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 center dot THF gives s-PhobPH(BH3) and similarly a-PhobPH gives a(5)-PhobPH(BH3)center dot 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 (BuBr)-Bu-n 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 Delta G(double dagger) (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) v(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 a-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, [PhobPH(2)](+), 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 a-donation being a(7)-PhobPR > s-PhobPR > a(5)-PhobPR. The calculated He-8 steric parameter shows that the bulk of the isomers increases in the order a(7)-PhobPR < s-PhobPR < a(5)-PhobPR.

The crystal structures of [a-PhobP(CH<INF>2</INF>OH)<INF>2</INF>][s-PhobP(CH<INF>2</INF>OH)<INF>2</INF>]Cl<INF>2</INF>, cis-[PtCl<INF>2</INF>(a<INF>5</INF>-PhobPCH<INF>2</INF>OH)<INF>2</INF>], trans-[PtCl<INF>2</INF>(s-PhobPBu)<INF>2</INF>], and trans-[PtCl<INF>2</INF>(a<INF></INF>-PhobPBu)<INF>2</INF>] are reported.