Phenols alkenylation

A greatly enhanced chemoselective formation of phenol is observed for alkoxy(alkenyl)carbene complexes compared to alkoxy(aryl)carbene complexes. This behaviour reflects the ease of formation of the rf-vinylketene complex intermediate E starting from alkenylcarbene complexes for aryl complexes this transformation would require dearomatisation. 

Wulff et al. examined the necessary reaction conditions for a,fi-unsaturated aminocarbene complexes to react in a benzannulation reaction [23]. The reaction of dimethylamino(alkenyl)carbene complexes 18 with terminal alkynes in non-coordinating and non-polar solvents afforded phenol products in acceptable yields (Scheme 12). 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

A second group of aromatic substitution reactions involves aryl diazonium ions. As for electrophilic aromatic substitution, many of the reactions of aromatic diazonium ions date to the nineteenth century. There have continued to be methodological developments for substitution reactions of diazonium intermediates. These reactions provide routes to aryl halides, cyanides, and azides, phenols, and in some cases to alkenyl derivatives. 

Occolowitz, J.L. Mass Spectrometry of Naturally Occurring Alkenyl Phenols and... 

Scheme 6 Alkenylation of phenols with alkenylbismuthonium salts [38]...

The alkenylation of phenols also proceeds smoothly in the presence of TMG (Scheme 6). The major products are not aryl alkenyl ethers but a-alkenylated cyclohexa-2,4-dienones. That is, C-alkenylation occurs exclusively at the ortho position of phenols. When 2-naphthol reacts with two equivalents of the alkenylbismuthonium salt, a,a-dialkenyl ketone is obtained in good yield as the sole alkenylated product. 

Ohe, Uemura et al. further developed this electrocyclization into the [3,3]-sigmatropy of a cyclopropane system substituted with ethynyl and an acyl or an alkenyl group [33]. Thus, treatment of cis-l-acyl-2-ethynylcyclopropanes 125 with 5 mol% Cr CO)5(thf) in the presence of EtgN at rt induced isomerization to phenol derivatives through the [3,3]-sigmatropy of the vinylidene intermediate 126 to give... 

Only a few benzo derivatives are known, and most of them have the oxygen attached to the aromatic rings, i.e. they are cyclic phenol ethers or esters. 3,4,5,6-Tetrahydro-2H-1-benzoxocin (136), mentioned above, can also be obtained by cyclization of o-(bromopentyl)phenol (139) with base. The cyclization is rapid in DMSO and does not require high dilution conditions. The yield of (136) is 30% the alkenyl phenol (140) is obtained in 57% yield (74JOC2598). The latter product is formed by an intramolecular E2 reaction and thus is independent of the concentration of (139). In 75% ethanol solution, more of (136) is formed (64%) and less of (140) (35%), even though the rate of cyclization is much lower than in DMSO (75JA4960). 

VO(acac)2/TBHP catalyzed epoxidation of 2-(2-alkenyl)phenols. highly... 

VO(acac)2/TBHP CATALYZED EPOXIDATION OF 2-(2-ALKENYL)PHENOLS. HIGHLY REGIO- AND DIASTEREOSELECTIVE OXIDATIVE CYCLIZATION TO 2,3-DIHYDROBENZOFURANOLS AND 3-CHROMANOLS... 

The procedures are very easy to reproduce and the mild and catalytic conditions are far superior to previously employed m-CPBA ", to perform, respectively, the epoxidation and the oxidative cyclization of 2-(2-alkenyl)phenols. 

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