Acid-catalysed rearrangements of 4-hydroxycyclohexa-2,5-dienones
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. A series of 4-aryl-4-hydroxycyclohexa-2,5-dienones has been synthesised along with one example of a 4-alkyl-4-hydroxycyclohexa-2,5-dienone. The acid-catalysed rearrangements of these compounds have been studied by product analysis. Rearrangement of the 2,6-di-t-butyl-4-aryl-4-hydroxycyclohexa-2,5-dienones (102), (103) and (123) in acetic anhydride-sulphuric acid, Thiele conditions) gave the ortho diacetates (163), (177), and (164), respectively. The formation of these products can be rationalised in terms of a mechanism involving an intermediate phenoxylium ion (193) generated by protonation of the hydroxy oxygen atom of the starting material followed by loss of water (Scheme XIX). In contrast, Thiele rearrangement of the 2,6-dimethyl- and 2,6-di-isopropyl-substituted compounds (124) and (125) afforded the meta diacetates (200) and (202), respectively. The mechanism accepted generally for dienone-phenol rearrangement of 4-hydroxycyclohexa-2,5-dienones (Scheme III) involving protonation of the carbonyl oxygen atom followed by 1,2-migration of a 4-substituent and deprotonation to give an aromatic product, explains the origin of the meta diacetates (200) and (202). The 4-hydroxy-4-methylcyclohexa-2,6-dienone (110d) gave the benzylic diacetate (208) under the same conditions. The rearrangement of 2,6-di-t-butyl-4-hydroxy-4-phenylcyclohexa-2,5-dienone (102) in boron trifluoride diethyl etherate-diethyl ether yielded the biphenylol (104), the benzo-1,3-dioxole (231) and the 4-(2-methylpropenyl)-4-phenylcyclohexa-2,5-dienone (226). In the same medium, rearrangement of 2,6-di-t-butyl-4-hydroxy-4-methylcyclohexa-2,5-dienone (110d) gave a dimeric product (229). The biphenylol (104) and the -o-benzoquinone (187) were the products of rearrangement of 4-hydroxycyclohexa-2,5-dienone (102) catalysed either by aqueous perchloric acid or by trifluoromethanesulphonic acid. The 4-methyl- substituted compound (110d) gave the dimers (269), (273), and (278), as well as the benzyl alcohol (272) when its rearrangement was catalysed by aqueous perchloric acid. The polymeric perfluorosulphonic acid resin, Nafion-H was found to catalyse the rearrangement of the 4-hydroxycyclohexa-2,5-dienone (110d), the products isolated, (269), (273), (278), and (272), being identical to those obtained from the aqueous perchloric acid-catalysed rearrangement of (110d).
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1980