Titanium tetrachloride carbonyl compound complexes

As previously mentioned, 1-alkynyltrialkylborates (18) have become increasingly important in the formation of carbon-carbon bonds via attack of electrophiles. However, such complexes cannot react with simple Qc,P-unsaturated carbonyl compounds such as methyl vinyl ketone, because of their weak electrophilicity. Recently it was ascertained that ,P-unsaturated carbonyl compounds react with 18 via a Michael-type reaction in the presence of titanium tetrachloride, and the usual alkaline hydrogen peroxide oxidation leads to the synthesis of 5-dicarbonyl compounds... 

The isolation of the initial aldol products from the condensation of the enolates of carbene complexes and carbonyl compounds is possible if the carbonyl compound is pretreated with a Lewis acid. As indicated in equation (9), the scope of the aldol reaction can also be extended to ketones and enolizable aldehydes by this procedure. The condensations with ketones were most successful when boron trifluoride etherate was employed, and for aldehydes, the Lewis acid of choice is titanium tetrachloride. The carbonyl compound is pretreated with a stoichiometric amount of the Lewis acid and to this is added a solution of the anion generated from the caibene complex. An excess of the carbonyl-Lewis acid complex (2-10 equiv.) is employed however, above 2 equiv. only small improvements in the overall yield are realized. 

The Mukaiyama version of the aldol reaction is well known a carbonyl-titanium tetrachloride complex reacts with a trimethylsilyl enol ether. Under these conditions there is no titanium enolate involved. Another procedure has been reported a trimethylsilyl enol ether reacts with titanium tetrachloride to give the titanium enolate addition of the carbonyl compound generates the aldol product (although with slightly lower diastereoselectivity than with Mukaiyama s procedure). (Z)-Enolsilanes from acyclic ketones react rapidly and stereospecifically with TiCU to form (Z)-configured CbTi enolates, while the ( )-isomers react slowly to afford low yields of mixtures of ( )- and (Z)-Cl3Ti enolates (Scheme 41). 

Titanium. Catalyses of hydrogenation of alkenes, alkynes, carbonyl-, and nitro-compounds have been described. The effect of the nature of the ligand L and of the alkene to be reduced on reactivity in catalytic hydrogenation by Ti(7r-C5H5)2L2 has been quantitatively studied. The dependence of rate constants on solvent for reduction of decene in the presence of Ti(7r-C5H5)Me+ is interpreted in terms of electrostatic interaction between the active ionic species and the solvent. There is also a thermochemical report relevant here, and that is of a determination of the heats of mixing of cyclohexene and of hex-l-ene with titanium tetrachloride. The heats of mixing are close to zero, which implies very small heats of complex formation between these alkenes and titanium. ... 

In order to obtain high selectivities in the reduction of linear carbonyl compounds, very often chelating reagents are added to fix the substrate in a specific conformation. Titanium complexes are known to be versatile reagents in such a reduction process, especially with dicarbonyl compounds [41]. Thus, the stereoinduction in the reduction of diketones alternates with the distance of the two carbonyl groups from each other [42]. The 1,2-induction in the reduction of diacetyl or benzil 91 is independent of the addition of titanium tetrachloride and predominantly yields the meso products 92 (displayed in Scheme 3.20). 

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