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

The reaction of 1,2-ferrocenediylazaphosphinines ( 1a, R 1 = H; 1b, R 1 = Me; 1c, R 1 = Ph) with M(CO) 6 (M = Mo, W), MX(CO) 5 (M = Mn, Re; X = Br, Cl), and [Pd(η 3 -C 3 H 5 )Cl] 2 shows a strong tendency to adopt an unusual chelating bidentate coordination through nitrogen and the carbonyl oxygen, yielding M(η 2 -N,O)(CO) 4 ( 2, M = Mo; 3, M = W), M(η 2 -N,O)(X)(CO) 3 ( 6, M = Mn, X = Br; 7, M = Re, X = Cl), and [Pd(η 2 -N,O)(η 3 -C 3 H 5 )]BF 4 (after treatment with AgBF 4 ) ( 8 ), respectively. X-ray crystallographic structure determinations of 3a (R 1 = H) and 8a (R 1 = 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(η 2 -N,O)( η 3 -C 3 H 5 )(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 CHCH 2 OAc (R = Pr, Ph) led exclusively to the formation of achiral linear product ( E )-RCH CHCH 2 Nu 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.