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

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 a 5 -PhobPH in which the H lies over the five-membered ring. The isomeric a 7 -PhobPH has been detected but isomerizes to a 5 -PhobPH rapidly in the presence of water. s-PhobPH is more basic than a-PhobPH by about 2 pK a units in MeOH. Treatment of s-PhobPH with BH3·THF gives s-PhobPH(BH3) and similarly a-PhobPH gives a 5 -PhobPH(BH3). Isomerically pure s-PhobPCl and a 5 -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 a 5 -PhobPBu is prepared by treatment of a-PhobPLi with nBuBr and a 7 -PhobPBu is prepared by quaternization of a-PhobPH with nBuBr followed by deprotonation. From the rates of conversion of a 7 -PhobPBu to a 5 -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) 1 J PSe for PhobP(Se)Bu; (ii) ν(CO) for trans-[RhCl(CO)(PhobPBu)2], (iii) 1 J PtP for the PEt3 in trans-[PtCl2(PEt3)(PhobPBu)]. In each case, the data are consistent with the order of σ-donation being a 7 -PhobPBu > s-PhobPBu > a 5 -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)(a 5 -PhobPBu)(dppe)][BPh4], and [Pt(CH3)(a 7 -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 − cor