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[5 + 1 + 2 + 1] vs [5 + 1 + 1 + 2] Rhodium-Catalyzed Cycloaddition Reactions of Vinylcyclopropanes with Terminal Alkynes and Carbon Monoxide: Density Functional Theory Investigations of Convergent Mechanistic Pathways and Reaction Regioselectivity Ifenna I. Mbaezue and Kai E. O. Ylijoki* Department of Chemistry, Saint Mary’s University, Halifax, Nova Scotia, Canada B3H 3C3 S Supporting Information *

ABSTRACT: The mechanism of the Rh-catalyzed [5 + 1 + 2 + 1] cycloaddition of VCPs, terminal alkynes, and CO to yield hydroxydihydroindanones has been investigated by ωB97XD/ SDD-6-31G* DFT calculations. The study has revealed the existence of two convergent cycloaddition pathways, [5 + 1 + 2 + 1] and [5 + 1 + 1 + 2], differing by only 2 kcal/mol. Both metalcatalyzed and off-metal electrocyclization/aromatization mechanisms for the final bicyclization reaction were investigated, with the traditional acid-catalyzed pathway dominating. The catalytic cycle is limited by the rate of alkyne insertion, which is consistent with mechanistic studies of the related [5 + 2] cycloaddition reaction. Monosubstituted alkyne insertion regioselectivity is kinetically controlled, with insertion via the unsubstituted alkyne terminus predominating. The alkyne insertion process does not appear to be sensitive to steric effects, with both the propyne and phenylacetylene model alkynes showing similar selectivities (TS energy differences of ca. 5 kcal/mol in both cases for the two regioisomeric transition states). C−C bond activation of disubstituted vinylcyclopropanes was investigated, suggesting that activation of the least substituted C−C bond is preferred unless an electron-withdrawing group is present on the VCP, in which case the most substituted bond will be cleaved.

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INTRODUCTION Cycloaddition chemistry has been a vital tool in organic synthesis since the development of the Diels−Alder reaction.1 Yet, despite their broad application, classic pericyclic cycloadditions are somewhat limited in scope. Since metal-mediated cycloaddition reactions are not subject to the orbital symmetry restrictions that limit pericyclic processes, they have opened several new formal cycloaddition manifolds.2 Rhodium3 has been widely applied as a catalyst for several cycloaddition processes, including [2 + 1],4 [2 + 2],5 [3 + 2],6 [4 + 1],7 [4 + 2],8 [5 + 1],9 [6 + 2],10 [2 + 2 + 1],11 [2 + 2 + 2],12 [3 + 2 + 2],13 [4 + 2 + 2],14 and [5 + 2 + 1]15 manifolds (Scheme 1). A widely studied Rh-catalyzed cycloaddition process is the [5 + 2] reaction between tethered vinylcyclopropanes (VCPs) and alkynes, yielding bicyclo[5.3.0]-fused ring systems (eq 1).16 This process has since been expanded to intra- and intermolecular reactions of VCPs with alkynes,17 alkenes,18 and allenes.19 Later work has shown that ruthenium,20 iron,21 nickel,22 and iridium23 are also effective catalysts for intramolecular [5 + 2] cycloaddition. In an extension to the [5 + 2] cycloaddition reaction dubbed the [5 + 1 + 2 + 1] cycloaddition, it was shown that a CO atmosphere24 results in hydroxydihydroindanones through CO insertion, generating five new C−C bonds in a single step (eq 2).25 Dihydroindanones are classically derived from reduction of indenones, © XXXX American Chemical Society

Scheme 1. Examples of Products Obtained from Rh-Catalyzed Cycloaddition Manifolds

which can be obtained through various multistep methods including cationic rearrangements (eq 3),26 Cope rearrangements (eq 4),27 chelotropic extrusion of CO (eq 5),28 and Friedel−Crafts acylation (eq 6).29 Indanone compounds have Received: April 27, 2017

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DOI: 10.1021/acs.organomet.7b00323 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

6-31G* on all other atoms), with solvent corrections (toluene, ε = 2.3741) applied via the polarizable continuum model.36 The final DFT level optimizations were performed with the Gaussian09 (Revision C.01) software suite.37 Vibrational analyses were conducted to confirm the nature of all stationary points and to produce thermal corrections (enthalpy and entropy) for 298 K, 1 bar, gas phase. To account for entropy overestimation in gas-phase calculations, the solution Gibbs energy was corrected by applying free volume theory, such that ΔG°sol = ΔG°gas − X, where X = 2.6, 5.0, and 7.3 kcal/mol for bi-, tri-, and tetramolecular processes, respectively.38 AIM analyses were performed with the AIMAll software suite39 using .wfx files generated from the Gaussian optimized structures.

