On the mechanism of the dehydroaromatization of hexane to benzene by an iridium pincer catalyst

Abstract

The developments in the area of transition-metal pincer complexes have opened up new avenues for conversion of saturated hydrocarbons to more useful aromatic compounds under homogeneous reaction conditions. In the backdrop of an interesting series of conversions of unbranched alkanes to benzene, toluene and xylene (known as the BTX family aromatics) reported by Goldman and co-workers (Nature Chem.2011, 3, 167), we herein present a comprehensive mechanistic picture obtained by using density functional computations. The reaction involves an iridium–PCP-pincer-catalyzed dehydroaromatization of hexane to benzene (in which PCP = η³-C₆H₃(iPrP)₂-1,3) by using Tert-butylethylene (TBE) as a sacrificial acceptor. The most energetically preferred pathway for a sequence of dehydrogenations is identified to begin with a terminal C H bond activation of n-hexane leading to the formation of hex-1-ene. Although the initial dehydrogenation of n-hexane was found to be endergonic, the accompanying exoergic hydrogenation of TBE to Tert-butylethane (TBA) compensates the energetics to keep the catalytic cycle efficient. Subsequent dehydrogenations provide a hexa-1,3-diene and then a hexa-1,3,5-triene. The pincer bound triene is identified to undergo cyclization to furnish cyclohexadiene. Eventually, dehydrogenation of cyclohexa-1,3-diene offers benzene. In the most preferred pathway, the Gibbs free energy barrier for cyclization leading to the formation of cyclohexa-1,3-diene is found to exhibit the highest barrier (21.7 kcal mol⁻¹).