Arylalkenes

Hodgson et al. have demonstrated that arylalkenes 139 and dienes 140 can readily be prepared from simple terminal epoxides in a highly stereoselective manner by employing LTMP as base in combination with aryl and vinyllithiums as nucleophiles at 0 °C (Scheme 5.31) [41]. Without addition of LTMP, secondary alcohols... 

Dimerization of a number of arylalkenes catalyzed by vitamin Bj2 and Ti(III) as reductant has been examined (Shey et al. 2002). Mechanisms were examined including the requirement of a reductant, and a reaction was proposed that involved the formation of radicals at the benzylic carbon atoms. 

Shey J, CM McGinley, KM McCauley, AS Dearth, BT Young, WA van der Donk (2002) Mechanistic investigation of a novel vitamin Bjj-catalyzed carbon-carbon bond forming reaction, the reductive dimerization of arylalkenes. J Org Chem 67 837-846. 

Scheme 2.11 Hydroboration of arylalkenes catalysed by copper NHC complexes...

Conjugated arylalkenes undergo ally lie amination on treatment with /V-phcnylhydro-xylamine under the influence of iron phthalocyanin, e.g. equation 71. The yields from aliphatic olefins are very poor 1-octene gives only 3% of the amine 193198. 

An interesting reaction that has been developed over the past decade is the application of (2 + 2)-cycloaddition reactions to the synthesis of cyclophanes49. One of the earliest examples of this is the selective conversion of the bis(arylalkenes) 112 into the adducts 113. The yield of product is dependent to some extent on the chain length separating the aryl groups and the best yield of 41% is obtained when the separation includes four methylene units (n = 4). Lower yields are recorded with the other derivatives. Mixtures of products are formed when the m-isomers 114 are used. This affords 115 and 116. The yields of these are better than those obtained from the p-isomers 11250 51. Nishimura and coworkers52 have examined the ease with which such cyclobutanes, e.g. 115, n = 2,... 

Selected examples of the reaction of carbenes with alkyl and arylalkenes R R3C=CR3R4 Reaction conditions % yield... 

Palladium salts will attack C-H bonds in functionalised aromatics such as acetoaniline to form palladium-carbon bonds that subsequently undergo insertion of alkenes [31], (3-Hydride elimination gave styryl derivatives and palladium hydride, which requires re-oxidation of palladium by benzoquinone. The reaction can be regarded as a combined Murai reaction (C-H activation, if electrophilic) and a Heck reaction (arylalkene formation), notably without the production of salts as the cross-coupling reactions do. An example is shown in Figure 19.15. 

The reactions are performed treating diarylteUurium dichlorides or aryltellurium trichlorides with excess olefin (5-10 mol equiv) in the presence of Pd(II) chloride-sodium acetate in refluxing acetic acid or acetonitrile. The yields are moderate to good with the dichlorides and poor with the trichlorides. Minor amounts of biaryls are the usual by-products beyond acetoxylated arylalkenes (formed by the addition of acetic acid to the olefins). 

Detailed mechanistic information concerning an intramolecular arylalkene cycloaddition yielding cyclobutane derivatives via a radical cation process gener-... 

A hydrogen-transfer reaction involving the olefin and the addition product of the benzylic carbanion and olefin may accompany the side-chain alkylation reaction. The result is that alkanes and arylalkenes are produced 19). This hydride-tiansfer reaction may take place by elimination of a hydride ion from the carbanion adduct followed by addition of the hydride ion to the olefin [Reaction (6)]. The amount of this side reaction probably depends largely on the severity of reaction conditions used. 

Arylalkenes can undergo various reactions when treated with sodium, since compounds such as a-methylstyrene are both olefins with allylic hydrogens and styrenes, both of which are reactive. The reaction of a-methylstyrene with sodium has been reported by Bergmann et al. (6i) to yield tetramers. More recent work by Kolobielski and Pines (56) has shown that dimers and products derived from dimers are formed when this compound is heated with a sodium-benzyl-sodium catalyst. Some of the major products were cumene (VII), p-terphenyl (VIII), and 1-methyl-1,3-diphenyl-cyclopentane (IX). 

Arylalkenes [23] and alkenes with electron withdrawing substituents [24] can be bis-alkylated across the alkene bond by electrochemical reaction with dflialoal-kanes giving 3- to 6-membered carbocyclic products in good yields. ITie best reaction conditions use an undivided cell with a nickel cathode and a sacrificial aluminium anode in dimethylformamide or N-methylpyrrolidone containing a tetraalkylammonium salt. Anodically generated aluminium ions are essential for the reaction. 1,2-Disubstituted alkenes, regardless of their stereochemistry, are converted to the tranj-substituted cycloalkane. 

Cyclo-coupling between arylalkenes and an aliphatic ester function is achieved by electrolysis in tetrahydrofuran using cathode and anode both of magnesium in an undivided cell. The first electron addition is to the arylalkene. The bond forming steps involves nucleophilic attack by radical-anions or dianions derived from the alkene. Magnesium ions generated at the anode are essential to the process. The... 

---