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RESULTS AND DISCUSSION Reaction Mechanism. The [5 + 1 + 2 + 1] cycloaddition reaction consists of three formal phases: cycloaddition, cyclization, and aromatization. Wender and co-workers proposed a mechanism whereby VCP, alkyne, and 2 equiv of CO couple to produce an alkoxycyclonona-2,6-diene-1,4-dione (eq 2).25 Subsequent tautomerization, 6π-electrocyclization, and elimination of alcohol yields the final hydroxydihydroindanone product. This proposal formed the initial framework for our DFT investigation, where VCP complex INT 1 is taken as the initial catalyst species (Figures 1−3).40 The catalytic cycle begins with C−C bond activation via a very low 1.7 kcal/mol barrier, yielding the trigonal-pyramidal INT 2, which then relaxes to the alternative trigonal-pyramidal complex INT 3. These initial steps are in accordance with the previously determined lowest energy path for C−C bond activation in [5 + 2] cycloaddition reactions.40 In the absence of CO, alkyne coordinates and inserts at INT 3, resulting in [5 + 2] cycloaddition reactions.40 CO coordination at INT 3 will therefore suppress the [5 + 2] pathway, diverting the reaction to the [5 + 1 + 2 + 1] mechanism.41 The CO complex INT 4 has multiple isomeric forms related through Berry pseudorotation; through calculation of all possible CO-insertion TSs, it was determined that the most favorable pathway proceeds via CO insertion into the axial Rh−C(sp3) bond via TS 4-5. The resulting square pyramidal complex INT 5 is unsaturated, allowing coordination of an additional ligand. From INT 5 the potential energy surface diverges into two alternative pathways for cycloaddition: in the first, alkyne coordinates (INT 6a) to proceed via the [5 + 1 + 2 + 1] mechanism, or CO coordinates (INT 6b) to proceed via an alternate [5 + 1 + 1 + 2] process. From INT 6a,42 the [5 + 1 + 2 + 1] mechanism invokes sequential insertion of alkyne and a final equivalent of CO. Alkyne insertion via TS 6a-7a proceeds through a high-energy TS, connecting to the square-pyramidal alkyne insertion complex INT 7a. To complete the cycloaddition sequence, a second equivalent of CO coordinates in a barrierless process to yield octahedral complex INT 8a, followed by insertion at the allyl terminus through high-energy TS 8a-11 to yield INT 11. From INT 11, reductive elimination via TS 11-12 yields the key Rh cyclononadiendione complex INT 12. The alternative [5 + 1 + 1 + 2] cycloaddition begins from octahedral INT 6b, wherein CO has coordinated in place of alkyne. CO insertion proceeds via transition state TS 6b-7b, yielding INT 7b. To preserve the experimentally observed insertion order, the second CO insertion must occur at the allyl terminus of the carbon chain. The unsaturated INT 7b can Berry pseudorotate to INT 8b, freeing a coordination site for alkyne coordination cis to both termini of the carbon chain (INT 9b). While alkyne insertion into either carbon terminus is possible, only insertion into the axial terminus via TS 9b-10b

found utility as cyclooxegenase inhibitors, peroxisome proliferator-activator receptor γ (PPARγ) agonists, and topoisomerase inhibitors.30 Despite the utility and incredible mechanistic complexity, the [5 + 1 + 2 + 1] cycloaddition has not received the same level of attention afforded the [5 + 2] reaction. Since a mechanistic understanding of multicomponent reactions is critical for the design and control of new synthetic manifolds, here we address this shortcoming though a computational investigation of the [5 + 1 + 2 + 1] cycloaddition process. Our work has revealed two convergent cycloaddition pathways. We have also investigated metal-catalyzed cyclization and aromatization reactions, alkyne insertion regiocontrol, and selectivity of C−C bond activation in disubstituted VCPs.

