Carbon monoxide transition metal adducts

Few examples of stereoselective transition metal catalyzed carbonylative multicomponent cycloadditions leading to other than five-membered rings have been reported1 72. Typical is the reaction of 3,3-dimethylcyclopropene with carbon monoxide, catalyzed by a palladium(O) complex, to stereoselectively form hexamcthyltrishomotropone as a tetracyclic adduct l73. 

Carbon monoxide is an extremely weak Lewis base towards conventional Lewis acids. It does not complex with the boron trihalides, although it does yield a weak adduct H3B CO with diborane. On the other hand it forms numerous complexes with transition elements. The source of this difference is that in the latter complexes not only is the weakly donating a-orbital of CO involved, but also the 7C orbitals which can function as acceptors. The conventional explanation is that a synergic effect exists in which the n interaction removes electron density from the metal, allowing a donation from the ligand to be enhanced. 

In spite of the usefulness of these complexes, it is generally not possible to cause the satisfactory reaction with transition metals in the metallic state [86] under mild conditions due to their poor reactivity. We have reported that activated metallic nickel, prepared by the reduction of nickel halide with lithium, underwent oxidative addition of benzylic halides to give homocoupled products [45]. We reported that carbonylation of the oxidative adducts of benzylic halides to the nickel proceeded smoothly to afford symmetrical 1,3-diarylpro-pan-2-ones in moderate yields, in which the carbonyl groups of alkyl oxalyl chlorides served as a source of carbon monoxide [43] see Equation 7.5. 

Electrocatalysis by transition metal complexes is elegantly illustrated by the work of DuBois and colleagues.2 In the absence of CO2, the palladium (II) complex undergoes two-electron reduction, but when CO2 is present, the one-electron reduction product binds CO2 (DMF as solvent) (Figure 5.9). With added acid, carbon monoxide is produced catalytically from carbon dioxide. This chemistry likely involves the protonated carbon dioxide adduct (hydroxycarbonyl complex) shown in Figures 5.9 and 5.10. Catalytic turnover numbers greater than 100 have been reported for this and related compounds. Some hydrogen is produced in parallel, evidently via a hydride complex. 

---