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.
The complex cations [M(η3‐R2All)(PPFPz{3‐tBu})]+ (M = PdII, R2All = 1,3‐diphenylallyl, 1,3‐dicyclohexylallyl, indenyl; M = PtII, R2All = 1,3‐diphenylallyl; PPFPz‐{3‐tBu} = 3‐tert‐butyl‐1‐{1‐[2‐diphenylphosphanyl‐ferrocenyl]ethyl}‐1H‐pyrazole)have been prepared as salts with PF6– or SbF6–. They have been characterized by NMR spectroscopy in solution and by X‐ray crystallography in the solid state. Their reactions with sources of nucleophilic and “naked” fluoride have been investigated by multinuclear NMR spectroscopy. The PdII complexes did not undergo any nucleophilic substitution with concomitant release of allyl fluorides. The dicyclohexylallyl fragment was released as a 1,3‐diene by elimination, but with other allyl complexes nonspecific decomposition reactions predominated. The complex [Pt(η3‐1,3‐Ph2C3H3)(PPFPz{3‐tBu})]PF6 underwent an anion exchange with Me4NF to give [Pt(1,3‐Ph2C3H3)(PPFPz{3‐tBu})]F which existed as a mixture of interconverting allyl isomers in solution at ambient temperature. For the bromide salt, [Pt(η3‐1,3‐Ph2C3H3)(PPFPz{3‐tBu})]Br, allyl isomerization was slow at ambient temperature. Precursors of Pt0 reacted with bromo‐1,3‐diphenylprop‐2‐ene to give [Pt2(μ‐Br)2(η3‐1,3‐Ph2All)2] and precursors of Pd0 underwent oxidative additions with bromo‐ and fluoro‐1,3‐diphenyl‐2‐propene to give 1,3‐diphenylallyl complexes of PdII. Therefore, the nucleophilic attack of fluoride on the allyl fragment of PdII complexes is endergonic, and the high energy barrier of this step is difficult to overcome in a catalytic allylic fluorination reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
A simple three-step synthesis of chiral dibenzo[a,e]cyclooctenes (dbcot) starting from commercially available dibenzosuberenone was developed. These compounds give highly stable and robust rhodium(I) and iridium(I) diene complexes of the type [M(Rdbcot)(L1)(L2)] (M = Rh, Ir; L1, L2 = MeCN, Cl, diamine, chloride). The complex [Rh((R)-Phdbcot)((R)-(+)-1,1‘-binaphthyl-2,2‘-diamine)]+OTf- could be obtained in enantiomerically pure form and catalyzes the enantioselective 1,2-addition of PhB(OH)2 to α,β-unsaturated ketones with good activity and acceptable enantiomeric excess (62%). The iridium complex [Ir(Phdbcot)(MeCN)2]+OTf- catalyzes the hydrogenation of dimethylitaconate with good activity, while the rhodium complexes are almost inactive. Likewise, the complex [Ir(Phdbcot)(H2NCH2CH2NH2)]+OTf- serves as a rather efficient catalyst precursor with an activity 4 orders of magnitude higher than for the analogous rhodium complex. These experiments further establish the use of dienes as steering ligands in catalysis.
Geladen! Die Kombination von MeBABAR‐Phos und Methyltriflat ergibt ein aminostabilisiertes Phosphiranyliumion (siehe Bild; C grau, N blau, P orange), das verschiedene nucleophile Additionsreaktionen eingeht. Dabei bilden sich P‐substituierte Phosphirane sowie ein Phosphiranyliumkation, das durch ein N‐heterocyclisches Carben stabilisiert wird.
BABAR-Phos is a very stable polycyclic phosphirane which is easily synthesized in few steps from dibenzosuberenone. BABAR-Phos is remarkably stable and is not oxygenated with O2 nor does it react with sulfur in boiling toluene. BABAR-Phos can be used as a ligand in homogenous catalysis. Substituents at the carbon of the PC2 heterocycle can be introduced and asymmetric BABAR-Phos were prepared. The coordination chemistry of rhodium complexes containing these as ligands was investigated.
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