6.6 Dimethyl 2,4 cyclohexadienone

This preparation of 2,6,6-trimethyl-2,4-cyclohexadienone is based upon the published procedure of the submitters,2 and it is the only preparation of the 2-substituted-2,4-cyclohexadienones. The simpler 6,6-dimethyl-2,4-cyclohexadienone is more conveniently prepared by the method of Alder.4... 

The third set of molecules that bring out the uniqueness of zeolites in the context of asymmetric induction is of 2,4-cyclohexadienones (6,6-dimethyl-2,4-cyclohexadienones 38, and 2,2-dimethyl-1,2-dihydronaphthalenones 39) [279,282,287,289,293-295]. The basic chromophore in these molecules is the conjugated dienone that undergoes oxa-di-rr-methane rearrangement to give a bicyclic product (Scheme 21). According to the accepted mechanism, the chirality is introduced into the system at the first step, yielding the diradical intermediate... 

Dimethyl-2,4-cyclohexadienone photorearranges to bicyclic [3.1.0] product via oxa-di-rr-methane rearrangement (Scheme 41). This achiral molecule photorearranges in solution to the chiral bicyclic product as a racemic mixture. This molecule within NaY zeolites in the presence of chiral inductors yields products with respectable ee. The best numbers are 50% (ephedrine as a chiral inductor at — 55°C) and 28% (pseudoephedrine at room temperature) [294]. These numbers are significant when one recognizes that in solution only a racemic mixture is obtained. 

Dimethyl-2,4-cyclohexadienones 102 undergo photoisomerization to 103 by cleavage of the 1,6-bond from their n,Jt singlet excited states [59]. 

Upon substitution of the 4-position (114.b, R3 = Me), the cyclohexadi-enone complex 118 was also isolated from the product mixture obtained with dimethyl acetylenedicarboxylate. Only the cyclohexadienone complex... 

It is seen from Table 7 that all these carbenes in solution react nonstereospeci-fically, indicating triplet character. For some carbenes such as CF3CH, PhCH, (CH3)sSiC(C02C2H5), 2,6-di-t-butyl-carbena-cyclohexadienone and 4,4-dimethyl-carbena-cyclohexadienone, the degree of non-stereospecifity is quite small. Other carbenes usually contain 7t-bonds, aromatic rings or heavy atoms as functional groups. 

Surprisingly 6-(p-hydroxyphenyl)- and l,4-dimethyl-6-(p-hydroxy-phenyl)-dihydrodiazepinium salts react readily with bromine, giving 6-(3-bromo-4-hydroxyphenyl) and 6-(3,5-dibromo-4-hydroxyphenyl) derivatives (86LA1380). It is thought that in these cases the intermediate is a cyclohexadienone rather than a doubly charged species as shown above (86LA1380). 

Cook, K. L., Waring, A. J. Kinetics of the dienone-phenol rearrangement of 4,4-dimethyl-2,5-cyclohexadienones. J. Chem. Soc., Perkin Trans. 21973, 88-92. 

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