Benzene, 1, -hexenyl

A valence isomerization reaction similar to that encountered with the tropylium ion has been observed when a variety of substituted benzenes are photolyzed in strong acid. The pentamethylbenzenium ion (18) photolyzed at —78° to give a single product, the pentamethylbicyclo-[3,l,0]hexenyl cation (19), in excess of 80% conversion (Childs et cd., 1968). 

Many examples exist for Pd-catalyzed cross-couplings of alkenylzirconocenes with simple carbocyclic aryl or alkenyl halides, whereas few precedents are seen for the coupling of alkenylzirconocenes with heteroaryl halides. Undheim and coworkers reported a Pd-catalyzed cross-coupling of 2,4-dichloropyrimidine with alkenylzirconocene [50]. Hydrozirconation of hexyne readily took place at room temperature with zirconocene chloride hydride in benzene. The resulting hexenylzirconocene chloride (76) was then coupled with 2,4-dichloropyrimidine at the more electrophilic 4 position, giving rise to 2-chloro-4-[( )-l-hexenyl]pyrimidine (77). 

Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene39, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadieny 1 cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

The combination of ortho metallation and meta nucleophilic acylation was used to prepare a key intermediate in a synthesis of deoxyfrenolicin (42), as outlined in Scheme 11. The complex of anisole is orf/io-metallated with n-butyllithium and quenched with chlorotrimethylsilane the resulting [(o-(tri-methylsilyl)anisole)Cr(CO)3] (43) is then metallated again, converted to the arylcuprate, and coupled with ( )-2-hexenyl bromide to give the complex of l-trimethylsilyl-2-methoxy-3-(2-hexenyl)benzene (44). Addition of the carbanion from the cyanohydrin acetal of 4-pentenal, followed by the standard iodine oxidation and subsequent hydrolysis of the cyanohydrin acetal to regenerate the carbonyl group... 

This result was confirmed and extended by Oliver and coworkers in a series of reports dealing with the intramolecular attack of a variety of organometallics7 (Al, Mg, Li, Zn, etc.) onto unactivated alkenes. When 4 was prepared from di(5-hexenyl)mercury, it cyclized to 5 in less than 1 h at 25 °C in diethyl ether and these cyclization reactions were thought to be promoted by metal-alkene complexation8, that required metals bearing empty orbitals. Moreover, the rate of cyclization is highly solvent-dependent at 25 °C 4 takes 8 days to cyclize in pentane, 96 h in benzene and less than an hour in diethyl ether. 

SYNS BENZENE.ACETIC ACID, 3-HEXENYL ESTER, (Z)- P,Y-HEXENYL a-TOLUATE... 

Coupling partner RY E-1-hexenyl boronic acid or phenyl boronic acid (1.1 equiv) Additives 2 equiv NaOEt or KOH Conditions dioxane or benzene, 80 °C... 

C15H22 1 -(1,5-dimethyl-4-hexenyl)-4-methy benzene 644-30-4 565.39 50.201 2 28772 C15H2402 2,6-di-tert-butyl-4-methoxyphenol 489-01-0 523.15 46.113 2... 

A solution of 0.136 g (0.5 mmol) of ( >5-oxo-3-hexenyl l-bcnzylaziridinecarboxylatc in 3 mL of benzene is subjected to flash vacuum pyrolysis at 330 °C. The reaction mixture is concentrated in vacuo and purified by flash chromatography to give the product as a single isomer yield 0.109 g (80%). 

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