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X
CO
CO
X
CO
P
R
N
Co
Co
CO
P
X
X
Ph
R
X
X
N
X
X
CO
CO
P
P
OC
CO
Co
Co
OC
CO
R = alkyl
X = Ph, F, OCH(CF 3 ) 2 ,
pyrrolyl, BINOL
R
Ph
X
X
N
X
X
O
1h
P
P
OC
CO
Co
Co
Ph
CO
OC
Ph
< 83% yield
2h (<17% ee)
Scheme 6.19 Preparation of C 2 -symmetric dicobalt-alkyne clusters using bis(phos-
phoramidite) ligands that enforce bridged coordination, and their subsequent PKR.
6.5.5 C-Chiral Bidentate (P,S) Ligands
Since 2000 Riera and Verdaguer's group has carved out a niche in the area of asymmetric
intermolecular PKRs, through the introduction of a series of bidentate ligands that combine
phosphorus and sulfur coordination. These ligands incorporate two key features that we
introduced earlier in this chapter: a bidentate scaffold containing a single-atom linker, to
enforce bridged coordination; and a sulfide moiety for hemilabile coordination to cobalt,
which is beneficial for reactivity (as previously demonstrated in the context of chiral
auxiliaries). 34
A relevant precedent of this work was found in a paper from Mays describing the coor-
dination of the alkylthiomethyl diphenylphosphines 27 with electron-deficient acetylenes
such as 26 35 (Scheme 6.20). The ligands 27 can be considered as achiral analogs of the lig-
ands subsequently developed by the Riera and Verdaguer group. An X-ray diffraction study
of the resulting dicobalt-alkyne complex 28a confirmed the bridged coordination of these
ligands. Although these complexes have not been evaluated for the PKR, the reversibility
of the sulfide coordination in solution inspired development of the chiral P,S ligands and
could foreshadow success of these scaffolds as PKR ligands.
Ph
PS R
Ph
RS
Ph
Ph
Ph
Ph
R
OC
CO
27a R = Me
P
PS
CO
OC
CO
27b R = Ph
CO
- CO
CO
OC
OC
Co
Co
Co
Co
Co
Co
OC
CO
OC
CO
OC
CO
+ CO
Toluene
60 °C
Toluene, 40 °C
- CO
MeOOC
COOMe
COOMe
MeOOC
COOMe
MeOOC
28a R =Me
28b R =Ph
26
Scheme 6.20 Bridged coordination of achiral (P,S) bidentate ligands.
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