Electrophilic tungsten(II) methylene carbene complexes: Adduct formation, methylene transfer, and catalysis of aziridine formation from imines and ethyl diazoacetate

Full text of DOI:10.1021/ja9640206

J. Am. Chem. Soc. 1997, 119, 3171-3172
3171
Electrophilic Tungsten(II) Methylene Carbene
Complexes: Adduct Formation, Methylene
Transfer, and Catalysis of Aziridine Formation
from Imines and Ethyl Diazoacetate
germane here are the unusually robust methylene carbene
complexes [Cp*(NO)(L)RedCH2)][PF6] (L ) PPh3 or P(OPh)3)
formed from a rhenium methyl precursor and triphenylcarbe-
9
c
nium (trityl) hexafluorophosphate.
We now report [Tp′(CO)(PhC2Me)WdCH2][PF6] (2) (Tp′ )
hydridotris(3,5-dimethylpyrazolylborate)), an electrophilic car-
bene complex that binds nucleophiles, transfers the methylene
fragment to olefins to form cyclopropanes, and serves as a
catalyst for aziridine formation from imines and EDA (ethyl
diazoacetate).
T. Brent Gunnoe, P. S. White, J. L. Templeton,* and
Luis Casarrubios
W. R. Kenan, Jr., Laboratory
Department of Chemistry
Trityl cation reacts with Tp′(CO)(PhC2Me)W-Me11 (1) in
CD2Cl2 at -78 °C to form methylene carbene complex 2 (eq
UniVersity of North Carolina
Chapel Hill, North Carolina 27599-3290
1
). The NMR signature for a methylene complex includes two
ReceiVed NoVember 20, 1996
1
From Fischer’s original heteroatom carbene synthesis to use
as carbene transfer mediators and as ROMP3 and olefin
metathesis catalysts,4 metal carbenes have been important
exhibits in the organometallic showcase. Schrock’s early
2
5
methylene breakthrough riveted attention on the MdCH2 entity,
6
,7
and during the ensuing two decades both nucleophilic and
electrophilic8 MdCH2 moieties have been generated. Most
-10
2
1
doublets (12.33, 12.04 ppm; JHH ) 12 Hz) in the H NMR
spectrum and a low-field triplet for the carbene carbon (308.2
ppm, JCH ) 140 Hz, JWC ) 110 Hz, 14% W, I ) 1/2) in
the C NMR spectrum. The absence of methylene rotation on
(
1) Fischer, E. O.; Maasb o¨ l, A. Angew. Chem., Int. Ed. Engl. 1964, 3,
80.
2) For metallocarbene reviews, see: (a) Hegedus, L. S. Transition Metals
1
1
183
5
(
13
in the Synthesis of Complex Organic Molecules; University Science
Books: Mill Valley, CA, 1994; p 151. (b) Brookhart, M.; Studabaker, W.
B. Chem. ReV. 1987, 87, 411. (c) Gallop, M. A.; Roper, W. R. AdV.
Organomet. Chem. 1986, 25, 121. (d) Doyle, M. P. Chem. ReV. 1986, 86,
the NMR time scale is compatible with optimal back-bonding
from the metal dπ orbitals to the carbene, alkyne, and carbonyl,
with the carbene orientation orthogonal to the W-CO axis. The
9
19. (e) D o¨ tz, K. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 587. (f) D o¨ tz,
-1
IR spectrum exhibits a CO stretch at 2073 cm in CH2Cl2 (cf.
K. H.; Fischer, H.; Hoffman, P.; Kreissl, F.; Schubert, U.; Weiss, K.
-
1
12
Transition Metal Carbene Complexes; Verlag Chemie: Deerfield, FL, 1983.
2073 cm for [Tp′(CO)W(PhC2H)2][BF4]).
The cationic
(g) Schrock, R. R. Science 1983, 219, 13. (h) Brown-Wensley, K. A.;
complex 2 shows no signs of decomposition in solution at room
temperature over a period of several hours and persists for days
in the solid state.
