Carbonylation photocatalysis

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

In this chapter, we focus on these imique photochemical properties of rhenium(I) diimine carbonyl complexes (Fig. 1), especially photochemical reactions and photocatalysis. 

In the bidentate structure, the oxygen atoms of the carboxyl group equally interact with the metal cation, whereas in the monodentate only one oxygen atom is coordinated to a cation preserving the carbonylic nature of the surface species. This structural difference may be relevant in photocatalysis as the excitation of the carbonyl group is believed to play a role in the photolytic mechanism to be discussed later. 

For iron carbonyl, sequential photolabilization of two CO s is implicated by the observation that photocatalysis is efficient even at -18° while thermal catalysis by the dihydride H2Fe(CO)4 is very inefficient at this temperature [57]. Thus, a sequence of steps such as Scheme I has been suggested as the catalytic mechanism. High quantum yields have been demonstrated for such sequential CO substitution by (E-)cyclooctene for irradiation of Fe(CO)5 solutions in hexanes [64]. The Fe(ri2-olefin)2(CO)3 complexes are extremely labile thermally as required for a likely participant in photocatalysis of alkene hydrogenation or isomerization (see below). 

Scheme I Proposed mechanism for photocatalysis of alkene hydrogenation by iron carbonyl...

Scheme II Proposed mechanisms for photocatalysis of norbornadiene hydrogenation by the group VI carbonyls M(CO)6...

The substituted iron carbonyls Fe(CO)4PPh3 and Fe(CO)3(PPh3)2 have also been examined as photocatalysts for hydrosilation however, the qualitative reactivity features were found to be similar to those of Fe(CO)s [74]. In addition the polymer anchored derivatives Fe(CO)4(PPh2-poly) and Fe(CO)3(PPh2-poly)2 (where PPh2-poly is the polystyrene bound diphenyl phosphine) proved to be effective photocatalysts for lx)th hydrosilation and hydrogenation [74]. Photocatalysis of alkene hydrosilation is also effected by metal carbonyl clusters [61]. 

Photocatalysis of alkene isomerizations and related processes has also been noted for a number of other common carbonyls including Fe(CO)s, Fe(CO)4PPh3, Mo(CO)6, W(C0)6, Ru(CO)4PPh3 and Ru3(CO)i2 [61,74,79-82], In some cases quantum yields substantially in excess of unity have been reported consistent with the photolytic formation of a catalytically active species capable of numerous turnovers. The photocatalysis of 1-pentene hydrogenation by Fe(CO)s is accompanied by isomerization of the substrate to 2-pentene [57], Such a pathway could occur via a -hydride elimination pathway comparable to the reverse of step 2 in Scheme I, but a more likely pathway would be via a C-H insertion reaction of the intermediate Fe(CO)3(alkene) to give an alkyl hydride intermediate as in eq. 20. 

Another interesting photocatalytic process involving iron carbonyl is the addition of the cyclic tetrafluorodisilane to cyclohexadiene [86]. Thermally this occurs at 100° C to give the tricyclic product of 1,4 addition while photocatalysis at ambient temperature with Fe(CO)5 gave the bicyclic product of 1,2 addition.(eq. 22)... 

Photocatalysis of benzene carbonylation (eq. 24) has been reported for the complexes Ru(CO)3(PPh3)2 and Ru(CO)4PPh3 in benzene under CO (500-800 torr) using a pyrex filtered 200 W Hg/Xe lamp [96]. 

Others have reported analogous photocatalyzed polymerizations when W(CO)6 is irradiated in halocarbon solvents in the presence of alkynes and halocarbene complexes have been suggested as the key intermdiates [110]. Halocarbene intermediates have also been implicated in the photocatalysis of olefin metathesis reactions by the group VI carbonyls M(CO)6 in carbon tetrachloride [111,112]. 

Aldehydes and ketones are useM building blocks in organic synthesis. The direct a-C-H substitutions of carbonyl compounds are well known. However, selective P-C(sp )-H functionalization remains rare. The MacMillan group introduced Site activation model by dual aminocatalysis and photocatalysis, opening up a practical synthetic route to P-substituted aldehydes and ketones (Scheme 3.25). With this novel strategy, radical-radical coupling of enaminyl radical with electron-poor cyanobenzene radical anion can elegantly produce P-aiylated aldehydes and ketones [74]. A recombination of enaminyl radical with imine anion radical was also developed [75]. In the presence of Michael acceptors, radical addition of enaminyl radical to electron-deficient alkenes affords P-alkylated aldehydes [76]. 

One of the most widely studied examples of photocatalysis by organometallic complexes is the photocatalytic hydrogenation of dienes by Group VI transition metal carbonyls M(CO>6 (M = Cr, Mo, W). One of the earliest discoveries was the finding that Cr(CO)6 is a good catalyst for the light-induced hydrogenation of 2,3-dimethylbutadiene and 1,3-cyclohexadiene to 2,3-dimethyl-2-butene and cyclohexene respectively ... 

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