Imidozirconium Complex with Allylic Electrophiles
A R T I C L E S
LanL2DZ29 level of theory. The energies are reported as
electronic energies plus unscaled zeropoint energy corrections.
All stationary points were confirmed by frequency calculations
and transition states were characterized by exactly one imaginary
vibrational frequency. IRC calculations proved that the calcu-
lated transition state structures connected those of the products
and reactants.
Scheme 4. Results of a Hammett Study
In agreement with the kinetics experiments, the calculations
predict that the THF-adduct starting material SM is favored over
the substrate-bound complex SC-OMe by 1.2 kcal/mol (Figure
5). We were also able to identify a six-membered closed
transition state TS-OMe for the substitution step that was
calculated to be 22 kcal/mol higher in energy than the initial
ground state. In this transition state structure TS-OMe, the
carbon-oxygen bond is elongated by 25% relative to the same
bond in SC-OMe (1.855 vs 1.493 Å), while the carbon-
nitrogen bond is elongated by 46% relative to same bond in
P-OMe (2.213 vs 1.511 Å). Similarly, bond lengths in the allyl
fragment in the transition state are closer to the bond lengths in
SC-OMe than to those in the product. These differences in
the relative bond lengths in the ground states and the transition
state indicate a relatively early transition state, which is
consistent with an exothermic reaction. Overall, these results
suggest that the substitution reaction is best described as a
concerted, asynchronous [3,3]-sigmatropic rearrangement with
an early transition state.
first-order rate constants for the reactions with TBS allyl ether
and (E)-1-(tert-butyldimetylsiloxy)-3-deuterioprop-2-ene (kH/kD
) 0.88 ( 0.01 at 20.6 °C).
Some insight into the substitution step of the reaction can be
obtained from the syn stereoselectivity and the high regiose-
lectivity observed in the substitution reaction. In the literature,
syn stereochemistry of allylic substitutions has been interpreted
as a strong indication of a cyclic transition state.26 The most
well-documented and studied example is the copper-promoted
substitution of allylic electrophiles, in which the stereochemistry
of the substitution has been directly linked to the ability of the
leaving group to interact with copper.2 The exclusive formation
of the SN2′ product has also been used as a strong indication of
the closed transition state in a related allylic substitution
reaction.12 Furthermore, the Lewis acidity of the metal center
in 1 also favors interaction with the leaving group that would
lead to a closed transition state. Similarly, the relatively modest
nucleophilicity of 1, which is evident from the inability of 1 to
react with electrophiles as reactive as benzyl chloride, supports
the notion of a closed transition state which is available only in
reactions with allylic substrates.
To probe the direct interaction between the leaving group
and the zirconium center necessary for the closed transition state
of the substitution step, we performed a Hammett study (Scheme
4). The competition experiments were done using an excess (10
equiv) of the equimolar mixture of para-substituted allyl ethers
and the relative rates of the reactions were extracted from the
product distribution at the end of the reactions. The negative F
value is consistent with a closed transition state and the need
for an electron-donating leaving group to coordinate strongly
to zirconium, a property that would not be expected for the direct
SN2 mechanism.
This reaction is mechanistically closely related to [3,3]-
sigmatropic rearrangements of imidates30 and phosphorimi-
dates,31 which have been the most reliable and selective methods
for transformation of allylic alcohols into rearranged allylic
amines. The major difference is that one of the covalent bonds
in the closed transition state is replaced by a dative bond between
the metal and the leaving group. This general strategy has been
applied in the rearrangement of allylic alcohols catalyzed by
metal oxo complexes8-10 and, more recently, in the reaction of
allylic alcohols catalyzed by titanium imido complexes.32
To explore the origins of the observed differences in reactivity
of E- and Z-substituted allylic ethers, we modeled the reactions
of both E and Z isomers of 1-methoxy-2-butene (Figure 6). As
expected, no significant difference in the stability of the
zirconium-bound substrates (less than 0.5 kcal/mol) was pre-
dicted. However, the activation energy corresponding to TS-
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision A.01; Gaussian, Inc.: Wallingford, CT, 2004.
Reaction MechanismsTheoretical Approach. In order to
gain more insight into the nature of the substitution step, we
performed a DFT study of a model reaction between methyl
allyl ether and a TMS-substituted zirconium imido complex.
All calculations were performed in Gaussian0327 at the B3LYP28/
(28) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648; (b) Vosko, S. H.; Wilk,
L.; Nusair, M. Can. J. Phys. 1980, 58, 1200; (c) Lee, C.; Yang, W.; Parr,
R. G. Phys. ReV. B 1988, 37, 785; (d) Krishnan, R.; Binkley, J. S.; Seeger,
R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650.
(29) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(30) (a) Overman, L. E. Acc. Chem. Res. 1980, 13, 218; (b) Overman, L. E. J.
Am. Chem. Soc. 1976, 98, 2901.
(26) (a) Tseng, C. C.; Paisley, S. D.; Goering, H. L. J. Org. Chem. 1986, 51,
2884; (b) Young, W. G.; Webb, I. D.; Goering, H. L. J. Am. Chem. Soc.
1951, 73, 1076.
(31) Chen, B.; Mapp, A. K. J. Am. Chem. Soc. 2005, 127, 6712.
(32) Ramanathan, B.; Odom, A. L. J. Am. Chem. Soc. 2006, 128, 9344.
9
J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008 4463