On the mechanism and synthetic applications of the thermal and alkaline degradation of c-18 castor oil

Abstract

The C-18 backbone of castor oil fragments, thermally to C-11 + C-7 by a Π²s + Π²s + o²s process and with hot alkali to c-10 + c-8 via unique sequence involving a primary reaction which is associated with three different types of redox systems as well as with uncoupled oxidation, the overall change amounting to a milieu of hydride transfer, Π-migration, retro-Michael, retro-aldol, Meerwein-Pondorff-Verley as well as Cannizzaro type of reactions. These findings which constitute the core of the present work, are of significance, not only with respect to the understanding of two of the most important reactions of castor oil, but also in the utilization of this knowledge to channelize and optimize the products desired. Further, they add a distinct facet to mechanistic organic chemistry. Convincing evidence for the concerted nature of the C-18→ C-ll + C-7 change of castor oil is the clean transformation of methyl 12-hydroxy octadec 9- ynoate to the novel and useful allenic ester, methyl undeca 9,10- dienoate. Model studies with diversely α-substituted γδ-unsaturated alcohols have shown that the Π²s+ σ²s + σ²s (retroene) process is assisted by a Cβ←Cα polarization. The utility of the retro-ene reaction of γδ -unsaturated alcohols has been demonstrated with a novel procedure for the PhCHO→ PhCOD change. The mechanism proposed in the present work for +he C-l8 → C-10 + C-8 change of castor oil with hot alkali provides a rationale for the formation of products at diverse redox levels. In the present work, the mechanism of the complex processes associated with the hot alkali fragmentation of castor oil has been probed using "castor soap", a standard recipe for which has been developed. Neat castor soap decomposes above 240° to give, hydrogen gas (:), 2-octanol (49%), 2-octanone (24%) and sebacic acid (12%). A very noteworthy finding was that the course of the above can be changed by the addition of external redox acceptors whereby the formation of 2-octanol and hydrogen are suppressed. Thus, non-enolizable ketones are reduced with cast or soap [tetraphenylcyclopentadienone → tetraphenylcyclopentadiene (12%) + tetraphenylcyclopentadienone (35%); fluorenone → fluorene (34%); benzophenone → benzhydrol (44%)], hydrazones are converted to hydrocarbons [ fluorenone hydrazone → fluorene (70%); benzophenone hydrazone → diphenylmethane (50%)]and, even with calculated quantities of castor soap, amounting to the required 6e transfer, excellent yields (81-95%) of aromatic amines were obtained from a variety of aromatic nitro compounds. The easily available castor soap has good practical potential. Additionally, the delineation of the multifacetted pathways associated with the castor oil → sebacic acid change with hot alkali could be used to advantage to optimize conditions relating to products at diverse redox levels, the redox partners could be changed and the theme of co-existence of several redox systems could be transplanted to other substrates.