AROMATIC SIGMATROPIC HYDROGEN-SHIFTS IN 2-VINYL-PHENOLS AND 2-ALLYL-PHENOLS
AROMATIC SIGMATROPIC HYDROGEN-SHIFTS IN 2-VINYL-PHENOLS AND 2-ALLYL-PHENOLS
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DOI：
10.1002/hlca.19770600624
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发表时间：
1977-01-01
影响因子：
1.8
通讯作者：
HANSEN, HJ
中科院分区：
文献类型：
--
作者：
HANSEN, HJ
Aromatic Sigmatropic Hydrogen‐Shifts in 2‐Vinyl‐ and 2‐Allyl‐phenolsIt is shown by deuterium labeling experiments that 2‐vinylphenols, on heating at 142,5°, undergo aromatic [1,5]‐H‐shifts wherebyo‐quinone methides are formed as intermediates(Scheme 7). Thus, heating of 2‐isopropenylphenol (6) in a D2O/dioxane mixture leads to a rapid deuterium incorporation into the methylidene group of the isopropenyl moiety(Table 1)whereas its methyl group shows only a slow uptake of deuterium. The latter exchange process can be attributed to intermolecular reactions(Scheme 8). The quinone methide intermediates (e.g.26,Scheme 8) can be regarded as vinyl homologues of alkyl ketones. Therefore,26can exchange hydrogen in both methyl groups by an acid‐ and base‐catalysed mechanism. Indeed, when6is heated in D2O/pyridine or D2O/CH3COOD/dioxane, an almost statistical incorporation of deuterium into the methylidene and the methyl group of the isopropenyl moiety is observed(Table 3).As a consequence of thermally induced [1,5]‐H‐shifts, 2‐(1′‐propenyl)‐phenols undergo rapid (E,Z) isomerization with first order kinetics on heating above 140° in decane solution. Activation parameters are given inTable 4. The observed primary +++++ H/D isotope effect of 3.3 in the (E,Z) isomerization of phenol8is in +++ment with intramolecular H/D‐shifts in the rate determing step (Scheme 9+++Table 5). As expected aromatic sigmatropic [1,5]‐H‐shifts in 2‐(1′‐propenyl)‐+++ are much faster than aromatic homosigmatropic [1,5]‐H‐shifts in 2‐(2′‐+++++)phenols (Scheme 1andTable 6). The structurally comparable phenols +++ (Z)‐10and (E)/(Z)‐14(Scheme 3) showk([1,5])/k(homo‐[1,5]) ≈ 2300 at ++++ .