Addition of a nucleophile is classic behavior for an electro-
philic Fischer carbene. Here addition of 1 equiv of PMe3 to a
CH2Cl2 solution of 2 results in the formation of [Tp′(CO)(PhC2-
Me)W-CH2-PMe3][PF6] (3), characterized by a CO stretch
Buchwald, S. L.; Cannizzo, L.; Clawson, L.; Ho, S.; Meinhardt, D.; Stille,
J. R.; Straus, D.; Grubbs, R. H. Pure Appl. Chem. 1983, 55, 1733. (i)-
Schrock, R. R. Acc. Chem. Res. 1979, 12, 98. (j) Cardin, D. J.; Cetinkaya,
B.; Lappert, M. F. Chem. ReV. 1972, 72, 545. (k) Cotton, F. A.; Lukehart,
C. M. Prog. Inorg. Chem. 1972, 16, 487.
(
3) For reviews of ROMP (ring opening metathesis polymerization), see
refs 3-5 in Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2039.
(4) For reviews of olefin metathesis, see: (a) Ivin, J. J. Olefin Metathesis;
-1
1
at 1888 cm in CH2Cl2 (Scheme 1). In the H NMR one of
the diastereotopic CH2 protons appears as a doublet of doublets
Academic Press: New York, 1983. (b) Calderon, N.; Lawrence, J. P.;
Ofstead, E. A. AdV. Organomet. Chem. 1979, 17, 449. (c) Grubbs, R. H. In
ComprehensiVe Organometallic Chemistry; Wilkinson, G. W., Stone, A.,
Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; Vol. 8, p 499. (d)
Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100
and references therein.
2
2
(1H, 0.79 ppm, JHH ) 14 Hz, JPH ) 22 Hz). The second
methylene proton lies between 1.5 and 1.6 ppm and is obscured
by methyl peaks. A COSY was used to assign the resonances
of the second methylene proton. A definitive triplet of doublets
(5) (a) Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577. (b) Guggen-
berger, L. J.; Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6578. (c) Schrock,
R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389.
1
1
(
20.1 ppm, JCH ) 125 Hz; JPC ) 33 Hz) is observed for the
13
methylene carbon in the C NMR.
(6) For representative examples of other nucleophilic methylene carbenes,
see: (a) Tour, J. M.; Bedworth, P. V.; Wu, R. Tetrahedron Lett. 1989, 30,
927. (b) Cannizzo, L. F.; Grubbs, R. H. J. Org. Chem. 1985, 50, 2316. (c)
The carbene complex reacts with a variety of electron rich
olefins to form cyclopropanes (Scheme 1). For example,
addition of styrene, 4-methylstyrene, or cyclohexene results in
stoichiometric CH2 transfer to form cyclopropane products in
3
Cannizzo, L. F.; Grubbs, R. H. J. Org. Chem. 1985, 50, 2386. (d) Schwartz,
J.; Gell, K. I. J. Organomet. Chem. 1980, 184, C1. (e) Petasis, N. A.;
Bzowej, E. I. J. Am. Chem. Soc. 1990, 112, 6392. (f) Tebbe, F. N.; Parshall,
G. W.; Reddy, G. S. J. Am. Chem. Soc. 1978, 100, 3611. (g) Pine, S. H.;
Zahler, R.; Evans, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 1980, 102,
1
3
73%, 57%, and 51% yield, respectively.
A new development in aziridine synthesis is metal-catalyzed
carbene transfer to imines. Addition of EDA to imines in the
presence of a catalytic amount of Cu(OTf)2 results in aziridine
3
270. (h) Hartner, Jr., F. W.; Schwartz, J.; Clift, S. M. J. Am. Chem. Soc.
983, 105, 640.
1
(
7) Numerous papers have been published concerning group VIII and
8 6
group IX d and d carbenes that display nucleophilic behavior; see: Roper,
W. R. In AdVances in Metal Carbene Chemistry; Schubert, U., Ed.; Kluwer
Academic Publishers: Norwell, MA, 1989; p 27 and references therein.
(10) Some representative examples of electrophilic methylene car-
benes: (a) Jolly, P. W.; Pettit, R. J. Am. Chem. Soc. 1966, 88, 5044. (b)
Guerchais, V.; Astruc, D. J. Chem. Soc., Chem. Commun. 1985, 835. (c)
Barefield, E. K.; McCarten, P.; Hillhouse, M. C. Organometallics 1985, 4,
1682. (d) Davison, A.; Krusell, W. C.; Michaelson, R. C. J. Organomet.
