Titanium complexes borohydrides

The diastereoselectivity of reduction of a series of symmetrical diketones to the corresponding diols revealed an intriguing dependance on the separation of the carbonyl groups, but the selectivity was not generally useful. However, in the case of 1,3-diketone (71) lithium borohydride alone produced the anti isomer (70) with 91% diastereoselectivity, but prior addition of titanium tetrachloride gave the syn-diol (72) with 96% diastereoselectivity via a chelated intermediate analogous to the crystalline complex between the diketone and TiCU (Scheme 11). ... 

The reverse regiocontrol, giving 1,2-diols, is observed with DIBAL-H (diisobutylaluminum hydride). The remarkable effect of titanium tetraisopropoxide as an additive to lithium borohydride has also been reported. In this reaction benzene is a better solvent than THF, probably because a Ti complex using both oxygens in epoxy alcohols is formed in benzene before the hydride attack. Other metal hydrides used include sodium hydrogen telluride (NaHTe) and an ate complex derived from DIBAL-H and butyllithium, both of which reduce epoxides to alcohols, although they have been tested with only a small number of examples. In the former case the reaction may proceed via a 2-hydroxyalkyltellurol intermediate. 

Vinyl halides add to allylic amines in the presence of Ni(cod)2 where cod=l, 5-cyclooctodine, followed by reduction with sodium borohydride. Aryl iodides add to alkynes using a platinum complex in conjunction with a palladium catalyst. A palladium catalyst has been used alone for the same purpose, and the intramolecular addition of a arene to an aUcene was accomplished with a palladium or a GaCl3 catalyst, " AUcyl iodides add intramolecularly to aUcenes with a titanium catalyst, or to alkynes using indium metal and additives. The latter cyclization of aryl iodides to alkenes was accomplished with indium and iodine or with Sml2. " ... 

The major reactions in this section are those involving an V-oxide oxygen. Deoxygenation of 2,3-disubstituted quinoxaline 1,4-dioxides is achieved under mild conditions by treating with hexa-chlorodisilane, TMS-1, TFAA-sodium iodide, or titanium tetrachloride-zinc dust <81H(i6)4ii>. Quinoxaline and phenazine V,A( -dioxides are also deoxygenated under mild conditions by sodium hypophosphite catalyzed by Pd/C or by treatment with sodium iodide in the presence of pyri-dine/sulfur trioxide complex <83JHC1735>. Deoxygenation of 6-chloro-2(l//)-quinoxalinone 4-oxide is effected particularly by sodium borohydride or sodium hydrosulfite <85H(23)143>. 

Carbonyl compounds include isobut3raldehyde, phenyl isopropyl ketone, glyoxylic acid and pyruvic acid. Diaryl ketones do not react. Modifications of the method consist in the use of a borohydride exchange-resin, of sodium triacetoxyborohydride NaBH(OAc)3 or in treating a mixture of a ketone and an amine with an equivalent of titanium(IV) chloride and Hiinig s base (ethyldiisopropylamine) in dichloromethane, followed by a methanolic solution of sodium cyanoborohydride. The borane-pridine complex and hydrogen telluride are excellent replacanents for sodium cyanoborohydride. 

Preparation.—Hydration of an alkene to the anti-Markovnikov alcohol can be accomplished with a titanium tetrachloride-sodium borohydride reagent [equation (1)] a boron complex of low-valent titanium may be involved. 

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