Ir bonding complexes

We are currently trying to answer specifically the question of whether ir-bonded complexes do occur in certain cases where insertion reactions are observed. I think they do because I believe that the same factors which favor stabilization of this type of transition state will also tend to favor formation of 7r-bonded olefin complexes, which are only slightly removed from this. At the moment Bern Tinker is examining the insertion of olefins in mercuric complexes to see whether there is any indication of 7r-bonded intermediates. In his paper, Dr. Heck referred to some unpublished work relevant to this theme. I would certainly be interested in anything more he can tell us about that. 

The epoxy alctrfiol (97), a key intermediate in the synthesis of maytansine, has been prepared through Ti-catalyzed epoxidadon of (95 equation S6). The alcohol (95) exists predominantly in confoimation (162), with the allylic hydrogen at C-4 and the ir-bond very nearly eclipsed. The oxygens of the alcohol and silyl ether which are located below the plane the ir bond complex with Ti this complex blocks the approach of the epoxidizing reagent fitom the a-fu and hence the P-epoxide is fmmed. It is of interest to note that the ir-facial selectivity resulting fiom this route is the opposite of the ir-facial selectivity observed in MCPBA epoxidadon (see equadon 33). 

Hexachlorobenzene was very slowly photolyzed by direct or acetone-sensitized mechanisms (Choudry and Hutzinger, 1984), but it was photodecomposed much more rapidly (10-6(K)x) in the presence of various amine and indole derivatives (Hirsch and Hutzinger, 1989). Pentachlorobenzene was the only product identified in the amine experiments. Although no mechanistic speculations were engaged in by the authors, it appears plausible that electron transfer within ir-bonded complexes in the excited state could account for the observed rate enhancements. 

J. A. Davies, Comp. Organomet. Chem. II, 1995, 9, 291-390. PaUadium-Carbon ir-Bonded Complexes. 

Nickel-Carbon ir-Bonded Complexes, Pages 133-196, D. Zargarian... 

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