1,2-ferrocenediylazaphosphinines: An unusual coordination behavior and application to allylic alkylation

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)]BF ₄ (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.