Nonconjugated alkenic

Uncatalyzed addition reactions of Grignard reagents with nonconjugated alkenes and aUcynes are of limited use in synthesis. However, carbon-carbon double bonds substituted by a leaving group, such as an acetate, are susceptible to be displaced by organomagnesium compounds presumably by an addition-elimination pathway. A few examples have been reported (equation 64). 

PhCHjCH=C(CH,)j-f PhCH==CHCH(CH,)2. The major product has the C==C conjugated with the benzene ring and therefore, even though it is a disubstituted alkene, it is more stable than the minor product, which is a trisubstituted nonconjugated alkene. 

Fairly selective reduction of certain alkenes can be achieved using the sodium-hexamethylphosphoramide (HMPA)-tm-butyI alcohol system,193 despite the general trend, that nonconjugated alkenes are usually quite resistant to the dissolving metal reduction method (see Section 11.3.2). In the case of 9(10)-octalin, the transformation leads to a nearly equilibrium product distribution ... 

The most commonly observed dimerization is that of alkenes to form cyclobutane derivatives. Nonconjugated alkenes such as cyclopentene and norbornene are dimerized in the presence of a sensitizer, whereas conjugated alkenes dimerize directly dimerizations of the second type have been observed in dienes, phenyl-ethylenes, and a,/9-unsaturated carbonyl, cyano, and nitro derivatives. The precise structure and stereochemistry of many of these dimers is uncertain, although it is known to be influenced by the solvent, by the presence and nature of substituents, and by the use of a sensitizer. The structures of dimers formed in solid-state irradiations are often determined by crystal structure. 

Bromine normally adds anti to a nonconjugated alkene. For example, cis-2-butene gives exclusively the D,L-2,3-dibromobutanes (Equation 7.11), whereas lranj-2-butene gives only the corresponding meso compound (Equation 7.12).29 Similarly, 4-1-butylcyclohexene gives only the trans dibromides (18 and 19).30... 

Scheme 4.56). The two-bond, F-F coupling constant observed in such conjugated systems is much smaller than that of the nonconjugated alkene systems. 

A stereochemical study of the synthesis of unsaturated 1,4-aminoalcohols via the reaction of unsaturated 1,4-alkoxyalcohols with chorosulfonyl isocyanate revealed a competition between an retentive mechanism and an SnI racemization mechanism, with the latter having a greater proportion with systems where the carbocation intermediate is more stable.254 An interrupted Nazarov reaction was observed, in which a nonconjugated alkene held near the dienone nucleus undergoes intramolecular trapping of the Nazarov cyclopentenyl cation intermediate.255 Cholesterol couples to 6-chloropurine under the conditions of the Mitsunobu reaction the stereochemistry and structural diversity of the products indicate that a homoallylic carbocation derived from cholesterol is the key intermediate.256 l-Siloxy-l,5-diynes undergo a Brpnsted acid-promoted 5-endo-dig cyclization with a ketenium ion and a vinyl cation proposed as intermediates.257... 

A large number of [2 + 2] photocycloadditions ofvinylarene compounds have been reported in the literature [37], and these are particular suitable for the synthesis of cyclophanes. Based on the fact that conjugation between the olefinic and aromatic 11-systems is rather low, these reactions can be compared to corresponding reactions with nonconjugated alkenes. 

Malik, C.K., Vaultier, M., and Ghosh, S. (2007) Copper(I)-catalyzed [2 + 2] photocycloaddition of nonconjugated alkenes in room-temperature ionic liquids. Synthesis, 1247-1250. 

The [2 + 2]-photocycloaddition of nonconjugated alkenes was described in Chapter 5. This strategy can be used for synthesizing macrocyclic rings by using a long linker between the two alkene moieties (Scheme 9.47). Thus, bicyclic cyclobutanes can be obtained starting from enol ethers and cinnamates by means of electron-transfer sensitizers, such as DCN or DCA [80], and triplet photosensitizers such as benzo-phenone (BP) [81]. 

Di-7i-methane rearrangements of acyclic and monocyclic 1,4-dienes usually occur in the singlet excited state and, therefore, the reaction takes place on direct irradiation. This is an important limitation since nonconjugated alkenes absorb below 250 nm. Consequently, the DPM reaction in acyclic and monocyclic dienes is restricted to compounds in... 

Removal of a proton from C4 forms an extended enol, which can be protonated at C2 or C4. Proto nation at C4 is thermodynamically favoured as it leads to the conjugated alkene. But protonation at C2 is kinetically favoured, and this leads to the nonconjugated alkene. The geometry of the new alkene depends on the conformation of the chain when the first (deprotonation) step occurs. It is thought that this is the best conformation for the previous reaction, the dehydration step, and that no rotation of the chain occurs before the isomerase gets to work. 

Gas electron diffraction of 2,5-dimethylthiophene shows one of the C—H bonds of the methyl substituents to be cis with respect to the S—C bond of thiophene <1993JST(301)107>. The C(2)—Me distance is smaller than a normal C—C single bond and quite close to the C—C bond length in nonconjugated alkenes. The C—S bond length is larger than in thiophene as is also the C—S—C angle. 

Rearrangements of nonconjugated alkenes have also been observed. Thus, a triterpenoid (partial structure 38) afforded the alkene (39 32% yield) upon reduction (Barton modification), a result attributed to initial base-induced isomerization of (38) to the corresponding a,3-unsaturated ketone followed by rearrangement during reduction. ... 

The Cope elimination has been used less widely than the Hofmann elimination of quaternary ammonium hydroxides, but in some cases has advantages in terms of the ease of manipulation and lack of product isomerization, e.g. nonconjugated alkenes can be obtained without double Ixmd migration, as in equation (2). ... 

Table 3.3 contains examples of nonconjugated alkenes with the major site of reaction indicated. This table also includes the examples of functionalized nonconjugated alkenes. 

D. N. J. White and M. Bovill,/. Chem. Soc., Perkin Trans. 2, 1610 (1977). Molecular Mechanics Calculations on Alkanes and Nonconjugated Alkenes. 

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