Stereochemistry and Mechanism of the Ring-Opening Reaction of

Jul 31, 2012 - Chemistry Department, Reed College, Portland, Oregon 97202, United States. •S Supporting Information. ABSTRACT: The chemical shifts o...
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Stereochemistry and Mechanism of the Ring-Opening Reaction of Cyclopropylenones with LiCu(Me)2 Charles P. Casey,*,† Mark C. Cesa,‡ and Alan J. Shusterman*,§ †

Department of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States INEOS Nitriles, 150 W. Warrenville Road, MS F-7, Naperville, Illinois 60563, United States § Chemistry Department, Reed College, Portland, Oregon 97202, United States ‡

S Supporting Information *

ABSTRACT: The chemical shifts of the diastereotopic hydrogens of the ethyl group of the cyclopropane ring-opening product of conjugate addition of LiCu(CH3)2 to cyclopropyl enone 1 were computed. Comparison of computed and observed 1H NMR chemical shifts of the diastereotopic hydrogens of the ethyl group of 2 established the stereochemistry of the ring-opening product as 2d-b. This provides evidence that the reaction proceeds by conjugate addition of the cuprate to the enone, followed by ring-opening of the cyclopropylmethyl copper species and reductive elimination. he conjugate addition of organocuprates to α,β-unsaturated carbonyl compounds is one of the most useful C−C bond forming reactions.1 In spite of the complexity of organocuprates, which have been shown by NMR to exist as a complex mixture of aggregates that vary with the solvent and LiX additives,2 great progress has been made in determining the major outlines of the mechanism of the conjugate addition of organocopper reagents to unsaturated carbonyl compounds (Scheme 1).3 π-Complexes of organocuprates to unsaturated

T

Reductive elimination from Cu(III) leads to C−C bond formation. The observation of primary 13C kinetic isotope effects is consistent with rate-limiting C−C bond formation via reductive elimination from Cu(III).6 Kinetic studies are also consistent with rate-limiting conversion of the π-complex to product.7 Computational studies support this mechanism.3a In the 1970s, an electron-transfer mechanism for conjugate addition of cuprates to enones involving a radical anion intermediate had been suggested based in part on the ringopening of cyclopropylenone substrates (Scheme 3).8

Scheme 1 Scheme 3

In 1979, we set out to test for intermediacy of a radical anion in the ring-opening of cyclopropyl enones accompanying the conjugate addition of cuprates. We proposed a stereochemical test for the intermediacy of radical anions using deuteriumlabeled compound 1d (Figure 1).9 If the reaction of cuprate with 1d proceeded by radical anion I (path a), then ringopening would lead to radical anion II, loss of stereochemical information by rotation about the CHD−C bond of II, and formation of both diastereomers of the ring-opened product

carbonyl compounds have been directly observed, and the reversibility of their formation has been established by lowtemperature NMR spectroscopy.2a,3b,4 Both σ- and π-allyl Cu(III) intermediates have been observed (Scheme 2).5 Scheme 2

Special Issue: Copper Organometallic Chemistry Received: June 11, 2012 Published: July 31, 2012 © 2012 American Chemical Society

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group of 2 to establish the stereochemistry of the ring-opening product as 2d-b. This provides plausible evidence that the reaction proceeds as shown in path c, by conjugate addition of the cuprate to the enone, followed by ring-opening of the cyclopropylmethyl copper species and reductive elimination. In addition, we have used comparison of computed and observed 1 H NMR chemical shifts to confirm the stereochemistry of the starting cyclopropylenone 1d and of the conjugate addition product 3 (Scheme 5). Scheme 5

Figure 1.

