A novel intermediate in allylic amination catalyzed by iron salts

Article abstract of DOI:10.1021/ja960088+

Our initial probe of the aminations catalyzed by iron salts excluded the intermediacy of free ArNO, suggesting that a coordinated organonitrogen species could be the active aminating agent. To elucidate the mechanism of these latter reactions we report herein (a) the isolation and first structural elucidation of a metal complex of a C-nitroso dimer (1), and (b) evidence that this novel compound is the key aminating agent in allylic aminations catalyzed by iron salts. We have established with 1 the first structurally verified metal complex of a C-nitroso dimer and its unprecedented reactivity and selectivity for the allylic N-functionalization of olefins. Its involvement as the aminating agent in FeX_2,3-catalyzed allylic amination also has been strongly implicated.

Full text of DOI:10.1021/ja960088+

J. Am. Chem. Soc. 1996, 118, 3311-3312
3311
plexes13 and salts14 catalyze the allylic amination of olefins by
arylhydroxylamines (eq 1). These catalytic reactions display
A Novel Intermediate in Allylic Amination
Catalyzed by Iron Salts
Radhey S. Srivastava, Masood A. Khan, and
Kenneth M. Nicholas*
unusual and synthetically attractive ene-reaction-type regiose-
lectivity (resulting in double-bond transposition). Mechanistic
studies of the reactions catalyzed by LMo(VI)O215 and (phthal)-
Fe(II)16 point to the intervention of PhNO, a proven enophile,17
as the active aminating agent, accounting for the observed
regioselectivity. However, our initial probe of the aminations
catalyzed by iron salts14 excluded the intermediacy of free
ArNO, suggesting that a coordinated organonitrogen species
could be the active aminating agent. To elucidate the mecha-
nism of these latter reactions we report herein (1) the isolation
and first structural elucidation of a metal complex of a C-nitroso
dimer and (2) evidence that this novel compound is the key
aminating agent in allylic aminations catalyzed by iron salts.
Department of Chemistry and Biochemistry
UniVersity of Oklahoma, Norman, Oklahoma 73019
ReceiVed January 11, 1996
In contrast to hydrocarbon oxidation which provides several
important industrial and laboratory processes for the production
of oxygenated compounds,1 the direct synthesis of organoni-
trogen compounds from hydrocarbons remains an attractive but
largely exlusive goal.2-6 The paucity of direct nitrogenation
reactions and mechanistic questions regarding the few known
ones has, in turn, stimulated interest in the chemistry of
organonitrogen metal complexes, including those having amido,7
imido,8 and C-nitroso9 ligands. To date, however, examples
of N-transfer from such complexes to hydrocarbons are rare.10
Following early reports by Sharpless and Mares of stoichio-
metric allylic amination of olefins and acetylenes by LnMoO-
(η2-RNO)9b,c and X(dNTs)2 (X ) S,Se),11 we and others
Scheme 1
12
recently have found that LnMo(VI)O2 and Fe(II,III) com-
To identify the intermediate iron complex(es) in the FeCl2,3
-
(1) Reviews: Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis;
Wiley: New York, 1992; pp 138-161, 172-174, 237-264. Sheldon, R.
A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds;
Academic Press: New York 1981; pp 271-249.
catalyzed aminations, preparative reactions of the iron chlorides
with PhNHOH and PhNO were conducted. The reaction of
FeCl2 with PhNO (1:2) in CHCl3 (20 °C) or dioxane (80 °C)
produced azoxybenzene and a dark red brown product 1 (ca.
50% yield following CH2Cl2/hexane recrystallization) whose IR
spectrum18 suggested the presence of coordinated PhNO (Scheme
1). Compound 1 was also formed (along with PhNO) and azo-
and azoxybenzene) when FeCl3 and PhNHOH (20 °C, CHCl3)
were combined, and most importantly, 1 also could be isolated
from the FeCl2-promoted amination reaction of 2-methyl-2-
pentene (2-MP) by PhNHOH (80 °C, dioxane, 2 h). The
structure of 1‚1.5(CH2Cl2) was established by X-ray diffraction19
and is shown in Figure 1 along with key bond lengths and
angles. Complex 1, {Fe[Ph(O)NN(O)Ph]3}[FeCl4]2, is thus
(2) Among the few useful hydrocarbon nitrogenation reactions are
propylene ammoxidation (ref 3), butadiene hydrocyanation (ref 4), and olefin
aziridination (ref 5) and hydroamination (ref 6).
(3) Review: Grasselli, R. K. J. Chem. Ed. 1986, 63, 216.
(4) Arthur, P.; Pratt, B. C. U.S. Patent 2,571,099, 1951. Drinkard, W.
