Benzylation of Arenes with Benzyl Halides under Promoter-Free and Additive-Free Conditions

Temperature-Assisted Generation of Arylmethyl Radicals from Bis(arylmethyl)tin Dichlorides: Efficient Reagents for C s p 3 ${{{\bf C}}_{{{\bf s p}}^{3}}}$ - C s p 2 ${{{\bf C}}_{{{\bf s p}}^{2}}}$ Bond-Forming Reactions

The oxidative-addition reaction between an arylmethyl chloride (RCH₂ Cl; R=1-C₁₀ H₇ , 2,4,6-Me₃ C₆ H₂ , 4-MeC₆ H₄ , 3-MeC₆ H₄ , C₆ H₅ , 4-ClC₆ H₄ ) and tin powder in boiling toluene produces bis(arylmethyl)tin dichlorides, [(RCH₂ )₂ SnCl₂ ] in good yields. At 160 °C in mesitylene bis(1-naphthylmethyl)tin dichloride undergoes Sn-C homolytic cleavage to generate two 1-naphthylmethyl radicals (1-C₁₀ H₇ CH₂ ⋅) which were trapped by TEMPO (C₉ H₈ NO⋅). Subsequently, the radicals (RCH₂ ⋅) produced in this manner were utilized for efficient substitution reactions with electron-rich arenes (R'H; R'=2,4,6-Me₃ C₆ H₂ , 1,2,4,5-Me₄ C₆ H, 1,2,3,4,5-Me₅ C₆ ) to obtain a variety of unsymmetrical diarylmethanes (RR'CH₂ ). The addition of one equivalent of iodine (I₂ ) to the reaction mixture resulted in a significant increase in the yields of coupled products.

Conversion of Aryl Aldehydes to Benzyl Iodides and Diarylmethanes by H3PO3/I2

For the first time, H₃PO₃ was used as both the reducing reagent and the promotor in the reductive benzylation reactions with aryl aldehydes. By using a H₃PO₃/I₂ combination, various aromatic aldehydes underwent iodination reactions and Friedel-Crafts type reactions with arenes via benzyl iodide intermediates, readily producing benzyl iodides and diarylmethanes in good yields. Intramolecular cyclization reactions also took place, giving the corresponding cyclic compounds. This new strategy features easy-handling, low-cost, and metal-free conditions.