Phosphorus-Mediated sp2-sp3 Couplings for Selective C–H Fluoroalkylation of Complex Azines

Abstract

Fluoroalkyl groups profoundly affect the physical properties of pharmaceuticals and influence virtually all metrics associated with their pharmacokinetic and pharmacodynamic profile.<sup>1-4</sup> Drug candidates increasingly contain CF<sub>3</sub> and CF<sub>2</sub>H groups, and the same trend in agrochemical development shows that the effect of fluoroalkylation translates across human, insect, and plant life.<sup>5,6</sup> New fluoroalkylation reactions have undoubtedly stimulated this uptake; however, methods that directly convert C–H bonds into C–CF<sub>2</sub>X (X = F or H) groups in complex drug-like molecules are rare.<sup>7-13</sup> For pyridine, the most common aromatic heterocycle in pharmaceuticals,<sup>14</sup> only one approach, via fluoroalkyl radicals, is viable for pyridyl C–H fluoroalkylation in the elaborate structures encountered during drug development.<sup>15-17</sup> Here, we have developed a new set of bench-stable fluoroalkylphosphines that directly convert the C–H bonds in pyridine building blocks, drug-like fragments, and pharmaceuticals into fluoroalkyl derivatives. No pre-installed functional groups or directing groups are required; the reaction tolerates a variety of sterically and electronically distinct pyridines and is exclusively selective for the 4-position in most cases. The reaction proceeds via initial phosphonium salt formation followed by <i>sp</i><sup>2</sup>-<i>sp</i><sup>3</sup> phosphorus ligand-coupling, an underdeveloped manifold for C–C bond formation.