Abstract
A range of twelve stabilised phosphonium ylides containing tetrahydrofuran, tetrahydropyran or 2,2-dimethyl-1,3-dioxolane rings have been prepared and fully characterised, including one X-ray structure determination of each type. The X-ray structures confirm the P=C and C=O functions to be syn and all the compounds undergo thermal extrusion of Ph3PO to give the corresponding alkynes. In some cases there is also competing loss of Ph3P to give different carbene-derived products and evidence has been obtained for the generation of 2-phenyloxete in this way. Raising the pyrolysis temperature leads in several cases to new secondary reactions of the alkyne products involving a sequence of alkyne to vinylidene isomerisation, intramolecular CH insertion, and retro Diels Alder reaction.
Highlights
View Article OnlineStructure and pyrolysis of stabilised phosphonium ylides containing saturated oxygen heterocycles† Cite this: Org. Biomol
Thermal extrusion of Ph3PO from suitably substituted β-oxophosphonium ylides in the so-called “intramolecular Wittig reaction” is a versatile method of alkyne synthesis.[1]
Over the 25 years many further examples appeared, but using conventional pyrolysis the scope was limited to examples where R1 was an electron withdrawing group.[4]. This restriction was overcome by the use of flash vacuum pyrolysis (FVP) which allowed synthesis of purely aliphatic and terminal alkynes from ylides with R1 = alkyl or H.5
Summary
Structure and pyrolysis of stabilised phosphonium ylides containing saturated oxygen heterocycles† Cite this: Org. Biomol. A range of twelve stabilised phosphonium ylides containing tetrahydrofuran, tetrahydropyran or 2,2dimethyl-1,3-dioxolane rings have been prepared and fully characterised, including one X-ray structure determination of each type. In some cases there is competing loss of Ph3P to give different carbene-derived products and evidence has been obtained for the generation of 2-phenyloxete in this way. Raising the pyrolysis temperature leads in several cases to new secondary reactions of the alkyne products involving a sequence of alkyne to vinylidene isomerisation, intramolecular CH insertion, and retro Diels Alder reaction
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