Unconjugated -alkenes

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

In the present section are discussed, in turn, the most important subclasses of alkene oxides that are known to be substrates of EH. The sequence begins with epoxides of unconjugated alkenes and ends with epoxides of complex, conjugated cycloalkenes of biochemical and pharmacological interest. 

In addition to the unfunctionalized alkene epoxides discussed in the previous subsection, various other types of epoxides exist that are also derived from unconjugated alkenes but that share two additional features, i. e., being characterized by the presence of one or more functional group(s) and having biological significance. Thus, the present subsection examines epoxy alcohols, epoxy fatty acids, allylbenzenes 2, 3 -oxides, as well as alkene oxide metabolites of a few selected drugs. 

Carbon-carbon double bonds in unconjugated alkenes usually exhibit weak to moderate absorptions due to C=C stretehing in the range 1660-1640 em V Disubstituted, trisubstituted and tetrasubstituted alkenes usually absorb near 1670 cm The more polar carbon-carbon double bonds in enol ethers and enones usually absorb strongly between 1600 and 1700 crn k Alkenes conjugated with an aromatic ring absorb strongly near 1625 crn k... 

C=C Stretching Vibrations Unconjugated Linear Alkenes The C=C stretching mode of unconjugated alkenes usually shows moderate to weak absorption at 1667-1640 cm1. Monosubstituted alkenes, that is, vinyl groups, absorb near 1640 cm-1, with moderate intensity. Disubstituted trans-alkenes, tri- and tetraalkyl-substituted alkenes absorb at or near 1670 cm-1 disubstituted ds-alkenes and vinylidene alkenes absorb near 1650 cm-1. 

These reactions may show considerable selectivity. Corey and Chaykovsky19 give an example with the terpene carvone 80. The ylid 78 is made with NaH and reacts only with the enone and not with the unconjugated alkene. The product is one diastereoisomer 81 as the ylid has added to the opposite side of the ring to the only substituent. It also has retained the stereochemistry of the cis alkene but that is inevitable as 3/6 ring fusion must be cis. 

A conjugated alkene is always preferred to an unconjugated alkene. 

IR spectroscopy provides us with evidence for polarization in C-C bonds conjugated to C-O bonds. An unconjugated ketone C=0 absorbs at 1715 cm-1 while an unconjugated alkene C-C absorbs... 

The polarization of the C=C bond is also evident in the 13C NMR spectrum, with the signal for the sp2 carbon atom furthest from the carbonyl group moving downfield relative to an unconjugated alkene to about 140 p.p.m., and the signal for the other double bond carbon atom staying at about 120p.p.m. 

Selenoxides normally show a preference for elimination towards the least-substituted carbon (Scheme 173, c), and for the formation of conjugated over unconjugated alkenes. 

Reactions of simple, unconjugated alkenes with electrophiles... 

Simple, unconjugated alkenes are nucleophilic and react with electrophiles. 

Unsaturated ketones have been prepared in moderate yield through the acylation of ir-allylnickel complexes with activated 2-pyridyl carboxylates. But, isomerization of the initially formed unconjugated alkene resulted in mixtures of products and limited the value of the method. Substituted ir-allylnickel complexes derived from crotyl bromide or cinnamyl bromide were acylated in 79% and 50% yields, respectively, without formation of the a,3-unsaturated ketone. ... 

The manganese dioxide/chlorotrimethylsilane reagent system gives a cleaner, trans-stereoselective chlorination of unconjugated alkenes in high yield136. This increased selectivity towards addition has been taken as evidence of negligible involvement of free chlorine atoms and a non-radical-chain mechanism has been proposed. 

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