Taming functionality: easy-to-handle chiral phosphiranesw

The phosphiranes are heterocycles which possess a threemembered phosphorus-containing ring which is highly strained as a result of the small sum of bond angles at the phosphorus atom ( P (P) o2601). The resultant greater s-character in the pyramidalised structure is considered to manifest itself in the form of lowered HOMO and LUMO energy levels, conferring upon the ligand a lower s-donor, but better p-acceptor electronic profile. Such characteristics are desirable because they offer the opportunity to synthesise ligand libraries for catalysis with a different window of activity from that offered by the more typical PR3 reagents. These intriguing molecules are still overlooked because of their tendency to be unstable: Phenylphosphirane I (Fig. 1) is stable at 0 1C for a month but undergoes decomposition at higher temperatures. Bulky substituents around the phosphorus lend stability to the ring; thus II is a distillable liquid and III is a colourless crystalline solid. The other strategy one can employ to stabilise a phosphirane (apart from metal coordination) is to incorporate the three-membered ring into an elaborate polycyclic structure. This niche approach was used to prepare the elegant BABARPhos IV series of phosphiranes which resist decomposition. Phosphirano-[1,2]-thiaphospholes V have [a]-annelated bicyclic structures, isomerising in their tungsten complexes over the temperature range 25–80 1C. We recently reported an electronic stabilising effect whereby increasing the amount of p-conjugation on the backbone of primary phosphines allowed us to prepare the novel air-stable, chiral derivatives (S)-1a and (R)-1b. We were keen therefore to use these precursors to prepare chiral phosphiranes, of which there are very few reports, and to examine their properties, structure and reactivity. Following the synthetic method of Kubiak and co-workers, (S)-1a and (R)-1b were treated with two equivalents of methyllithium followed by dichloroethane, and thus we were able to prepare the asymmetric compounds (S)-2a and (R)-2b in high yields (Scheme 1). The P NMR spectroscopic data for the two phosphiranes are consistent with those found for other members of this class, showing a characteristic resonance at very high field (d= 235.0 ppm for (S)-2a, 235.4 for (R)-2b). The H NMR spectrum shows four distinctive peaks for the protons on the heterocycle, with a complicated fine-structure arising from coupling to each other, coupling to the nearest aromatic proton, and coupling to the phosphorus nucleus. The two proton signals at lower field (endo protons) both show NOE correlations to the aromatic proton in the 3-position. Thus the phosphirane unit is rotating in solution, an observation further rationalised by quantum chemical calculations.w COSY NMR experiments revealed that the J-coupling of the phosphorus nucleus to the two exo protons is significantly larger (18.0/18.1 Hz in (S)-2a/(R)-2b) than the coupling to the endo protons (1.8/2.0 Hz in (S)-2a/(R)-2b). The latter smaller coupling was thereby not fully resolvable in the P NMR spectra. The C NMR spectrum also demonstrates the diastereotopic nature of the phosphirane-ring carbon atoms; two individual resonances are observed although the phosphorus–carbon coupling constants within the phosphirane ring are equivalent (JPC = 40.3 Hz for (S)-2a; JPC = 40.1 Hz for (R)-2b). Two important features of our novel phosphiranes lie in their high thermal stability and their excellent resistance to air-oxidation. Remarkably (S)-2a and (R)-2b showed no sign of decomposition when heated overnight in refluxing toluene (under nitrogen). Nor was there any evidence of decomposition when the compounds were left open to the atmosphere in chloroform solution for seven days. In fact, in the solid state they can be stored in air over several months without problems. Treatment of the phosphiranes by peroxides in reagent-grade tetrahydrofuran does oxidise the compounds, but this process takes place via a different mechanistic process than the oxidation by air. A full rationale for this behaviour will be provided