Reactivity optical isomerization

Methyldichlorosilane was by far the most reactive in hydrosilation of 1,1-disubstituted olefins. Trialkylsilanes did not add at all, even at 120°C. Trichlorosilane gave complicated results involving isomerization of olefins and dimerization of a-methylstyrene, and products were not optically active. 2-Methylbutene-2 and trichlorosilane gave two adducts, 2-meth-ylbutyltrichlorosilane and 3-methylbutyltrichlorosilane. The latter required isomerization of the olefin. 2,3-Dimethylbutene-l gave one adduct in 70% yield, and it was optically slightly active [0.8% (R) isomer]. 

Although various transition-metal complexes have reportedly been active catalysts for the migration of inner double bonds to terminal ones in functionalized allylic systems (Eq. 3.2) [5], prochiral allylic compounds with a multisubstituted olefin (Rl, R2 H in eq 2) are not always susceptible to catalysis or they show only a low reactivity [Id]. Choosing allylamines 1 and 2 as the substrates for enantioselective isomerization has its merits (1) optically pure citronellal, which is an important starting material for optically active terpenoids such as (-)-menthol, cannot be obtained directly from natural sources [6], and (2) both ( )-allylamine 1 and (Z)-allylamine 2 can be prepared in reasonable yields from myrcene or isoprene, respectively, The ( )-allylamine 1 is obtained from the reaction of myrcene and diethylamine in the presence of lithium diethylamide under Ar in an almost quantitative yield (Eq. 3.3) [7], The (Z)-allylamine 2 can also be prepared with high selectivity (-90%) by Li-catalyzed telomerization of isoprene using diethylamine as a telomer (Eq. 3.4) [8], Thus, natural or petroleum resources can be selected. 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

The related (Z)-lithium dialkenylcuprates (147) derived from acetylene itself also react well with epoxides to provide a useful route to (2)-homoallylic alcohols the lack of reactivity with esters allows an easy access to lactones (148) by condensations between epoxy esters and this type of cuprate (Scheme 29). Likewise, the lower homologs (149) and (151), both of which are relatively easy to prepare in optically active forms, can be readily converted into homoallylic and bishomoallylic alcohols (150) and (152) respectively. An ester unit can also be incorporated into the cuprate functions thus, addition of a mixed lithium cuprate, RCuYLi , to ethyl propiolate gives the cuprates (153), which add to epoxides to give unexpectedly the (Z)-crotonates (154). Such isomerization is not uncommon with vinyl carbanions in general, and is obviously a limitation when isomeric mixtures are produced. 

The azepines (92) undergo photoisomerization, a typical reaction of such trienes, into the bicyclic derivatives (93). Substitution dependent reactivity has been observed on irradiation of the differently substituted dienes (94) and (95). The diester (94) undergoes the more usual isomerization, as observed for (92), to yield the cyclobutene derivative (96). The mono-ester, as the optically active compound (+)-(95), however, affords a good yield (77%) of the bicyclobutane (+)-(97). In this case crossed addition occurs within the diene. 

The conventional free radical copolymerization of 3-pinene and MMA or St, initiated by AIBN, yielded copolymers with Mw of about 11 600 and 25 400, respectively and MWD of 1.5 and 1.7, respectively [56]. When 2,3,4,5,6-pentafluorostyrene (PFS) was used as comonomer, and benzoyl peroxide as the initiator, a PFS-rich copolymer incorporating isolated isomerized 3-pinene units distributed between poly(PFS) segments was obtained [57]. This feature is related to the low reactivity of the 3-pinene free radicalar towards its monomer, that is to a reactivity ratio close to zero. These PFS-[3-pinene copol5miers were shown to combine the typical high water contact angles of perflourinated polymers (hydrophobicity) with the optical activity of polyOPESf) [57]. A recently published study indicated that the radical random copolymerization of both a- and 3-pinene with styrene under microwave irradiation yields materials with Mn values considerably higher than those obtained under conventional conditions, but still with very low conversions [18]. 

These chiral alkylboronic esters are exceptionally promising intermediates for C-C bond formation reaction in the synthesis [8, 9] of a-chiral aldehydes, P Chiral alcohols, a-chiral acids, and a-chiral amines. Brown et al [10], in a real breakthrough, discovered that LiAlH readily converts these relatively inert boronic esters to a very high reactive lithium monoalkylborohydrides R BHjLi (5) of very high optical purity. The optically active monoalkylborane (R BH2) is generated, when required, by a convenient reaction with trimethylsilyl chloride [6]. Consequently, the desired B-R -9-BBN is prepared conveniently by hydroboration of 1,5-cyclooctadiene with RBHj (prepared in situ), and the desired stable 1,5-isomer is obtained by thermal isomerization. The whole sequence is illustrated in Scheme 9.1. 

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