Butadiene, reactions with silenes

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

Conlin and Bobbitt used 1,2-divinyl-tetramethyldisilane 89 to generate a simple silene 90 used to explore competing [2 + 2] and [2 + 4] cycloaddition reactions with butadiene (see Scheme 13), where the -91 and Z-91 isomers of the [2 + 2] adduct predominated over the [2 + 4] cycloadduct 9245. 

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. 

Silene Reactions with Organic Dienes. The second example of reactions wdll concern Diels-Alder reactions of silenes. Certainly, as was already mentioned, the diene additions to silaethenes occur much faster than those to ethenes. Over and above that, in most eases other reactions operate in addition. As is shown in Scheme 10, the [4+4] cycloaddition of silaetiienes with 2,3-dimethyl-l,3-butadiene (DMB) leads - independently of tiie direction of silaethene addition to DMB - to one and the same [4+2]... 

Deprotonation of lb or Ic with methyllithium in presence of 2,3-dimethyl-l,3-butadiene gives the silacyclohexenes 9a and 9b, the expected [2+4] cycloaddition products of the respective silene and the diene (Eq. 6). Attempts to perform the same reaction with la failed. 

Reactions favoring [2 + 2] cycloaddition tended to be those that had strongly electronegative groups on the sp2-hybridized silicon but only H and the neopentyl group on the sp2-hybridized carbon atom. Butadiene and cyclohexadiene generally favored [2 + 2] cycloaddition with these silenes. The [2 + 2] adducts with cyclohexadiene appear to be kinetic products, since they cleanly isomerized to the Diels-Alder adducts over time.182... 

These studies proved that the reactions of 22 with butadienes and propenes take place both regioselectively and stereoselectively and are accelerated by electron-donating groups on propenes and butadienes (e.g., 2-methylpropene in relation to propene) and retarded by increasing bulkiness of substituents in 1,4- or 1,3-positions. As in the case of alkenes and silenes, the reactions of 22 occur in a concerted way and are HOMO (dienes or enes)-LUMO (dienophiles or enophiles) controlled.31 However, some small differences are observed between germene 22 and the analogous... 

In some cases the ene reaction of the silene with a diene competes with the Diels-Alder addition118,140,210,243"246,264. Thus, 54 undergoes a 4 + 2 cycloaddition with 1,3-butadiene but gives mixtures of products with its methyl derivatives (Table 4). In some cases the only observed products are those of an ene reaction, for instance in the reaction of 54 with Me2C=CH-CH=CMe2244 and in the addition of 2,3-dimethylbutadiene to the silene 75 with an activated allylic hydrogen (equation 134)140. 

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