Mechanistic Insights into Cyclopropenes Involved Carbonylative

Article Cite This: J. Org. Chem. 2018, 83, 12734−12743

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Mechanistic Insights into Cyclopropenes-Involved Carbonylative Carbocyclization Catalyzed by Rh(I) Catalyst: A DFT Study Ping Dai, Abosede Adejoke Ogunlana, and Xiaoguang Bao* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China

J. Org. Chem. 2018.83:12734-12743. Downloaded from pubs.acs.org by REGIS UNIV on 10/19/18. For personal use only.

S Supporting Information *

ABSTRACT: Computational studies were carried out to provide mechanistic insights into the Rh(I)-catalyzed activation of cyclopropenes and the detailed mechanistic pathways of [3+2+1] carbonylative carbocyclization of tethered ene- and yne-cyclopropenes. Computational results suggest that it is more favorable for the cyclopropene moiety of tethered ene-cyclopropenes to initially undergo heterolytic cleavage of a C−C σ-bond to form a vinyl Rh(I) carbenoid intermediate than to proceed through homolytic C−C σbond cleavage to generate a rhodacyclobutene intermediate. The yielded vinyl Rh(I) carbenoid intermediate could undergo cyclization to generate a Rh(III) metallacyclobutene intermediate, which could further lead to a thermodynamically more stable six-coordinated Rh(III) metallacycle intermediate in the presence of additional CO. Afterward, it is more feasible for the yielded six-coordinated Rh(III) metallacycle to sequentially undergo CO migratory insertion, cyclization, and reductive elimination to furnish the final cyclohexenone product. The origin of stereoselectivity of the product was also discussed. The proposed mechanistic pathway can also be applied to the Rh(I)-catalyzed carbonylative carbocyclization of tethered yne-cyclopropenes and vinyl cyclopropenes to produce phenol derivatives. The main mechanistic difference for the vinyl cyclopropene substrate is that the conversion of Rh(I) carbenoid intermediate to the Rh(III) metallacycle proceeds via intramolecular 6π electrocyclization.

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C σ-bond cleavage of cyclopropene mediated by appropriate transition metal catalysts could lead to the formation of either a metallocyclobutene intermediate (II) or a metal vinylcarbenoid intermediate (III, Scheme 1).3,4 The conversion of II to III has been proposed in the literature.5 The utilization of the metal vinylcarbenoid intermediate, derived from cyclopropenes in an atom-economic manner, to undergo C−H insertion,6 cycloaddition,7 and cyclopropanation,8 has been well documented. However, little attention has been paid to the potential reverse conversion of III to II and the important role of the metallacycle intermediate in the transformations of cyclopropenes, although the metallacycle intermediate was proposed in some cases of cyclopropenes-involved reactions.9 Recently, the rhodium-catalyzed carbonylative carbocyclization, denoted by [2+2+1] carbocyclization (Pauson−Khand reaction), has been greatly developed to effectively construct carbocyclic molecules containing carbonyl functional group.10 In addition, the Rh(I)-catalyzed intermolecular [5+2+1] carbocyclization, employing vinylcyclopropene as the fivecarbon component, was reported by Wender and co-workers.11 The Rh(I)-catalyzed intramolecular [5+2+1] cycloaddition with the ene-vinylcyclopropane substrates was developed by Yu’s

INTRODUCTION

The versatile transformations of cyclopropenes in organic synthesis have attracted significant research interest due to the presence of a highly strained unsaturated three-membered ring.1 In general, transition metal-catalyzed cyclopropene involved reactions can be classified into two categories, π-bond activation and σ-bond activation. A typical transition metal-catalyzed πbond activation of cyclopropene derivatives via migratory insertion could lead to intermediate I, from which functionalized cyclopropane derivatives could be obtained (Scheme 1).2 A C− Scheme 1. Transition Metal-Catalyzed π- and σ-Bond Activations of Cyclopropenes

Received: August 22, 2018 Published: September 24, 2018 © 2018 American Chemical Society

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DOI: 10.1021/acs.joc.8b02178 J. Org. Chem. 2018, 83, 12734−12743

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The Journal of Organic Chemistry group.12 Furthermore, Wang and co-workers13 expanded the Rh(I)-catalyzed Pauson−Khand reaction using tethered eneand yne-cyclopropenes to realize the [3+2+1] carbonylative carbocyclization, leading to bicyclohexenones and phenols, respectively (Scheme 2).

