PHOTOCHEMICAL-REACTIONS OF GROUP-6 METAL-CARBONYLS IN CATALYTIC TRANSFORMATION OF ALKENES AND ALKYNES

This review considers some results that illustrate one of the differen t categories of photocatalysis where light is used to generate a therm ally active catalytic species C from a catalytically inactive precurso r, the nominal catalyst N. In photocatalytic reactions presented here, the nominal catalyst N can be a Group 6 metal carbonyl, M(CO)(6), and substrates are alkenes or alkynes. This paper summarizes the most int eresting approaches in each of three photocatalytic reactions: polymer ization of alkynes; isomerization of alkenes; metathesis of alkenes. S pecial attention is given to catalytically active intermediate species formed in photocatalytic systems and to the reaction mechanism. With the application of the low-temperature method formation of alkyne comp lexes of the type [(eta(2)-RC=CH)M(CO)(5)], [R(H)C=C=M(CO)(5)] and [(e ta(2)-RC=CH)M(CO)(4)], intermediate compounds proposed for the photoca talytical polymerization of terminal alkynes have been shown. The bis( alkene) tetracarbonyl complexes of tungsten are good prototypical cata lysts for isomerization and metathesis of olefin. The rearrangement of these compounds to pi-allyl hydrides was monitored by H-1 NMR. The [( CO)(4)W(mu-Cl)(3)W(SnCl3)(CO)(3)] formed in the photochemical reaction of W(CO)(6) and SnCl4 can be regarded as a prototypical catalyst for the metathesis of alkenes. (C) 1997 Elsevier Science S.A.