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On the Rapid Oxidation of Allene-Containing Phosphines Feng Cai, Neela D. Thangada, Ende Pan, and Joseph M. Ready* Department of Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9038, United States S Supporting Information *
ABSTRACT: Allene-containing phosphines have recently been shown to serve as effective ligands in transition metalcatalyzed enantioselective reactions. Surprisingly, (2allenylphenyl)diphenyl phosphines rapidly oxidize when exposed to air, whereas many other triaryl phosphines are stable under ambient conditions. Here we describe experiments designed to understand the origin of this behavior. Stereochemical probes and an isolated phosphonium complex support the hypothesis that phosphines can cyclize onto pendant allenes and that the resultant zwitterion undergoes rapid oxidation with molecular oxygen.
Scheme 1
We recently invented chiral ligands based on the allene scaffold for asymmetric catalysis.1 The overarching hypothesis was that the large chiral cavity formed by the allene would accommodate a variety of chemical transformations and facilitate differentiation of diastereomeric transition states. For instance, bisphosphine 1 forms complexes with Rh(I) that catalyze the enantioselective addition of aryl boronic acids to activated ketones. X-ray crystallography revealed that bisphosphine 1 acts as a tridentate ligand featuring coordination to both phosphines and one double bond of the allene. One restriction of this class of allenes, however, appeared to be the requirement for electron-withdrawing groups on the phosphines. For instance, attempts to form bis(diphenylphosphine) 2 failed, as it was rapidly oxidized to the corresponding bisphosphine oxide 3. Fortunately, this allene acts as a catalyst itself, as optically active phosphine oxides promote the addition of SiCl4 to mesoepoxides to provide chlorohydrins in high optical purity.2 Nonetheless, we were perplexed by the facile oxidation of allene-containing phosphines and designed experiments to probe the origin of this reactivity.
of triphenyl phosphine under an oxygen atmosphere for 65 h.4 Phosphine 2 is more hindered than PPh3, but how increased sterics would accelerate oxidation is unclear. In any event, phosphine 2 is sterically similar to BINAP, which requires no special handling precautions. Alternatively, the presence of an olefin adjacent to the phosphine might account for rapid oxidation. To test this possibility, we prepared vinyl-substituted triaryl phosphine 5 and found that it was stable to chromatography and storage under ambient atmosphere. The available evidence suggested that an interaction of the phosphine with the allene might be responsible for the facile oxidation of the latter. In this scenario, intramolecular nucleophilic addition might yield a cyclic zwitterion, 6, which is more reactive toward oxygen (eq 2). Consistent with this explanation, several research groups have explored nucleophilic phosphines as catalysts for annulations involving allenes.5 These transformations frequently involve addition of the phosphorus to the sp carbon of the allene. One characteristic of these reactions, however, is the requirement for electrondeficient allenes such as allenoates. Whether the intramolecularity of the addition envisioned to provide 6 would remove the requirement for a carbonyl remained unclear.
Aerobic oxidation of phosphines could involve the formation of a peroxy phosphine 4 followed by reaction with a second equivalent of phosphine or could involve a radical chain process featuring peroxy and alkoxy radicals (Scheme 1).3 Regardless of which mechanism is operative, the susceptibility of allenecontaining phosphines toward oxidation is surprising. Many triaryl phosphines are commercially available and, qualitatively, are frequently used on the benchtop without precaution. More quantitatively, Buchwald and co-workers noted
