Platinum Assisted Tandem P–C Bond Cleavage and P–N Bond Formation in Amide Functionalized Bisphosphine o‑Ph₂PC₆H₄C(O)N(H)C₆H₄PPh₂‑o: Synthesis, Mechanistic, and Catalytic Studies

The reactions of amide functionalized bisphosphine o-Ph₂PC₆H₄C­(O)­N­(H)­C₆H₄PPh₂-o (1) with platinum salts are described. Treatment of 1 with [Pt­(COD)­Cl₂] yielded a chelate complex, [PtCl₂{o-Ph₂PC₆H₄C­(O)­N­(H)­C₆H₄PPh₂-o}κ²-P,P] (2), which on subsequent treatment with LiHMDS formed a novel 1,2-azaphospholene-phosphine complex [Pt­(C₆H₅)­Cl­{o-C₆H₄{C­(O)­N­(o-PPh₂(C₆H₄))­P­(Ph)}}κ²-P,P] (3) involving a tandem P–C bond cleavage and P–N bond formation. The same complex 3 on passing dry HCl gas afforded the dichloro complex [PtCl₂{o-C₆H₄{C­(O)­N­(o-PPh₂(C₆H₄))­P­(Ph)}}κ²-P,P] (5). Complex 2 upon refluxing in toluene or treatment of 1 with [Pt­(COD)­Cl₂] in the presence of a base at room temperature resulted in the pincer complex [PtCl­{o-Ph₂PC₆H₄C­(O)­N­(C₆H₄PPh₂-o)}κ³-P,N,P] (4). Reaction of 1 with [Pt­(COD)­ClMe] at room temperature also afforded the pincer complex [PtMe­{o-Ph₂PC₆H₄C­(O)­N­(C₆H₄PPh₂-o)}κ³-P,N,P] (6). Mechanistic studies on 1,2-azaphospholene formation showed the reductive elimination of LiCl to form a phosphonium salt that readily adds one of the P–C bonds oxidatively to the in situ generated Pt⁰ species to form a chelate complex 3. The analogous palladium complex [PdCl₂{o-C₆H₄{C­(O)­N­(o-PPh₂(C₆H₄))­P­(Ph)}}κ²-P,P] (7) showed excellent catalytic activity toward N-alkylation of amines with alcohols with a very low catalyst loading (0.05 mol %), and the methodology is very efficient toward the gram-scale synthesis of many N-alkylated amines.