2d-a and 2d-b. If back-side nucleophilic attack of the cuprate on the cyclopropyl carbon occurred (path b) to give ring-opened intermediate III-a, then stereochemical information would be preserved and a single diastereomer of 2d-a would be formed with inversion of stereochemistry at the cyclopropyl carbon. If conjugate addition of copper to the enone to form intermediate IV occurred (path c), followed by ring-opening of the cyclopropylmethyl copper intermediate, then stereochemical information would again be preserved and a single diastereomer of 2d-b would be formed but with retention of stereochemistry at the cyclopropyl carbon. We found that the addition of LiCu(CH3)2 to 1d led to stereospecific formation of a single diastereomer of 2d, along with a similar amount of conjugate addition product 3d.9 The NMR spectrum of 2d showed the resonance for a single diastereotopic H of the ethyl group. The stereospecificity of the ring-opening of the cyclopropane ring ruled out path a since rapid rotation in II would have led to equal amounts of both 2d-a and 2d-b. However, since we were not able to assign the chemical shifts of diastereotopic H’s of the ethyl group in 1979, we were unable to distinguish between path b and path c. Nevertheless, we did not hesitate to speculate that the reaction occurred by back-side nucleophilic ring-opening of the cyclopropane (path b). In 1984, Bertz suggested that nucleophilic ring-opening of the cyclopropane was unlikely since two activating groups on the cyclopropanes are usually required for nucleophilic opening by cuprates (Scheme 4).10 In addition, Bertz found that

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RESULTS The 1H NMR chemical shifts of the diastereotopic hydrogens of the ethyl group of 2 in CDCl3 appear at δ 1.33 and 1.48, a difference of 0.15 ppm. In 2d formed in the reaction of 1d with LiCu(CH3)2, only one of the diastereotopic hydrogens was seen, at δ 1.33, establishing the stereospecificity of the reaction. To determine the stereochemistry of the reaction, assignment of the relative chemical shifts of the diastereotopic hydrogens is required. Chemical shifts can be predicted from molecular structure by first-principles calculations. Because the diastereotopic ethyl hydrogens occupy different chemical environments, a different chemical shift would be predicted for each hydrogen, and matching these values to the experimental shifts should be a straightforward task. Unfortunately, several factors limit the accuracy of calculated chemical shifts. First, the shifts are derived from molecular models whose properties can only approximate those of the real molecules they represent. Surveys of calculated chemical shift show that excellent agreement is often found between calculation and experiment, but errors of ca. 0.2 ppm are routine.11 Fortunately, the present study requires only the accurate prediction of the relative shifts of the diastereotopic ethyl hydrogens. The fact that these hydrogens are bonded to a common carbon in the same molecule makes the cancellation of model-derived errors plausible and likely. Another, potentially larger problem is the fact that 2 undoubtedly exists as a rapidly equilibrating mixture of conformers. The experimental shifts for 2 are Boltzmannweighted averages of the shifts associated with the various conformers present in the experimental sample, so accurate shift prediction requires (1) modeling errors to be relatively independent of conformation, (2) the accurate identification of experimentally relevant conformers and an accurate assessment of their energies and populations, and (3) a reliable estimate of solvent (CDCl3) effects on conformer structure and energy. A molecular mechanics search of the conformational space of 2 using the automated methods provided in Spartan’1012 produced six structures. These structures were reoptimized in chloroform by minimizing the sum of the EDF2/6-31G* energy13 and the SM8 solvation energy for chloroform.14 The six optimized structures adopt half-chair conformations with a planar alkene group and a quasi-staggered arrangement of the

Scheme 4

conjugate addition of LiCu(CH3)2 to cyclohexenone was >1000 times faster than ring-opening of diethyl 1,1-cyclopropanedicarboxylate. Bertz suggested a mechanism involving Cu addition syn to the cyclopropane, followed by cyclopropylmethyl−homoallyl rearrangement with retention of stereochemistry, and then reductive elimination from a Cu(III) intermediate (path c, Figure 1). Here we report the comparison of computed and observed 1 H NMR chemical shifts of the diastereotopic H’s of the ethyl 7850

