Styrenes trans-/3-alkyl substituted styrene

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

The reaction of 3-chloro-3-(2-thienyl)-3//-diazirine with alkyl-substituted alkenes gave predominantly the trans-isomer of 5 while with acrylates, crotonates and styrene, the c/s-isomer predominated. The reaction of chloro(2-thienyl)carbene with (Z)- and ( )-but-2-ene proceeded with the retention of alkene stereochemistry. 

Regarding the scope of the reaction, it was found that electron-rich substrates like di-, tri- and tetra-substituted alkenes were giving moderate to good yields of their corresponding epoxides. Styrene and styrene derivatives were also demonstrated to react smoothly, whereas mono-alkyl-substituted substrates were completely un-reactive under these conditions. The basic reaction medium used was very beneficial for product protection, and hence acid sensitive epoxides were formed in good yields. Different additives were screened in order to improve this epoxidation system, and it was found that the addition of sodium acetate was beneficial for reactions performed in t-BuOH. Similarly, the addition of salicylic acid improved the outcome of the reaction performed in DMF. The use of these additives efficiently reduced the number of hydrogen peroxide equivalents necessary for a productive epoxidation (Table 2.5). The reaction is not completely stereospecific, since the epoxidation of as-4-octene yielded a cis/trans mixture ofthe product (1 1.45 without additive and 1 1.1 in the presence of 4mol% salicylic acid). 

Of course, the cleavage reactions of both 2-aryl- and 2-alkyl aziridines are stereoselective because only the trans-diastereomers of the corresponding regioisomers 1 and 2 are formed. In other words, 2-aryl substituted aziridines showed opposite regioselectivity to 2-alkyl aziridines. With bicyclic aziridines, exclusive formation of the trans diastereomer is observed in the case of symmetric bicyclic aziridines. Unsymmetrical aziridines such as styrene, octene, and undecene aziridines produce a minor amount of the other regioisomer. 

In contrast to SMOs, which exclusively catalyze the formation of (S)-epoxides, CPOs are able to yield chiral (P)-epoxides from styrene derivatives, which could serve as a stereocomplementary method (Scheme 13.11). However, the substrate spectrum appeared to be very limited [85,109]. Two ds-disubstituted aryl-substituted alkenes, czs-a-methylstyrene and 1,2-dihydronaphthalene, were effectively epoxi-dized in the presence of H Oj, and the later underwent spontaneous hydrolytic ring opening to afford the corresponding trans-diol. The enantiomeric excesses of the products were as high as 96% and 97% ee, respectively [83]. Terminal alkenes such as styrene and derivatives with a halide substituent on the benzene ring [109] or an alpha-alkyl substituent [85] could be accepted by CPO, and underwent epoxidation using f-BuOOH or HjOj as the oxidant, but the (P)-epoxides were achieved with only low to medium enantiopurity [83,85,109]. 

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