Elimination antiperiplanar

When the p position of the substrate has only one proton, the stereoisomeric alkene resulting from antiperiplanar elimination will be obtained (exclusively, in most cases). 

The mechanism of nitrosation of MeCOCH2 Na+ with MeONO to give MeCOCH=NOH has been studied by HF and MP3 ab initio methods pericyclic rearrangement of first-formed adduct gives an intermediate complex, [MeCOCH2NO(OMe)] Na+, from which the product is obtained by antiperiplanar elimination of MeOH. 

Ab initio methods using the 6-31 + G basis sets have been used in a theoretical study of competing, S n2 and El reactions of NCCH2CH2CI with HO and HS in the gas phase. The antiperiplanar elimination transition state, which is favoured over those for n2 and El(gauche) reactions, is more lcB-like than that for the slower El anti) reaction of ethyl chloride. 

Studies by Jarvie and coworkers37 showed that solvolysis of erythro- Me3SiCHBrCHBr-CH3 led predominantly to cw-l-bromopropene, via an antiperiplanar elimination. This... 

The reaction is thought to proceed as shown in Figure B4.2. After hydroboration and addition of hydroxide anion to the boron atom, an iodonium species is formed which creates an electrophilic terminus for alkyl migration. (The migration occurs with retention of configuration of the alkyl group.) Finally an antiperiplanar elimination reaction gives the (Z)-alkene. 

Preparative electroreduction of various vicinal dibromides at a stirred mercury cathode in the presence or absence of added proton source produced quantitative yields of olefins119. With meso- (142 meso) and dr,/-2,3-dibromobutane (142 dt) trans- and cis-2-butene, respectively, were formed stereo specifically (equations 79 and 80). The absence of monobromide products suggests concerted antiperiplanar elimination or a very shortlived carbanion. 

An apparently similar, antiperiplanar elimination with loss of the isoleucine 4-pro-S proton, to generate a double-bond of opposite geometry in alkaloidal necic acid fragments, has been observed13 (cf. Vol. 11, p. 2). 

Both synthetic and biosynthetically prepared samples of stereospecifically labeled isoleucine have been used to investigate the biosynthesis and catabolism of the amino acid described in Sections VIIl.D and VIII.E. They have also been used to show that biosynthesis of 3-ethylidene-L-azetidine-(Z)-carbo-xylate 216 from isoleucine involves dehydrogenation with antiperiplanar elimination of H-3 and H-4 (187). 

At some point in the near future you should study the Eliwinatio ejce/vise in the CD-ROM that accompanies this book. This uses WebLab to examine the consequence of synperiplanar or antiperiplanar eliminations and thus allows you to predict the stereochemistry of the alkcne formed in an elimination. 

In plants and microorganisms cinnamic acid is formed from L-phenylalanine by phenylalanine ammonia-lyase (PAL). This enzyme catalyzes the antiperiplanar elimination of the pro 3S-hydrogen atom and of the NHg-group to yield trans-cinnamic acid (Fig. 294). Most PAL preparations deaminate also L-tyrosine, but to a smaller extend. In some organisms a special tyrosine ammonia-lyase exists. 

Scheme 735. A representation of bimolecular elimination pathways for the loss of HCl from (+)-2-chlorohexane [H3C(C1)CH(CH2)3CH3] to prodnce 1-hexene [H2C=CH(CH2)3CH3] and (E)- and (Z)-2-hexene [( ,Z)-CH3CH=CH(CH2)CH3].The base RO is an alkoxide anion. The ratio of products is a function of the alkoxide in the particnlar alcohol from which it is derived. Only antiperiplanar elimination is shown (see Bartsch, R. A. Bnnnett, J. F. J. Am. Chem. Soc., 1968,90,408 and Bartsch, R. A. Bnnnett, J. F. J. Am. Chem. Soa, 1969,91,1376).

As shown in Scheme 7.38 for menthyl chloride [(li ,3i ,45 )-3-chloro-4-(l-methylethyl)methyl-cyclohexane], the isomer in which all of the substituents are (preferentially) equatorial, antiperiplanar elimination can only occur when the ring flexes into the less stable conformation where all groups are axial. As shown in the scheme, there is only one proton that is both axial and p to the chlorine. Thus, there is only one E2 elimination product on reaction with sodium hydroxide in ethanol. 

This result Indicates that the dehydrogenation of the intermediate (2S)-2-methylbutanoic acid involves an antiperiplanar elimination of the hydrogen at C-2 and the pro-R hydrogen at C-3). 

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