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that 2d-b is the product obtained from the cuprate reaction with 1d and that CC bond-breaking and bond-making occur with retention of configuration at the cyclopropyl carbon (Scheme 5). Because the calculations suggest that several conformations have appreciable equilibrium populations and the relative shifts of HA and HB may be reversed in certain conformations, we reoptimized the geometries of all six conformers and recalculated their populations using several other methods (EDF2/6-31G**,13a B3LYP/6-31G*,13b MP2/6-31G*,13c T1 thermochemical procedure;15 results are reported in the Supporting Information). Generally speaking, the choice of method was unimportant. All of the methods consistently predicted that conformers 2-Eq-1 and 2-Ax-1 would dominate an equilibrium mixture with a combined population that ranged between 56% and 68%. The calculated population of conformer 2-Ax-2, the only significant conformer in which the relative shifts of HA and HB were reversed, was always considerably smaller (20−26%). Furthermore, an EDF2/6-31G* calculation of the solvation energies of all six conformers revealed that these energies, which ranged between 31.4 and 32.1 kJ/mol, were not sensitive to conformation. The possibility that other methods, or other geometries, might predict radically different chemical shifts was also examined. Chemical shifts were calculated using geometries and wave functions obtained with several other methods (EDF2/6-31G**, B3LYP/6-31G*, EDF2/6-31G*//B3LYP/631G*, EDF2/6-31G*//EDF2/6-31G*+SM8). While the predicted shifts of the diastereotopic protons varied slightly from one set of models to the next, the relative shifts did not (see Supporting Information). The chemical shift of HB was consistently predicted to be 0.16−0.20 ppm higher in frequency than that of HA. Another way to detect systematic errors in the predicted chemical shifts of the diastereotopic ethyl hydrogens is to compare the entire calculated spectrum with the experimental spectrum of 2. Boltzmann-averaged shifts were calculated for every alkyl proton in compound 2 using the models listed in Table 1. The predicted values are compared with the experimental shifts in Table 2. (A single shift had been reported9 for each methylene group (C-2, C-6), so the calculated shifts in Table 2 are reported twice, an average shift for the two methylene protons and individual shifts for each proton in parentheses.) The maximum error in the calculated shifts is 0.121 ppm, while the average absolute error is only 0.07 ppm. In addition, visual inspection of the experimental spectrum (Figure 2, in ref 9; reproduced in Supporting Information of this paper) shows that the C-2 methylene signal is virtually a single broad peak, while the C-6 methylene is an AB quartet with signals located at δ 2.21 and 2.49. By comparison, the calculated shifts for the C-2 protons are nearly identical, δ 2.72 and 2.74, and the calculated shifts for the C-6 protons are δ 2.24 and 2.44, a remarkably good match with the experimental signals. These results suggest that the calculated shifts for 2, all of which are conformationweighted averages, provide a good account of the experimental

remaining ring carbons (Figure 2). Three conformers contain a pseudoequatorial ethyl group (2-Eq), while the other three

Figure 2. Six conformers of 2.

contain a pseudoaxial ethyl group (2-Ax). The three conformers in each set differ with respect to the CH3CCH2CH3 dihedral angle (rotation about the ring carbon−Et bond). The Boltzmann populations of the pseudoaxial conformers collectively exceed those of the pseudoequatorial conformers by a substantial margin, 62% versus 38%, but several conformers of both types are predicted to have substantial populations at 298 K (Table 1). Table 1. Calculated Structural Parameters, Relative Energies, and Boltzmann Populations of Conformers of 2 conformer

dihedral anglea (deg) CH3CCH2CH3

Erel (kJ mol−1) in CHCl3a

% in CHCl3 (298 K)a

179 66 −40 177 58 −69

0.09 2.46 9.80 0 0.17 3.41

27.2 10.5 0.5 28.3 26.4 7.1

2-Eq-1 2-Eq-2 2-Eq-3 2-Ax-1 2-Ax-2 2-Ax-3 Boltzmann average a

EDF2/6-31G* + SM8 geometry and energy. geometry and chemical shift.

b

δ HAb (ppm)

δ HBb (ppm)

1.243 1.738 1.874 1.412 1.624 1.663 1.476

1.822 1.581 1.356 1.732 1.432 1.387 1.635

EDF2/6-31G*

The proton chemical shifts of each conformer were calculated in Spartan’10 using structures obtained from an EDF2/6-31G* optimization without solvation. The calculated shifts of the diastereotopic ethyl protons HA and HB are listed in Table 1. Although the absolute shifts, and even the relative shifts, of these protons are highly sensitive to conformation, the chemical shift of HB is predicted to be at higher frequency than that of HA for the two most important conformers (2-Eq-1 and 2-Ax-1), and this trend is maintained when Boltzmannaveraged shifts are calculated. The averaged shifts for HA and HB are calculated to be δ 1.476 and 1.635, respectively. To our surprise, the calculated difference in shifts, 0.16 ppm, almost exactly duplicates the experimental value. These data indicate

Table 2. Comparison of Experimental and Computed Chemical Shifts of 2 position

C(2)H2

C(6)H2

Me@C(4)

