Stille and Suzuki Cross Coupling Reactions of o-Nitrophenyl Triflates: A Versatile Route to a Variety of Heterocycles

Abstract

The cross coupling reactions of selected o-nitrophenyl triflates with arylstannane and arylboron substrates are reported.The resultant 2-nitrobiphenyls provide ready access to a variety of substituted heterocycles.We recently reported the versatility of o-nitrophenyl triflates as intermediates in the synthesis of 2- hydroxyquinolines.' Their success as substrates for palladium catalysed Heck reactions prompted us to investigate their potential in Stille and Suzuki cross coupling reactions as the mechanisms proceed i ~w the same initial palladium insertion step2 The three o-nitrophenyl triflates (1-3) which were selected for the study were readily prepared from the corresponding o-nitrophenols3 and include appropriate functionality to compare the effects of electron donating and withdrawing groups on the cross coupling reactions OMe C02MePreliminary investigations employing phenylboronic acid (413) and o-nitrophenyl triflate (1) as simple substrates indicated that optimal conditions for the Suzuki cross coupling of these substrates involved the use of Pd(PPh3)a as catalyst, K 8 0 4 as base and DME as solvent.However, in the corresponding Stille reaction employing phenyltrimethyltin, Pd(PPh,)4 was found to afford poorer results than the Pd~dba3.CHCl~iP(o-tol)~ catalyst system.Dioxane was employed as the solvent, and as is standard in Stille reactions involving triflate~,'~ lithium chloride was employed as an additive.