Fischer Carbene Photochemistry

Fischer-type carbenes, 8, 218 iV-heterocyclic carbenes, 8, 219 non-heteroatom carbenes, 8, 217 overview, 8, 197—268 photochemistry, 1, 252 with Re(I), 5, 900 with rhodium, 7, 184 rotation, dynamic NMR studies, 1, 420 in Ru and Os half-sandwich preparations, 6, 575 silver-catalyzed insertions, 9, 568 with silver(I), 2, 203 as spectator ligands, 1, 66... 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

Reactions A carbyne can couple with another carbyne to give an alkyne or alkyne complex.For instance, Br(CO)4Cr CPh reacts with Ce(IV) to give free PhC CPh. Carbynes also have extensive photochemistry. In the Fischer series, the carbyne carbon is electrophilic and subject to nucleophilic attack, for example, by PMcj, pyridine, RLi, or isonitrile (= Nu) to give a carbene of the type L M=CR(Nu). Alternatively, the nucleophile may attack the metal in L (CO)M=CR and produce a ketenyl complex... 

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