Diarylalkanes

Under even more drastic conditions (150 °C, 4 hours), the same reaction mixture afforded the cyclic sulfone disulfonyl chloride 21 (45%). Attempted chlorosulfonations of a-chloro and a,a -dichlorodiphenylmethane by reaction with chlorosulfonic acid failed, possibly because die substitution of the a-hydrogens destroys the activating hyperconjugative effect on the phenyl rings.  

2-Diphenylethane (bibenzyl) 22 reacted with excess chlorosulfonic acid (4.5 equivalents) at room temperature (24 hours) to give the 4,4 -disulfonyl chloride 23 (79%) (Equation 8). 2 

Diphenylethane 22 is more reactive towards chlorosulfonic acid than diphenyl-methane 18, probably as a result of increased hyperconjugative electron release from the ethylene bridge bond. When diphenylethane 22 was heated with a large excess of chlorosulfonic acid (20 equivalents) at 100 °C (4 hours), the cyclized product 24 was isolated in excellent (90%) yield (Equation 8). The cyclization to the seven-membered ring sulfone 24 was more easily achieved than the analogous conversion of diphenylmethane 18 to the six-membered ring sulfone 21 

2-Diphenylpropane 27 reacted with chlorosulfonic acid (three equivalents) in thionyl chloride at room temperature (7 days) to yield the 4,4 -disulfonyl chloride 28 (Equation 10). However, when 27 was heated with large excess of chlorosulfonic acid (20 equivalents) at 150 °C (4 hours) and subsequently treated with excess dimethylamine, the compounds 29-31 were isolated die cyclized product 31 was only obtained in low yield (Equation 10). 

Attempted chlorosulfonation of stilbene (1,2-diphenylethene) or 1,4-diphenyl-butadiene with chlorosulfonic acid failed to give identifiable products, probably as a result of acid-catalysed pol)mierization of the substrates in the highly acidic 

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Diarylalkanes are important structural motifs that can be found in a variety of pharmaceuticals, agrochemicals and fine chemicals. Examples are papaverin, avrainvilleol and beclobrate (Scheme 2). Traditionally, 1,1-diarylalkanes can be prepared from benzyl halides and the corresponding arenes under Friedel-Crafts-type conditions. 

When optically pure (S)- 1-phenylethanol Id was treated with p-xylene only racemic 1,1-diarylalkane 4a was isolated (Scheme 6). This strongly implies a carbocation as the reactive intermediate in the Bi(OTf)3-catalyzed Friedel-Crafts alkylations. Mechanistically, it is not clear whether Bi(III), in situ generated TfOH, or both Lewis and Brpnsted acids together are involved in the catalytic cycle... 

Besides the high efficiency of this route, many styrenes are readily available. This widens the product scope for 1,1-diarylalkanes and would additionally complement the previously described benzyl-alcohol-based Friedel-Crafts-type alkylations. 

Similar to Rueping s procedure, Hua and coworkers developed a BiCl3-catalyzed synthesis of 1,1-diarylalkanes also starting from electron-rich arenes and styrenes [68]. They found that styrenes 37 could be transformed to the substituted cyclopentanes 39 if catalytic amounts BiCl3 were applied (Scheme 30). This reaction is believed to proceed via an intermolecular ene-reaction between styrene and the carbocationic intermediate I, followed by an intramolecular Friedel-Crafts alkylation of the resulting intermediate II. 

This then readily undergoes cleavage to produce benzene and a dialkylbenzene. The initial R+ cation initiating the reaction might arise from some impurity present in the reaction mixture. Consistent with this mechanism is the observed very low reactivity of methylbenzenes due to the necessary involvement of the primary benzyl cations. At the same time 1,1-diarylalkanes undergo cleavage with great ease. 

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