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COMPUTATIONAL METHODS

Stationary point geometries were first obtained at the PM3 semiempirical level of theory31 as implemented in the Spartan’16 computational software package.32 These geometries were used as initial points for density functional theory (DFT) geometry optimization at the B3LYP33/6-31G* level. Improved geometries and energies were then obtained from these coordinates by optimization at the ωB97XD34/ SDD35-6-31G* level (Stuttgart/Dresden ECP on rhodium, with B


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Figure 1. ωB97XD/SDD-6-31G* potential energy surface (Gibbs energies in kcal/mol).

results in the experimentally observed regiochemistry (vide infra). The resulting INT 10b can relax to INT 11, converging with the [5 + 1 + 2 + 1] energy surface. A final reductive elimination via TS 11-12 concludes the cycloaddition process. When the two cycloaddition pathways are compared, the turnover-limiting step for both manifolds is alkyne insertion, which is in agreement with the previously reported [5 + 2] cycloaddition reaction.40 For the [5 + 1 + 1 + 2] manifold, the highest energy barrier is INT 6b → TS 9b-10b (38.2 kcal/mol). Excluding excess CO, the highest barrier for the [5 + 1 + 2 + 1] manifold is CO insertion via INT 8a → TS 8a-11 (34.0 kcal/mol), which is lower than that for the [5 + 1 + 1 + 2] pathway. However, given that an excess of CO is present in the reaction system, it is likely that the CO complex INT 6b acts as a catalyst resting state for the [5 + 1 + 2 + 1] reaction path. Therefore, the turnover-limiting step for the [5 + 1 + 2 + 1] pathway is best described as INT 6b → TS 6a-7a (36.2 kcal/mol), which differs by only 2 kcal/mol in comparison to the alternative [5 + 1 + 1 + 2] mechanism. The small difference in energy barriers suggests that the two pathways can be competitive. To complete the reaction process from INT 12, VCP-assisted dissociation of the cyclononadienone liberates the organic product (INT 13). Subsequent tautomerization and disrotatory 6π-electrocyclization (TS 14-15) leads to bicyclic INT 15

(Figures 4 and 5). From INT 15, multiple aromatization pathways can be proposed. We first located TS 15-17, which corresponds to a concerted elimination of MeOH. The energy barrier for this process (INT 15 → TS 15-17, 43.0 kcal/mol) is higher than that expected for a reaction conducted at 60 °C. Alternative lower energy pathways require the invocation of acid catalysis, either adventitious acid or acid introduced during workup, which is reasonable given that the proposed keto−enol tautomerization (INT 13 → INT 14) will also require such mediation.43 Two acid-catalyzed mechanisms can be proposed, both of which present reasonable alternatives to the concerted elimination mechanism. The lowest energy path invokes protonation at the methoxy group to generate INT 16. Attempts to locate a subsequent tautomeric intermediate were not successful, with all compounds spontaneously eliminating MeOH to yield the aromatized compound INT 17. Final deprotonation results in the experimentally obtained dihydroindanone product. Regioselectivity of Alkyne Insertion. Nonsymmetric alkynes introduce an aspect of regioselectivity during alkyne insertion at TS 6a-7a and TS 9b-10b. Experimentally, insertion occurs to place the alkyne substituent at the 7-position of the dihydroindanone framework (eq 2). To examine insertion regioselectivity in our mechanistic proposal, propyne and phenylacetylene were chosen as model nonsymmetric alkynes. C


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Figure 2. Intermediates from the potential energy surface in Figure 1. Key bond lengths are indicated in Å.

The barrier for propyne insertion was modeled for both the [5 + 1 + 2 + 1] and [5 + 1 + 1 + 2] pathways, with the relative energies plotted in Figure 6. Propyne and phenylacetylene can coordinate in two different orientations, corresponding to INT 6c/INT 6c′ and INT 9c/INT 9c′ for propyne and INT 6c Ph/INT 6c′ Ph and INT 9c Ph/INT 9c′ Ph for phenylacetylene. The two regioisomers for alkyne coordination in both pathways are very close in energy, with only INT 9c Ph and INT 9c′ Ph differing by more than 0.5 kcal/mol. In the [5 + 1 + 2 + 1] pathway, to place the substituent at the 7-position the alkyne must insert from the unsubstituted end. For both the propyne and phenylacetylene models, insertion via the unsubstituted end is the most energetically favorable pathway (31.0 vs 26.2 kcal/mol in propyne and 30.5 vs 25.4 kcal/mol in phenylacetylene), for an average difference of 5.0 kcal/mol, agreeing with the experimental observations (Figure 7). In the [5 + 1 + 1 + 2] pathway, insertion of acetylene at either the axial or equatorial positions from

