Nitrogen transfer from a nitridomanganese(V) complex: Amination of silyl enol ethers

Full text of DOI:10.1021/ja953659r

J. Am. Chem. Soc. 1996, 118, 915-916
915
Scheme 1
Nitrogen Transfer from a Nitridomanganese(V)
Complex: Amination of Silyl Enol Ethers
J. Du Bois,† Jason Hong,† Erick M. Carreira,*,† and
Michael W. Day‡
Contribution No. 9155, Arnold and Mabel Beckman
Laboratory for Chemical Synthesis and Beckman
Institute, X-Ray Crystallography Laboratory
California Institute of Technology
(λ g 290 nm) of the corresponding [porphyrinato]Mn(III) azide.
Isolation of nitrido-Mn(V) adduct and subsequent treatment
of a solution of this complex with cyclooctene (11 equiv) and
TFAA (1.2 equiv) afforded the N-trifluoroacetylated aziridine
product. Following this example, we have prepared the nitrido-
Mn(V) species (1) derived from (salen)MnN3. The choice of
salen as a ligand followed from the work of Arshankow and
Poznjak, who had previously prepared (salen)CrVN‚H2O by
photolysis of the (salen)CrIIIN3‚2H2O complex.9,10 Although
formation of (salen)MnVN (1) was possible by the Arshankow-
Poznjak protocol, isolated yields of the desired manganese
nitride were low (<35%).
An alternative method for the preparation of 1 was developed
which offered significant improvement over the photolysis
procedure. Oxidation of both Cr(III) and Mn(III) porphyrin
complexes with either NaOCl or PhIO in the presence of
NH4OH had been reported to yield the corresponding Cr(V)
and Mn(V) nitrides.11,12 In a similar fashion, treatment of a
methanolic suspension of (salen)MnIIICl with NH4OH (15 M,
15 equiv) and aqueous NaOCl (Clorox bleach, 6 equiv) provided
nitride 1 as an emerald green solid. With this procedure,
multigram quantities of 1 could be prepared; however, the low
solubility of this compound in most organic solvents (CH2Cl2,
EtOAc, CH3CN, Et2O) made its isolation and purification
difficult. It was possible to circumvent such problems by
synthesizing the ligand in which the ethylenediamine backbone
of salen was replaced with 2,3-diamino-2,3-dimethylbutane.13
Condensation of this diamine with salicylaldehyde (2 equiv)
furnished the H2saltmen ligand as a yellow crystalline solid
(96%).7 The nitrido-Mn(V) complex (2) derived from H2-
saltmen was prepared in a single operation by first reacting Mn-
(OAc)2‚4H2O with a solution of the ligand in methanol to give
an air-oxidized (saltmen)Mn(III) intermediate.14 Subsequent
treatment of the resulting dark brown solution with NH4OH and
Clorox bleach afforded the desired Mn(V) nitride 2 (Scheme
1). Following purification, the product was isolated as a dark
green microcrystalline solid (in up to 20 g scale) in overall yields
which consistently ranged from 80 to 85%.15
Pasadena, California 91125
ReceiVed October 31, 1995
Metal-catalyzed oxygen atom-transfer reactions are of fun-
damental interest in both chemistry and biology.1 Detailed
investigations of the mechanism of oxygen atom transfer in
biological systems have led to the generation of synthetic models
capable of performing analogous oxidation chemistry.2 This
work has fueled the development of valuable synthetic meth-
odology specifically with regard to alkene epoxidation, a
reaction for which numerous protocols exist.3 In contrast, fewer
methods are available for the related metal-mediated nitrogen
atom-transfer reaction, despite the utility of such technology in
synthesis.4,5 Herein, we describe the preparation of two novel
nitridomanganese(V) salen-derived complexes that can be
activated for nitrogen transfer with trifluoroacetic anhydride
(TFAA).6,7 When this activation process is conducted in the
presence of certain silyl enol ethers, rapid formation of
N-trifluoroacetyl R-amino ketone products is observed (eq 1).
