D-isomerism

Fig. 9. Xylenes separation via Mitsubishi Gas—Chemical Co. HF-BF extraction—isomerization process (107). A = extractor B = decomposer C = separator D = isomerization reactor E = heavy ends tower F = raffinate tower G = separator H = light ends fractionator ...

D. Isomerization of Organophosphorus Betaines I Containing a Hydrogen Atom in the a-Position to the Phosphonium Center... 

D. Isomerization with Palladium and Other Group VIII Metals. 38... 

Fig. 13.11. Regioselective generation of ketone enolates, I the effects of different substituents in the Aland appositions. Enolate D is formed in THF at -78 °C with LDA irrespective of whether a substoichiometric amount or an excess of LDA is used. However, if one employs slightly less than the stoichiometric amount of LDA (so that a trace of the neutral ketone is present), then, upon warming, the initially formed enolate D isomerizes quantitatively to enolate C with its more highly substituted C=C double bond. It should be noted that LDA removes an axially oriented a-H from the cyclohexanone this is because only then does the resulting lone pair of electrons receive optimum stabilization by the adjacent C=0 bond. With the kinetically preferred deprotonation leading to the enolate D the axial of-H is transferred to the base (via transition state B), but not the equatorial of-H (via transition state iso-B.)...

The first such reaction published in 1908 by Ciamician and Silber was the light induced carvone —> carvonecamphor isomerization, corresponding to type b [1]. Between 1930 and 1960 some examples of photodimerizations (type c) of steroidal cyclohexenones and 3-alkylcyclohexenones were reported [2-5]. In 1964, Eaton and Cole accomplished the synthesis of cubane, wherein the key step is again a type b) photocycloisomerization [6]. The first examples of type a) reactions were the cyclopent-2-enone + cyclopentene photocycloaddition (Eaton, 1962) and then the photoaddition of cyclohex-2-enone to a variety of alkenes (Corey, 1964) [7,8]. Very soon thereafter the first reviews on photocycloaddition of a,(3-unsaturated ketones to alkenes appeared [9,10]. Finally, one early example of a type d) isomerization was communicated in 1981 [11]. This chapter will focus mainly on intermolecular enone + alkene cycloadditions, i.e., type a), reactions and also comprise some recent developments in the intramolecular, i.e., type b) cycloisomerizations. 

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