Alkylidenation of carbonyl compound

Titanium-based reagents generated by the reduction of gem-dihalides with low-valent metal species are widely used for the alkylidenation of carbonyl compounds (Scheme 14.28). As in the case of methylidenation, the system gem-dibromide/TiCl4/Zn/TMEDA... 

Alkylidenation of Carbonyl Compounds with Dialkyltitanocenes and Related Complexes... 

Petasis and co-workers extended the above methylenation procedure to the alkylidenation of carbonyl compounds by using dialkyltitanocenes [lie, 62]. Like methylidenation with dimethyltitanocene, the Petasis alkylidenation is believed to proceed via the formation of titanocene-alkylidenes through a-elimination of dialkyltitanocenes. This assumption is supported by the isolation of the cy-clometalated product 32, which is indicative of the intermediary formation of titanocene-benzylidene 34 by thermolysis of dibenzyltitanocene 33 bearing a tert-butyl group on the Cp ring (Scheme 4.30) [83]. 

Following the same strategy as used for methylenation, general alkylidenation of carbonyl compounds would be realized by preparation of the corresponding gem-dizinc species. The preparation of these gem-dizinc species, however, is con-... 

Scheme 5.17. Alkylidenation of carbonyl compounds with gem-dizinc reagents and /J-TiClj.

Following the same strategy with methylenation, any alkylidenation of carbonyl compounds would be realized by preparation of the corresponding gem-dizinc species. The preparation of these gem-dizinc species, however, presents some difficulty, compared to the preparation of bts(iodozmcio)methane. The simple reduction of gem-dihaloalkane carrying /3-hydrogen will suffer from /8-elimination and result in the formation of elimination product. In addition, depending on the substrate, the intermediary a-haloalkylzinc compound is less stable than a-halo-methyl zinc to take a route to a-elimination (Scheme 8.17). 

BalLini and coworkers have developed a new strategy for alkenyladon of carbonyl compounds based on the Michael adthdon followed by eliminadon of HNO fsee Secdon 7 3 A variety of 2-alkylidene 1,4-tholes have been conveniently prepared, in two steps, by the Michael adthdon of a nltroalkane to the appropriate enedione derivadves under basic conthdons, followed by chemoselecdve reducdon v/ith LLAlHl CEq 4 123 ... 

The isomerization of a-alkylidene cyclic carbonyl compounds to a,/3-unsaturated cyclic carbonyl compounds was achieved by using catalytic amounts of RhCl(PPh3)3 and Et3SiH (Scheme 48).87... 

As noted above, titanocene-alkylidenes can be prepared using various methods and starting materials. Like the methylidene complex, higher alkylidene complexes are useful for the transformation of carbonyl compounds to highly substituted olefins. Ketones and aldehydes are converted into substituted allenes by treatment with titanocene-alkenylidenes prepared by olefin metathesis between titanocene-methylidene and substituted allenes (see Scheme 14.7) [17]. Titanocene-alkenylidene complexes can also be prepared from... 

Athough transition metal alkylidene complexes are successfully used for the alkenation of carbonyl compounds, various 1,1-bimetalloalkanes, often prepared by the hydrometal-lation of alkenyl organometallics, are also useful reagents for the alkenation of carbonyl compounds. 

Preparation of alkylidene cyclopropanes is feasible by alkylation of lithium carbenoids followed by base-promoted elimination (equation 13or by addition of carbonyl compounds, silylation and reductive elimination (equation 132). Similarities with the chemistry displayed by sulphur-substituted cyclopropanes (Section VI.D.l) are obvious. 

Alkylidene- and vinylcyclopropanes. The reagent effects cyclopropyl idenation of carbonyl compounds including esters and lactones. Reaction with alkynes gives vinylcyclopropanes on acidic workup. 

When reacting alkenes with triethylsilane it is necessary to keep in mind that the PdCl2/Et3SiH combination also promotes the double bond isomerization of monosubstituted aliphatic olefins and a-alkylidene cyclic carbonyl compounds are isomerized to a,/3-unsaturated cyclic carbonyls with tris(triphenylphosphine) rhodium chloride. ... 

One of the most versatile procedures for the olefination of carbonyl compounds utilizes a thioacetal-titanocene(II) system [40, 92]. The organotitanium species generated by the desulfurization of thioacetals with the triethyl phosphite complex of titanocene(II) 44 react with a variety of multiple bonds, such as those of alkenes [93], alkynes [94], and nitriles [95], as well as carbonyl functions. The reactivities observed in these reactions indicate that the active species involved are titanocene-alkylidene complexes 48 (Scheme 4.40). 

The alkylidene complex 61 readily converts a wide variety of carbonyl compounds into the corresponding olefins in good yields, with the following order of reactivity aldehydes > ketones > formates > esters > amides (Table 4.20) [134]. Replacement of two neopentoxo groups by chloro, bromo, or iodo ligands reduces... 

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