Methylenation titanium-zinc

Methylene-2-azetidonones, 536 Methylene bromide-zinc-titanium(IV) chloride, 337-338... 

M ETHYLENATION Dimethoxymethane. Dimethyl methylphosphonate. Methylene bromide-Zinc-Titanium(IV) chloride. Methylenetriphenylphosphorane. N-Methylphenylsulfonimidoylmethyllithium. Titanium(O). N,N,P-Trimethyl-P-phenylphosphinothioic amide. Trimethylsilylmethyllithium. 

Another titanium-based reagent for the methylenation of carbonyl compounds is that prepared from dibromomethane/zinc/titanium tetrachloride and related systems (Scheme 14.25) [48]. These systems transform a wide variety of carboxylic acid derivatives to terminal olefins in the same way as titanocene-methylidene does. 

The role of titanium salt is to activate the carbonyl compounds as Lewis acid. As described above, bis(iodozincio)methane (3) is nucleophilic enough to attack the carbonyl group of aldehydes or ce-alkoxyketones. In the reaction with simple ketones or esters, however, the addition of titanium salt is necessary to facilitate the nucleophilic attack. Instead of this Lewis acid activator, simple heating may induce the nucleophilic attack. Treatment of 2-dodecanone with 3 without titanium salt at higher temperature, however, does not improve the yield of alkene (Scheme 13). The reason for the low reactivity of 3 at higher temperature comes from the structural change of 3 into the polymeric methylene zinc 4 through the Schlenk equilibrium shown in equation 740. 

Diiodomethane-Zinc-Titanium(IV) isopropoxide, 115 Iodomethyltrimethylsilane, 315 Tributyl(iodomethyl)tin, 314 a-METHYLENE-y-BUTYROLACTONES (see Un-saturated Lactones)... 

In addition to zinc oxide, other sensitizers of photoxidation may be employed. The addition of sodium nitrite in the presence of moisture turns starch into lower dextrins when the material is irradiated with 355-nm light.256 Ruthenium(IV) and titanium(IV) oxides as well as platinum oxide, all in conjunction with aqueous solutions of starch, eventually produce (in the presence of alkali) hydrogen, carbon dioxide, and photoxidation products.257 The use of Methylene Blue has also been reported. These studies provide evidence that only the sensitizer, and not the starch, is sensitized.258... 

A 2-methoxyethoxymethyl ether (MEMOR) is normally prepared under non-acidic conditions in methylene chloride solution or under basic conditions. The MEM ether group can be removed in excellent yield with trifluoroacetic acid (TEA) in dichloromethane (1 1). The MEM group can also be removed by treatment with zinc bromide (ZnBr2), titanium chloride (TiCU) or bromocatechol borane. When MEM-protected diols are treated with zinc bromide (ZnBr2) in ethyl acetate, 1,3-dioxane is formed and a mechanism of this reaction is given in Scheme 1.23. 

Friedrich et al. [45] discovered that a catalytic amount of titanium(IV) chloride as a Lewis acid greatly facilitates cyclopropanation reactions of alkenes by the system CFl2Br2-Zn-CuCl. The Lewis acid catalyst might bind to the oxygen atom of the allylic alcohol present as the (iodomethyl)zinc alkoxide, and thus increase the electro-philicity of the methylene group [46]. 

It is possible to exclusively methylenate a ketone in the presence of an aldehyde by precomplexing the aldehyde (e.g. 76) with Ti(NEt2)4, followed by treatment with the usual methylene zinc/TiCU reagent (equation 17). Takai also studied the chemoselective methylenadon of aldehydes (78) in the presence of ketones, and found the use of diiodomethane, zinc and titanium isopropoxide or trimethylaluminum to be effective (equation 18). ... 

When the methylenation reagent is generated using zinc, CI>2Br2 or CD2CI2, and titanium tetrachloride, it can be utilized to synthesize the deuterated analogs (82). In addition, Trost has demonstrated that a label may be incorporated in a ketone (81) using H2 Cl2 (Scheme 19). ... 

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