Chiral Phosphane Alkenes (PALs): Simple Synthesis, Applications in Catalysis, and Functional Hemilability

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Abstract
A simple synthesis of a chiral phosphane alkene (PAL) involves: 1) palladium‐catalyzed Suzuki coupling of 10‐bromo‐5H‐dibenzo[a,d]cyclohepten‐5‐ol (1) with phenylboronic acid to give quantitatively 10‐phenyl‐5H‐dibenzo[a,d]cyclohepten‐5‐ol (2); 2) reaction of 2 with Ph2PCl under acidic conditions to give a racemic mixture of the phosphane oxide (10‐phenyl‐5H‐dibenzo[a,d]cyclohepten‐5‐yl)diphenylphosphane oxide (PhtroppoPh, 3), which is separated into enantiomers by using high‐pressure liquid chromatography (HPLC) on a chiral column; 3) reduction with trichlorosilane to give the enantiomerically pure phosphanes (R)‐ and (S)‐(10‐phenyl‐5H‐dibenzo[a,d]cyclohepten‐5‐yl)diphenylphosphane (PhtroppPh, 4). This highly rigid, concave‐shaped ligand serves as a bidentate ligand in RhI and IrI complexes. Catalysts prepared from [Rh2(μ2‐Cl)2(C2H4)4] and (S)‐4 have allowed the efficient enantioselective 1,4‐addition of arylboronic acids to α,β‐unsaturated carbonyls (Hayashi–Miyaura reaction) (5–0.1 mol % catalyst, up to 95 % ee). The iridium complex (S,S)‐[Ir(PhtroppPh)2]OTf ((S,S)‐6; OTf=SO3CF3) has been used as a catalyst in the hydrogenation of various nonfunctionalized and functionalized olefins (turnover frequencies (TOFs) of up to 4000 h−1) and moderate enantiomeric excesses have been achieved (up to 67 % ee). [Ir(PhtroppPh)2]OTf reversibly takes up three equivalents of H2. The highly reactive octahedral [Ir(H)2(OTf)(CH2Cl2)(H2‐PhtroppPh)2] could be isolated and contains two hydrogenated monodentate H2‐PhtroppPh phosphanes, one CH2Cl2 molecule, one triflate anion, and two hydrides. Based on this structure and extensive NMR spectroscopic studies, a mechanism for the hydrogenation reactions is proposed.

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