Synthesis, structure, and reactivity studies of an η2-N2-titanium diazoalkane complex. Generation and trapping of a carbene complex intermediate

Full text of DOI:10.1021/ja9614981

J. Am. Chem. Soc. 1996, 118, 8737-8738
8737
Synthesis, Structure, and Reactivity Studies of an
η2-N2-Titanium Diazoalkane Complex. Generation
and Trapping of a Carbene Complex Intermediate
Jennifer L. Polse, Richard A. Andersen,* and
Robert G. Bergman*
Department of Chemistry,
UniVersity of California
Berkeley, California 94720
ReceiVed May 6, 1996
Organic diazoalkanes have long been used as synthons for
metal carbene complexes.1-4 It is generally assumed that an
intermediate diazoalkane complex is involved in the carbene
transfer process.3 Diazoalkane complexes have also been
implicated as intermediates in the metal-catalyzed cyclopropa-
nation of olefins by organic diazoalkanes.2,3 Unfortunately,
while there are numerous examples of isolable transition metal
diazoalkane complexes, they are typically quite resistant to N2
loss;3,4 as a result there are few isolable diazoalkane complexes
known to generate metal carbene complexes.5-9 We report here
on the synthesis of a titanocene diazoalkane complex which
undergoes facile N2 loss and on the mechanism of its reaction
with olefins to give titanacyclobutane complexes.
Treatment of a benzene solution of Cp* Ti(C2H4)10 (Cp* )
2
C5Me5) with 1 equiv of Me3SiCHN2 results in an immediate
color change from lime to forest green. Removal of the solvent
gives a green powder, which may be recrystallized from hexanes
at -50 °C to give Cp* TiN2CHSiMe3 (1) in 60-70% yield
2
(Scheme 1). The modest isolated yield reflects the high
solubility of the complex rather than the formation of any
byproducts. When the reaction is performed in C6D6 and
1
Figure 1. ORTEP drawings of (a) 1 and (b) 6. Selected bond distances
(Å) and angles (deg) for 1: Ti-N1, 1.979(2); Ti-N2, 2.012(2); N1-
N2, 1.276(3); N2-C21, 1.342(3); C21-H, 0.96 (3); C21-Si, 1.840-
(3); N1-Ti-N2, 37.27(9); N1-N2-C21, 131.3(2); N2-C21-Si,
122.6(2); Cp*1-Ti-Cp*2, 140.72(2). Selected bond distances (Å) and
angles (deg) for 6: Ti-C23, 2.192(4); Ti-C21, 2.155(4); C23-C22,
1.557(5); C21-C22, 1.551(5); C23-Si, 1.873(4); C23-Ti-C21, 71.2-
(1); Ti-C23-C22, 86.1(2); Ti-C21-C22, 87.6(2); C23-C22-C21,
109.0(3); Cp*1-Ti-Cp*2 135.78(3).
followed by H NMR spectroscopy, 1 equiv of free C2H4 is
produced in addition to 1, which is the only observable titanium-
containing product and is formed in >98% yield. Retention of
the nitrogen was confirmed by elemental analysis.11
Because of the wide variety of coordination modes possible
for diazoalkane complexes, it is often difficult to predict their
structures on the basis of spectroscopic data. We therefore
undertook an X-ray diffraction study to determine the coordina-
tion mode of the diazoalkane fragment in 1. An ORTEP
diagram and selected bond lengths and angles are shown in
Figure 1a.11 The Me3SiCHN2 group is bound to the titanium
in a side-on fashion through the two nitrogen atoms. Although
side-on coordination is far less common than end-on or bridging
coordination modes, it is not unprecedented.7,12,13 The bonding
situation in 1 is similar to that in an olefin adduct. The relatively
long N-N bond distance (1.276(3) Å) compared with that of
Scheme 1
(1) For a very recent example, see: Schwab, P.; Grubbs, R. H.; Ziller,
J. W. J. Am. Chem. Soc. 1996, 118, 100.
(2) Mizobe, Y.; Ishii, Y.; Hidai, M. Coord. Chem. ReV. 1995, 139, 281.
(3) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1978, 17, 800.
(4) Sutton, D. Chem. ReV. 1993, 93, 995.
(5) Messerle, L.; Curtis, M. D. J. Am. Chem. Soc. 1980, 102, 7789.
(6) Curtis, M. D.; Messerle, L.; D'Errico, J. J.; Butler, W. M.; Hay, M.
S. Organometallics 1986, 5, 2283.
(7) Nakamura, A.; Yoshida, T.; Cowie, M.; Otsuka, S.; Ibers, J. A. J.
