Alkenes, methyl-branched

The addition reaction of allyltriorganosilanes to 1-alkenes in the presence of anhydrous aluminum chloride as catalyst at room temperature gives regiospecific allylsilylated products, in which the silyl group adds to the terminal carbon and the allyl group adds to the inner carbon of the double bond [Eq. (3)]. Compared with the starting alkenes, the products of the allylsilylation reaction possess two additional carbon atoms in addition to a (triorganosilyl)methyl branch at the carbon (3 to the double bond. 

There are four pairs of 1- and 2-alkenes with branching in a substituent group for which we can examine double bond migration enthalpies49 4-methyl-l- and 4-methyl-2-pentene 5-methyl-1- and 5-methy 1-2-pentene 4-methyl-l- and 4-methyl-2-hexene and 4,4-dimethyl-1- and 4,4-dimethyl-2-pentene. Each of the 2-enes exists in cis and tram forms. The first two pairs, which are homologous, have Ci to trans-C2 isomerization enthalpies of ca —13 (for two measurements of trans-4-methyl-2-pentene) and —14.4 kJmol-1, and Ci to cis-C2 isomerization enthalpies of —9.6 and —10.4 kJmol-1. The third pair, not homologous with any other, has a Ci to trans-C2 isomerization enthalpy of... 

Several alkene isomers vary structurally in the position of a methyl branch on their parent 1-alkene chain. Generally, moving the methyl group from C3 to positions further from the double bond results in an exothermic enthalpy of isomerization. That is, the isoalkyl-1-alkenes are the most stable isomers and the 3-methyl-1-alkenes are (presumably) the least stable. Because of the problematic 5-methyl-1-hexene data and the lack of data for 3-methyl-1-heptene, nothing more quantitative can be said other than each methyl re-positioning down the chain results in about 1-2 kJmol-1 stabilization. A similar change in branching position from 3-methyl-n-alkanes to 2-methyl-n-alkanes releases about 3 kJmol-1. 

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

The soluble and a unique microsomal fatty acid synthase (FAS) which are involved in producing the 18-carbon fatty acyl precursors to hydrocarbons have been purified to homogeneity and characterized (Gu et al., 1997). It appears that the soluble FAS synthesizes the straight chain fatty acids involved in n-alkane and alkene formation, whereas the microsomal FAS produces the precursors for the methyl-branched hydrocarbons (Blomquist et al. 1995). 

The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular layer of the cuticle. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called cuticulin . The presence of hydrocarbons in this wax of insects was suggested by Chibnall et al. (1934) and Blount et al. (1937), and over the next few decades the importance of hydrocarbons in the cuticular wax of insects was established (Baker et al., 1963 and references therein). The first relatively complete chemical analyses of the hydrocarbons from any insect, the American cockroach, Periplaneta americana (Baker et al., 1963), occurred after the development of gas-liquid chromatography (GLC). The three major components of the hydrocarbons of this insect, //-pen taco sane, 3-methylpentacosane and (Z,Z)-6,9-heptacosadiene, represent the three major classes of hydrocarbons on insects, n-alkanes, methyl-branched alkanes and alkenes. Baker and co-workers (1963) were able to identify n-pentacosane by its elution time on GLC to a standard and its inclusion in a 5-angstrom molecular sieve. 3-Methylpentacosane... 

The hundreds of different cuticular hydrocarbon components reported on insects can be divided into three major classes, n-alkanes, methyl-branched components and unsaturated hydrocarbons. There are reports of methyl-branched alkenes (see below), but these are rare. The hydrocarbon components on the surface of insects are usually complex mixtures comprised of anywhere from a few to up to hundreds of different components in some species. 

While the vast majority of alkenes reported on the surface of insects are straight-chain molecules, there have been a few reports of methyl-branched alkenes. Warthen and Uebel (1980) found (Z)-2-methyl-24-hexatriacontene in the hydrocarbons of the house cricket, Acheta domesticus. Carlson and Schlein (1991) reported 19,23-dimethyltriacont-l-ene and other homologs in tsetse flies. Howard et al. (1990) found a homologous series of... 

The differential importance of alkenes, and linear and methyl-branched alkanes in the nestmate recognition system of the paper wasp Polistes dominulus was tested using... 

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