Diels-Alder type cycloadditions with 2,3-dimethyl-1,3-butadienes

Cations (56) and (57) react rapidly with 2,3-dimethyl-l,3-butadiene in CH2CI2 to give the bicyclic cationic prodncts (58) and (59). The qnantitive formation of (58) and (59) imply a concerted process, possibly a Diels Alder type cycloaddition... 

A Diels-Alder type [4+2] cycloadditions of 4,5-dihydropyridazine, prepared in situ from its trimer, with 2-methyl- and 2,3-dimethyl-1,3-butadienes (65, R = H, Me R = Me) afforded a complex reaction mixture, from which 6-methyl- and 6,7-dimethyl-3,4,4n,5-tetrahydro-8//-pyrido[l,2-ftjpyridazines (66, R = H, Me R =Me) could be isolated (97CEJ1588). With 1,3-butadiene (65, R = R =H) only a mixture of endo and exo isomers 67 and 68 (R = R =H) was obtained. 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

Other types of sila systems also give di-n-methane products. In the case of (19), irradiation at 10 K in an argon matrix or at 77 K in a 3-methylpentane glass affords the di-n-methane product (20), which on further irradiation or upon heating reverts to the starting material (equation 21). Photolysis of (19) at room temperature gives instead retro Diels-Alder fragmentation to tetramethyldisilene, whose intermediacy was established by means of [4 + 2] cycloaddition with 2,3-dimethyl-1,3-butadiene. ... 

Reaction of benzynes with acyclic dienes generally gives Diels-Alder products in poor yields because of competitive [2 + 2] cycloadditions and ene-type processes (Scheme 113). The product distribution, obtained from 2,3-dimethyl-l,3-butadiene (484)," is more characteristic of a two-step mechanism than of a concerted addition. Without pursuing this argument, it appears that the extremely poor yield of [4 + 2] cycloadduct (485) results from the very fast reaction of benzyne with the transoid diene (484). Bond formation at C-1 coupled with a hydrogen transfer (- 487) or cyclobutane ring closure (- 486) seems to occur rapidly before diene (484) can adopt the s-cis conformation. 

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