In one of the earliest examples of nitrogen atom-transfer
chemistry, Groves and Takahashi elegantly described the use
of nitrido[meso-tetrakis(2,4,6-trimethylphenyl)porphyrinato]-
manganese(V), (TMPMnN), for the aziridination of cis-cy-
clooctene.8 Formation of TMPMnN was effected by irradiation
† Arnold and Mabel Beckman Laboratory for Chemical Synthesis.
‡ Beckman Institute, X-Ray Crystallography Laboratory.
(1) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of
Organic Compounds; Academic: New York, 1981. (b) Feig, A. L.; Lippard,
S. J. Chem. ReV. 1994, 94, 759. (c) Collman, J. P.; Zhang, X.; Lee, V. J.;
Uffelman, E. S.; Brauman, J. I. Science 1993, 261, 1404. (d) Ostovic, D.;
Bruice, T. C. Acc. Chem. Res. 1992, 25, 314 and references therein.
(2) (a) Mansuy, D.; Battioni, P.; Battioni, J.-P. Eur. J. Biochem. 1989,
184, 267. (b) Holm, R. H. Chem. ReV. 1987, 87, 1401 and references
therein. (c) Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem. Soc.
1986, 108, 2309.
The (saltmen)Mn(N) complex 2, like the parent 1, is
remarkably stable to both air and H2O. 1H and 13C NMR spectra
recorded for the two complexes show sharp resonances in the
usual range for chemical shifts, consistent with a diamagnetic
complex of low-spin d2 configuration. Infrared spectroscopic
(3) (a) Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric
Synthesis; Ojima, I., Ed.; VCH: New York, 1993; p 103. (b) Jacobsen, E.
N. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York,
1993; p 159. (c) Jorgensen, K. A. Chem. ReV. 1989, 89, 431.
(4) For metal-catalyzed aziridination with PhIdNTs, see: (a) Evans, D.
A.; Faul, M. M.; Bilodeau, M. T. J. Am. Chem. Soc. 1994, 116, 2742 and
references therein. (b) Li, Z.; Quan, R.; Jacobsen, E. N. J. Am. Chem.
Soc. 1995, 117, 5889 and references therein. (c) Noda, K.; Hosoya, N.;
Irie, R.; Ito, Y.; Katsuki, T. Synlett 1993, 469. (d) O’Connor, K.; J.; Wey,
S.-J.; Burrows, C. J. Tetrahedron Lett. 1992, 33, 1001.
(5) For a review on available aziridination and amination methods, see:
Kemp, J. E. G. In ComprehensiVe Organic Synthesis; Ley, S. V., Ed.;
Pergamon: Oxford, U.K., 1991; Vol. 7, p, 469. See also: Vedejs, E.; Sano,
H. Tetrahedron Lett. 1992, 33, 3261.
(6) For a review of nitrido metal complexes, see: (a) Dehnicke, K.;
Strahle, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 955. (b) Nugent, W.
A.; Haymore, B. L. Coord. Chem. ReV. 1980, 31, 123. See also: Nugent,
W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley-Interscience:
New York, 1988.
(7) Salen ) N,N′-ethylenebis(salicylideneaminato); saltmen ) N,N′-
(1,1,2,2-tetramethylethylene)bis(salicylideneaminato).
(9) Arshankow, S. I.; Poznjak, A. L. Z. Anorg. Allg. Chem. 1981, 481,
201. (Salen)Cr(N) has also been prepared by intermetal nitrogen atom
transfer from a nitrido(octaethylporphyrinato)chromium to (salen)CrCl:
Neely, F. L.; Bottomley, L. A. Inorg. Chim. Acta 1992, 192, 147.
(10) A nitrido-Cr(V) complex derived from 1,2-bis(2-pyridinecarboxa-
mido)benzene has also been prepared by photolysis of the corresponding
CrIIIN3: Che, C.-M.; Ma, J.-X.; Wong, W.-T.; Lai, T.-F.; Poon, C.-K. Inorg.
Chem. 1988, 27, 2547.