addition, the origin of the trans stereoselectivity of the bicyclohexenone product (2) remains unclear. Meanwhile, in-depth mechanistic studies on the transition metal-catalzyed cyclopropenes-involved reactions have been performed by some groups.14 For example, Lee and co-workers investigated the mechanism of Au(I)-catalyzed rearrangement of cyclopropenes, in which the gold carbene intermediate was proposed to control the product formation.14b Xia and Huang14e proposed a formal 1,1-halometalation mechanism for the PdCl2or CuI-catalyzed cycloisomerization of 3-acylcyclopropenes via regiochemical-controlled C−C σ-bond cleavage. The Zncatalyzed alkene cyclopropanation through zinc vinyl carbenoids generated from cyclopropenes was studied by Vicente et al.14f In this work, computational studies were carried out to provide mechanistic insights into the Rh(I)-catalyzed activation of cyclopropenes and the detailed mechanistic pathways of [3+2+1] carbonylative carbocyclization of tethered ene- and yne-cyclopropenes.13 The conversion of Rh(I) vinylcarbenoid intermediate to Rh(III) metallocyclobutene intermediate in the presence of CO and the important role of the metallacycle intermediate in the formation of the carbonylative carbocyclization products were revealed. The in-depth mechanistic insights provided could be applied to the Rh(I)-catalyzed carbonylation of vinyl cyclopropenes to produce phenol derivatives.15

Scheme 2. Rh(I)-Catalyzed [3+2+1] Carbonylative Carbocyclization with Tethered Ene- and Yne-cyclopropenes in the Presence of CO

Although complicated, the mechanistic pathways of the Rh(I)-catalyzed carbonylative carbocyclizations involving cyclopropenes could proceed via the formation of either a metallocyclobutene intermediate (A, path a) or a metal vinylcarbenoid intermediate (B, path b) after a C−C σ-bond activation of the three-membered ring of 1 (Scheme 3). From intermediate A, a subsequent CO insertion might occur to afford intermediate C in the presence of CO. Next, the terminal alkene moiety of 1 could undergo alkene insertion to yield a cyclized intermediate E (path a1). Alternatively, the alkene insertion to yield intermediate D from A might take place prior to the CO insertion (path a2). After the formation of E, reductive elimination (RE) could follow to produce the carbonylative carbocyclization product 2. On the other hand, if the metal vinylcarbenoid intermediate B were to be a feasible intermediate, the sequential alkene insertion and CO insertion could also afford the key intermediate E (path b1). Alternatively, the CO insertion into the carbenoid site of B to generate the ketene intermediate F followed by alkene insertion to form E cannot be excluded (path b2). On the basis of the possibilities stated above, the detailed mechanistic pathway for the cyclopropenes-involved carbonylative carbocyclization promoted by the Rh(I) catalyst needs further clarification. In

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COMPUTATIONAL METHODS

All of the computations were carried out using the M06 density functional method,16 which has been widely applied to rhodiumcatalyzed reactions.17 The LANL2DZ basis set in conjunction with the LANL2DZ pseudopotential18 was used for the Rh atom. The 6-31G(d) basis set19 was used for other atoms in the geometry optimizations. Vibrational frequency analyses at the same level of theory were performed on all optimized structures to characterize stationary points as local minima or transition states. Further, intrinsic reaction coordinate (IRC)20 computations were carried out to confirm that transition states connect to the appropriate reactants and products. The gas-phase Gibbs free energies for all optimized structures were obtained at 298.15 K and 1 atm. To consider solvation effects, single-point energy computations using the SMD model21 with 1,2-dichloroethane as solvent, being consistent with experimental conditions, were performed on the basis of the optimized gas-phase geometries of all species. On the basis of the optimized geometries, the M06 functional with larger basis sets (LANL2TZ(f) for Rh and 6-311++G(d,p) for other atoms) was utilized for single-point energy calculations on stationary points. The solution-phase Gibbs energy was determined by adding the solvation single-point energy and the gas-phase thermal correction to the Gibbs

Scheme 3. Possible Mechanistic Pathways for the Rh(I)-Catalyzed [3+2+1] Carbonylative Carbocyclization of Tethered Enecyclopropenes

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Figure 1. Energy profiles for the two types of σ-bond cleavage of the cyclopropene moiety of 1 and subsequent conversion of the Rh(I) vinylcarbenoid intermediate to the Rh(III) metallocyclobutene intermediate. Bond lengths are shown in angstroms. 12736


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Figure 2. Energy profiles for the formation of carbonylative carbocyclization products from INT6. Bond lengths are shown in angstroms. energy obtained from the vibrational frequencies. The translational entropy in solution was corrected using the method proposed by Whitesides et al.22 Unless otherwise specified, the solution-phase Gibbs energy was used in the present discussions. The Gaussian 09 suite of programs23 was used to carry out all of the calculations. The 3D structures of the studied species were shown using the CYLView software.24