INT 9b would lead to the same cycloaddition product upon reductive elimination. However, given that insertion from the unsubstituted end of a monosubstituted alkyne is lower in energy, only insertion at the axial position will lead to the experimentally observed regiochemistry. The experimentally observed regioselectivity is again predicted for the [5 + 1 + 1 + 2] pathway, with the energy barriers being 21.3 vs 16.5 kcal/mol for propyne and 20.0 vs 17.8 kcal/mol for phenylacetylene, for an average difference of 3.5 kcal/mol. In all cases examined, the product of alkyne insertion from the unsubstituted end results in a higher energy product than that resulting from insertion at the substituted end, likely as a result of the formation of a secondary Rh−C bond. These observations strongly suggest that the regiochemistry is a result of kinetic rather than thermodynamic control. When the activation energies for the kinetically preferred monosubstituted alkyne insertion steps are compared with those determined for the insertion of acetylene, the barriers are consistent (27.0, 26.2, and 25.4 kcal/mol for the D


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Figure 3. Transition state structures from Figure 1. Key distances are indicated in Å.

Figure 4. Possible concerted and acid-catalyzed cyclization/aromatization pathways (ωB97XD/6-31G*, Gibbs energies in kcal/mol).

[5 + 1 + 2 + 1] pathway and 17.5, 16.5, and 17.8 kcal/mol for the [5 + 1 + 1 + 2] mechanism), suggesting that the reaction rate is relatively insensitive to the nature of the alkyne. Selectivity of C−C Activation in 1,2-Disubstituted VCPs. A second aspect of regioselectivity that has been well studied in intramolecular metal-catalyzed [5 + 2] cycloaddition16 but was not an aspect of the original [5 + 1 + 2 + 1] cycloaddition report is selectivity of C−C activation in 1,2-disubstituted VCPs. The lowest energy pathway for the C−C activation in intermolecular pathways proceeds via coordination

Figure 5. Intermediates and transition state structures from Figure 4. Key distances are indicated in Å.

of the VCP to the unsaturated metal catalyst to form a metal alkene complex (INT 1). This complex is further stabilized with a weak agostic interaction between rhodium and the proximal cyclopropane carbon, as shown via AIM analysis (Figure 8). We began our investigation of C−C activation selectivity with the 2-methyl VCP complex (1a−d, Table 1). Depending on the E


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Figure 6. Comparison of alkyne insertion transition state energies for monosubstituted alkynes (ωB97XD/SDD-6-31G*, Gibbs energy in kcal/mol), with the [5 + 1 + 2 + 1] mechanism shown in black and the [5 + 1 + 1 + 2] mechanism in blue.

Rh and proximal cyclopropane carbon (a stabilizing interaction observed in all other methyl-substituted VCP complexes as well as INT 1 (Figure 8)); instead, a weaker Rh−H interaction occurs with the trans methyl group (Figure 9). The energy difference is also preserved in the TS energies, with activation via TS 1c-2c being 3.5 kcal/mol lower in energy. If the fact that 1d is 3.1 kcal/mol higher in energy than 1c is removed from the TS 1d-2d energy barrier, the two processes are nearly the same energetically (ca. 2.5 kcal/mol), suggesting that the preference for activation of the least substituted bond in the trans-disubstituted VCP is driven by the enhanced stability of 1c. These results are consistent with intramolecular [5 + 2] cycloaddition synthetic studies, where cis- and trans-disubstituted VCP substrates yield seven-membered-ring products arising from cleavage of the least substituted cyclopropane bond, with exclusive selectivity. It should be noted that intramolecular [5 + 2] cycloaddition is believed to occur through a mechanism that differs significantly from intermolecular variants.40

geometry of the 1,2-disubstituted VCP (cis vs trans) and the C−C bond to be activated, there are four possible geometries for this alkene complex. For cis-substituted complexes 1a,b, there is very little difference in the relative stability, suggesting that the two coordination modes are in equilibrium. The TS energy difference between the two modes of activation (least substituted bond 1a vs more substituted bond 1b) is only 0.9 kcal/mol, suggesting that both pathways are readily accessed at the reaction temperature. The relative stability of the products differs by 3.2 kcal/mol, with 2b being only slightly lower energy than TS 1b-2b, suggesting that the activation is readily reversible. As such, it is expected that any preference in C−C activation would occur at the least substituted bond in cis-disubstituted VCP complexes. The preference for cleavage of the least substituted bond is retained in the case of transdisubstituted VCPs. The stability of the two alkene coordination complexes 1c and 1d differ by 3.1 kcal/mol, suggesting a clear preference for 1c. This preference is likely due to the inability of complex 1d to form a stabilizing bond between the F


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Figure 7. Alkyne insertion transition state structures from Figure 6. Key distances are indicated in Å.

