Substitution on Alkenes

Heat of hydrogenation values (AH°) for several alkenes. The effect of alkyl substitution is evident. Also, the stability of trans double bonds remains relatively constant, but significant destabilization of the cis alkenes is seen as the R group size increases. Derived from data in Turner, R. B., Jarrett, A. D., Goebel, R, and Mallon, B. J. Heats of Hydrogenation. IX. Cyclic Acetylenes and Some Miscellaneous Olefins. /. Am. Chem. Sac, 95, 790(1973). 

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Vinyl substitutions on alkenes not having their double bonds conjugated with carbonyl groups often proceed more rapidly and give better product yields when the reactions are conducted in the presence of an unhindered secondary amine. Conjugated and nonconjugated dienes are usually only minor products in these cases. The major products normally are allylic amines obtained by nucleophilic attack of the secondary amine upon the ir-allylpalladium intermediates. Since allylic amines may be quatemized and subjected to the Hoffmann elimination, this is a two-step alternative to the direct vinyl substitution reaction.90... 

The general formula for alcohols is R — OH, where the R group can represent the alkyl, alkenyl, or alkynal groups. In the case of substitution on alkenes and alkynes, only saturated carbons may be substituted. For example, the following compounds are all alcohols ... 

Trifluoromethyl substitution on alkenes favors cycloadditions with various dipoles. Yields are generally high and, with reactive dipoles, reactions can be conducted with trisubstituted alkenes. Despite the zwitterionic character of the dipole, these [34-2] cycloadditions are generally concerted and the geometry of the starting alkenes is conserved however, when no stereochemical information is expected, a stepwise process cannot be ruled out. 

Reaction of diphenylsulfoxide with triflic anhydride affords, as precedently mentioned, the corresponding sulfoxonium triflate, a powerful reagent for numerous reactions, such as a-arylation of carbonyls, enolate additions,or nucleophilic substitution on alkenes (102). ... 

The reactivity of most singlet carbenes with alkenes is dominated by the electrophilic character of the carbene (the empty p orbital). Thus, the more electron rich the alkene, the faster the carbene addition. Increasing alkyl group substitution on alkenes increases the rate of addition. This trend parallels the reactivity for the addition of other electrophiles with alkenes, such as acids, Xj, and borane. Dialkylcarbenes are less selective than dihalocarbenes, whereas carbenes with neighboring O or N atoms are resonance stabilized (see margin) and are highly selective. This trend tracks the reactivity-selectivity principle (see Chapter 7), where the more stable carbenes are the more selective. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

General reactivity trends for alkenes were established for hydrozirconahon by way of qualitative studies terminal alkene > internal alkene > exocyclic alkene > cyclic alkene trisubshtuted alkene. The rate of hydrozirconahon decreases with increasing substitution on the alkene. This property was used for selechve monohydrozir-conation of conjugated and non-conjugated polyene derivahves (Scheme 8-8) [84-86]. 

As for alkenes, the rate of hydrozirconation of alkynes decreases with increasing substitution on the alkyne. An unsymmetrical diyne reacts with 1 preferentially at the less-substituted triple bond [85]. 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

The competition between formation of bicyclo[3.2.0] and bicyclo[3.1.1] products is determined by substitution on the alkene. 

Scheme 32 Dependence of enyne cyclizations on alkene substitution...

Hyperconjugation does not seem to have much effect on alkene reactivity towards bromine, since (16) applies whatever the number of alkyl groups on the double bond. However, only cis-olefins are involved in this correlation. To include geminally substituted olefins, an additional term is necessary, as in (19) where d is unity for the pem-disubstituted compounds and zero for... 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

The rate also decreases with an increase in the chain length of the alkene molecule (hex-l-ene > oct-1-ene > dodec-l-ene). Although the latter phenomenon is attributed mainly to diffusion constraints for longer molecules in the MFI pores, the former (enhanced reactivity of terminal alkenes) is interesting, especially because the reactivity in epoxidations by organometallic complexes in solution is usually determined by the electron density at the double bond, which increases with alkyl substitution. On this basis, hex-3-ene and hex-2-ene would be expected to be more reactive than the terminal alkene hex-l-ene. The reverse sequence shown in Table XIV is a consequence of the steric hindrance in the neighborhood of the double bond, which hinders adsorption on the electrophilic oxo-titanium species on the surface. This observation highlights the fact that in reactions catalyzed by solids, adsorption constraints are superimposed on the inherent reactivity features of the chemical reaction as well as the diffiisional constraints. 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

Pfaltz and co-workers (28) also showed that the semicorrin-derived catalyst is remarkably effective in intramolecular cyclopropanation reactions (28). Cycliza-tion of co-alkenyl diazoketones to form six-membered rings proceeds in high selectivity, while the analogous five-membered rings are somewhat more sensitive to substitution on the pendant alkene, Eqs. 16 and 17. 

More specifically, 3,5-di-ferf-butylphenyl substitution on the 3,3 -position of the binaphthol backbone (260) provided overall best yields and selectivities. Using catalyst 260, the authors expanded the scope of substrates to include aliphatic and aromatic nitro-alkenes, and a-substituted P-ketoesters, while maintaining good yields and enantiomeric ratios (Scheme 71). 

Being aware of the fact that a hetero-substituted carbon-carbon double bond is convertible into a carbonyl group, one can use a-hetero-substituted lithio-alkenes 2 as nucleophilic acylation reagents 142 and 143, which display the umpoled d reactivity, provided that the carbanionic character is effective. Depending on the hetero-snbstitnent X, the conversion of the vinyl moiety into a carbonyl gronp can be effected either by hydrolysis or by ozonolysis. The former procednre has been applied preferentially in the case of lithiated vinyl ethers, whereas the latter has been nsed in particnlar for cleavage of the double bond in such products that result from the reaction of hthiated vinyl bromides with electrophiles (Scheme 17). 

Pyridinium bromide, useful for bromine addition to alkenes and a-substitution on aldehydes and ketones, is dangerous to handle. The polymer analog (XXXXI) is easy to handle... 

The FOZ of A -alkenes singly substituted on position 3 with an alkyl group or with various open-chain and ring ketones (299) can be structurally characterized by H and NMR spectroscopy. Thus, with one exception the proton denoted as H shows a singlet at 5 = 4.98 to 5.04 ppm whereas H and H" show a singlet and a triplet, respectively. 

Scheme 15 Twofold Heok oouplings on acceptor-substituted terminal alkenes to yield geminally diarylated alkenes. ...

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