Rh(I) center to form a rhodacyclobutene intermediate. The located transition state (TS) is shown as TS1 in Figure 1, in which the C1···C3 bond distance is lengthened to 1.85 Å, and the C1···Rh and C3···Rh distances are stabilized to 2.23 and 2.37 Å, respectively. The greater s component of C1 than C3 is mainly responsible for the shorter distance with Rh. The homolytic cleavage of a C−C σ-bond of the cyclopropene moiety leads to the rhodacyclobutene intermediate INT3, in which the fourmembered ring is almost coplanar. The heterolytic cleavage of a C−C σ-bond of the cyclopropene moiety would result in a metal vinylcarbenoid intermediate. The optimized TS is demonstrated as TS2 in Figure 1, in which the C1···C3 bond distance is lengthened to 1.89 Å while the C1···Rh distance is shortened to 2.16 Å. In contrast to TS1, the C3···Rh distance (2.86 Å) in TS2 is significantly lengthened, implying the interaction between C3 and Rh could be negligible. In addition, sp2 character is found for C3 in TS2, in which the dihedral angle H−C4−C3−C2 is almost

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RESULTS AND DISCUSSION Mechanistic Insights into the Rh(I)-Catalyzed Carbonylative Carbocyclization of Tethered Ene- and Ynecyclopropenes. The Rh(I)-catalyzed σ-bond cleavage of the cyclopropene moiety of tethered ene-cyclopropenes (1) was computationally studied first. Both homolytic and heterolytic cleavage of C−C σ-bond of the three-membered ring of 1 were considered. The homolytic cleavage can occur via the oxidative addition of a C−C σ-bond of the cyclopropene moiety onto the 12737


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The Journal of Organic Chemistry planar (178°), while an sp3 character remains for C3 in TS1, in which the corresponding dihedral angle in TS1 is 143°. Thus, the heterolytic cleavage of the C−C σ-bond of the cyclopropene moiety results in the formation of the vinyl Rh-carbenoid intermediate (INT2). The predicted energy barriers of homolytic and heterolytic cleavage of the σ-bond of cyclopropene are 17.6 and 15.1 kcal/ mol, respectively, indicating that the heterolytic cleavage of the σ-bond is more favorable to occur. The yielded vinyl Rhcarbenoid intermediate (INT2) is also more thermodynamically stable than the rhodacyclobutene intermediate (INT3). It should be noted that a five-coordinated structure is found in INT3 and there remains a vacant site to form a more stable sixcoordinated, 18e, octahedral structure. The terminal olefin group in INT3 could coordinate with Rh center to form a thermodynamically more stable intermediate INT4. Interestingly, the yielded vinyl Rh-carbenoid intermediate, INT2, and the six-coordinated rhodacyclobutene intermediate, INT4, are almost the same in energy.25 In addition, computational studies reveal that the two intermediates, INT2 and INT3, could undergo interconversion via TS3. Thus, the cyclopropene moiety of substrate 1 readily undergoes heterolytic C−C σ-bond cleavage to form the vinyl Rh-carbenoid intermediate, followed by the formation of a six-coordinated intermediate INT4. In the presence of external CO, it should be noted that one additional CO can coordinate with INT3 to form a thermodynamically more stable intermediate INT5 (ca. 3 kcal/mol lower in energy than INT2). In addition, the formed six-coordinated INT4 could undergo a ligand exchange step with CO to afford INT5 because the CO ligand binds more readily with the Rh(III) center than does the terminal olefin group. After the formation of INT5, one of the coordinated CO may undergo migratory insertion to the adjacent sp2 carbon to afford the intermediate INT6. The located TS is shown as TS4 in Figure 1, in which the C1···C5 bond distance is shortened to 2.08 Å. The predicted ΔG⧧ is only ca. 3 kcal/mol, indicating that the migratory insertion of CO to the adjacent sp2 carbon to form INT6 is a very facile process. Alternatively, the CO migratory insertion to the neighboring sp3 carbon was also considered. However, the located transition state (TS10) is much higher in energy than TS4 due to the conjugative effect of the inserted CO with the C1C2 double bond found in TS4 while absent in TS10. Thus, the CO insertion into the neighboring sp2 carbon to form INT6 occurs readily as compared to its insertion into the sp3 carbon. The yielded INT6 could undergo isomerization to form the corresponding ketene intermediate (INT7).26 The optimized TS is shown as TS5 in Figure 2. Subsequently, the Rh(I) catalyst could switch its coordination to the terminal olefin group of 1 to yield thermodynamically more stable complexes, INT8 or INT10, which are lower in energy than INT7 by 7.8 or 5.9 kcal/ mol, respectively. Afterward, the two structural conformers, INT8 and INT10, could undergo the Rh(I)-mediated [2+4] cyclization27,28 followed by reductive elimination to afford the corresponding trans and cis carbonylative carbocyclization products, respectively. Starting from INT8, the TS of cyclization is shown as TS6, in which the C3···C6 bond distance is shortened to 2.17 Å. Consequently, a Rh(III) intermediate, INT9, is formed. The predicted Gibbs energy barrier of this step is 24.1 kcal/mol, and the formed INT9 is thermodynamically stable by 2.8 kcal/mol relative to INT8. The yielded INT9 would readily undergo a reductive elimination step via TS7 to furnish the final trans stereoisomerized product. It should be noted that the

