Heck Reaction of Protected Alk-1-en-3-ol, -4-ols, -5-ol or -6-ol with Aryl Bromides Catalysed by a Palladium Complex Derived from a Tetraphosphine

Abstract

The tetraphosphine cis,cis,cis-1,2,3,4-tetrakis(diphenyl­phosphinomethyl)cyclopentane in combination with [Pd(C₃H₅)Cl]₂ affords a very efficient catalyst for the Heck reaction of protected alk-1-en-3-ols, -4-ols, -5-ols or -6-ols with aryl bromides. With 3-benzyloxyhex-1-ene or 4-benzyloxypent-1-ene, the (E)-1-aryl-3-benzyloxyhex-1-ene or (E)-1-aryl-4-benzyloxypent-1-ene derivatives were regio- and stereoselectively obtained. The protected ethylenic­ primary alcohols 4-benzyloxybut-1-ene, 4-tetrahydropyranyloxybut-1-ene, 5-tetrahydropyranyloxypent-1-ene and 6-tetrahydropyranyloxyhex-1-ene led in general to the (E)-1-aryl-4-benzyloxybut-1-ene or (E)-1-aryltetrahydropyranyloxyalk-1-enes as major isomers. However, with these substrates, the migration of the double bond was observed, and mixtures of isomers were obtained in most cases. The selectivities of the reactions depend both on electronic and steric factors. With electron-poor aryl bromides higher selectivities in favour of 1-arylalk-1-enol derivatives were generally obtained than with electron-rich aryl bromides. In the presence of sterically congested aryl bromides, larger amounts of products resulting from the migration of the double bond were observed. For example from 2,4,6-triisopropylbromobenzene and 6-tetrahydropyranyloxyhex-1-ene, 6-(2,4,6-triisopropylphenyl)hexanal was obtained as major product after deprotection. For most of these reactions, similar reactions-rates were observed with electron-poor and electron-rich aryl bromides. Several reactions can be performed with as little as 0.1% catalyst.

Heck reaction - tetraphosphine ligand - alkenols - aryl bromides

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