Silenes 2+4 cycloaddition reactions with diene

The [4 + 2] reaction with dienes has been frequently used to establish the intermediacy of a silene in the sila-Peterson olefination reaction39,107-112-1 r fulSj further examples for cycloadditions can be found in Section I.A.5. 

Recent study of silenes has clearly established their ability to act as dienophiles in [2 + 4] Diels-Alder-type reactions involving 1,3-dienes. However, it has also been clearly demonstrated that products of the ene reaction commonly accompany the [2 + 4] cycloadducts and may on occasion be the major products. In addition, unlike those in carbon chemistry, [2 + 2] cycloadditions are often observed to occur in competition with the above processes—not only from reactions of silenes with dienes... 

In this section we will summarize bimolecular reactions of silenes with alkenes, alkynes and dienes which might be regarded nominally as cycloaddition reactions. 

Ene-additions of alkenes and dienes to silene 6 are considerably slower than [2 + 4]-cycloadditions. cA-Substitution in the ene component of the reaction causes a small acceleration in rate relative to fraws-substitution, as illustrated in Table 2 by the relative rate constants for reaction of 6 with cis- and rraws-2-butene. Reaction with cis, trans-2,4-hexadiene produces only a single adduct (66 equation 51), corresponding to selective ene-reaction with the cA-methyl group in the diene. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

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. 

It was also well established that silenes could take part as the dienophile in Diels-Alder reactions. In many cases, particularly with unsymmetric dienes such as isoprene, the reactions were not clean because, in addition to formation of the [2+4] cycloadduct 61, the possibility exists for the formation of it regioisomer 62, products of an ene reaction 63, and conceivably the [2+2] cycloaddition product 64, as shown in Eq. (23). Wiberg... 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

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. 

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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