(11) (a) Buchler, J. W.; Dreher, C.; Lay, K.-L. Z. Naturforsch., B: Anorg.
Chem., Org. Chem. 1982, 37B, 1155. (b) Buchler, J. W.; Dreher, C.; Lay,
K.-L.; Lee, Y. J. A.; Scheidt, W. R. Inorg. Chem. 1983, 22, 888.
(12) Hill, C. L.; Hollander, F. J. J. Am. Chem. Soc. 1982, 104, 7318.
(13) For the preparation of 2,3-diamino-2,3-dimethylbutane, see: Sayre,
R. J. Am. Chem. Soc. 1955, 77, 6689.
(14) The nature of this dark brown Mn(III) species has not been
established. For an analogous reaction in which the resulting Mn(III)-
salen complex was isolated and characterized following treatment with LiCl,
see: Zhang, W.; Jacobsen, E. N. J. Org. Chem. 1991, 56, 2296.
(15) A detailed experimental procedure for the preparation of both H2-
saltmen and 2 is provided in the supporting information.
(8) (a) Groves, J. T.; Takahashi, T. J. Am. Chem. Soc. 1983, 105, 2074.
(b) Groves, J. T.; Takahashi, T.; Butler, W. M. Inorg. Chem. 1983, 22,
884.
0002-7863/96/1518-0915$12.00/0 © 1996 American Chemical Society

916 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Communications to the Editor
of cyclooctene with TMPMnN and TFAA (Vide supra), may
proceed via initial formation of a reactive N-trifluoroacetylimi-
domanganese species with subsequent transfer of the CF3CON
group to the olefin substrate.8a,21 Although currently limited
to certain enol silanes, the amination method described has
several appealing features, which include (i) the facile prepara-
tion of large quantities of the starting Mn-nitrido reagent 2;
(ii) the use of the silyl enol ether substrate as the limiting reagent
(1 equiv); and (iii) the mild reaction conditions employed.
Additionally, the trifluoroacetyl residue serves as a useful amine
protecting group which may be cleaved under mild conditions.
Figure 1. ORTEP diagram of (salen)Mn(N) 1 displaying 50%
probability ellipsoids.
analysis established the MntN stretching frequency at 1047
cm-1 for 1 and 2, similar to that reported for the analogous
nitridomanganese porphyrin species (∼1050 cm-1).8a,11b The
structures of 1 and 2 were confirmed by single crystal X-ray
analysis (ORTEP for 1 shown in Figure 1).16 X-ray data show
both compounds to be monomeric, each having a MnsN bond
length of 1.51 Å, consistent with the assignment of a formal
MntN triple bond.17 To the best of our knowledge, 1 and 2
represent the first non-porphyrin nitridomanganese(V) com-
plexes to be synthesized and crystallographically characterized.18
Preliminary investigations into the use of 2 (or 1) as a
nitrogen-transfer reagent suggested that electron-rich alkenes,
such as ketone silyl enol ethers, would serve as optimal
substrates for amination. Treatment of a solution of 2 (2 equiv),
1 equiv of a Me3Si enol ether, and pyridine (3 equiv) in CH2-
Cl2 with TFAA (2.4 equiv) at reduced temperature (ca. -30
°C) led to rapid consumption of the starting materials and
provided the corresponding N-trifluoroacetylated R-amino ke-
tone (eqs 2-5).19,20 This reaction, like the analogous reaction
We have described the facile preparation of two novel
nitridomanganese(V) salen-derived complexes that serve as
reagents for the amination of electron-rich enol ethers, and in
doing so, we have demonstrated that reactions involving
activation and transfer of nitrogen from metal nitrides are not
unique to porphyrin-derived species. The ease with which both
the structural and electronic properties of the salen ligand may
be varied by simple substitutions of either the salicylaldehyde
or ethylenediamine moieties should allow for the preparation
of other nitrido-metal reagents that display increased reactivity.
Efforts along these lines are currently in progress.
