1,2-Ferrocenediylazaphosphinines:  An Unusual Coordination Behavior and Application to Allylic Alkylation

Abstract

The reaction of 1,2-ferrocenediylazaphosphinines (1a, R1 = H; 1b, R1 = Me; 1c, R1 = Ph) with M(CO)6 (M = Mo, W), MX(CO)5 (M = Mn, Re; X = Br, Cl), and [Pd(η3-C3H5)Cl]2 shows a strong tendency to adopt an unusual chelating bidentate coordination through nitrogen and the carbonyl oxygen, yielding M(η2-N,O)(CO)4 (2, M = Mo; 3, M = W), M(η2-N,O)(X)(CO)3 (6, M = Mn, X = Br; 7, M = Re, X = Cl), and [Pd(η2-N,O)(η3-C3H5)]BF4 (after treatment with AgBF4) (8), respectively. X-ray crystallographic structure determinations of 3a (R1 = H) and 8a (R1 = H) show the formation of a five-membered metallacycle with the distance of the metal−carbonyl oxygen bond being shorter than that of the metal−nitrogen bond in both compounds. The complexes 2 and 3 further undergo oxidative addition with allyl iodide to yield the corresponding M(II) complexes of the type [M(η2-N,O)( η3-C3H5)(I)(CO)2] (4, M = Mo; 5, M = W). Complexes 2−5 and 8 were employed as catalysts for nucleophilic allylic substitution of allyl acetates as a probe for both regio- and enantioselectivities of the reaction. All reactions involving unsymmetrical allyl acetates (E)-RCHCHCH2OAc (R = Pr, Ph) led exclusively to the formation of achiral linear product (E)-RCHCHCH2Nu regardless of the type of catalysts, the ligand, or the allyl substrate employed. One exception to the above statement is the observation that Mo- and W-based catalysts (2−5) are totally inactive toward the allylic substitution of cinnamyl acetate (R = Ph). Asymmetric allylic alkylation of a symmetrically 1,3-disubstituted substrate, PhCHCHCH(OAc)Ph, is accomplished only by Pd-catalysts (8) with enantiomeric excesses up to 50% ee