Bicyclization enynes

TITANIUM-CATALYZED ENANTIOSELECTIVE ENYNE BICYCLIZATION-CARBONYLATION... 

Since the advent of the stoichiometric Zr-promoted enyne bicyclization-carbonylation [19,20], development of its catalytic and enantioselecti ve version has been an attractive but elusive target. Although its catalytic but racemic version has recently been developed in the form of Zr-cataly zed cyclic carboalumination [39], there does not appear to be any successful report on Zr-catalyzed enantioselective enyne bicyclization-carbonylation, the closest process being that discussed in... 

TABLE 4.7. Ti-Catalyzed Enantioselective Enyne Bicyclization-Carbonylation... 

The enyne bicyclization-carbonylation tandem reaction is of great interest to synthesize various organic compounds. Possible mechanisms have been proposed as follows (Scheme 27). 

Prior to the development of enyne bicyclization reactions promoted by Zr and other Group IV metals, the Co-catalyzed enyne bicyclization-carbonylation reaction (the Pauson-Khand reaction ) was known. This reaction is discussed in Volume 5, Chapter 9.1. In the Pauson-Khand reaction, the overall transformation is the conversion of enynes into bicyclic enones, and the organometallic bicyclic intermediates are usually neither readily available nor isolated. The use of Co2(CO)s, an 18-electron species, necessitates relatively high reaction temperatures. These and other limitations suggested the desirability of developing alternative enyne bicyclization reactions. 

The results summarized in Scheme 25 indicate the following. First, lx)th Ni and Pd catalysts can induce and catalyze the desired bicyclization. Second, the product yields observed with Pd catalysts tend to be modest and lower than those observed with Ni catalysts. Both classes of catalysts have, however, been used in subsequent studies. The comparative usefulness of Ni and Pd catalysts in a given case should probably be experimentally determined. One significant factor affecting the yield of cascade bicyclization is the stereochemistry of the initial allylmetallation. If this step leads to tra i-l,2-disubstituted cyclopentane derivatives, they would not be converted to the desired bicycles, unless the stereochemistry is corrected under the reaction conditions. This stereochemical problem does not exist in cases of the enyne reaction. Presumably for this reason, the bicyclization yields appear to be more favorable in the enyne bicyclization. The cyclic allylmetaUation-Type II acylmetallation cascade mechanism shown in Scheme 26 has been suggested and generally accepted. 

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