(16) Crystal data for (salen)MnN (1): emerald green crystals were
deposited by slow evaporation of a CH2Cl2 solution of 1; space group P21/
c; cell constants a ) 9.496(3), b ) 12.313(3), and c ) 12.857(4) Å; â )
103.61°; V ) 1461.1(7) Å3, Z ) 4. A total of 4023 observations were
collected (Mo KR, 2θmax ) 44°, -9 e h e 9, 0 e k e 12, -13 e l e 13)
and merged to 1781 unique reflections (Rmerge ) 0.038, GOFmerge ) 1.05).
The structure was solved by direct methods (SHELXS-86) and refined
ansiotropically (SHELXL-93) to R ) 0.061 with GOF ) 1.304. Additional
information is given in the supporting information.
(17) A Mn-N distance of 1.51 Å has been reported for the (5,15-
dimethyl-2,3,7,8,12,13,17,18-octaethyl-5H,15H-porphyrinato)nitridomanga-
nese(V) compound, see ref 11b.
(18) The preparation of a nitrido-Mn(V) phthalocyanine complex has
been reported; however, its structure was only established by UV and
resonance Raman spectroscopy. Grunewald, H.; Homborg, H. Z. Anorg.
Allg. Chem. 1992, 608, 81. A stable Mn(V) oxo complex has been
crystallographically characterized, see: Collins, T. J.; Powell, R. D.;
Slebodnick, C.; Uffelman, E. S. J. Am. Chem. Soc. 1990, 112, 899.
(19) Following is a general procedure for the amination of the silyl enol
ethers shown in eqs 2-5: A solution of 2 (1.0 mmol, 2 equiv) in 3.0 mL
of CH2Cl2 was cooled to -78 °C. Pyridine (1.5 mmol) was added, followed
by a solution of the silyl enol ether (0.5 mmol) in 2.0 mL of CH2Cl2. Freshly
distilled TFAA (1.2 mmol) was then added dropwise to the dark green
mixture. The solution was allowed to warm slowly from -78 °C to 23 °C
over a 3-4 h period. During this time, the reaction mixture turned dark
brown. Silica gel (800 mg) and Celite (800 mg) were added, along with
25 mL of n-pentane. The dark brown slurry was stirred vigorously at 23
°C for 30 min before being filtered through a 20 mm × 40 mm plug of
silica gel using Et2O (4 × 10 mL) as eluent. Concentration of the filtrate
under reduced pressure afforded a pale yellow residue, which was purified
by chromatography on silica gel. In one case (eq 5), a higher yield of the
desired product was obtained if the reaction was initially run at -30 °C
(3:2 H2O/ethylene glycol-dry ice) and only a catalytic amount of pyridine
(62 µmol) was employed.
Acknowledgment. J.D.B. is grateful to the National Science
Foundation for a predoctoral fellowship. We kindly thank Prof. Harry
Gray and Mr. Don Low for many helpful discussions. We are grateful
to Dr. John Greeves (UC Irvine) for obtaining high-resolution mass
spectral data. This research has been supported by generous grants
from the NSF, Merck, Pfizer, and Upjohn.
Supporting Information Available: Experimental procedures for
the preparation of H2saltmen and 2 are included, spectral and analytical
data for N-trifluoroacetyl R-amino ketone products, and crystallographic
data for 1 (13 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 donwloaded
from the Internet; see any current masthead page for ordering
information and Internet access instructions.
JA953659R
1
(20) The camphor-derived product (eq 3) has been shown by H NMR
difference NOE experiments to be the exo isomer. In addition, the product
was found to be identical (1H and 13C NMR, IR) to authentic material which
was prepared according to a known procedure for a related compound:
Chittenden, R. A.; Cooper, G. H. J. Chem. Soc. C, 1970, 49.
(21) For mechanistic studies on the reaction of TFAA with nitrido-Cr
and Mn porphyrin complexes, see: (a) Bottomley, L. A.; Neely, F. L. Inorg.
Chem. 1990, 29, 1860. (b) Bottomley, L. A.; Neely, F. L. J. Am. Chem.
Soc. 1988, 110, 6748.