Me@C5

Me@Et

experiment (ppm) computed (ppm)

2.83 2.730 (2.72, 2.74)

2.35 (2.21, 2.49) 2.338 (2.24, 2.44)

1.72 1.732

1.03 1.093

0.81 0.931

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addition product.9 These models had suggested that each stereoisomer of 3 might adopt two boat-like conformations in which the methyl group and the methine proton attached to C5 could occupy either a pseudoequatorial or pseudoaxial position. The observation of two small coupling constants (J = 5.8, J′ = 2.6 Hz) from the C-5 methine proton at δ 2.17 to the C-4 methylene protons of the conjugate addition product 3 established the equatorial orientation of the C-5 methine proton. This observation ruled out 3-cis as the addition product because the 3-cis conformer with an axial C-5 methyl group has a destabilizing eclipsing interaction between the methyl group and the cyclopropane ring. Inspection of the two conformers of 3-trans did not lead to a clear conformational preference, so an axial C-5 methyl group was deemed a viable option; this was supported by the observed couplings between the C-4 and C-5 protons. Our conclusion that the normal conjugate addition product 3-trans results from attack of cuprate from the side opposite the cyclopropane ring is similar to Marshall’s finding for addition of LiCu(CH3)2 to 6 (Scheme 8).

data. It seems very unlikely that the prediction that HB appears at higher δ than HA is incorrect. Computation of NMR Spectra Confirms Structural Assignment of Deuterated Cyclopropylenone 1d. Deuterium was introduced into 1d as shown in Scheme 6. Scheme 6

This relied on obtaining pure exo bromide 4-exo and the replacement of bromine by deuterium with retention of stereochemistry. Pure 4-exo was obtained from a 4:1 mixture of 4-exo:4-endo by selective destruction of 4-endo by treatment with Ag+. The disrotatory solvolytic ring-opening of cyclopropyl halides is known to take place with “back-side orbital assistance” and concerted formation of allylic cations.16 Solvolysis of 4-endo produces a stable allylic cation, whereas solvolysis of 4-exo would produce a highly strained cation (Scheme 7).

Scheme 8

Scheme 7 A reanalysis of the conformational preferences of 3-trans and 3-cis was carried out using the same procedure that was employed for ring-opened product 2 (see above). Two minimum-energy conformers were obtained for each diasteromer of 3 (Table 3). Three conformers, 3-trans-1, 3-transTable 3. Calculated Relative Energies, Boltzmann Populations, and Structural Parameters of Conformers of 3trans and 3-cis The calculated chemical shifts for the cyclopropyl protons in 4-exo, 4-endo, 5, and 1 provide strong support for the original stereochemical assignment of 1d (Figure 3). EDF2/6-31G* optimization in each case yielded a single minimum-energy structure containing a nearly planar six-membered ring. All of the calculated shifts are larger than the experimental shifts (average absolute error = 0.14 ppm), but the calculated shifts reliably duplicate the assignments that had been made earlier; that is, the shift is larger for the proton in the endo positions of 4-exo (relative to 4-endo) and 5, but it is smaller in 1. Stereochemistry of Me Conjugate Addition Product 3-trans. The stereochemistry of Me conjugate addition product 3-trans had been assigned on the basis of its NMR spectrum and examination of Dreiding models of the possible methyl addition products.9 The NMR spectrum and Dreiding model of 3-cis, which was prepared by an independent pathway, had shown that 3-cis was distinct from the conjugate

conformer

Erel (kJ mol−1) in CHCl3a

% in CHCl3 (298 K)a

dihedral angle (deg)b H−C5− C4−Hendo

dihedral angle (deg)b H−C5− C4−Hexo

3-trans-1 3-trans-2 3-cis-1 3-cis-2

0 3.41 0 4.49

79.8 20.2 85.9 14.1

48 62 168 81

69 180 52 38

a

EDF2/6-31G* + SM8 geometry and energy. geometry.

b

EDF2/6-31G*

2, and 3-cis-1, adopt boat-like structures, but 3-cis-2 adopts a half-chair structure. Nevertheless, the two groups bonded to C5 adopt different orientations with respect to the two C-4 methylene protons in all four conformers. The dihedral angles in 3-trans-1 between the C-5 methine proton and the C-4 methylene protons are 48° and 69°, consistent with the two relatively small coupling constants (J = 5.8 Hz, J′ = 2.6 Hz)

