The Prins and Carbonyl Ene Reactions

The addition of aldehydes and ketones to alkenes in the presence of Brønsted acids is usually called the Prins reaction.1,2 Addition of the protonated carbonyl compound to the alkene gives a β-hydroxy carbocation which reacts with a nucleophile such as chloride, water or acetate to give (1), adds to a second molecule of aldehyde to give (2) or loses a proton to give homoallylic alcohol (3; Scheme 1). Formaldehyde and electron-deficient carbonyl compounds undergo concerted thermal ene reactions with alkenes at temperatures ranging from 100 to 200 °C to give homoallylic alcohols (equation 1; R1 = H, CCl3, or CO2R).3–7 A wide variety of carbonyl compounds undergo Lewis acid catalyzed reactions with alkenes to give homoallylic alcohol ene adducts in excellent yield with high selectivity, although in some cases γ-chloro alcohols are formed as by-products.8,9 Recent investigations suggest that at least some of these Lewis acid catalyzed ene reactions are stepwise, rather than concerted, but that the intermediate bears more resemblance to a π-complex than to the open carbocation intermediate in the Prins reaction.