Reaction of an Enantiomerically Pure Phosphaalkene-Oxazoline with MeM Nucleophiles (M = Li and MgBr): Stereoselectivity and Noninnocence of the P-Mesityl Substituent

Abstract

The addition of alkyl nucleophiles (MeM, M = Li, MgBr) across the PC bond of an enantiomerically pure phosphaalkene-oxazoline followed by protonation of the C anion affords phosphines with three chirality centers. The formation of palladium(II) complexes of the resultant phosphines permitted structural characterization of the products by X-ray diffraction. The choice of nucleophile has a profound effect on the product distributions. For instance, the Grignard reagent adds in a diastereoselective manner to give one major phosphine product with P-and C-stereocenters. In contrast, addition of methyllithium has proven not only to be less stereoselective but also affords a fascinating cyclic phosphine product. Both the Grignard and RLi reactions involve proton transfer from the o-Me of the P-Mes substituent even though the products are quite different in each case. ■ INTRODUCTIONPhosphaalkene chemistry strikingly parallels that of its structural relatives, the alkenes and imines, in diverse areas ranging from ligand development for transition metals to functional groups in polymer science. 1 Increasingly, researchers are developing sophisticated compounds containing PC bonds that display unique properties, bonding, reactivity, and catalytic activity. 2 To this end, we have disclosed the synthesis and catalytic utility of enantiomerically pure phosphaalkenes of general structure A, a phosphaalkene-oxazoline hybrid (PhAkOx) (Chart 1). 3−5 PhAk-Ox complements existing acyclic (B and C) and cyclic (D and E) structural motifs that incorporate PC bonds within an enantiomerically enriched organic framework. 6Building on our work on the polymerization of MesPCPh 2 (Scheme 1), 7−9 we have successfully incorporated an enantiomerically pure phosphaalkene-oxazoline (PhAk-Ox, 1) into copolymers with styrene using radical methods of initiation. 5 Given that the anionic polymerizations of MesP CPh 2 can be living, thereby providing access to block copolymers with controlled architectures, 9 the possible extension of this methodology to the chiral PhAk-Ox monomer is intriguing. One can envisage stereoselective polymerizations of 1 that afford fascinating main chain chiral (or helical) polymers that possess novel physical properties and potential applications in asymmetric catalysis or in chiral separations. Thus far, homopolymers of PhAk-Ox (1) have not been

Phosphaalkene-oxazoline copolymers with styrene as chiral ligands for rhodium(i)

“…2) which reveal signals attributable to a -P (Ph)CMe 2 Ox (-P -: 13 C: δ = 50.0/49.5; 1 H: δ = 4.2, 3.0, 2.6). 29 For comparison, model 3 30 showed similar signals for the P-(Ph)CMe 2 Ox ( 13 C: δ = 51.0; 1 H: δ = 4.2) with assignments aided by 13 C{ 1 H}, 13 C-1 H HSQC, 13 C APT and 1 H{ 31 P} NMR spectra. In addition, signals assigned to an -Ar 2moiety were also observed ( 13 C: δ = 36.5; 1 H: δ ≈ 1.6).…”

Streamlined Preparation and Coordination Chemistry of Hybrid Phosphine–Phosphaalkene Ligands

“…Future investigations will target synthesizing an expanded library of new hybrid ligands, with the goal of exploiting the innate reactivity of the PC bond to enhance catalytic reactions and bond activation events. The low-lying π* component of the PC bond suggests these hybrid phosphine–phosphaalkene ligands may harness redox-active/noninnocent behavior, which will be the focus of upcoming reactivity studies.…”