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Scheme 4.52 Synthesis and 31 P NMR data for [Rh(CO)2dppb]BF4 and related complexes.
Rh +
O
C
O
C
BF 4-
dppb
(1 equiv)
DCE
Ph Ph
P
Rh
P
Ph Ph
326
31 P NMR:
δ = 22.1 (d, J (Rh-P) = 123 Hz)
+ - SbF6 Ph Ph
P
Rh
P
Ph Ph
BF4- CO (1 atm)
O
C
Ph Ph
P
Rh
DCE, 80
O C
Ph
P
Ph
o + + BF4- C
323 324 325
31 P NMR:
δ = 23.5 (d, J (Rh-P) = 144 Hz)
Ph Ph
P P
Rh
Ph
Ph
P
Ph Ph
31 P NMR:
δ = 21.4 (d, J (Rh-P) = 119.7 Hz)
P
Ph Ph
327
31 P NMR:
δ = 20.6 (d, J (Rh-P) = 136 Hz)
With 325 in hand, it was tested in promoting the cyclocarbonylation reaction of 74c. As
expected, reaction did not occur under full atmosphere of CO. Similar phenomenon of CO
impeding the rate of the Rh(I)- or Ir(I)-catalyzed cyclocarbonylation reaction has been observed
by Narasaka and Shibata, and is attributed to competitive binding of CO and the substrate to the
catalyst. 223, 229 Therefore, using partial pressure of CO to catalyze the reaction was attempted. It
was soon discovered that the reaction can be driven to completion under partial pressure of CO
(0.2 atm), and heating to 80 °C. ee In this manner, using 5 mol % of catalyst resulted in 87% yield
of the α-alkylidene cyclopentenone (Scheme 4.53). It was found that either the pre-isolated
catalyst or a freshly prepared solution can be used without noticeable difference in reactivity and
yield. The product was obtained as a mixture of diastereomers 288c : 287c = 2.2 : 1 ratio which
were separated by semi-preparative HPLC (normal-phase, 35% EtOAc/hexanes). This is
ee The partial pressure of CO is approximate, and it was created by injecting a measured volume of CO from a
syringe into a sealed reaction vessel containing a solution of substrate and catalyst. Reactions were most commonly
performed on ~20 mg scale in a 15 mL test tube (3 mL of CO was injected).
134
+ BF4-