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

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 Ph(2)PCl under acidic conditions to give a racemic mixture of the phosphane oxide (10-phenyl-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane oxide ((Ph)troppo(Ph), 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 ((Ph)tropp(Ph), 4). This highly rigid, concave-shaped ligand serves as a bidentate ligand in Rh(I) and Ir(I) complexes. Catalysts prepared from [Rh(2)(mu(2)-Cl)(2)(C(2)H(4))(4)] and (S)-4 have allowed the efficient enantioselective 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyls (Hayashi-Miyaura reaction) (5-0.1 mol % catalyst, up to 95% ee). The iridium complex (S,S)-[Ir((Ph)tropp(Ph))(2)]OTf ((S,S)-6; OTf=SO(3)CF(3)) 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((Ph)tropp(Ph))(2)]OTf reversibly takes up three equivalents of H(2). The highly reactive octahedral [Ir(H)(2)(OTf)(CH(2)Cl(2))(H(2)-(Ph)tropp(Ph))(2)] could be isolated and contains two hydrogenated monodentate H(2)-(Ph)tropp(Ph) phosphanes, one CH(2)Cl(2) molecule, one triflate anion, and two hydrides. Based on this structure and extensive NMR spectroscopic studies, a mechanism for the hydrogenation reactions is proposed.

Intramolecular Alkyl Phosphine Dehydrogenation in Cationic Rhodium Complexes of Tris(cyclopentylphosphine)

“…III, see above, R = Ph). [17,18,[35][36][37][38][39][40][41] Aspects of the work reported here have been communicated previously. [28] Results and Discussion ] as a halide abstracting reagent in a biphasic CH 2 Cl 2 /H 2 O solvent system.…”

“…Related complexes based on the tropp Ph ligand set have been previously reported by Grütz-macher, although in these cases a preformed phosphine alkene ligand is used. [17,40,41] The synthesis of a suitable pre- Adding an excess of nbd to 7 affords the nbd adduct [Rh- Acceptorless dehydrogenation: Up to this point all the dehydrogenations reported have been facilitated by a hydrogen acceptor, either norbornene or tert-butylethene. The systems under discussion here also can undergo acceptorless alkyl dehydrogenation, by changing the supporting ligand set from nbd to dppe.…”

“…Examples of intramolecular dehydrogenation from alkyl phosphines [9][10][11][12] (e.g. I, [13,14] II, [15,16] III [17,18] ) and betadiketimates (IV [19][20][21] ) are well-established; and other coordinated ligands, such as ethers, [22] alkoxides, [23] N-heterocyclic carbenes [24] and functionalised cyclopentadienyls [25] also undergo dehydrogenation. The intramolecular dehydrogenation of alkyl groups can also result in the addition of useful double-bond functionality that can be utilised for further elaborations in a synthetic scheme.…”

Chiral Olefins as Steering Ligands in Asymmetric Catalysis

“…[61] Alcohol 96 underwent an Arbuzov-like rearrangement to provide phosphane oxide 97. The enantiomers were resolved by preparative HPLC on a chiral stationary phase.…”

“…[61] At room temperature under an atmosphere of 1 bar H 2 , the iridium complex 178 takes up three equivalents of dihydrogen to yield the remarkable Ir III -dihydride complex 179 (Scheme 51) in which the previously coordinated olefins are hydrogenated. The addition of three equivalents of an external olefin fully regenerates the starting complex 178.…”