Correspondence methylene

Reduction of tosylhydrazone derivatives to the corresponding methylene analogs with metal hydride complexes provides an excellent solution to this problem. Soon after the discovery of this reaction it was recognized... 

Dimethyl ketals and enol ethers are stable to the conditions of oxime formation (hydroxylamine acetate or hydroxylamine hydrochloride-pyridine). Thioketals and hemithioketals are cleaved to the parent ketones by cadmium carbonate and mercuric chloride. Desulfurization of thioketals with Raney nickel leads to the corresponding methylene compounds, while thioenol ethers give the corresponding olefin. In contrast, desulfurization of hemithioketals regenerates the parent ketone. ... 

A characteristic feature of picolines and many azoles is the well-known ability of methyl (and corresponding methylene) groups to undergo condensation of the aldol, crotonic, and Michael type. This is especially pronounced in the quaternary salts of these heterocycles where it occurs under comparatively mild conditions. Such condensations are not unknown for alkylisoxazoles. Lampe and Smolinska were the first to describe the condensation of the quaternary alkyl iodides of 3,5-dimethyl- (96) and 3-methyl-5-phenylisoxazole with... 

Another important synthetic method for the reduction of ketones and aldehydes to the corresponding methylene compounds is the Woljf-Kishner reduction. This reaction is carried out under basic conditions, and therefore can be applied for the reduction of acid-sensitive substrates it can thus be regarded as a complementary method. The experimental procedure for the Clemmensen reduction is simpler however for starting materials of high molecular weight the Wolff-Kishner reduction is more successful. 

It is interesting to see that amines can be deaminated to give the corresponding methylene compounds with low-valent titanium (TiCE/Li/THF). " ... 

The cycloadduct of an a,a -dioxo-thioketone with antracene,84 treated at low temperature with the Nysted reagent and TiCl4, transforms a ketone group to the corresponding methylenic group. This product is thermically decomposed via retro Diels-Alder reaction into the unstable a, p-unsaturated-a -oxo derivative (Scheme 10). 

Accordingly, diazobenzene and nitrous acid are related to each other in the same way as are phenylhydrazine and hydroxylamine the latter two substances, in fact, yield the same reaction products with carbonyl compounds as the former two do with the corresponding methylene derivatives. 

Heteroatom-substituted carbene complexes are less electrophilic than the corresponding methylene, dialkylcarbene, or diarylcarbene complexes. For this reason cyclopropanation of electron-rich alkenes with the former does not proceed as readily as with the latter. Usually high reaction temperatures are necessary, with radical scavengers being used to supress side-reactions (Table 2.16). Also acceptor-substituted alkenes can be cyclopropanated by Fischer-type carbene complexes, but with this type of substrate also heating is generally required. 

Reduction of aldehydes and ketones to the corresponding methylene compounds using amalgamated zinc and hydrogen chloride. 

Wolff-Kishner reduction of ketones that bear other functional groups sometimes give products other than the corresponding methylene compound. Some examples are... 

Bislactones 34 were prepared similarly from the corresponding methylene bisketolactones.75,76... 

Deoxygenation of carbonyl compounds (6, 98 7, 54 8, 79-80). This easily prepared borane is as effective as catechol borane for reduction of tosylhydrazones of carbonyl compounds to the corresponding methylene compounds. 

The reduction of tosylhydrazones can also be performed with sodium borodeuteride in boiling methanol or dioxane, but the mechanism of this reaction (in boiling dioxane at least) is radically different from that of the lithium aluminum deuteride reductions.82 With sodium borohydride the first step is apparently hydride attack on the carbon atom of the C=N bond which is probably concerted with the elimination of the tosylate anion (110 - 111). Migration of the hydrogen from nitrogen to C-3 in (111) concerted with expulsion of nitrogen, provides the corresponding methylene derivative (100).82... 

Reduction of steroid tosylhydrazones Steroid tosylhydrazones can be reduced to the corresponding methylene compounds in 55-75% yield by NaBFLt in HOAc without reduction of alkoxycarbonyl substituents. 

The tetrahydropyrazine ring in the 5-methyluracil cation-radical is nonaromatic spin delocalization in the corresponding methylene radical is impossible. Correspondingly, this cation radical does not expel the proton at all (Rhodes et al. 1988). 

Higher Vinylogs of Cyanine Dyes. Pentamethinecyanine dyes can be obtained analogously to trimethinecyanine dyes from the methyl-substituted heterocyclic quaternary salts by condensation. Heptamethinecyanine dyes are obtained by treating the quaternary salts of 2- or 4 -m e t h y 1 - A - h c t c r o cy c 1 e s or the corresponding methylene compounds with pentamethinestreptocyanine dyes. 

In the first case, the reaction produces a methylene-diphenol (MDPh), i.e molecules in which two phenolic rings are linked by a methylene group. In the second case, the reaction produces an aromatic ether, i.e., a molecule in which two aromatic rings are linked by a dimethyl-ether group. However, these compounds are relatively unstable and rapidly decay (producing a formaldehyde molecule) to the corresponding methylene-diphenols (MDPh). Thus, the first reaction step is the rate limiting step, whereas the second one determines the final product and the total stoichiometry of the reaction. 

Clemmensen-type reduction.1 Aromatic ketones can be reduced to the corresponding methylene compounds with ammonium formate on transfer hydrogenation in acetic acid catalyzed by 10% Pd/C. The reduction is usually complete in 10-30 minutes at 110°. Halo and nitro substituents can be reduced under these conditions, and a,p-unsaturated carbonyl groups are reduced to saturated carbonyl groups. 

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