or OA/RE INTOA/RE TS

reductive eliminationTS:
TS transition state
INT:
INT intermediate

A

E

DC

E

E

[(acac)2OsMe2H]-[(acac)2OsPh2H]-[(acac)2PtMe2H]+[(acac)2PtPh2H]+

E

E

B

A

(Analized with DFT(Gaussian03 B3LYP/DZP) and AIM)

Thank you8