Chem. 1974, 72, C7. (e) Brandt, S.; Helquist, P. J. Am. Chem. Soc. 1979,101,
6473. (f) O’Connor, E. J.; Brandt, S.; Helquist, P. J. Am. Chem. Soc. 1987,
109, 3739. (g) Mattson, M. N.; Bays, J. P.; Zakutansky, J.; Stolarski, V.;
Helquist, P. J. Org. Chem. 1989, 54, 2467. (h) Riley, P. E.; Capshew, C.
E.; Pettit, R.; Davies, R. E. Inorg. Chem. 1978, 17, 408. (i) Flood, T. C.;
DiSanti, F. J.; Miles, D. L. Inorg. Chem. 1976, 15, 1910.
(11) Complex 1 was synthesized according to a published procedure:
Caldarelli, J. L.; Wagner, L. E.; White, P. S.; Templeton, J. L. J. Am. Chem.
Soc. 1994, 116, 2878.
(12) Feng, S. G.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1992,
114, 2951.
(
8) (a) Brookhart, M; Liu, Y. In AdVances in Metal Carbene Chemistry;
Schubert, U., Ed.; Kluwer Academic Publishers: Norwell, MA, 1989, p
51. (b) Kegley, S. E.; Brookhart, M. Organometallics 1982, 1, 760. (c)
Studabaker, W. B.; Brookhart, M. J. Organomet. Chem. 1986, 310, C39.
d) Brookhart, M.; Tucker, J. R.; Flood, T. C.; Jensen, J. J. Am. Chem.
Soc. 1980, 102, 1203. (e) Brookhart, M.; Nelson, G. O. J. Am. Chem. Soc.
977, 99, 6099.
9) (a) Kiel, W. A.; Lin, G.; Bodner, G. S.; Gladysz, J. A. J. Am. Chem.
Soc. 1983, 105, 4958. (b) Wang, Y.; Gladysz, J. A. Chem. Ber. 1995, 128,
13. (c) Patton, A. T.; Strouse, C. E.; Knobler, C. B.; Gladysz, J. A. J. Am.
2
(
1
(
2
Chem. Soc. 1983, 105, 5804. (d) Merrifield, J. H.; Lin, G.; Kiel, W. A.;
Gladysz, J. A. J. Am. Chem. Soc. 1983, 105, 5811. (e) Wong, W.; Tam.
W.; Gladysz, J. A. J. Am. Chem. Soc. 1979, 101, 5440. (f) Buhro, W. E.;
Etter, M. C.; Georgiou, S.; Gladysz, J. A.; McCormick, F. B. Organome-
tallics 1987, 6, 1150. (g) Buhro, W. E.; Patton, A. T.; Strouse, C. E.;
Gladysz, J. A.; McCormick, F. B.; Etter, M. C. J. Am. Chem. Soc. 1983,
(13) Yields for cyclopropanation reactions were determined by isolation
of the cylcopropane on an alumina column, followed by GC analysis with
a known amount of the internal standard undecane.
1
05, 1056.
S0002-7863(96)04020-6 CCC: $14.00 © 1997 American Chemical Society

3172 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Communications to the Editor
Scheme 1. Reactions of the Methylene Carbene 2
14
formation. Copper(I) catalysts with chiral bis(dihydrooxazole)
Figure 1. ORTEP diagram for [Tp′(CO)(PhC
2 2
Me)WsCH N(Me)dC-
1
5
16
ligands and methylrhenium trioxide have also been used to
produce aziridines from imines and EDA. Mechanistic specula-
tion for the copper(I) system postulates the formation of a copper
carbene complex followed by nucleophilic attack of the imine
on the carbene fragment.
Is carbene transfer to an imine possible in this system?
Reaction of [Tp′(CO)(PhC2Me)WdCH2][PF6] (2) with N-
benzylidenemethylamine results in formation of the isolable
imine adduct [Tp′(CO)(PhC2Me)WsCH2N(Me)dC(H)(Ph)]-
(
H)(Ph)][PF ] (4). Selected bond distances (Å) and angles (deg): W(1)-
6
C(1), 1.943(16); W(1)-C(3), 2.076(13); W(1)-C(4), 2.032(12); W(1)-
C(5), 2.237(13); C(5)-N(6), 1.468(17); C(1)-O(1), 1.183(19); W(1)-
C(1)-O(1), 178.3(11); W(1)-C(5)-N(6), 125.3(9); C(5)-N(6)-C(8),
1
12.9(11); C(5)-N(6)-C(7), 127.0(12).