C.; Lindsay, R. V. U.S. Patent 3,496,215, 1970. Parshall, G. W.; Ittel, S.
D. Homogeneous Catalysis, Wiley: New York, 1992; pp 42-46.
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D. M. J. Am. Chem. Soc. 1993, 115, 5328. Lowenthal, R. E.; Masamune,
S. Tetrahedron Lett. 1991, 32, 7373. Li, Z.; Quan, R. W.; Jacobsen, E. N.
J. Am. Chem. Soc. 1995, 117, 5889. Atkinson, R. S.; Fawcett, J.; Williams,
P. J. Tetrahedron Lett. 1995, 36, 3241. Noda, K.; Hosoya, N.; Irie, R.
Synlett 1993, 7, 469. Perez, P. J.; Brookhart, M.; Templeton, J. L.
Organometallics 1993, 12, 261.
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114, 275. Gagne, M. R.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 4108.
(7) Bryndza, H.; Tam, W. Chem. ReV. 1988, 88, 1163. Fryzuk, M. D.;
Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1. Glueck, D. D.;
Bergman, R. G. Organometallics 1991, 10, 1479. Koelliker, R.; Millstein,
D. Angew. Chem., Int. Ed. Engl. 1991, 30, 707. Ge, Y-W.; Peng, F.; Sharp,
P. R. J. Am. Chem. Soc. 1990, 112, 2632. Joslin, F. L.; Johnson, M. P.;
Mague, J. T.; Roundhill, D. M. Organometallics 1991, 10, 41. Matsunga,
P. T.; Hess, C. R.; Hillhouse, G. L. J. Am. Chem. Soc. 1994, 116, 3665.
Rahim, M.; Bushweller, C. H.; Ahmed, K. J. Organometallics 1994, 13,
4952.
-
found to consist of tetrahedral Fe(III)Cl4 anions and a six-
coordinate dication having iron(II) bound through the oxygens
of three azobenzene dioxide ligands. Remarkably, while
C-nitroso compounds are well known to exist as azo dioxide
dimers in the solid state,20 1 proVides the first crystallographi-
cally established metal complex haVing a C-nitroso dimer (azo
dioxide) ligand.21 The distinctly distorted, nonoctahedral cation
(8) (a) Nugent, W. A.; J. A. Mayer Metal-Ligand Multiple Bonds;
Wiley: New York, 1988; Chapter 6, pp 220-287. (b) Holm, R. H. Chem.
ReV. 1987, 87, 1401. Dyer, P. W.; Gibson, V. C.; Clegg, W. J. Chem.
Soc., Dalton Trans. 1995, 3313. Wigley, D. E.; Morrison, D. L. Inorg.
Chem. 1995, 34, 2610. Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am.
Chem. Soc. 1995, 117, 974. Cummins, C. C.; Schrock, R. R.; Davis, W.
M. Inorg. Chem. 1994, 33, 1448. McGrane, P. L.; Livinghouse, T. J. Org.
Chem. 1992, 57, 1323. Brunner, H.; Meier, W.; Wachter, J. J. Organomet.
Chem. 1989, 362, 95.
(9) (a) Cameron, M.; Gowenlock, B. G.; Vasapollo, G. Chem. Soc. ReV.
1990, 19, 355. (b) Liebeskind, L. S.; Sharpless, K. B.; Wilson, R. D.; Ibers,
J. A. J. Am. Chem. Soc. 1978, 100, 7061. (c) Muccigrosso, D. A.; Jacobson,
S. E.; Apgar, P. A.; Mares, F. Ibid. 1978, 100, 7063.
(13) Johannsen, M.; Jorgensen, K. A. J. Org. Chem. 1994, 59, 214.
(14) Srivastava, R. S.; Nicholas, K. M. Tetrahedron Lett. 1994, 35, 8739.
(15) Srivastava, R. S.; Nicholas, K. M. J. Org. Chem. 1994, 59, 5365.
(16) Johannsen, M.; Jorgenson, K. A. J. Org. Chem. 1995, 60, 5979.
(17) Keck, G. E.; Webb, R. R.; Yates, J. B. Tetrahedron 1981, 37, 4007.
Knight, G. T. J. Chem. Soc., Chem. Commun. 1970, 1016. Banks, R. E.;
Haszeldine, R. N.; Miller, P. J. Tetrahedron Lett. 1970, 4417.
(18) For 1: IR (KBr, cm-1) 3099 (w), 3077 (w), 1681 (m), 1651 (m),
1600 (m), 1481 (s), 1462 (s), 1374 (s), 1161 (s), 1076 (m), 1020 (m) 962
(s), 922 (m), 762 (s), 686 (s); UV-vis (nm) 215, 280, 300; MS (FAB) 214
(Ph2N2O2+), 269 [Fe(PhNO)2+].