Figure 3. 7852

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Figure 4. based on MMFF94 force field energies.17 Equilibrium geometries were calculated for the resulting conformers using the EDF2/6-31G* density functional method.13 These geometries were used for subsequent NMR calculations and geometry optimizations based on other quantum mechanical methods. The effects of chloroform solvation on molecular geometry and energy were simulated by combining EDF2/6-31G* total energies with solvation energies from the SM8 implicit solvation model.14 NMR chemical shifts were calculated using an efficient algorithm based on gauge-including atomic orbitals (GIAO)18 and are reported here without additional corrections or scaling.

observed for the conjugate addition product. The corresponding dihedral angles in 3-cis-1 of 52° and 168° are consistent with the combination of small and large coupling constants (J = 5.5 Hz, J′ = 11.2 Hz) observed for the C-5 epimer of the addition product. Unfortunately, while the dihedral angles obtained for the lowenergy conformers support the original interpretation of observed coupling constants, both conformers need to be considered in predicting coupling constants. For example, calculating a Boltzmann-averaged coupling constant between H-5 and H-4exo in 3-trans would require averaging a very small coupling constant for 3-trans-1 with a very large one for 3trans-2. Because the large coupling constant is associated with the high-energy conformer, a small change in conformer energy will lead to a large change in the average value. The calculated proton chemical shifts of 3-trans and 3-cis provide an independent check on our structural assignments. The shifts of all of the protons in 3-trans and 3-cis were calculated as Boltzmann-weighted averages over the two conformations using the same methodology employed for the ring-opened product, 2 (see Supporting Information). Excellent agreement was found between the calculated shifts of 3-trans and the observed shifts of the conjugate addition product (average absolute error = 0.065 ppm, linear correlation coefficient r2 = 0.998) and also between the calculated shifts of 3-cis and the C-5 epimer of the addition product (average absolute error = 0.081 ppm, r2 = 0.991). The alternative assignments, in which the spectrum of 3-trans was compared to that of the C-5 epimer (average absolute error = 0.160, r2 = 0.954) and the spectrum of 3-cis was compared to that of the addition product (average absolute error = 0.130, r2 = 0.933), gave markedly inferior results. Moreover, unlike the situation with averaged coupling constants, the chemical shift comparisons were not affected by assuming higher energies for conformers 3-trans-2 and 3-cis-2. Therefore, the identification of the conjugate addition product as 3-trans appears secure.

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DISCUSSION Computations of the relative NMR chemical shifts of the diastereotopic hydrogens of the ethyl group of 2 are in excellent agreement with the experimentally observed chemical shift differences. The computations predict that the chemical shift of HB will appear 0.16 ppm to higher δ than that of HA, while the experimental spectrum of 2 has a Δδ of 0.15 ppm. The computed difference in chemical shift was robust and did not vary significantly with changes in basis set or method of computing energy or chemical shift. Further confidence in the computations was gained by the close agreement between the calculated and observed chemical shifts of other alkyl proton resonances of 2. As a consequence, we have assigned the stereochemistry of the product of LiCu(CH3)2 addition to 1d as 2d-b, the product of ring-opening with retention of stereochemistry shown as path c in Figure 1 and Scheme 5. This stereochemistry is consistent with conjugate addition of LiCu(CH3)2 to the enone from the face syn to the cyclopropane ring, followed by a cyclopropylmethylcopper to homoallylcopper rearrangement that results in front-side ring-opening of the cyclopropane ring to give intermediate III-b and then by reductive elimination from this Cu(III) intermediate to eventually give 2d-b following aqueous workup. This is essentially the mechanism suggested by Bertz10 in 1984. While coordinatively saturated cyclopropylmethyl transition metal complexes such as Cp(CO)2FeCH2(c-CHCH2CH2) are known,19 cyclopropylmethyl to homoallyl metal rearrangements of coordinatively unsaturated intermediates have often been invoked.20 Ring-opening has been seen in reactions of

COMPUTATIONAL METHODS

All computations were performed using standard algorithms and options available in Spartan’10.12 The conformational space of each molecule was searched with respect to internal rotation of substituents and twisting of six-membered rings using automatic search procedures 7853