Am. Chem. Soc. 1977, 99, 2108.
uncomplexed diazoalkanes (1.12-1.13 Å)14 indicates substantial
π-back-bonding from the b2 orbital of the titanocene fragment
to the diazoalkane LUMO, which is N-N antibonding.15
However, the N-N distance is still indicative of a partial N-N
double bond.16
(8) Cowie, M.; McKeer, I. R.; Loeb, S. J.; Gauthier, M. D. Organome-
tallics 1986, 5, 860.
(9) Clauss, A. D.; Shapley, J. R.; Wilson, S. R. J. Am. Chem. Soc. 1981,
103, 7387.
(10) Cohen, S. A.; Auburn, P. R.; Bercaw, J. E. J. Am. Chem. Soc. 1983,
105, 1136.
(11) Typical HMQC and NOESY experiments, details of the X-ray
structures of 1 and 6, representative kinetic data, and full spectroscopic
and analytical data for 4-7 are provided as supporting information.
(12) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Am.
Chem. Soc. 1982, 104, 1918.
(14) Patai, S. The Chemistry of Diazonium and Diazo Groups; Wiley:
New York, 1978; Part 1 and 2 and references therein.
(15) Jorgensen, W. L.; Salem, L. The Organic Chemist’s Book of
Orbitals; Academic Press: New York, 1973; p 126.
(13) Schramm, K. D.; Ibers, J. A. Inorg. Chem. 1980, 19, 2441.
S0002-7863(96)01498-9 CCC: $12.00 © 1996 American Chemical Society

8738 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Communications to the Editor
Compound 1 is thermally unstable in both the solid and
solution states. It decomposes in the solid state with a half-life
of ∼12 h to give an insoluble red powder. This powder has
not been identified, but on the basis of its low solubility it is
presumed to be polymeric in nature. In benzene and toluene
solution 1 decomposes to the fulvene complex Cp*FvTiCH2-
SiMe3 (2) with a half-life of 12 h at 25 °C (Scheme 1). Complex
2 was identified on the basis of its NMR spectral data, elemental
analysis,11 and independent synthesis from Cp*FvTiCl17 and
Me3SiCH2MgCl. This reaction is assumed to proceed Via initial
N2 loss to give a transient carbene complex (8) which then
undergoes hydrogen transfer from the Cp* ligand. The forma-
tion of a carbene complex intermediate during thermolysis of 1
is strongly suggested by a mechanistic study of the reaction of
1 with alkenes (Vide infra). In related work, Bercaw and co-
the R group, and Ha. No NOEs were observed between the
ring protons and the Cp* on the opposite side of the ring from
them. This is consistent with a trans arrangement of the two
ring substituents as shown in Scheme 1.
The stereochemical assignment has been confirmed by X-ray
crystallography in the case of 6. An ORTEP diagram along
with selected bond lengths and angles is shown in Figure 1b.11
As expected from the NMR experiments, the two substituents
are on adjacent carbons, and are trans to one another. The Ti-C
and C-C distances are similar to those observed in other
titanacyclobutane complexes.21 The ring is puckered with a
dihedral angle of 28° between the C21TiC23 and C21C22C23
planes. Although the vast majority of structurally characterized
titanacyclobutanes are planar,21 puckering of the ring has been
observed previously in an R,â-disubstituted titanacyclobutane
derived from Tebbe’s reagent and a norbornene diester.25 The
distortion in this case is probably a result of the extreme steric
congestion between the metal center and the two ring substit-
uents.
The metallacyclobutane complexes are thermally stable up
to 75 °C in solution. Heating benzene solutions of 4-7 at
temperatures at or above 75 °C gives complicated product
mixtures. The major organometallic product is 2, suggesting
that cycloreversion of the metallacycle to the transient (tri-
methylsilyl)methylidene complex is the principal thermal de-
composition route. Curiously, 4-7 are far more thermally stable
than the R,â-disubstituted metallacycles reported by Grubbs in
the parent Cp system.24 This greater stability is clearly
electronic in nature, since the Cp* system is far more sterically
encumbered than the parent system.
