Less substituted alkene

Furthermore, the regioselective hydrogenolysis can be extended to internal allylic systems. In this case, clean differentiation of a tertiary carbon from a secondary carbon in an allylic system is a problem. The regioselectivity in the hydrogenolysis of unsymmetrically substituted internal allylic compounds depends on the nature and size of the substituents. The less substituted alkene 596 was obtained from 595 as the main product, but the selectivity was only... 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

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. 

A novel aspect of the Hofmann elimination is its regioselectivity. Elimination in alkyltrimethylfflnmonium hydroxides proceeds in the direction that gives the less substituted alkene. 

Markovnikov s rule is used to predict the regiochemistry of HX (electrophilic) addition reactions. The rule states that HX adds to an unsymmetrical alkene mainly in the direction that bonds H to the less substituted alkene carbon and X to the more substituted alkene carbon. 

The large base t-BuO helps to give the less substituted alkene (10) and also helps elimination... 

For an elimination reaction where there is more than one possible aUcene that can be formed, we have names for the different products based on which alkene is more substituted and which is less substituted. The more substituted alkene is called the Zaitsev product, and the less substituted alkene is called the Hofmann product. Usually, the Zaitsev product is the major product ... 

However, there are many exceptions in which the Zaitsev product (the more-substituted alkene) is not the major product. For example, if the reaction above is performed with a stericaUy hindered base (rather than using ethoxide as the base), then the major product will be the less-substituted alkene ... 

When doing this type of sequence, there are a few important things to keep in mind. In the hrst step (elimination), we have a choice regarding which way to eliminate do we form the more substituted alkene (Zaitsev product) or do we form the less substituted alkene (Hofmann product) ... 

We can control which product we get by carefully choosing our base. If we use a strong base (like methoxide or ethoxide), then we will get the more substituted alkene. However, if we use a strong, sterically hindered base, such as tert-butoxide, then we will get the less substituted alkene. 

In competition, the less substituted alkenes displace more highly substituted alkenes (this can be virtually 100%, for example, 2,3-dimethylcy-clohexene versus 1,2-dimethyl cyclohexene.)... 

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

Figure 6.3 Energy profile E2 reaction to more- or less-substituted alkenes...

It is a general observation that, where different alkene products can arise through E2 elimination, the more-substituted alkene predominates. 2-Menthene contains a double bond with two alkyl substituents, whereas the double bond in 3-menthene has three substituents. The more-substituted alkene is termed the Saytzeff product the less-substituted alkene is termed the Hofmann product. We recommend you disregard the proper names, and think of the products in terms of more-substituted alkene and less-substituted alkene . 

This E2 elimination (Table 7-3) give the less substituted alkene (Hofmann product) rather than the more substituted alkene (Saytzeff product Section 6.3). 

The less substituted alkene is formed (a) HjC=CH2, not H2C==CHCH3 (b) CH3CH2CH2CH=CHj, not CH3CHjCH=CHCH3. 

In the related Shapiro reaction, two equivalents of an alkyl lithium are used and the less substituted alkene is formed. 

This reaction is conducted in a solvent that won t separate the ion pairs. This necessitates an eclipsed / syn orientation for elimination to take place. If there is a choice of eclipsed states, elimination will take place during the most stable, which generally gives the less substituted alkene. 

Useful for homogeneous reduction of alkenes. As a consequence of the reagent bulk, it is understandable that the reactivity of alkene reduction is dependent on substitution the less-substituted alkenes react faster. Also, reduction occurs from the less-hindered face in a cis-stereochemistry. Many other functional groups are tolerated by conditions. 

Few examples of the Prins reaction of homologous aldehydes and ketones are known.73,74 Acetaldehyde was shown to react smoothly with isobutylene under mild conditions, whereas other, less substituted alkenes were found to be very unreactive.73... 

With tertiary halides, bimolecular elimination usually occurs if isomeric alkenes can result, the proportions formed depend on the steric requirements of the pyridine because formation of the more substituted alkene (Saytsev Rule) is more sensitive to steric hindrance than formation of the less substituted alkene (Hofmann Rule). Pyridine and r-amyl bromide give 25% of 2-methylbut-l-ene (less... 

Electrochemical reduction of a rr-allylpalladium complex results in preferred formation of the more substituted alkene (equation 323).307 Conversely, reduction by Smh, followed by protonation, provides the less substituted alkene predominantly (equation 324).305... 

Quaternary ammonium hydroxides undergo Hofmann elimination when they are heated. A point to be considered here concerns the regioselectivity of Hofmann eliminations it is the less hindered / proton that is removed by the base giving the less substituted alkene. 

The transition state for the reaction resembles the product more than the reactant and so the factors that stabilise the product also stabilise the transition state and make that particular route more likely. However, the opposite preference is found when potassium ferf-butoxide is used as base, and the less substituted alkene is favoured. 

This elimination reaction follows Hofmann s rule, so the less substituted alkene should be the major product. 

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