Regioisomeric alkene

Desulfonylation of equally substituted allylic sulfones with NaBH4 and LiBHEt3 usually yields a mixture of regioisomeric alkenes[406,407]. However, the regioselective attack of the less substituted side of the unsymme-trically substituted allylic system with LiEtjBH has been utilized for the removal of the allylic sulfone group in synthesis of the polyprenoid 658[408],... 

Besides direct nucleophilic attack onto the acceptor group, an activated diene may also undergo 1,4- or 1,6-addition in the latter case, capture of the ambident enolate with a soft electrophile can take place at two different positions. Hence, the nucleophilic addition can result in the formation of three regioisomeric alkenes, which may in addition be formed as E/Z isomers. Moreover, depending on the nature of nucleophile and electrophile, the addition products may contain one or two stereogenic centers, and, as a further complication, basic conditions may give rise to the isomerization of the initially formed 8,y-unsaturated carbonyl compounds (and other acceptor-substituted alkenes of this type) to the thermodynamically more stable conjugated isomer (Eq. 4.1). 

If 1-cyclopentylethyl bromide were used, a mixture of regioisomeric alkenes would be formed, with the desired vinylcyclopentane being the minor component of the mixture. 

The substrate is a tertiary alkyl bromide and can undergo SN1 substitution and El elimination under these reaction conditions. Elimination in either of two directions to give regioisomeric alkenes can also occur. 

Formation of Regioisomeric Alkenes by /3-Elimination Saytzeff and Hofmann Product(s)... 

If the /1-elimination of H/Het from R —Het can, in principle, afford regioisomeric alkenes whose C=C double bonds (Figure 4.7) contain a different number of alkyl substituents, they are differentiated as Hofmann and Saytzeff products the Hofmann product is the alkene with the less alkylated double bond, and the Saytzeff product is the alkene with the more alkylated double bond. Because C=C double bonds are stabilized by alkyl substituents, a Hofmann product is, in general, less stable than its Saytzeff isomer. Accordingly, eliminations of H/Het from Rv(,f —Het, which exhibit product development control, furnish a Saytzeff product with some regioselectivity. 

When the carbenium ion intermediate of an El elimination can be deprotonated to give two regioisomeric alkenes, generally both of them are produced. If these alkene isomers are Saytzeff and Hofmann products, the first one is produced preferentially because of product development control. This is illustrated in Figure 4.34 by the El elimination from ferf-amyl... 

Aza-annulation of enamine substrates with acrylamide (20) occurred under milder reaction conditions. Treatment of 19 with 20 in dioxane at reflux (100 °C) resulted in the efficient formation of 4b with reported yields of 70%7 and 83%8 (eq. 4). Subsequent study of this reaction led to the quantitative formation of 4b and 5b reported as a 50 50 mixture of the regioisomeric alkenes.9 This methodology was also extended to aza-annulation of the cyclopentanone enamine with 20 (72%).9 The analogous reactions have been performed by the combination of 20 with the benzyl imine (77%)8 or the propyl imine (18%)10 of cyclohexanone (lb). Limitations arose when the acrylamide was / -substituted or had a substituent at the [3-position (crotylanilide).9 When reagents with these features were used, the reaction did not proceed. 

Explain why the two diastereomeric amine A -oxides 1 give, on heating, two different major regioisomeric alkene products. 

We can use the same ideas when we think about El eliminations that can give more than one regioisomeric alkene. Here is an example. The major product is the alkene that has the more substituents because this alkene is the more stable of the two possible products. 

Two regioisomeric alkenes possible Two stereoisomeric alkenes possible OH... 

The two regioisomeric alkenes formed via the E2 transition state of the tertiary halide 2-bromo-2-methylbutane are both more substituted than the alkenes (ethylene and propene) formed from ethyl and isopropyl bromide, respectively. 

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