Carbon monoxide bond formation, mechanism

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

Allyl methylcarbonate reacts with norbornene following a ruthenium-catalyzed carbonylative cyclization under carbon monoxide pressure to give cyclopentenone derivatives 12 (Scheme 4).32 Catalyst loading, amine and CO pressure have been optimized to give the cyclopentenone compound in 80% yield and a total control of the stereoselectivity (exo 100%). Aromatic or bidentate amines inhibit the reaction certainly by a too strong interaction with ruthenium. A plausible mechanism is proposed. Stereoselective CM-carboruthenation of norbornene with allyl-ruthenium complex 13 followed by carbon monoxide insertion generates an acylruthenium intermediate 15. Intramolecular carboruthenation and /3-hydride elimination of 16 afford the -olefin 17. Isomerization of the double bond under experimental conditions allows formation of the cyclopentenone derivative 12. 

The principal difficulty with metal only mechanisms is to find a plausible way of activating the oxygen molecule. The problem is discussed in Section 5.2.2 formation of the Oj ion, in which the 0-0 bond is considerably weakened (see Table 5.2), appears possible on small particles,130,131 and the suggestion that the adsorption of oxygen and of carbon monoxide is mutually supportive opens a possible route for a metal-only mechanism. 

The possible reaction mechanism for the formation of 31 is shown in Scheme 15. Insertion of alkyne 14 into silazirconacyclopropane 3 gives silazirconacyclopentene 22. Then, insertion of carbon monoxide into the carbon-zirconium bond in silazirconacyclopentene 22 gives silazirconacyclohexenone 34, whose carbonyl oxygen would coordinate to zirconium metal. Then the zirconium carbon bond migrates onto silicon to afford oxazirconacyclohexene 36 via 35 [26]. Deuterolysis of 36 would afford 31-D2, which has two deuteriums. 

By the nature of its molecular mechanism, the carbonyl-insertion reaction represents a typical reaction mode of o alkyltransition metal complexes. Formation of the new C—C cr-bond takes place during a 1,2-alkyl-migration step, transforming an alkylmetal carbonyl moiety [cts-M(CO)R] into an acylmetal unit (M—COR) (89). In general, (s-cir-diene)-zirconocene complexes 5 appear to exhibit a substantial alkylmetal character (90). Therefore, it is not too surprising that some members of this class of compounds [in contrast to most other dienetransition metal complexes (97)] react with carbon monoxide with C—C bond formation (45). However, as demonstrated by X-ray structural data for 5 (Tables V... 

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