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anched and linear carboxylic acid is possible. Van Leeuwen and coworkers found that use of
PPh3 leads to predominant formation of the branched product. As shown in Table 4.7, switching
to bidentate phosphine ligands leads to gradual reversal of the selectivity, and with dppb (1,4-
bis(diphenylphosphino)butane) the linear product is favored. In addition to the product ratios, the
efficiency of the reaction was also dependent on the nature of the diphosphine ligand (see
conversion column in Table 4.7). The authors associated these effects to the increasing bite angle
of the diphosphine ligand.
Table 4.7 Effect of the phosphine ligand on the Pd-catalyzed hydroxycarbonylation of styrene
reported by van Leeuwen.
319
4 mol % [PdCl 2(PhCN) 2]
phosphine, a CO, 100-150 o C
oxalic acid, dimethoxyethane
20-24h
COOH
320 - branched 320 - linear
COOH
phosphine bite angle (deg) % conversion branched : linear ratio
PPh 3 96 84 : 16
dppe 78 5 47 : 53
dppp 86 28 23 : 77
BINAP 93 42 30 : 70
dppb 98 95 17 : 83
a P : Pd ratio = 4 : 1
To examine whether double-bond selectivity in the Rh(I)-catalyzed allenic cyclocarbonylation
reaction can be influenced with diphosphine ligands, terminally unsubstituted allenes 74
(Scheme 4.50) were chosen to minimize steric bias, and avoid formation of E and Z isomers in
the reaction. Preliminary results of this investigation are presented here.
A preliminary screen of common bidentate phosphine ligands (dppm, dppe, BINAP and
dppb) was first preformed. It was found that the reactive species prepared by addition of dppb to
10 mol % [Rh(CO)2Cl]2 followed by AgBF4 (in 2 : 1 : 2 molar ratio) effected the transformation
131