Alkenes regioselectivity

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Distortionless enhancement by polarization transfer (DEPT), 725-6 Disubstituted alkenes, regioselectivity, 842-4 o-Ditoluidine, glucose determination, 632, 634 o,o -Dityrosine, low-density lipoprotein, 610 DMD see Dimethyldioxirane DMDO see Dimethyldioxirane DNA... 

With these nonstabilized carbanions, attack occurred almost exclusively at the less-substituted terminus of the alkene, regioselectivity opposite that observed with stabilized carbanions. This regioselectivity... 

Azasulfenylation of alkenesf s The adducts of dimethyl(methylthio)-sulfonium tetrafluoroborate (1) with alkenes react slowly but smoothly with various nitrogen nucleophiles to give products of overall rrarw-addition to the alkene. Regioselectivity depends on the substitution pattern of the alkene and on the nucleophilicity of the attacking reagent, and is subject to some control. The mechanism of this azasulfenylation is not certain it may involve an episulfonium ion. 

Another approach for the ring expansion of epoxides uses low-valent iron complexes which open epoxides under reductive conditions, as reported by Hilt et al. [106]. The iron complexes are reduced and after coordination of the epoxide to the iron center an electron transfer initiates the radical-type ring opening of the epoxide. Under formal insertion of an alkene, regioselective formation of tetrahy-drofurans was observed (Scheme 9.46). The reaction is applicable to a broad range of acceptor-substituted alkenes bearing another double or triple bond system in conjugation with the inserted carbon-carbon double bond. 

Preparatively, it is important that mineral acids, carboxylic acids, and terf-carbenium ions can be added to alkenes via carbenium ion intermediates. Because of their relatively low stability, primary carbenium ions form more slowly in the course of such reactions than the more stable secondary carbenium ions, and these form more slowly than the even more stable tertiary carbenium ions (Hammond postulate ). Therefore, mineral and carboxylic acids add to unsymmetrical alkenes regioselectively to give Markovnikov products (see Section 3.3.3 for an explanation of this term). In addition, these electrophiles add most rapidly to those alkenes from which tertiary carbenium ion intermediates can be derived. 

Pt-nanocluster catalyzed hydrosilylation has also been explored with aromatic alkenes using PMHS. As shown in the results summarized in Table 3, substrate dependent (alkene) regioselectivity pattern was displayed for the hydrosilylation... 

The radical addition of perfluoroalkyl halides to teraiinal alkynes is performed under conditions similar to those used for alkenes. Regioselective attack of the radical at the less-hindered carbon is observed, e.g. formation of 1. The monoaddition product so formed is often a mixture of E- and Z-vinylic halides (see Table 5). 

As before, the reaction is best with monosubstituted alkenes regioselectivity is not high with 1,2-disubstituted alkenes, and the yields are less impressive (eqnation 12). ... 

Electronic effects on alkene regioselectivity in the Pauson-Khand reaction have also been observed. The regioselectivity observed in cycloadditions of norbomen-2-ones has been interpreted as arising from an electronic preference for attachment of the 5 C-S of the alkene to an alkyne carbon rather than cobalt in the bond-forming insertion step (equation 13). In these systems electronic and steric effects have been separated by carrying out identical reactions with the corresponding norbomen-2-ols, in which the... 

In 1913, Kishner observed in one instance that under standard Wolff-Kishner reduction conditions, 2-hydroxy-2,6-dimethyloctan-3-one underwent eliminative reduction upon treatment with hydrazine hydrate and base at elevated temperatures to afford 2,6-dimethyloctan-2-ene (Scheme 7). This same reaction was later found to occur in the case of a-methoxy ketones and has since been referred to as the Kishner eliminative reduction. The reaction entails initial formation of the hydrazone and elimination of the a-substituent to afford the intermediate alkenyldiazene, which subsequently collapses to the desired alkene. Given the facile transformation of ketones into a-halo ketones, these conditions have been used to introduce alkenes regioselectively in the 2a-halocholestan-3-one series as shown in Scheme 8. Yields of 2-cholestene parallel the resistance of the a-halogen to undergo competitive elimination reactions. 

Allyl sulfones have been used in a large number of valuable syntheses, but notably in the synthesis of alkenes, dienes and polyenes. Thus a-alkylallyl tolyl sulfones afford terminal alkenes regioselectively when treated sequentially with 2 equiv. of tin hydride and concentrated hydrochloric acid (Scheme 117, entry Allyltin derivatives, which are intermediates in this transformation, can be isolated if the acidic treatment is omitted (Scheme 117, entry a). 

Key Words Platinum complexes. Epoxidation, Pentafluorophenyl, Hydrogen peroxide. Terminal alkenes, Regioselectivity, Enantioselectivity. 2008 Elsevier B.v. 

There are essentially two selectivity problems when dealing with 1.3-dipolar cycloaddition reactions to alkenes regioselectivity and stereoselectivity. The issue of regiocontrol has received extensive coverage116 and will not be further discussed here. This chapter will deal with the problem of stereochemistry of the cycloaddition which can be addressed at three different levels. 

Functionalized alkenes. Regioselective hydroformylation of functionalized alkenes has been extensively studied. The rhodium complex with... 

Given that the propagation involves the addition of unsymmetrical alkenes, regioselectivity is an important issue. As a general rule, is less-substituted carbon is more preferred to produce a bond. However, the more-substituted carbon is not totally discriminated. Thus, for a given monomer, different modes of bonding are observed and their percentages depend on the nature of substituent... 

In Summary Oxymercuration-demercuration is a synthetically useful method for converting alkenes regioselectively (following the Markovnikov rule) into alcohols or ethers. Carbocalions are not involved therefore, rearrangements do not occur. 

---