Arenes alkoxy-: reduction

Scheme 1 shows the desired Heck reaction of alkoxy-DSB 1 with 2. The formation of 3 is accompanied by two destructive pathways the reductive debromination of 1 to 4 as a side reaction and the protodesilylation to 5 as a subsequent reaction. Particularly the latter limits the reaction conditions in terms of time and temperature. The phosphine is a decisive factor in this system consisting of three reactions a fine-tuning of the reaction conditions is possible via electronic and steric effects of the substituents in the phosphine electron-rich trialkylphosphines 6 and 7 strongly favor the reduction. Fast coupling reactions were observed with tris-o-tolylphosphine 8, the chelating diphosphine dppe 9 being even more efficient in terms of turnover, yield, and suppression of side reactions. Compared with Heck reactions of polycyclic or electron-deficient arenes with 2 [21, 22], the yield of 3 is only moderate. The reactivity of bromo-distyrylbenzenes 1 and 12 -14 in the coupling reaction is controlled by the substituents on the opposite side of the n-system (Fig. 1, Table 2) a compensation for the electron-donating alkoxy groups by a cyanide (13) or exchange of donors with electronically neutral alkyl side chains strongly improves the yields.

The inclusion of ethanol (8% v/v) in the reaction solvent (dichloromethane or acetonitrile) used for photolysis of the charge transfer complexes of tetranitro-methane with alkoxy or dialkoxyarenes leads to stabilisation of alkoxytrinitro-methylarenes. Reduction in the nucleophilicity of the trinitromethanide ion as well as changes in the regioselectivity of trinitromethanide ion attack on the arene radical cations and stabilisation of the adducts also result. ... 

---