Titanium-catalysed Reduction Reactions

Kuroda and Tarui [498] developed a spectrophotometric method for molybdenum based on the fact that MoVI catalyses the reduction of ferric iron by divalent tin ions. The plot of initial reaction rate constant versus molybdenum concentration is rectilinear in the range 0.01-0.3 mg/1 molybdenum. Several elements interfere, namely, titanium, rhenium, palladium, platinum, gold, arsenic, selenium, and tellurium. 

A very high stereoselectivity was observed in the reduction of 4-tert-butylcyclohexanone to the m-alcohol (> 95%), which is the industrially relevant product. The observed high selectivity to the thermodynamically unfavorable cis-alcohol was explained by a restricted transition-state for the formation of the trans-alcohol within the pores of the zeolites (Scheme 5). This reaction was found not only to be catalysed by Al-Beta, van der Waal et al. reported the catalytic activity of aluminum-free zeolite titanium beta (Ti-Beta) in the same reaction.74 Again, a very high selectivity to the cis-alcohol was observed indicating similar steric restrictions on the mechanism. Kinetically restricted product distributions were also reported for the 2-,3- and 4-methylcyclohexanone the cis, trans- and ds-isomers being the major products, respectively. In this case the tetrahedrally coordinated Ti-atom was assumed to behave as the Lewis acid metal center. Recent quantum-chemical calculations on zeolite TS-1 and Ti-Beta confirm the higher Lewis acidic nature of the latter one.75... 

A Leimbruger-Batcho-type amino-enamine intermediate is likely to be involved on reduction of the base-catalysed condensation product of an ortho-nitro araldehydes with nitromethane. Reduction, traditionally with metal/acid combinations, but now with reagents such as palladium/carbon with ammonium formate and formic acid, iron with acetic acid and silica gel, or titanium(III) chloride, gives the indole. The arylacetaldehyde precursors can also be generated by Heck reactions on vinylidene carbonate. ... 

The catalytic alkylation of imines catalysed by titanium(iv) alkoxides in the presence of one equivalent of allylmagnesium bromide was described by Samuelson. A mechanistic study was carried out and the authors found that using a stoichiometric amount of titanium alkoxide the reduction and coupling of imines took place but using as little as 0.1 mol% of the catalyst, the allylation reaction occurs due to the large ratio of imine to titanium. 

Organotitanium and -zirconium chemistry already has an established place in organic synthesis and many reactions are covered elsewhere in the Practical Approach series. Examples include reductive coupling of carbonyl compounds with low valent titanium to form 1,2-diols or alkenes methylenation of ester carbonyl groups with titanocene methylidene (Cp2Ti=CH2) zirconium-catalysed methylalumination of alkynes and hydrozirconation of alkynes and alkenes with the Schwartz reagent, Cp2ZrHCl. ... 

Titanium(ii)-chloride-induced reductive cyclization of one of the photo-adducts (43) of isoprene and methyl 2,4-dioxopentanoate produced a mixture of diastereo-meric diols (44), which are useful precursors in the synthesis of iridoids. The Lewis-acid-catalysed reaction of unsaturated carbonyl compounds such as (45) shows a strong dependence on the strength and quantity of Lewis acid used. 

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