1.4- dioxane hydride transfer

The same regioselective and stereospecific reactions are observed in decalin systems. The 3/3-formate 605 is converted into the a-oriented (j-allylpalladium complex 606, and the hydride transfer generates the fra .s-decalin 607, while the cis junction in 610 is generated from the 3tt-formate 608 by attack of the hydride from the /3-side (609). An active catalyst for the reaction is prepared by mixing Pd(OAc)2 and BU3P in a 1 I ratio with this catalyst the reaction proceeds at room temperature. The reaction proceeded in boiling dioxane when a catalyst prepared from Pd(OAc)2 and BujP in a 1 4 ratio was used[390]. 

The disproportionation of 1,3-cyclohexadiene to benzene and cyclohexene in dioxan is catalysed by KOt-Bu (El-Kholy et al, 1977) according to reactions (31)-(33). The hydride transfer step (32) was found to be rate-determining. The presence of 18-crown-6 increased the overall rate of disproportionation by... 

Attempts to measure kn for the following cyclic acetals and ethers were unsuccessful because of the too low value of kn (below 10 mole"" 1 s at 25 °Q 2-Trichk>ioinethyi l,3 x-olane, 1,3,5-trioxane, tetrahydropyran, and 1,4-dioxane. The non-planar and puckered structure of the last three compounds can hinder a sufficiently close approach to the central atom of the triphenylmethylium cation necessary for hydride transfer to proceed. ... 

Lau, S.Y.W., Andersen, N.G. and Keay, B.A. (2001) Optimization of palladium-catalyzed polyene cyclizations suppression of competing hydride transfer from tertiary amines with DABCO and an unexpected hydride transfer from 1,4-dioxane. Org. Lett., 2,181. ... 

A trinuclear cobalt(I) complex, PhCCo3(CO)9, can also catalyse the reduction of nitro compounds in the presence of hydroxide ion at room temperature under a normal pressure of CO [49]. Satisfactory results were obtained under phase transfer conditions. The catalyst and the aromatic nitro compounds were dissolved in benzene under carbon monoxide and an aqueous solution of sodium hydroxide containing cethyltrimethylammonium bromide was added. At a substrate/cat =10 ratio, ca. 60-80 % of amine was obtained in a 18 h reaction. The reaction also proceeded in a homogeneous phase (methanol-water, methanol, dioxane-water) but with lower conversions (less than 45 %). Cobalt complexes such as MeCCo3(CO)9 and MeCo(CO)4 were also active, but less effective. At the end of the reaction, the catalyst was recovered only in part (ca. 15 %). In the organic phase, an IR absorption at 1891 cm, attributable to [Co(CO)4] anion, was observed. Strangely enough, the preformed [Co(CO)4] anion has not been tested as catalyst. The active species was supposed to be the hydride cluster anion reported in Scheme 6. 

Sodium and potassium borohydride can be dissolved in water and then phase-transferred into a nonpolar solution, but such solvents as benzene or toluene offer little advantage over alcohols which commonly dissolve organic substrates. One application in which the two-phase technique does afford special convenience is the two-phase reduction of acid chlorides, usually a heterogeneous reaction conducted in dioxane solution [2]. The hydride reductions and several other techniques are discussed in this chapter. 

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