A new chiral phosphine−phosphite ligand,
(R)-2-(diphenylphosphino)-1,1‘-binaphthalen-2‘-yl
(S)-1,1‘-binaphthalene-2,2‘-diyl phosphite
[(R,S)-BINAPHOS,
(R,S)-2a], was synthesized. Its Rh(I)
complex was prepared,
and its structure has been characterized by 1H and
31P NMR spectroscopy. Using Rh(I) complexes of
(R,S)-2a and
its enantiomer, highly enantioselective hydroformylation of styrene has
been performed (94% ee, iso/normal = 88/12). The catalyst system was also effective for a variety of other
olefins. Some other phosphine−phosphite ligands
bearing 1,1‘-binaphthyl and biphenyl backbones, such as
(S)-3,3‘-dichloro-6-(diphenylphosphino)-2,2‘,4,4‘-tetramethylbiphenyl-6‘-yl (R)-1,1‘-binaphthalene-2,2‘-diyl phosphite
[(S,R)-BIPHEMPHOS, (S,R)-5a],
(R,R)-2a, (R,S)-2b,
(R)-2c, and (R)-5b, were tested for
asymmetric hydroformylation. The results indicate that the sense
of enantioface
selection for the prochiral olefins is mainly determined by the
absolute configuration of the phosphine site, for
example, the
(R)-2-(diphenylphosphino)-1,1‘-binaphthalen-2‘-yl group in
(R,S)-2a. The relative configurations
of
the two biaryl groups in the phosphine−phosphites play crucial roles
in the degree of the enantioselectivities, that
is, the (R*,S*)-isomer generally gives products in high
ee's and the (R*,R*)-isomer does in low ee's.
Treatment of
Rh(acac)[(R,S)-2a] with a 1:1 mixture
of carbon monoxide and hydrogen gave a hydridorhodium complex,
RhH(CO)2[(R,S)-2a], as a single
species. Trigonal bipyramidal structure is suggested for this
complex, in which the
hydride and the phosphite moiety are located at the apical positions
and the phosphine and the two carbonyls occupy
the equatorial positions. The interchange of the phosphine and the
phosphite sites with each other through rapid
pseudorotations has not been observed in
RhH(CO)2[(R,S)-2a].
The structural deviations of the monohydride
complexes from an ideal trigonal bipyramid seem to be larger in
(R*,R*)-isomers than in the corresponding
(R*,S*)-isomers. The existence of only one active species involved in the
Rh(I)−(R,S)-2a-catalyzed hydroformylation
has
been manifested by the plot of
ln([R]/[S]) of the hydroformylation
product vs the reciprocals of the reaction
temperatures. The higher thermodynamic stability of
Rh(acac)[(R,S)-2a] than its
diastereomer
Rh(acac)[(R,R)-2a]
is demonstrated in relation to the restricted configuration of
(R)-2c to (R,S)-2c in its
complex formation with Rh(I).
