Allylic Amination Catalyzed by Iron Salts
J. Am. Chem. Soc., Vol. 119, No. 14, 1997 3303
Table 1. Allylic Amination Catalyzed by Iron Salts
Scheme 1
interest in the chemistry of organonitrogen metal complexes,
including those having amido,12 imido,13 and C-nitroso14 ligands.
To date, however, examples of N-transfer from such species to
hydrocarbons are rare.15 In early work Sharpless16 and Mares17
reported examples of stoichiometric R-amination of alkenes and
cyclohexanone by C-nitroso complexes (molybdooxaziridines),
LnMoO(η2-RNO). More recently, on the basis of these early
communications, we developed molybdenum(VI)-catalyzed al-
lylic aminations using arylhydroxylamines as aminating agents
(Scheme 1).18 Both the stoichiometric and catalytic versions
display distinctive ene reaction-like regioselectivity, occurring
with transposition of the carbon-carbon double bond. There-
after, apparently similar iron-catalyzed allylic aminations were
reported by Jorgensen’s group utilizing iron phthalocyanine
complexes19 and by our group employing simple Fe(II,III)
salts.20
21
Mechanistic studies of the reactions catalyzed by LMoVIO2
and (phthal)FeII 22 point to the intermediacy of PhNO, a proven
enophile,23 as the active aminating agent, accounting for the
observed regioselectivity and alkene relative reactivity. The role
of the catalyst in these reactions thus is to serve as a redox
agent, oxidizing the starting hydroxylamine to the reactive
nitroso arene and then reducing the derived allylhydroxylamine.
In contrast, our initial probe of the aminations catalyzed by iron
salts20 excluded the intermediacy of free ArNO, suggesting that
a coordinated organonitrogen species could be the active
aminating agent. Recent follow-up studies led to the identifica-
tion of a novel iron azo dioxide complex {Fe[Ph(O)NN(O)-
a GC yield, naphthalene standard (isolated yield after chromatogra-
phy. b Allylamine/(allylamine + aniline + azobenzene + azoxyben-
zene), by GC. c A ) FeCl2‚4H2O, B ) FeCl3‚6H2O; C ) Fe(ClO4)2‚
6H2O; D ) Fe(ClO4)3‚6H2O; 1a ) {Fe[Ph(O)NN(O)Ph]3}[FeCl4]2.
d 2.5:1 PhNHOH/alkene ratio.
Ph]3}[FeCl4]2 (1a)24 which appears to be the active aminating
agent in the reactions promoted by iron salts. This compound
could be isolated from catalytic amination reaction mixtures, is
an active amination catalyst with PhNHOH as aminating agent,
and reacts readily with alkenes to produce the corresponding
allylamines even in the presence of a diene trapping agent for
free PhNO. We now report the full details of these preliminary
synthetic, structural, and mechanistic studies of allylic amination
catalyzed by iron salts. Included as well are new kinetic and
reactivity investigations of 1a, including the isolation of alkene
adducts 3 derived from 1 which appear to be on the catalytic
amination pathway as well.
(12) Bryndza, H.; Tam, W. Chem. ReV. 1988, 88, 1163. Fryzuk, M. D.;
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D. Angew. Chem., Int. Ed. Engl. 1991, 30, 707. Ge, Y.-W.; Peng, F.; Sharp,
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Results and Discussion
Synthetic Survey. Slow addition of phenylhydroxylamine
(1.5 mM in dioxane) to a heated (70-100 °C) dioxane solution
containing an excess of alkene and 10 mol % FeCl2,3‚nH2O or
Fe(ClO4)2,3‚nH2O produces after 8 h the corresponding N-
phenyl-N-allylamines in fair to good yield following chroma-
tography (Scheme 1, Table 1). Important features of these
reactions include (1) trisubstituted and 1,1-disubstituted alkenes
give the best yields; (2) unsymmetrical alkenes react with
virtually complete (>95%) regioselectivity to produce the
corresponding allyl amine derived from double-bond transposi-
tion; and (3) generally no other alkene-derived products are
observed. Features 1 and 2 are typical of ene-type reactions.25
The major competing process is the formation of N-containing
byproducts derived from phenylhydroxylamine decomposition,
i.e. aniline and azo- and azoxybenzene; the selectivity for allyl
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