Substituted alkenes, formation

In the reaction of a substituted ylide (r CH—PPh ) with an aldehyde R CHO, a stereochemical problem arises. Much work has been carried out in order to achieve control of either cis- or rrans-alkene formation. This work has been reviewed several times with always changing viewpoints (A. Maercker, 1965 L.D. Bergelson, 1964 M. Schlosser, 1970 H. Best-mann, 1979). 

Zaitsev s rule as applied to the acid catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly substituted alkene as the major product Because as was discussed in Section 5 6 the most highly substituted alkene is also normally the most stable one Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by elimination... 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

Comparison of the data for methoxide with those for t-butoxide in Table 6.4 illustrates a second general trend Stronger bases favor formation of the less substituted alkene. " A stronger base leads to an increase in the carbanion character at the transition state and thus shifts the transition state in the Elcb direction. A linear correlation between the strength of the base and the difference in AG for the formation of 1-butene versus 2-butene has been established. Some of the data are given in Table 6.5. 

The coupling of bromo- or iodobenzene to styrene yields regioselectively a mixture of E- and Z-stilbenes 12 and 13. An electron-withdrawing substituent at the olefinic double bond often improves the regioselectivity, while an electron-donor-substituted alkene gives rise to the formation of regioisomers. 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

ANSWER Let s first consider the expected regiochetnical outcome of the reaction. The reaction does not employ a sterically hindered base, so we expect formation of the more substituted alkene (the Zaitsev product) ... 

The nickel addition in chromium oxide decreased the formation of alkenes which was smaller than the one observed in the presence of just chromium oxide. It is to be remarked that the decrease of alkene formation was independent of the quantity of nickel in the catalyst. However, the catalytic activity for the fluorination reaction decreased when the nickel content increased. Thus the addition of nickel in small quantities allowed to increase the selectivity for the fluorination reaction. We could suggest that nickel substitute... 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

Where the nucleophile attacking the substituted alkene is a carbanion (cf. p. 284) the process is referred to as a Michael reaction its particular synthetic utility resides in its being a general method of carbon-carbon bond formation e.g. with (91) ... 

A bulky base such as potassium tert-butoxide in tert-butyl alcohol favors the formation of the less substituted alkene in dehydrohalgenation reactions. 

In the course of investigation into new C-C bond formation processes, Hiyama has developed an efficient nickel-catalyzed arylcyanation of alkynes.67 The addition reaction of an aryl-CN bond to alkyne affords aryl-substituted alkene nitrile in good yield. Good regioselectivity is reported in the case of unsymmetrical alkynes with two sterically different substituents. 

Crowe proposed that benzylidene 6 would be stabilised, relative to alkylidene 8, by conjugation of the a-aryl substituent with the electron-rich metal-carbon bond. Formation of metallacyclobutane 10, rather than 9, should then be favoured by the smaller size and greater nucleophilicity of an incoming alkyl-substituted alkene. Electron-deficient alkyl-substituents would stabilise the competing alkylidene 8, leading to increased production of the self-metathesis product. The high trans selectivity observed was attributed to the greater stability of a fra s- ,p-disubstituted metallacyclobutane intermediate. 

Previously acrylonitrile had proved to be inert towards transition metal catalysed cross- and self-metathesis using ill-defined multicomponent catalysts [lib]. Using the molybdenum catalyst, however, acrylonitrile was successfully cross-metathesised with a range of alkyl-substituted alkenes in yields of40-90% (with the exception of 4-bromobut-l-ene, which gave a yield of 17.5%). A dinitrile product formed from self-metathesis of the acrylonitrile was not observed in any of the reactions and significant formation (>10%) of self-metathesis products of the second alkene was only observed in a couple of reactions. 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

The reaction of heteroatom-substituted alkenes with electrophilic carbene complexes can lead to the formation of highly reactive, donor-acceptor-substituted cyclopropanes. This type of cyclopropane usually undergoes ring fission and rearrangement reactions under milder conditions than do unsubstituted cyclopropanes (Figure 4.22). 

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). 

The reaction is promoted by a variety of bases, usually in catalytic quantities only, which generate an equilibrium concentration of carbanion (92) it is reversible, and the rate-limiting step is believed to be carbon-carbon bond formation, i.e. the reaction of the carbanion (92) with the substituted alkene (91). Its general synthetic utility stems from the wide variety both of substituted alkenes and of carbanions that may be employed the most common carbanions are probably those from CHjfCOjEtlj—see below, MeCOCHjCOjEt, NCCH -COjEt, RCH2NO2, etc. Many Michael reactions involve C=C—C=0 as the substituted alkene. 

With more substituted alkenes, reaction under these conditions is often accompanied by double-bond migrations which eventually lead to the formation of an alky ltrichloro silane with a primary alkyl group.51 52... 

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