RATE ENHANCEMENT IN DIELS-ALDER REACTIONS

In many instances, Diels-Alder reactions give poor yields and/or require excessively harsh reaction conditions and long reaction times. There are several fairly straightforward techniques that increase the rate and, in some cases, the selectivity of Diels-Alder reactions. The three most important methods are catalysis by Lewis acids, rate enhancement in aqueous media and the use of high pressure (usually greater than 5 kbar). These three methods will be the focus of this section. 

Lewis acids are, in general, excellent catalysts for the Diels-Alder reaction.i Their catalytic activity lies in the ability of the Lewis acid to complex a heteroatom component of the diene or dienophile (usually the dienophile). Complexation with acrolein, for example, generates an ionic intermediate such as 108, and 

There are many synthetic applications of Lewis acid catalyzed Diels-Alder reactions. The conversion of 71 to 72 in Section 11.4.C.ii is one example. Another example is the reaction of 1,3-butadiene with 2-bromoacrolein, catalyzed by the chiral oxaborinane Lewis acid shown to give a 99% yield of 112 with 94% ee. Asymmetric Diels-Alder reactions will be explored further in Section 11.9. 

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