Photocarbonylation

Boese and Goldman investigated the photocarbonylation of cyclohexane 105 cocatalyzed by d8 metal complexes, such as 106, and aromatic ketones, like acetophenone 107 or benzophenone, as sensitizers (Fig. 27) [170]. The transformation furnished cyclohexanecarbaldehyde 108. The formation of 108 was... 

Primary, secondary, and tertiary alkyl chlorides as well as aryl chlorides undergo reductive dechlorination using Sml2, and in the presence of CO ketones are formed by photocarbonylation. ° ... 

Several examples of transition metal-catalyzed insertions of carbon monoxide and isocyanide into the C-H bond are known. The carbonylation of a C-H bond to an aldehyde requires photoirradiation conditions. Eisenberg et al. have found iridium-[45,46] or rhodium-catalyzed [47] photocarbonylation of benzene affording benzaldehyde, albeit with low efficiency [45-47]. They have also reported the photochemical carbonylation of benzene catalyzed by ruthenium(O) complexes [48]. 

The mechanism proposed for the photocarbonylation of benzene is presented in Scheme IV-24 [46b] (also see [46c-e]). 

Scheme IV,24, The mechanism of benzene photocarbonylation catalyzed by RhCl(COXPMe3)2.

Bond activation is the result of insertion of a metal fragment into a C—H bond, giving rise to oxidative addition as shown in Eq. (1). This process, if it can be commercially exploited, could be of use in the homologation of inert alkane feedstocks. The general topic of C-—H bond activation is extensively treated in Hill s book. Of special interest are the chapters by Crabtree, Jones, and Rothwell. Tanaka s summaries of his work in photocarbonylation using the [RhCl(CO)-(PMe3)2] catalyst are also important contributions. 

It is not clear what role is played by the K2CO3 base. The reactions were carried out in methanol solution, and it has previously been demonstrated that certain of the above carbonyls react rapidly with methoxide to form methoxycarbonyl adducts [98]. It is likely that such adducts play a significant role in the photocarbonylation processes. 

The system using the trimethylphosphine-substituted rhodium(I) complex RhCl(CO)(PMe3)2 has been studied in more detail. The photocarbonylation of benzene at 1 atm of CO in the presence of this complex gives benzaldehyde, along with other carbonylation products such as benzyl alcohol and benzophenone. Alkanes can also be photocarbonylated to aldehydes under mild conditions in the presence of RhCl(CO)(PMe3)2 with a high selectivity for carbonylation at the terminal position ... 

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