(19) Crystals of 1 (CH2Cl2/hexane) are triclinic P1, a ) 15.572(9) Å, b
) 16.308(5) Å, c ) 19.841(7) Å, R ) 88.87(3)°, â ) 86.80(4)°, γ ) 86.60-
(4)°, V ) 5021(4) Å3. Data were collected at 198 K with Mo KR (λ )
0.71073 Å) radiation and corrected for Lorentz, polarization, and empirical
absorption effects. The structure was solved by the heavy atom method
and refined by full-matrix least squares on F2 using all reflections
(SHELXTL 5.0); final conventional R1 ) 0.088 (WR2 ) 0.0206, GOF )
1.0) for 7928 “observed” reflections with I g 2σ(I). The asymmetric unit
contains two dications, four anions, and three CH2Cl2 molecules.
(20) Zuman, P.; Shah, B. Chem. ReV. 1994, 94, 1621 and references
cited therein.
(10) Examples of -NR transfer to unsaturated hydrocarbons are largely
restricted to additions to the CdC or CtC: Sharpless, K. B.; Patrick, D.
W.; Truesdale, L. K.; Biller, S. A. J. Am. Chem. Soc. 1975, 97, 2305.
Sharpless, K. B. J. Org. Chem. 1976, 41, 1976. Cummins, C. C.; Shaller,
C. P.; VanDuyne, G.; Wolczanski, P. T.; Chan, A. W. E.; Hoffmann, R. J.
Am. Chem. Soc. 1991, 113, 2985. Walsh, P. J.; Hollander, F. J.; Bergman,
R. G. Organometallics 1993, 12, 3705. For transfer to benzyl radicals,
see: Chan, D. M. T.; Nugent, W. A. Inorg. Chem. 1985, 24, 1422.
(11) Sharpless, K. B.; Hori, T. J. Org. Chem. 1976, 41, 176. Kresze,
G.; Braxmeier, H.; Munsterer, H. Org. Syn. 1987, 65, 159. Katz, T. J.;
Shi, S. J. Org. Chem. 1994, 59, 8297. Sharpless, K. B.; Hori, T.; Truesdale,
L. K.; Dietrich, C. O. J. Am. Chem. Soc. 1976, 98, 269.
(21) The formation of Pb(a), Sn(b,c), and Ti(c) complexes of dimeric
C-nitroso ligands has been proposed on the basis of IR or 13C NMR data:
(a) Williams, K. C.; Imhoff, D. W. J. Organomet. Chem. 1972, 42, 107.
(b) Williams, K. C.; Imhoff, D. W. Inorg. Nucl. Chem. Lett. 1973, 9, 227.
(c) Cameron, M.; Gowenlock, B. G. Polyhedron 1992, 11, 2781.
(12) Srivastava, A.; Ma, Y.; Pankayatselvan, R.; Dinges, W.; Nicholas,
K. M. J. Chem. Soc., Chem. Commun. 1992, 853.
0002-7863/96/1518-3311$12.00/0 © 1996 American Chemical Society

3312 J. Am. Chem. Soc., Vol. 118, No. 13, 1996
Communications to the Editor
Scheme 3
was found to catalyze the allylic amination of 2-MP by
PhNHOH (80 °C, 8 h, 81% yield) at an initial rate comparable
to (or somewhat faster than) that of the FeCl2-catalyzed reaction.
The following initial observations and features provide some
clues to possible mechanistic pathways: (1) the ene-reaction-
type regioselectivity; (2) the noninvolvement of free PhNO; (3)
the coordinative saturation of the complex ion {Fe(II)[Ph(O)-
NN(O)Ph]3}2+; and (4) the facile, induced exchange of coordi-
nated and free nitrosoarene with 1. The latter was demonstrated
by treatment of 1 with 3.5 equiv of 2-MeC6H4NO (20 °C,
dioxane, 2 h) after which a 2:3 PhNO/2-MeC6H4NO mixture
was detected by GC. Taking these features into consideration,
we suggest (Scheme 3) that amination by 1 may proceed via
initial dechelation of an azo dioxide ligand, generating an
electrophilic nitrosating species (e.g. 2) which transfers the
activated ArNO unit to an olefin (free or coordinated) by an
ene-reaction-type process; reduction of the resulting allyl
hydroxylamine by Fe(II) would generate the allyl amine.25
Interception of five-coordinate 2 by nitrosotoluene (in the
exchange experiments) could cause release of PhNO from the
dangling chelate arm and subsequent rechelation to form a mixed
nitroso dimer complex.
