Fischer carbene complex photochemical reaction

In attempts to understand the photochemical reactions of Fischer carbene complexes, several matrix isolation and flash photolysis studies have been conducted using both Cr and W (but not Mo) complexes [5-11]. Although the complexes studied and conditions used varied, several general conclusions were drawn ... 

Besides the thermal reactions of Fischer car-benes, photochemical transformations of such complexes also deserve attention. Since the discovery of McGuire and Hegedus [4] in the early 1980s that irradiation of chromium carbene complexes (4) with visible light in the presence of N-substituted benzaldimines (5) leads to (racemic) )ff-lactams of type 6, photolytic reactions of Fischer carbene complexes have been intensively investigated by L.S. Hegedus and his group in Fort Collins, Colorado [5]. 

It should be noted that a key element in most of these reactions is that they are initiated by the coordination of the substrate to the metal.Since Fischer carbene complexes are coordinatively saturated, the coordination of the substrate needs to be preceded by the loss of a ligand (see, e.g., equation 8, S = substrate). This ligand loss can be initiated thermally as well as photochemically. This loss of CO is usually the rate-limiting step which is a major reason why kinetic experiments give little information about the complex steps that follow the initial ligand loss. 

Actual metal carbene complex catalysts can be divided into two broad classes, Fischer-type and Schrock-type . The Fischer-type carbene complexes are low-valent and generally characterized by the presence of one or two heteroatoms (O, N, or S) bonded to the carbene carbon. Such complexes do not normally initiate the chain metathesis of olefins, since they are both coordinatively and electronically (18e) saturated. However, they can sometimes be activated for metathesis by heating, or by reaction with a cocatalyst, or photochemically. Some examples are listed in Table 2.1. 

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]. 

---