The Selective Hydroxylation of Cyclohexane by Alkyl Hydroperoxides a Challenge that Can be Solved ?

Abstract

The development of efficient routes towards cyclohexanone and cyclohexanol is one of the challenges for industrial researchers. Both chemicals are important feedstocks for the production of caprolactam and adipic acid. The latter two are precursors for Nylon-6 and Nylon-6,6

In our lab the hydroxylation of cyclohexane by cyclohexyl hydroperoxide (CHHP) and cumyl hydroperoxide (CumOOH) was studied. As catalysts sterically hindered metalloporphyrines (Metals = Co, Mn, Cr, Porphyrines = TPP, TDCIPP, TPyP¹) were used. A comparison is made with data from literature and the possible impact on industrial scale production of cyclohexanone is discussed briefly. To distinguish between formation of cyclohexanone via the decomposition of the CHHP and the oxidation of the formed alcohol experiments were performed with both 3,3,5,5,- tetramethylcyclohexanol and cyclohexanol-d₁₂. The results indicated that the added alcohols were not oxidized to the corresponding ketone. Based on both epoxidation experiments with styrene and cis-stilbene and the hydroxylation experiments it was concluded that for both MnTDCIPP and CrTDCIPP the reaction mainly goes via a heterolytic mechanism. A third indication for a non radical pathway with Mn porphyrines was found by using a radical trapping agent, galvincxyl (2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxyl, free radical). The addition of galvinoxyl did not influence the rate of reaction which is a strong indication of the absence of free radicals in the solution for the tested Mn porphyrin. With Co porphyrines the reaction is believed to proceed via a homolytic mechanism in which non sterically hindered porphyrines are readily converted to non-porphyrin species.

A comparison of the results for CumOOH and CHHP clearly indicated that CumOOH is a better hydroxylating agent than cyclohexylhydroperoxide. It is believed that this is due to the absence of an α-hydrogen in cumyl alcohol which makes it a less reactive molecule than cyclohexane. With a sterically hindered Mn porphyrin it was possible to hydroxylate with a high selectively cyclohexane (yield > 40 %). The formed cyclohexanol was almost not oxidized which resulted in very high alcohol: ketone ratios (>10). Despite these high selectivities there is still a strong competition between hydroxylation and normal decomposition of the peroxide.