Phosphines diphosphines, electron rich

The success of Burk s alkyl diphosphines spurred development of a number of other ligands with electron rich phosphines, such as Zhang s PennPHOS (7) [36-38], Marinetti s /Pr-CnrPHOS (8) [39], and Imamoto s BisP ligands (9) [40],... 

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.

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