Ryuhei Okamoto

Abstract: a,w-Diarylalkanes are attractive materials for chromophores and have been employed in fluorescent probes. Extensive spectroscopic investigations of their excimers, such as biradicals of a,w-diarylalkanes, have therefore been performed. a,w-Diarylalkanes are now also in demand as ligands for metal complexes, precursors of cyclophanes/calixarenes, polymerization initiators, and as agents for improving the viscosity index of lubricants, especially in high-temperature nuclear-radiation-resistant hydraulic fluids. In addition, a,w-diarylalkanes are important model compounds for elucidating the mechanism of coal liquefaction. Conventionally, a,w-diarylalkanes have been synthesized by coupling Grignard reagents with a,w-dihaloalkanes, Friedel–Crafts acylation of arenes using ClOC(CH2)nCOCl in AlCl3 with a subsequent reduction, Na-mediated Wurtz-Fittig reactions of aryl bromides and a,w-dibromoalkanes, and by reduction of the corresponding ketones and a,b-enones. However, these existing methods have major drawbacks: they are multistep reactions, have narrow substrate scope, and/or produce stoichiometric amounts of metal salts as waste. The development of a facile, versatile, and waste-minimizing method for preparing a,w-diarylalkanes from easily accessible chemicals is therefore highly desirable with regard to atom economy. It is well known that Ir and Ru complexes serve as efficient catalysts for hydrogen transfer from alcohols to aldehydes and this catalysis has been used in a alkylations of carbonyl and related compounds, and b alkylations (Guerbet reaction) of alcohols. 14] Furthermore, Ir, Ru, and Rh complexes are known to show efficient catalytic activity in the decarbonylation of aldehydes. Herein, we report a novel general synthetic method for producing a,w-diarylalkanes 2 from w-arylalkanols 1 by dehydrogenation/b-alkylation (step 1) and a subsequent dehydrogenation/decarbonylation (step 2), either by a direct one-step method (Scheme 1, route A) or a sequential twostep method (Scheme 1, route B). This reaction provides a simple, versatile, and clean route to a,w-diarylalkanes from easily available alcohols. 2-Phenylethanol (1a) was used as a model substrate to investigate the reaction conditions for the direct one-step synthesis of a,w-diarylalkanes from w-arylalkanols (Scheme 1, route A; Table 1). The reaction of 1a (2 mmol) in the presence of a catalytic amount of [(Cp*IrCl2)2] (1 mol%; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) and tBuOK (40 mol%) in p-xylene (0.5 mL) at 120 8C for 4 hours gave 1,3-diphenylpropane (2a) in 81% yield with high selectivity, along with a small amount (5%) of toluene (4a ; Table 1, entry 1). Scheme 1. Preparation of a,w-diarylalkanes (2) from 1. cod = 1,5-cyclooctadiene, dppe = 1,2-bis(diphenylphosphino)ethane.