Palladium-Catalyzed Suzuki–Miyaura Cross-Couplings with 2‑Diethylphosphonato-Substituted Aryl- and Naphthylboronate Esters as the Nucleophilic Partner: A Complementary Approach to the Synthesis of Biaryl Monophosphonates

This
paper reports the first examples of Suzuki–Miyaura
cross-couplings involving aryl- and naphthylphosphonate-based boronate
esters as the nucleophilic partner. A systematic comparison of the
performance of biaryl-like KITPHOS- and XPHOS-based systems revealed
that, between them, an electronically and sterically diverse range
of substrates can be coupled with remarkable efficiency to afford
high yields of the corresponding biaryl and heterobiaryl monophosphonates.
The use of an aryl- and naphthylphosphonate-based boronate ester as
the coupling partner presents an alternative and potentially complementary
pathway to existing couplings in which the aryl- or naphthylphosphonate
unit is typically introduced as the electrophile. The potential advantages
associated with the use of this new class of coupling partner were
clearly demonstrated by the palladium-catalyzed reaction between diethyl
[2-(4,4,5,5-tetra­methyl-1,3,2-dioxa­borolan-2-yl)­phenyl]­phosphonate
and 1-bromo-2-methoxy­naphthalene that gave the corresponding
biaryl monophosphonate in 56% yield, a marked improvement on the 6%
yield obtained from the reaction between 2-methoxy-1-naphthyl­boronic
acid and diethyl (2-bromo­phenyl)­phosphonate with the same
catalyst under the same conditions. The potential utility of this
new coupling combination was demonstrated by reducing one of the products,
2-methoxy-1-(2′-diethoxy­phosphoryl­phenyl)­naphthylene,
to the corresponding primary phosphine, which was subsequently converted
into a diastereoisomeric mixture of the R,R-hexane-2,5-diol-derived phospholane in reasonable yield.