Titanium dichloride dimethyl

Reductive alkylation of aldehydes and ketones essentially involves replacement of oxygen with two alkyl groups to furnish alkanes. The procedure can be employed advantageously for the conversion of ketones into em-dimethylated alkanes, a structural feature present in many natural products. Thus, several cyclic ketones have been transformed into m-dimethylated alkanes, employing mild conditions, with dimethyl-titanium dichloride (equation 68). m-Dimethylation of ketones has also been achieved by heating them with (CH3)3 A1 in the presence of AICI3 (equations 69 and 70). ... 

A rather special case of bimolecular termination was described recently in the literature by Chien (7). It concerns the polymerization of ethylene initiated by a soluble biscyclopentadienyl titanium dichloride-dimethyl aluminium chloride complex. Such a polymerization should be classified as a coordination polymerization and not as an ionic polymerization. Nevertheless, some similarity to anionic polymerization justifies its discussion at this place. It was shown that the termination is kineti-cally bimolecular, and it is postulated that it involves the reduction of two TiIV+ to TiIII+ complexes, proceeding simultaneously with the disproportion of 2 polymeric chains,... 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

Several inorganic atoms have also been methylated by dimethyl sulfate. Sodium iodide gives methyl iodide, organoxyfitaninm dichloride gives several highly colored organoxytitanium methyl sulfates, titanium tetrachloride yields titanium chlorosulfates or methyl sulfates, and mossy tin gives dimethyltin sulfate (36—39). 

Methylalumoxane (MAO) activated 6fa(cyclopentadienyl)zirconium dimethyl Alumoxide and trimethyl aluminum, and 6w(cyclopentadienyl)titanium diphenyl Methylalumoxane and his(cyclopentadienyl)zirconium dichloride... 

More recently, Petasis has shown that titanocene dimethyl, " which is easily generated from titanocene dichloride and methyllithium, also generates the same titanium methylidene upon thermolysis (Scheme 13.5, bottom). Thermolysis of titanocene dimethyl is a convenient way to generate the titanocene-methylidene complex because titanocene dimethyl is formed in high yield and is stable to water, -although it is sensitive to light and can undergo autocatalytic decomposition. This complex has been used in synthesis... 

The use of titanium species in organic synthesis is increasing notably. Ketones react with dimethyltitanium dichloride under mild conditions leading directly to the corresponding geminal dimethyl-substituted compounds (Scheme 12). As the geminal dimethyl substituent occurs in many natural products, this procedure could find many applications. 

Preparation by reaction of 2-fluorobenzoyl chloride with resorcinol dimethyl ether in the presence of aluminium chloride in refluxing ethylene dichloride for 30 min (78%) [1031] or refluxing hexane for 8 h [239], Titanium tetrachloride can also be used instead of aluminium chloride [239],... 

Like homopropargylic alcohols, homoallylic alcohols undergo with dimethyltitanium dichloride carbotitanation followed by elimination of titanium hydride. As a consequence of the yw-addition/syw-elimination mode, the configiuation of internal double bonds (as in 21 or 22) is inverted whereas terminal substrates (20) minimize steric repulsion and afford tra 5-3-penten-l-ols (Scheme 1-17). The optimum medium is a mixture of pentanes and the slowly evaporating dimethyl ether. Obviously the titanium species has to be coordinatively unsaturated to accomplish the critical carbometalation step. ° ... 

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