workers have described the thermolysis of Cp* TiMe2 to give
2
the fulvene complex Cp*FvTiMe (eq 1).18 A detailed labeling
study showed that methylidene complex 3 was the likely
intermediate, although it could not be trapped at the high
temperature needed for its generation.19
Compound 1 reacts with R-olefins H2CdCHR (R ) H, Ph,
Me, Et) over a period of 2 d at rt to give the deep red
metallacyclobutane complexes 4-7 (Scheme 1) in excellent
1
yield.11 The H NMR spectra of these materials show the
expected features, including inequivalent Cp* ligands. In the
case of substituted olefins the reaction is regio- and stereospe-
cific; NMR experiments indicate that only the trans-R,â-
The kinetics of the reaction of 1 with styrene in toluene-d8
1
have been examined by H NMR spectroscopy at a variety of
temperatures and styrene concentrations.11 These studies show
that the reaction is first order in metal complex and the rate is
independent of styrene concentration. From an Eyring plot (30-
60 °C) we have measured ∆Hq ) 22.5 ( 0.3 kcal/mol, ∆Sq )
-3.1 ( 1.0 eu, and ∆Gq ) 23.4 ( 0.4 kcal/mol (calculated at
25 °C). Since we observe no rate dependence on alkene
concentration, any mechanism that involves initial attack or
precoordination of the alkene may be ruled out. We have also
found that at 45 °C 1 rearranges to fulvene complex 2 with a
kobs (5.17 × 10-4 ( 0.10 × 10-4 s-1) identical to that for the
reaction with of 1 with styrene at 45 °C (5.02 × 10-4 ( 0.04
× 10-4 s-1) Our data are consistent with a mechanism in which
the rate-determining step is N2 loss to give carbene intermediate
8 (Scheme 1).
1
disubstituted diastereomer is formed (Scheme 1). The H and
13C{1H} NMR spectra of these materials were fully assigned
using a combination of COSY and HMQC experiments.11 In
addition to the resonances for the Cp*, R, and Me3Si groups,
the 13C{1H} NMR spectra of these compounds display a
methylene signal at ∼70 ppm, as well as methine peaks at ∼70
and ∼30 ppm. The downfield resonances are characteristic of
the R-carbons of titanacyclobutanes,20-24 indicating that the
complexes are R,â-disubstituted. The stereochemistry was
determined by 2D NOESY experiments.11 Cross peaks were
observed (see Scheme 1 for substituent labeling) between Cp*
a
and the Me3Si group, Hb, and Hc. Cp*b showed NOEs to Hd,
(16) For examples of other structurally characterized Ti diazoalkane
complexes, see: Kool, L. B.; Rausch, M. D.; Alt, H. G.; Herberhold, M.;
Hill, A. F.; Thewalt, U.; Wolf, B. J. Chem. Soc., Chem. Commun. 1986,
408 and ref 12.
(17) Fandos, R.; Meetsma, A.; Teuben, J. H. Organometallics 1991, 10,
2665.
(18) McDade, C.; Green, J. C.; Bercaw, J. E. Organometallics 1982, 1,
1629.
Acknowledgment. We thank the National Science Foundation
(Grant No. CHE-9526388) for generous financial support of this work.
We also thank Dr. F. J. Hollander, director of the University of
California Berkeley College of Chemistry X-ray diffraction facility
(CHEXRAY), for solving the crystal structures of 1 and 6.
(19) For sucessful trapping of a titanium carbene generated from
thermolysis of a dialkyltitanocene fragment, see: (a) Petasis, N. A.; Fu,
D.-K. J. Am. Chem. Soc. 1993, 115, 7208. (b) van der Heijden, H.; Hessen,
B. J. Chem. Soc., Chem. Commun. 1995, 145.
Supporting Information Available: Spectroscopic and analytical
data for complexes 1, 2, and 4-7, structural data for 1 and 6, HMQC
and NOESY spectra of 7, and representative kinetic data for the reaction
of 1 and styrene, including plots of concentration vs time, kobs vs
[styrene], and ln(k/T) vs 1/T (Eyring plot) (25 pages). See any current
masthead page for ordering and Internet access instructions.
(20) Lee, J. B.; Ott, K. C.; Grubbs, R. H. J. Am. Chem. Soc. 1982, 104,
7491.
(21) Lee, J. B.; Gajda, G. J.; Schaefer, W. P.; Howard, T. R.; Ikariya,
T.; Straus, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 1981, 103, 7358.
(22) Howard, T. R.; Lee, J. B.; Grubbs, R. H. J. Am. Chem. Soc. 1980,
102, 6876.
(23) Gilliom, L. R.; Grubbs, R. H. Organometallics 1986, 5, 721.
(24) Straus, D. A.; Grubbs, R. H. J. Mol. Catal. 1985, 28, 9.
JA9614981
(25) Stille, J. R.; Santarsiero, B. D.; Grubbs, R. H. J. Org. Chem. 1990,
55, 843.