Figure 1. X-ray structure of 1‚1.5(CH2Cl2). Selected bond lengths
(Å) and angles (deg): Fe(1)-O(1) 2.107(7), Fe(1)-O(2) 2.083(7), Fe-
(1)-O(3) 2.105(7), Fe(1)-O(4) 2.158(8), Fe(1)-O(5) 2.150(7), Fe-
(1)-O(6) 2.097(7), O(1)-N(1) 1.270(10), N(1)-N(2) 1.300(11), N(2)-
O(2) 1.299(10); O(1)-Fe(1)-O(4) 147.7(3), O(1)-Fe(1)-O(5) 86.1(3),
Fe(1)-O(1)-N(1) 115.9(6), O(1)-N(1)-N(2) 118.1(8), N(1)-N(2)-
O(2) 116.4(7), N(2)-O(2)-Fe(1) 116.4(6); Fe(2)-Cl(1) 2.173(4), Fe-
(2)-Cl(2) 2.213(4), Fe(2)-Cl(3) 2.189(4), Fe(2)-Cl(4) 2.177(4).
Scheme 2
In summary we have established with 1 the first structurally
verified metal complex of a C-nitroso dimer and its unprec-
edented reactivity and selectivity for the allylic N-functional-
ization of olefins. Its involvement as the aminating agent in
FeX2,3-catalyzed allylic amination also has been strongly
implicated. Studies are underway to establish the mechanism
by which 1 is formed and how it transfers the imido (or PhNO)
unit to the substrate and to develop enantioselective catalysts
based on 1.
of 1 features planar chelate rings and rather varied Fe-O (2.12
Å av) lengths, but relatively uniform O-N (1.28 Å av) and
N-N (1.29 Å av) bond lengths. Comparison of the N-N and
N-O lengths of 1 to typical sp2-sp2 single- and double-bond
values (N-O 1.42 Å, NdO 1.20 Å, N-N 1.36 Å, NdN 1.235
Å)22 and to those of azobenzene dioxide23 (N-O 1.268(4) Å,
N-N 1.321(5) Å) indicates electronically delocalized chelate
rings in 1 but rather modest differences in the structure of the
free and coordinated ligand.
Acknowledgment. We are grateful to the National Science Founda-
tion for support (CHE 9123751) and for an instrument grant (CHE
1256631) allowing purchase of the X-ray diffractometer. We also
appreciate helpful discussions with Prof. G. Richter-Addo.
Supporting Information Available: Experimental details for the
preparation of 1 and X-ray crystal structure determination and refine-
ment; X-ray structural drawings, tables of crystal data, atomic
coordinates, anisotropic thermal parameters, hydrogen coordinates, and
bond distances and angles (20 pages). This material is contained in
many libraries on microfiche, immediately follows this article in the
microfilm version of the journal, can be ordered from the ACS, and
can be downloaded from the Internet; see any current masthead page
for ordering information and Internet access instructions.
In addition to its detection in catalytic reaction mixtures,
several other experiments strongly implicate nitrosobenzene
dimer complex 1 as the actiVe aminating agent in allylic
aminations catalyzed by iron salts. Firstly, treatment of 1 with
2-MP (1:25) in dioxane results in smooth conversion, even at
room temperature (8 h) (rapidly at 80 °C), to the corresponding
allyl amine (83% yield) with the same distinctive regioselectivity
found in the FeCl2,3-catalyzed reactions (Scheme 2). Moreover,
as found in the FeCl2,3-catalyzed reactions,14 1 aminates olefins
without the intervention of free PhNO since heating 1 with an
equimolar mixture of 2-MP and 2,3-dimethylbutadiene (DMB)
produced allylic amination products exclusively, i.e. no Diels-
Alder adduct from DMB and PhNO was detected.24 Finally, 1
JA960088+
(24) Separate control experiments have established that (1) 2,3-dimeth-
ylbutadiene efficiently traps PhNO as a hetero-Diels-Alder adduct even
in the presence of 2-MP (60 °C, dioxane) and (2) this adduct is stable at 80
°C in the presence of FeCl2 and 2-MP.
(25) The feasibility of this step is indicated by the facile reduction of
PhNHOH to PhNH2 by FeCl2 (20 °C, dioxane). Other metal-based
reductants also convert RNHOH to RNH2 by an undetermined mechanism;
see for example ref 15 and refs in the following: March, J. AdVanced
Organic Chemistry, 4th ed.; Wiley: New York, 1992; p 1218.
(22) Pendergrass, D. B., Jr.; Paul, I. C.; Curtin, D. Y. J. Am. Chem. Soc.
1972, 94, 8730.
(23) Dieterich, D. A.; Paul, I. C.; Curtin, D. Y. J. Am. Chem. Soc. 1974,
96, 6372.