Säurekatalysierte Umlagerung von 4‐Allyl‐cyclohex‐2‐en‐1‐olen; Beispiele für ladungskontrollierte [3 s , 4 s ]‐Umlagerungen von cyclischen Allylkationen

The acid‐catalysed rearrangement of the cyclohex‐2‐en‐1‐ols 15 , d 3 ‐ 15 , 16 , 17 and 19 , the cyclohexa‐2,5‐dien‐1‐ols 20 and 21 , and also the allyl alcohols 22 and 23 (Scheme 3), using 98‐percent sulfuric acid/acetic anhydride 1:99 at room temperature, was investigated. From the rearrangement of 4‐allyl‐4‐phenyl‐cyclohex‐2‐en‐1‐ol ( 15 ), with reaction times greater than 2 hours a single product is obtained, 4‐allyl‐biphenyl ( 50 ) in 33% yield (Scheme 9). With reaction times below 2 hours the acetate 53 from 15 was isolated, and this could be converted into 50 . The reaction of 2′,3′,3′‐d 3 ‐15 in Ac 2 O/H 2 SO 4 lead to 1′,1′,2′‐d 3 ‐50 (Scheme 11). The rearrangement of 4‐allyl‐4‐methyl‐cyclohex‐2‐en‐1‐ol (16) (Scheme 14) yielded 39% of the corresponding acetate 60 and 30% of 4‐allyl‐toluene ( 6 ), which also resulted by a rearrangement of 60 under the reaction conditions. These rearrangements are all [3s,4s]‐sigmatropic reactions, which proceed via the cyclohexenyl cation a (Scheme 12, R = C 6 H 5 , CH 3 ). In Ac 2 O/H 2 SO 4 the allyl‐cyclohexadienes primarely formed subsequently undergo dehydrogenation to yield the benzene derivatives 6 , 50 and d 3 ‐ 50 . From the rearrangement of 4,4‐diphenyl‐cyclohex‐2‐en‐1‐ol ( 19 ) at 0° a reaction mixture is obtained which consists of the acetate 55 , 2,3‐diphenyl‐cyclohexa‐1,4‐diene ( 57 ) and o‐terphenyl ( 56 ) (Scheme 10). Both 55 and 57 are converted under the reaction conditions to o‐terphenyl ( 56 ). No 4‐(1′‐methylallyl)‐biphenyl is obtained from the rearrangement of 4‐crotyl‐4‐phenyl‐cyclohex‐2‐en‐1‐ol ( 17 ). In this case, apart from the corresponding acetate 64 , a single product 5‐(1′‐acetoxyethyl)‐1‐phenyl‐bicyclo[2.2.2]oct‐2‐ene ( 65 ) (Scheme 16) was obtained; under the reaction conditions the acetate 64 rearranges to 65 . The rearrangement of 4‐allyl‐4‐phenyl‐cyclohexa‐2,5‐dien‐1‐ol ( 20 ) gives, as expected, not only 4‐allyl‐biphenyl ( 50 ) but also 2‐ and 3‐allyl‐biphenyl ( 51 and 52 ) and biphenyl (Scheme 13). 4‐Benzyl‐4‐methyl‐cyclohexa‐2,5‐dien‐1‐ol (syn‐ and anti‐ 21 ) gave in Ac 2 O/H 2 SO 4 at 10° as rearrangement products 93% of 2‐benzyltoluene ( 97 ) and 7% of 4‐benzyl‐toluene ( 98 ) (Scheme 21). Hence [1,4]‐rearrangements in cyclohexadienyl cations, seems to occur only to a limited extent. The alicyclic alcohols 22 and 23 (Scheme 18) gave, in Ac 2 O/H 2 SO 4 , as main product the corresponding acetates 73 and 75 , as well as small amounts of olefins 74 and 76 formed by dehydration i.e. [3,4]‐rearrangements occur in these systems. Also no [3,4]‐rearrangements were observed in solvents reactions of either 4,4‐dimethyl‐hepta‐1, 6‐dien‐3‐yl tosulate (79; see Scheme 19) or its corresponding alcohol 24.