Rh(I)-mediated cyclization to afford INT9 is the rate-limiting step for the formation of the final product. The analogous pathway leading to the corresponding cis structure starting from INT10 was also performed. Computational results show that the initial trans structural conformer (INT8) is ca. 2 kcal/mol lower in energy than the cis conformer (INT10), indicating that INT8 is readier to form. For the subsequent cyclization process, being the rate-determining step, the trans structure is also ca. 2 kcal/ mol lower in energy than the corresponding cis isomer. The formation of a C3−C6 bond close to a staggered conformation is found in TS6, while a nearly eclipsed conformation is found in TS8. This difference in conformation is mainly responsible for the favored trans stereoselectivity. Consequently, the yielded trans intermediate INT9 can further furnish the final trans product 2, which is consistent with the experimental results.13 In the presence of CO, one might propose that additional CO might attack the carbenoid site of INT2 to form the ketene intermediate in a straightforward manner. The located TS of this pathway is shown as TS11 (Figure 1). The predicted energy barrier of this step is 13 kcal/mol, which is 3.6 kcal/mol higher than that of the intramolecular conversion to form INT3. Consequently, the formation of ketene intermediate via direct CO attack to the carbenoid site of INT2 is not a favorable route. Alternatively, the alkene insertion prior to CO attack based on INT2 was also considered.29 The TS of cyclization in a trans manner is shown as TS12 in Figure 1. The calculated energy barrier (20.6 kcal/mol) for this route is even higher, suggesting the alkene insertion to afford INT14 is also not a feasible pathway. Thus, the Rh(I) vinylcarbenoid intermediate might not play an important role in the formation of carbonylative carbocyclization products except for the conversion to form the six-coordinated Rh(III) metallacycle intermediate INT5 (Figure 1). In addition, the alkene insertion prior to the CO insertion might proceed from INT4. However, computational result shows that the predicted Gibbs energy barrier (34.8 kcal/ mol) is significantly higher for this possibility (Figure S1). Overall, the conversion of INT2 to the six-coordinated Rh(III) metallacycle intermediate, INT5, is more likely to occur in the presence of CO. Afterward, it is more feasible for INT5 to sequentially undergo CO migratory insertion, cyclization, and reductive elimination to furnish the final cyclohexenone product (Scheme 4). The aforementioned mechanistic insight could be applied to understand the Rh(I)-catalyzed carbonylative carbocyclization of tethered yne-cyclopropene (3) to form phenol derivative (4) (Scheme 5). Similar to 1, computational results suggest that it is more feasible for the cyclopropene moiety of 3 to undergo the Rh(I) catalyzed heterolytic C−C σ-bond cleavage to form the vinyl Rh(I) carbenoid intermediate (INT2a) (Figure S5). The afforded INT2a could undergo cyclization to generate the Rh(III) metallacyclobutene intermediate (INT3a). In the presence of an external CO, a six-coordinated Rh(III) metallacycle intermediate formed via CO coordination with the Rh(III) metallacycle intermediate might be responsible for the ensuing reactions. Thus, it becomes easier for the sixcoordinated Rh(III) metallacycle to undergo CO migratory insertion to provide an intermediate INT6a, which could subsequently be converted to a ketene intermediate. The ketene cyclization followed by RE then would produce INT10a, from which proton migration would finally occur to form the expected phenol derivative product (Figure S6). Mechanistic Insight into the Rh(I)-Catalyzed Carbonylative Carbocyclization of Vinyl Cyclopropene (5) To 12738


