Microwave-accelerated Wittig olefination of β-chloroacroleins

The modern interpretation of the Wittig reaction mechanism

“…Several excellent reviews on the topic have previously been written that contain extensive detail on the Wittig reaction mechanism. 2,3,4,5 M ore recent reviews do 25 not focus on the details of the mechanism. 6,7,8 The reaction (see Scheme 1) occurs between a carbonyl compound (aldehyde or ketone in general, 2) and a phosphonium ylide (1) to give alkene (3) with phosphine oxide (4) as the by-product.…”

“…Amongst the recent developments reported in the literature are: 5 -New methods for generation of the ylide. 18,19,20 -Reactions showing atypical stereoselectivity, 21,22,23,24,25,26,27 which can be induced e.g. by the use of phosphonium ylides with modified "spectator substituents" on phosphorus, 21,22,24,26 by the use of trialkylgallium base to 10 generate the ylide, 27 and by the addition of methanol at low temperature to the oxaphosphetane adducts from reactions of non-stabilised ylides.…”

“…22 -New reaction conditions. 25,28,29 -One-pot reaction from either phosphine 19,20,29,30,31 or alcohol 32 15 -Synthetic studies. 31,33 -Catalytic variants.…”

“…37 -Demonstration by us of a unified mechanism for the Li saltfree Wittig reaction of all phosphonium ylide ty pes from reactions of -heteroatom-substituted aldehydes. 38,39 25 -A study of the decomposition of the OPA intermediate (see later), which indicates that its decomposition may occur by a single step. 40 -Computational studies on the reaction mechanism 41,42,43 and on the stability of heteroatom-stabilised ylides including 30 phosphonium ylides.…”

“…M any stabilised ylides are air stable and can be isolated and 25 purified, so reactions of such ylides are often carried out using the pre-formed ylide, without the need for in situ generation. Wittig reactions of stabilised ylides derived from both triphenylphosphine (Chart 1 (i)) and trialkylphosphine (Chart 1 (j)) generally show very high selectivity for E-alkene in polar 30 aprotic solvents.…”