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organocuprates with cyclopropylmethyl tosylates21 and halides;22 these reactions probably proceed via ring-opening of a cyclopropylmethylcopper(III) intermediate at a rate faster than reductive elimination. The other major product observed in the reaction of cyclopropylenone 1d with LiCu(CH3)2 is the conjugate addition product 3d-trans (Scheme 5). The stereochemistry of 3d-trans was assigned as arising from conjugate addition to the enone from the face anti to the cyclopropane ring9 and was confirmed by the computation of the NMR spectra reported here. The question then arises: why isn't the product of methyl addition syn to the cyclopropane (3d-cis) also seen? We suggest that syn addition of LiCu(CH3)2 to the cyclopropylenone 1d does occur, but the Cu(III) intermediate IVsyn undergoes ring-opening ultimately leading to 2d-b much faster than it reductively eliminates to give 3d-cis. Similar amounts of 2d-b arising from syn addition and of 3dcis arising from anti addition are observed. This was not expected since examination of molecular models of 1d showed that the face anti to the cyclopropane was far less congested. Computations of π-complexes of Cu(CH3)2− to 1 confirmed that complex V-anti with Cu on the face anti to the cyclopropane was 25 kJ mol−1 more stable than complex Vsyn. Since computations of charged species are problematic, the neutral π-complexes of Ni(PH3)2 to 1 were also computed; a similar result was obtained with the complex anti to the cyclopropanes being 12.5 kJ mol−1 more stable than the syn complex. A plausible explanation of the formation of similar amounts of 2d-b and 3d-trans is shown in Figure 4. We suggest that formation of π-complexes V-anti and V-syn is fast and reversible and strongly favors V-anti. The small amount of Vsyn present undergoes ring-opening (possibly via reversible formation of Cu(III) intermediate IV-syn) much faster than it undergoes reductive elimination to give the unseen 3d-cis. In this mechanism, the observation of similar amounts of 2d-b and 3d-trans requires that the rate of ring-opening of the small amount of V-syn exceed the rate of reductive elimination from the predominant species V-anti by an amount similar to the Vanti:V-syn ratio at equilibrium.