Table 1. Yields and Product Ratios for Aziridine Catalysis
total
%
%
%
a
imine yield, % cis trans enamine azir:enamine cis:trans
-
1
1
[
PF6] (4) (νCO ) 1886 cm ). H NMR data include two
5
6
7
8
67.5
64.6
45.0
41.3 14.4
65.0
4.5
18.0
8.9
22.5
13.4
2.8:1
6.3:1
2.9:1
4.3:1
10:1
2.9:1
2
doublets (4.45, 2.00 ppm, JHH ) 12 Hz) assigned to the
diastereotopic methylene protons. The iminic carbon resonates
87.5b
1
as a doublet at 161.6 ppm ( JCH ) 170 Hz) while the methylene
71.2
27.7 30.1
1:1.1
1
carbon is observed as a triplet at 66.4 ppm ( JCH ) 135 Hz) in
a
the 13C NMR. A low-temperature (-120 °C) single-crystal
As determined by integration (versus an internal standard) of well-
1 b
resolved peaks in the H NMR. Isolated yield: reaction performed
in CH Cl with products purified by chromatography on silica gel (10:1
X-ray structure study provided metric details of 4 as shown in
Figure 1.
2
2
hexanes/ethyl acetate).
The addition of benzylideneaniline to [Tp′(CO)(PhC2-
Me)WdCH2][PF6] (2) yields the imine adduct [Tp′(CO)(PhC2-
Me)WsCH2N(Ph)dC(H)(Ph)][PF6]. Reaction of additional
arylimines 5-8 with the carbene complex 2 yielded similar
results (Scheme 1). These arylimine adducts 9-12 are not
stable enough to permit isolation. No aziridine products have
been detected in these reactions.
of aziridines from imines and EDA (Scheme 1). Yields and
ratios for cis and trans aziridines as well as for the formation
of enamine products are given in Table 1.
We believe that these catalytic reactions utilize a reactive,
electrophilic carbene complex simply as a Lewis acid to activate
imines toward nucleophilic attack by EDA. The discovery of
this catalytic sequence suggests another plausible mechanistic
pathway in other transition metal mediated carbene transfer
reactions.
1
7
A recent report notes that EDA can attack imines which have
1
8
been activated by simple Lewis acids (BF3, TiCl4, or AlCl3).
The two major products observed for these systems are
aziridines and enamines. In a similar fashion, we have found
that the methylene carbene complex 2 catalyzes the production
Acknowledgment. Special thanks are due to Dr. Julio P e´ rez for
informative discussions. This work was supported by the National
Science Foundation (Grant CHE-9208207).
(
14) Rasmussen, K. G.; Jørgensen, K. A. J. Chem. Soc., Chem. Commun.
995, 1401.
15) Hansen, K. B.; Finney, N. S.; Jacobsen, E. N. Angew. Chem., Int.
Ed. Engl. 1995, 34, 676.
16) (a) Zhu, Z.; Espenson, J. H. J. Org. Chem. 1995, 60, 7090. (b) Zhu,
Z.; Espenson, J. H. J. Am. Chem. Soc. 1996, 118, 9901.
17) For imines 5-8, (R)NdC(R1)(H): 5, R ) R1 ) phenyl; 6, R )
phenyl, R1 ) p-nitrophenyl; 7, R ) p-methoxyphenyl, R1 ) phenyl; 8, R
phenyl, R1 ) tert-butyl.
18) Casarrubios, L.; P e´ rez, J. A.; Brookhart, M.; Templeton, J. L. J.
Org. Chem. 1996, 61, 8358.
1
Supporting Information Available: Experimental procedures,
analytical data, and discussion of control reactions with [Ph C][PF ],
3 6
(
X-ray and crystal structure information for 4, including a labeled
ORTEP diagram, and tables showing crystallographic data and collec-
tion parameters, atomic positional parameters, complete bond lengths
and angles, and anisotropic temperature factors (13 pages). See any
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JA9640206