Photolytic alkene complex formation

Rhodium and iridium complexes of basic trialkylphosphines catalyze the dehydrogenation of alkanes to alkenes and H2, or the transfer dehydrogenation of alkanes in the presence of H2 receptors such as CH2=CHBu . For example, RhCl(CO)(PMe3)2 becomes active after photolytic CO dissociation, while the compounds RhClLy (L = PMe3, PPr 3 or PCy3) are thermally active. The initial step is the formation of a Rh111 dihydride 135... 

The reaction system (6-37) includes the thermal azo-extrusion of a cyclic azo compound to a cyclopropane derivative and the direct formation of cyclopropanes, catalyzed by metal complexes. Synthetic routes to cyclopropane derivatives became an important subject in the last two decades, and one frequently used method is the 1,3-dipolar cycloaddition of a diazoalkane to an alkene followed by thermal or photolytic azo-extrusion of the 4,5-dihydro-3//-pyrazole formed to the cyclopropane derivative (6-37 A). This route can be followed in many cases without isolation, or even without direct observation, of the 4,5-dihydro-3//-pyrazole. Therefore, it is formally very similar to cyclopropane formation from alkenes with diazoalkanes, in which a carbene is first formed by azo-extrusion of the diazoalkane (see Sect. 8.3). As shown in pathway (6-37 B), this step can be catalyzed by copper, palladium, or rhodium complexes (see Sects. 8.2, 8.7, and 8.8). There are cases where it is not clearly known whether route A or B is followed. Scheme 6-37 also includes... 

The probable role of tricarbonyl complexes, e.g., Fe(CO)3(alkene)2, in these isomerization cycles has been demonstrated by preparing these species photolytically from Fe(CO)5 [66,79]. Both Fe(CO)3(alkene)2 (and Ru(CO)3(alkene)2) proved active thermal catalysts for the isomerization of 1-pentene at 293 K. However, a competing dehydrogenation process which leads to the formation of stable and catalytically inactive (at 298 K) M(CO)3(Tl -l,3-pentadiene) complexes serves to deactivate these... 

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