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(2) For reviews of cuprate structure, see: (a) Gschwind, R. M. Chem. Rev. 2008, 108, 3029. (b) van Koten, G.; Jastrzebski, J. T. B. H. In The Chemistry of Organocopper Compounds, Part 1; Rappoport, Z.; Marek, I., Eds.; Wiley: Chichester, 2009; pp 23−143. (c) Gärtner, T.; Gschwind, R. M. In The Chemistry of Organocopper Compounds, Part 1; Rappoport, Z.; Marek, I., Eds.; Wiley: Chichester, 2009; pp 163−215. (3) For reviews of mechanism, see: (a) Yoshikai, N.; Nakamura, E. Chem. Rev. 2012, 112, 2339. (b) Woodward, S. Chem. Soc. Rev. 2000, 29, 393. (4) For π-complex formation, see: (a) Henze, W.; Gärtner, T.; Gschwind, R. M. J. Am. Chem. Soc. 2008, 130, 13718. (b) Krause, N.; Gerold, A. Angew. Chem., Int. Ed. 1997, 36, 186. (c) Bertz, S. H.; Carlin, C. M.; Deadwyler, D. A.; Murphy, M. D.; Ogle, C. A.; Seagle, P. H. J. Am. Chem. Soc. 2002, 124, 13650. (d) Bertz, S. H.; Smith, R. A. J. J. Am. Chem. Soc. 1989, 111, 8276. (e) Hallnemo, G.; Olsson, T.; Ullenius, C. J. Organomet. Chem. 1985, 282, 133. (f) Krause, N.; Wagner, R.; Gerold, A. J. Am. Chem. Soc. 1994, 116, 381. (5) For Cu(III) complexes, see: (a) Bertz, S. H.; Cope, S.; Murphy, M.; Ogle, C. A.; Taylor, B. J. J. Am. Chem. Soc. 2007, 129, 7208. (b) Bertz, S. H.; Cope, S.; Dorton, D.; Murphy, M.; Ogle, C. A. Angew. Chem., Int. Ed. 2007, 46, 7082. (c) Bartholomew, E. R.; Bertz, S. H.; Cope, S.; Dorton, D. C.; Murphy, M.; Ogle, C. A. Chem. Commun. 2008, 1176. (d) Gärtner, T.; Henze, W.; Gschwind, R. M. J. Am. Chem. Soc. 2007, 129, 11362. (e) King, A. E.; Huffman, L. M.; Casitas, A.; Costas, M.; Ribas, X.; Stahl, S. S. J. Am. Chem. Soc. 2010, 132, 12068. (6) Frantz, D. E.; Singleton, D. A.; Snyder, J. P. J. Am. Chem. Soc. 1997, 119, 3383. (7) (a) Krauss, S. R.; Smith, S. G. J. Am. Chem. Soc. 1981, 103, 141. (b) Canisius, J.; Gerold, A.; Krause, N. Angew. Chem., Int. Ed. 1999, 38, 1644. (8) (a) House, H. O. Acc. Chem. Res. 1976, 9, 59. (b) House, H. O.; Umen, M. J. J. Am. Chem. Soc. 1972, 94, 5495. (c) House, H. O.; Umen, M. J. J. Org. Chem. 1973, 38, 3893. (d) Marshall, J. A.; Ruden, R. A. J. Org. Chem. 1972, 37, 659. (e) House, H. O.; Snoble, K. A. J. J. Org. Chem. 1976, 41, 3076. (9) Casey, C. P.; Cesa, M. C. J. Am. Chem. Soc. 1979, 101, 4236. (10) Bertz, S. H.; Dabbagh, G.; Cook, J. M.; Honkan, V. J. Org. Chem. 1984, 49, 1739. (11) Lodewyk, M. W.; Siebert, M. R.; Tantillo, D. J. Chem. Rev. 2012, 112, 1839. (12) Spartan’10; Wavefunction, Inc.: Irvine, CA, 2010. (13) (a) Lin, C. Y.; George, M. W.; Gill, P. M. W. Aust. J. Chem. 2004, 57, 365 (EDF2). (b) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623 (B3LYP). (c) HeadGordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503 (MP2). (14) Marenich, A. V.; Olson, R. M.; Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Chem. Theory Comput. 2007, 3, 2011. (15) Ohlinger, W. S.; Klunzinger, P. E.; Deppmeier, B. J.; Hehre, W. J. J. Phys. Chem. A 2009, 113, 2165 (T1). (16) (a) Reese, C. B.; Shaw, A. J. Am. Chem. Soc. 1970, 92, 2566. (b) Schleyer, P.; von, R.; Su, T. M.; Saunders, M.; Rosenfeld, J. C. J. Am. Chem. Soc. 1969, 91, 5174. (c) Woodward, R. B.; Hoffmann., R. J. Am. Chem. Soc. 1965, 87, 395. (d) DePuy, C. H. Acc. Chem. Res. 1968, 1, 33. (17) Halgren, T. A. J. Comput. Chem. 1996, 17, 490. (18) Kussmann, J.; Ochsenfeld, C. J. Chem. Phys. 2007, 127, 054103. (19) San Filippo, J.; Silbermann, J.; Fagan, P. J. J. Am. Chem. Soc. 1978, 100, 4834. (20) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem. Rev. 2007, 107, 3117. (21) Posner, G. H.; Ting, J.-S.; Lentz, C. M. Tetrahedron 1976, 32, 2281. (22) (a) Hrubiec, R. T.; Smith, M. B. J. Org. Chem. 1984, 49, 385. (b) Kwon, T. W.; Smith, M. B. J. Org. Chem. 1989, 54, 4250.

ASSOCIATED CONTENT

S Supporting Information *

Tables giving energies, geometries, and chemical shifts of representative stationary points. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Professor Warren J. Hehre of the University of California-Irvine for helpful discussions. REFERENCES

(1) For use of organocuprates in organic synthesis, see: (a) Posner, G. H. Org. React. 1972, 19, 1. (b) Posner, G. H. Org. React. 1975, 22, 253. (c) Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135. (d) Modern Organocopper Chemistry; Krause, N., Ed.; Wiley-VCH: Weinheim, 2002. (e) The Chemistry of Organocopper Compounds, Parts 1 and 2; Rappoport, Z.; Marek, I., Eds.; Wiley: Chichester, 2009. 7854

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