Table 1. Comparison of C−C Bond Activation in 1,2-Disubstituted VCPs (ωB97XD/SDD-6-31G*, Gibbs Energy)

Figure 8. AIM analysis of INT 1 showing the additional stabilizing bond path between Rh and the cyclopropane.

We also investigated the effect of an electron-withdrawing substituent (i.e., aldehyde), shown in intramolecular [5 + 2] studies to result in a reversal of regioselectivity, with preferential activation of the most substituted C−C bond of the cyclopropane occurring when combined with [Rh(CO)2Cl]2 as catalyst. For trans-disubstituted 1g,h the relative stability difference between the two complexes is 2.3 kcal/mol, favoring the complex that leads to activation of the least substituted C−C bond. The formyl-substituted complexes do not exhibit the Rh−C stabilizing interactions observed for 1a−d and INT 1 (Figure 10). The corresponding C−C bond activation TSs also favor activation of the least substituted C−C bond by 0.9 kcal/mol, suggesting that cleavage of the least substituted C−C bond is slightly kinetically preferred. The resulting C−C activation complexes 2g,h differ markedly in stability, with 2g, arising from cleavage of the most substituted bond, being 4.5 kcal/mol lower in energy. Therefore, preferential cleavage of the most substituted bond would be thermodynamically driven, with the barrier to C−C reductive elimination from 2g being 12.5 kcal/mol, while that from 2h is only 4.8 kcal/mol, suggesting that the C−C activation from 1h is more reversible. For the cis-disubstituted case, we were unable to locate either

alkene complex 1f or a TS corresponding to C−C bond activation to form 2f: all attempts to optimize 1f led only to activated complex 2f, suggesting that the C−C bond activation is barrierless. The corresponding activation of the least substituted bond (1e to 2e) has a low barrier of 2.4 kcal/mol. The activated complexes 2e,f differ significantly in energy, with 2f being 9.8 kcal/mol lower in energy. When taken together, these data strongly suggest that for cis-disubstituted VCPs bearing electron-withdrawing groups, activation of the most substituted bond is preferred. G


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Figure 10. AIM analyses of the cis (1e) and trans (1g,h) formylsubstituted analogues of INT 1. Figure 9. AIM analyses of the cis (1a,b) and trans methyl-substituted analogues (1c,d) of INT 1.

Alternate Rh-Catalyzed Cyclization/Aromatization Sequence. We detailed an acid-mediated cyclization/aromatization sequence proposed for reaction completion (Figure 4) that appears consistent with experimental observations. We have also considered an alternate metal-catalyzed process (Figures 11 and 12). In Rh-diene complex INT 12 the two olefin units are held in proximity by the rhodium metal, permitting oxidative coupling via TS 12-18.44 This metal-catalyzed cyclization yields the η1,η1 square-pyramidal complex INT 18 through a highenergy barrier of 34.2 kcal/mol, generating the requisite bicyclic ring system. From Rh complex INT 18, the aromatization sequence proceeds via C−O bond activation of the dative methoxy group (TS 18-19) leading to Rh(IV) methoxy complex INT 19 via a turnover-limiting barrier of 44.4 kcal/mol, which is much higher than the barrier to alkyne insertion at TS 6a-7a or TS 9b-10b.45 Intramolecular deprotonation via TS 19-20 yields methanol complex INT 20. The catalytic cycle is closed through MeOH dissociation and tautomerization to yield η6-hydroxydihydroindanone complex INT 2246 followed by VCP-assisted dissociation of the organic product to regenerate the Rh-VCP complex INT 1. The C−O activation barrier is the slowest step in this proposed mechanistic cycle, and the barrier height of 44.4 kcal/mol is 8 kcal/mol higher in energy than the computed off-metal pathway (Figure 4). This energy difference suggests that this metal-catalyzed pathway is not competitive with the off-metal cyclization/aromatization sequence. [5 + 1] and [5 + 2 + 1] Cycloaddition. As previously noted, multicomponent rhodium-catalyzed reactions under

Figure 11. Rh-catalyzed cyclization/aromatization (ωB97XD/SDD-631G*, Gibbs energy).

