1006 Bull. Chem. Soc. Jpn. Vol. 82, No. 8, 1006–1008 (2009)
Short Articles
(a)
Cl
R
Sulfide-Bearing Phosphine Ligands:
Their Pd Complexes and Application
to Copolymerizations of
PPh2
PPh2
TMS
TMS
S
S
S
S
Olefins and CO
S Ph
R =
(b)
Ph
S
1
2
3
4
Ken Sakakibara and Kyoko Nozaki*
S
S
Ph
S
Department of Chemistry and Biotechnology,
Graduate School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
P
Pd Cl
CH3
P
Pd Cl
CH3
Ph
Ph
Ph
Ph
5
6
Received March 13, 2009; E-mail: nozaki@chembio.t.u-
TMS
tokyo.ac.jp
S
TMS
Ph
S
S
Various types of sulfide-phosphine ligands and their Pd
complexes were synthesized and applied to copolymerization
of olefins with carbon monooxide.
P
Pd Cl
CH3
P
Pd Cl
CH3
Ph
Ph
Ph
Ph
7
8
Figure 1. Phosphine-sulfide ligands 1-4 and Pd complexes
5-8. The stereochemistry represents relative configuration
but not absolute.
Sulfur ligands have been used much less than phosphorus
ligands, although the number of studies on catalytic systems
using sulfur ligands has increased notably in recent decades.1-3
Compared to phosphorus ligands, sulfur ligands have both
lower donor and lower acceptor characteristics. Moreover, the
sulfur atom has only two substituents to create a less hindered
environment than trivalent phosphorus. Thus, the use of sulfur
ligands in reactions catalyzed by transition metals is still
relatively unexplored.
Efficient carbonylation of MeOH by using mixed P-S
ligands has been reported.4,5 By comparing Ph2PCH2P(=S)Ph2
(dppms) with Ph2PCH2CH2PPh2 (dppe), Haynes reported the
acceleration of CO insertion to a methyl-rhodium bond, which
was explained to result from (1) the higher steric demand for
migration, and (2) the higher ·-bonding character of the sulfur-
rhodium bond to stabilize the transition state when the sulfur is
located trans to the carbonyl.6
Figure 2. ORTEP drawing of 5 (all H atoms are omitted for
clarity).
Thioether ligands have higher stability, allowing easier
storage and handling than phosphorus ligands. In addition, it is
easier to synthesize thioether-phosphine ligands and change
both their electronic states and steric states,2 compared with
dppms derivates.7 From these viewpoints, it would be valuable
to attempt to synthesize sulfide-bearing phosphine ligands
by assuming that thioether ligands possess the trans effect of
sulfur for CO insertion as well as dppms. Dilworth reported
phosphine-thioether ligands represented as [ortho-(alkylthio)-
phenylmethyl]diphenylphosphines which showed low activity
for ethylene/CO copolymerization.8 Here we report synthesis
of a new classes of mixed P-thioether ligands and their
application to alkene/CO copolymerization.9-12
Supporting Information). In particular, ligands 3 and 4 are
expected to accelerate CO insertion because of steric strain due
to bulky TMS groups, which is based on Haynes’ insight.6 The
metal complexes (S-P)PdMeCl 5-8 were synthesized from S-P
ligands 1-4 and PdClMe(cod) (Figure 1b). The metal com-
plexes (S-P)PdMeCl 5-8 were precursors of catalysts (vide
infra).
Single-crystal X-ray analyses of 5-8 revealed their structures
and their ORTEP views are shown in Figures 2-5. Judging
from their bond angles and bond lengths, all the local
geometries at the palladium atoms of 5-8 are square planar
with Pd, P, S, C, and Cl. The methyl groups and phosphorus
atoms of 5-8 are coordinated cis on the palladium centers. In
the case of 6, it should be noted that the pallada-cycle created
by coordination and the dithiane ring are trans to each other.14
On the other hand, due to the steric effect of the TMS group,
the pallada-cycle of 7 created by coordination and the dithiane
ring are cis to each other.
First, it was noted that o-(diphenylphosphino)benzyl chlo-
ride was a good precursor for mixed P-S ligands.13 The new
ligands 1-4 (Figure 1a) were prepared by the reaction of o-
(diphenylphosphino)benzyl chloride together with the lithiated
thioalkoxide, lithiated dithianes, and lithiated sulfide (See