Photo-induced decarbonylation

The standard method of synthesis involves the reaction of suitable transition metal halides with Fc-Li [49, 113, 171 — 180, 189, 190], although HgFcj has also been used as ferrocenyl transfer reagent [181 — 183]. A different approach to ferrocenyl complexes proceeds via thermal or photo-induced decarbonylation of the corresponding ferrocenoyl compounds [178,184]. All three routes have been applied to prepare the half-sandwich/sandwich complex CpFe(CO)2-Fc, which contains two different iron centers (Scheme 5-24). It is surprising that Fc-SnMej has rarely been used [185] for ferrocenyl transfer. 

This ruthenium complex is one of the most efficient of known homogeneous catalysts for hydrogenation, but has not found wide use because of the difficult preparation. Recently Geoffrey and Bradley found that it can be prepared efficiently by photo-induced decarbonylation of the relatively stable RuHCI(CO)-[P(C6H5)3]3 in benzene. The solutions can be used directly or the complex can be isolated in slightiy impure form. 

Cymantrene (6) demonstrates a considerable amount of organic chemistry just as does ferrocene. No organic photochemistry has been found in cy-mantrenyl systems, however, due to the customary complication of photo-chemically induced decarbonylation. E. O. Fischer55 has investigated this in some detail and three representative reactions are listed here. 

Decarbonylations can occur by either an ionic or a radical mechanism. The former mechanism is involved in reactions catalysed by acids, Lewis acids, or bases, and the latter in chain reactions induced thermally, photo-lytically, or by some other radical-forming process. 

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