CO can also lead to reaction products arising from [5 + 1] and [5 + 2 + 1] cycloaddition reactions. Given the related conditions, it can be conceived that similar processes can occur as alternate mechanistic pathways during the [5 + 1 + 2 + 1] cycloaddition reaction. Therefore, the following question H


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Figure 12. Intermediates and transition states from Figure 11. Key distances are indicated in Å.

arises: why does the reaction here proceed to [5 + 1 + 2 + 1] reaction products rather than the other possible pathways? From Figure 1, it is immediately apparent that [5 + 2] cycloaddition is suppressed by the CO atmosphere, with CO outcompeting alkyne for metal coordination. A competitive [5 + 1] cycloaddition would occur via reductive elimination from the CO insertion complex INT 5 or INT 6b (Figures 13 and 14). From the unsaturated complex INT 5, TS 5-23 leads to agostic bond stabilized [5 + 1] cycloaddition product INT 23. The barrier to [5 + 1] cycloaddition in this path is 24.8 kcal/mol, which is lower than the 29.3 kcal/mol barrier for alkyne insertion. The corresponding [5 + 1] cycloaddition barrier from INT 6b (TS 6b-24) is 30.3 kcal/mol, which is significantly higher than the 26.5 kcal/mol barrier for CO insertion. Given that products of [5 + 1] cycloaddition do not result under these conditions, support is given to the [5 + 1 + 1 + 2] cycloaddition proposal, where CO complexation and insertion outcompete other pathways.

Figure 14. Intermediates and transition states from Figure 13. Key distances are given in Å.

A [5 + 1 + 2] reaction could potentially occur via reductive elimination from INT 7a or INT 8a; attempts to locate corresponding TSs were not successful. Related [5 + 2 + 1] processes have been reported for VCP/alkyne/CO cycloadditions and have generally employed electron-deficient internal alkynes.15 It has previously been demonstrated that the [5 + 1 + 2 + 1] process is completely selective for terminal (preferentially aryl) alkynes and is in large part driven by solvent effects, where product insolubility enhances yields. Additional studies are required to elucidate the causes for suppression of the [5 + 2 + 1] mechanism.

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CONCLUSIONS The mechanism of the rhodium-catalyzed [5 + 1 + 2 + 1] cycloaddition of VCPs, terminal alkynes, and CO has been investigated using DFT calculations. The mechanistic proposal of a rhodium-catalyzed four-component cycloaddition to

Figure 13. [5 + 1] cycloaddition possibilities (ωB97XD/SDD-6-31G*, Gibbs energy). I


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produce a cyclononadiendione, followed by off-metal tautomerization, disrotatory 6π-electrocyclization, and aromatizationdriven elimination originally put forward by Wender et al. has been verified as energetically feasible. We have also shown that two energetically similar and convergent cycloaddition pathways lie on the potential energy surface, differing only in their order of component insertion. While the newly postulated [5 + 1 + 1 + 2] cycloaddition mechanism is slightly higher in energy that the [5 + 1 + 2 + 1] pathway, it provides an elegant rationale for the observed insertion order. Investigation of the reaction regioselectivity agrees with the experimental observation that terminal alkynes insert from the unsubstituted terminus. The alkyne insertion does not appear to be sensitive to steric effects, with the model propyne and phenylacetylene exhibiting similar activation energy barriers. An investigation of the regioselectivity of C−C bond activation in disubstituted VCPs predicts that the same regioselectivity will be observed as for intramolecular [5 + 2] cycloaddition reactions: cleavage of the least substituted C−C bond occurs preferentially, unless an electron-donating group is present on the VCP, in which case the selectivity will reverse. An alternate mechanistic possibility for metal-catalyzed cyclization/aromatization was explored but found to be energetically unfeasible in comparison to the off-metal pathway. Spectroscopic studies are currently underway to structurally characterize reaction intermediates and catalyst resting states.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00323. Thermodynamic parameters (PDF) Atomic coordinates for all stationary points (XYZ)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail for K.E.O.Y.: [email protected]. ORCID

Kai E. O. Ylijoki: 0000-0002-6985-1036 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Canada Foundation for Innovation (CFI), the Faculties of Science and Graduate Studies and Research of Saint Mary’s University, the SMUworks program, and ACEnet (SMUworks grant and ACEnet Research Fellowship to I.I.M.) is gratefully acknowledged. Computational facilities were provided by ACEnet, the regional high-performance computing consortium for universities in Atlantic Canada. ACEnet is funded by the CFI, the Atlantic Canada Opportunities Agency (ACOA), and the provinces of Newfoundland and Labrador, Nova Scotia, and New Brunswick.

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ABBREVIATIONS CO, carbon monoxide; DFT, density functional theory; ECP, effective core potential; INT, intermediate; LDA, lithium diisopropylamide; TS, transition state; VCP, vinylcyclopropane J


Article

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