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Anatomy of Phobanes. Diastereoselective Synthesis of the Three Isomers of n-Butylphobane and a Comparison of their Donor Properties

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posted on 04.03.2009, 00:00 by Monica Carreira, Manutsavin Charernsuk, Michael Eberhard, Natalie Fey, Roel van Ginkel, Alex Hamilton, Wilhelmus P. Mul, A. Guy Orpen, Hirihataya Phetmung, Paul G. Pringle
Three methods for the large scale (50−100 g) separation of the secondary phobanes 9-phosphabicyclo[3.3.1]nonane (s-PhobPH) and 9-phosphabicyclo[4.2.1]nonane (a-PhobPH) are described in detail. Selective protonation of s-PhobPH with aqueous HCl in the presence of a-PhobPH is an efficient way to obtain large quantities of a-PhobPH. Selective oxidation of a-PhobPH in an acidified mixture of a-PhobPH and s-PhobPH is an efficient way to obtain large quantities of s-PhobPH. The crystalline, air-stable phosphonium salts [s-PhobP(CH2OH)2]Cl and [a-PhobP(CH2OH)2]Cl can be separated by a selective deformylation with aqueous NaOH. a-PhobPH is shown to be a5-PhobPH in which the H lies over the five-membered ring. The isomeric a7-PhobPH has been detected but isomerizes to a5-PhobPH rapidly in the presence of water. s-PhobPH is more basic than a-PhobPH by about 2 pKa units in MeOH. Treatment of s-PhobPH with BH3·THF gives s-PhobPH(BH3) and similarly a-PhobPH gives a5-PhobPH(BH3). Isomerically pure s-PhobPCl and a5-PhobPCl are prepared by reaction of the corresponding PhobPH with C2Cl6. The n-butyl phobane s-PhobPBu is prepared by nucleophilic (using s-PhobPH or s-PhobPLi) and electrophilic (using s-PhobPCl) routes. Isomerically pure a5-PhobPBu is prepared by treatment of a-PhobPLi with nBuBr and a7-PhobPBu is prepared by quaternization of a-PhobPH with nBuBr followed by deprotonation. From the rates of conversion of a7-PhobPBu to a5-PhobPBu, the ΔG (403 K) for P-inversion is calculated to be 38.1 kcal mol−1 (160 kJ mol−1). The donor properties of the individual isomers of PhobPBu were assessed from the following spectroscopic measurements: (i) 1JPSe for PhobP(Se)Bu; (ii) ν(CO) for trans-[RhCl(CO)(PhobPBu)2], (iii) 1JPtP for the PEt3 in trans-[PtCl2(PEt3)(PhobPBu)]. In each case, the data are consistent with the order of σ-donation being a7-PhobPBu > s-PhobPBu > a5-PhobPBu. This same order was found when the affinity of the PhobPBu isomers for platinum(II) was investigated by determining the relative stabilities of [Pt(CH3)(s-PhobPBu)(dppe)][BPh4], [Pt(CH3)(a5-PhobPBu)(dppe)][BPh4], and [Pt(CH3)(a7-PhobPBu)(dppe)][BPh4] from competition experiments. Calculations of the relative stabilities of the isomers of PhobPH, [PhobPH2]+, and PhobPH(BH3) support the conclusions drawn from the experimental results. Moreover, calculations on the frontier orbital energies of PhobPMe isomers and their binding energies to H+, BH3, PdCl3, and PtCl3 corroborate the experimental observation of the order of σ-donation being a7-PhobPR > s-PhobPR > a5-PhobPR. The calculated He8 steric parameter shows that the bulk of the isomers increases in the order a7-PhobPR < s-PhobPR < a5-PhobPR. The crystal structures of [a-PhobP(CH2OH)2][s-PhobP(CH2OH)2]Cl2, cis-[PtCl2(a5-PhobPCH2OH)2], trans-[PtCl2(s-PhobPBu)2], and trans-[PtCl2(a7-PhobPBu)2] are reported.