Reactions Under Reduced CO Pressure

The rhodium-catalyzed PK reaction has many unique features as compared to the original cobalt-mediated version. For example, most of the rhodium-catalyzed reactions could be carried out under only 1 atm CO. Initially, these reactions were expected to proceed more effectively under higher CO pressures. However, experimental results demonstrated that the reaction was retarded by elevated CO pressure ( 1 atm.), while the reaction at a lower CO pressure (0.1 atm.) proved to be superior [13b]. For example, the reaction of la with [RhCl(CO)2]2 under a high pressure of CO (3 atm.) afforded 2a in 70% yield, but was not complete after 36 h. The analogous reaction under a mixed CO/Ar atmosphere (1 atm. CO/Ar=l 9) was complete within an hour to afford 2a in 90% yield. 

The advantage of these reaction conditions became more obvious with the enyne 11, in which the reacting olefin is attached to an electron-withdrawing substituent. As mentioned earher, the decarboxylation product 21 of the initially formed keto ester 21 was the major product, due to the prolonged reaction time (Eq. 3). Gratifyingly, when the reaction was carried out under a reduced pressure of CO, the reaction time was significantly reduced to facilitate the isolation of the keto ester 21 as the major product (Eq. 9). 

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