Titanium salts alkenes

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. 

Probably the most popular carbon-carbon double-bond-forming reaction involving sulphur proceeds via the elimination of a sulphur-oxygen species. This is illustrated by a synthesis of 1,5-unsaturated dicarbonyl compounds (236) which proceeds by phenylthioalkylation of enolates (236a), using the phenyl-thioalkene (237), followed by ozonolysis and elimination of the sulphoxide moiety to provide the double bond. An alternative method for double-bond formation is shown in the preparation of alkenes R CH=CHR by reductive cleavage of the sulphide (238) with titanium salts, and demonstrates the versatility of sulphur in such double-bond formations. In the latter example... 

Reactions of carbonyl compounds with these gem-dizinc species gave the corresponding heteroatom-substituted alkenes. In these reactions, the addition of a titanium salt was necessary. jS-TiCls was used, except in the case of reaction of a-boryl-substituted gem-dizinc, where TiCU was used instead (Table 5.4). 

As shown in Scheme 8.14, various ketones were converted into alkenes in good to moderate yields by i ts(iodozincio)methane (4)/ [bmim] [PF ]. Instead of 4, Nysted reagent was also examined for the methylenation of 2 in the presence of ionic liquid, but did not give the corresponding alkene. Although the titanium salt is still the best additive for the effective methylenation of ketones with 4, use of an ionic liquid shows the new possibility for an activation of gem-dizinc reagents [27]. 

Arco have developed an integrated process for the production of industrially important epoxides via an adapted AO process (Figure 1.13).33 34 A sulfonic acid substituted alkylhydroanthraquinone alkylammonium salt is reacted with molecular oxygen to form the alkylanthraquinone and hydrogen peroxide. The hydrogen peroxide is then reacted with an alkene in the presence of a titanium zeolite catalyst (TS-1 see Chapter 4). The epoxide product is then separated, and the anthraquinone salt recycled to a hydrogenator for reaction with... 

Alkene, Alkyne, Alkylidene,m and Carbonyl Complexes. While titanium al-kene complexes are unquestionably involved in polymerizations, relatively few have been isolated. Interactions of TiCL,(dmpe)2 and butadiene under reducing conditions give Ti(T7-C4H6)2(dmpe), which can be converted by CO and PF3 to Ti(CO)2(PF3)-(dmpe)2 and Ti(CO)3(dmpe)2. The latter reacts with K in the presence of biphenyl or naphthalene and then with CO to give Ti(CO) - species which are isolable as K(cryptate)+ salts 102... 

In Section 3.11.1.4 it was pointed out that salts of certain transition metals, lanthanides and actinides promote the hydroalumination reaction. Since such metal salts are introduced into the reaction in their high oxidation states it can be assumed that the metal ions are rapidly reduced to a lower oxidation state and that this state is the active catalyst. For nickel(II) salts, Wilke has shown conclusively that the active agent is a nickel(0)-alkene complex. Analogously, for titanium(IV) salts, such as TiCU, Ti(OR>4 and Cp2TiCl2, it is most likely that a titanium(III) state is involved. The possible role of such metal centers in accelerating hydroalumination will be considered in the next section. 

ALKENES Dimethyl(methylene)-ammonium salts. Methylene bromide-Zinc-Titanium(IV) chloride. Methylene bromide-Zinc-Trimethylaluminium. 

A combination of alkali metal salts, particularly potassium chloride, with low-valent titanium reagents generated from titanium chlorides with lithium or magnesium in either THF or DME are effective reagents for stereoselective McMurry coupling reactions of aldehydes and ketones to substituted alkenes (Figure 1.11). 

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