We report the use of biaryl phosphorinanes as ligands for Pd-catalyzed
cross-coupling reactions. A modular synthesis was developed that employs
a double conjugate addition of primary biaryl phosphines into 1,1,5,5-tetraalkyl
penta-1,4-diene-3-ones. Notably, this synthesis does not require the
use of copper, a known contaminant in structurally related biaryl
phosphane ligands. Using the synthetic strategy described above, we
synthesized a library of biaryl phosphorinanes, varying their substitution
about phosphorus and the steric and electronic nature of the biaryl
motif. We then benchmarked their performance as ligands in Pd-catalyzed
cross coupling reactions such as aryl sulfonamidation, aryl alkoxylation,
and aryl amination in the presence of soluble organic bases. In each
reaction studied, many ligands outperformed biaryl phosphanes known
to promote the given transformation. Detailed substrate scopes were
determined using high-throughput screening technology. Several biaryl
phosphorinanes and their corresponding Pd(II) oxidative-addition complexes
were extensively characterized using NMR spectroscopy and X-ray crystallography.
General observations support that biaryl phosphorinanes promote reductive
elimination and form robust catalysts with palladium. In many cases
the use of these biaryl phosphorinanes may be advantageous over the
use of biaryl phosphanes with respect to lower catalyst loadings,
shorter reaction times, and robustness.