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The Journal of Organic Chemistry Scheme 4. Proposed Mechanistic Pathway for the Rh(I)Catalyzed Carbonylative Carbocyclization of Tethered Enecyclopropenes

the butadienyl Rh(I) carbenoid intermediate (INT2b). The predicted ΔG⧧ is ca. 12 kcal/mol (Figure 3). The Rh(I)catalyzed homolytic σ-bond cleavage of the three-membered ring of 5 to form rhodacyclobutene intermediate INT3b, however, has a higher energy barrier (14.3 kcal/mol). The yielded INT3b is higher in energy than INT2b by 8.5 kcal/mol. The TS for conversion of INT2b to INT3b is located as TS3b, requiring an energy barrier of 9.2 kcal/mol. In the presence of CO, one additional CO coordination with INT3b leads to a sixcoordinated rhodacyclobutene intermediate (INT4b). However, INT4b is 2.1 kcal/mol higher in energy than the Rhcarbenoid intermediate INT2b, indicating the yielded Rh(III) metallacycle is thermodynamically unstable. In comparison with the thermodynamically stable six-coordinated Rh(III) metallacycle generated from substrate 1, the destabilizing effect (resulting from the formation of INT4b) could be attributed to the breaking of the conjugated butadienyl moiety of INT2b. Alternatively, the yielded INT2b could undergo 6π electrocyclization to form a rhodacyclohexadiene intermediate, INT6b.30 The TS is located as TS4b, in which the C4···Rh distance is shortened to 2.82 Å, while the C1···Rh distance is lengthened to 2.04 Å. The predicted Gibbs energy barrier of 6π electrocyclization to form INT6b is only 5.9 kcal/mol, which is ca. 3 kcal/mol lower than that leading to INT4b. In the presence of CO, the formed INT6b could bind with one additional CO to yield a six-coordinated Rh(III) metallacycle (INT7b). The formed INT7b is ca. 3 kcal/mol lower in energy than the Rh(I) carbenoid intermediate INT2b, indicating the conversion of INT2b to INT7b is thermodynamically favorable. It should be noted that the conversion of the Rh(I) carbenoid intermediate to the Rh(III) metallacycle intermediate in reaction 3 is different from that of reactions 1 and 2. Nevertheless, the feasible formation of the Rh(III) metallacycle intermediate, INT7b, is proposed to account for the subsequent conversions. Next, the yielded INT7b could undergo CO migratory insertion to either the adjacent sp2 carbon or the sp3 carbon. Both possibilities were considered computationally. The located TS of CO insertion into sp2 carbon is shown as TS5b in Figure 3, in which the C1···C5 bond distance is shortened to 2.10 Å. The predicted ΔG⧧ is only 4.6 kcal/mol, suggesting a facile step. The CO insertion into sp3 carbon, however, has a much higher energy barrier (ca. 23 kcal/mol). Structural examination shows that CO migratory insertion to adjacent sp2 carbon can lead to a conjugation-stabilized structure. Such a stabilizing effect is absent in CO insertion into sp3 carbon. Therefore, it is more feasible for INT7b to undergo CO migratory insertion to adjacent sp2 carbon to afford the intermediate INT8b. Subsequently, reductive elimination via TS6b could follow to furnish the cyclohexenone intermediate (INT10b).31 Finally, proton transfer32 and rearomatization of INT10b would produce the final phenol derivative 6 (Scheme 6). Additionally, computational studies were carried out to exclude a competitive mechanistic pathway, in which an external CO molecule inserts into the carbenoid site of INT2b to generate a ketene intermediate. The corresponding TS is located as TS8b in Figure 3. Computational results show that a Gibbs energy barrier of 8.7 kcal/mol is required to afford the ketene

Scheme 5. Proposed Mechanistic Pathway for the Rh(I)Catalyzed Carbonylative Carbocyclization of Tethered Ynecyclopropene (3) To Produce Phenol Derivative (4)

Form Phenol Derivative (6). The computational studies of the Rh(I)-catalyzed activation of cyclopropenes and the detailed mechanistic pathways of [3+2+1] carbonylative carbocyclization of tethered ene- and yne-cyclopropenes suggest that the sixcoordinated Rh(III) metallacycle plays an important role in the presence of external CO. In fact, the unveiled mechanistic insight is also applicable to the Rh(I)-catalyzed carbonylative carbocyclization of vinyl cyclopropene (5) to form phenol derivative (6) in the presence of external CO (reaction 3).15 Computational studies were carried out to explore the feasible activation mode of 5. Similarly, it is easier for 5 to undergo the Rh(I) catalyzed heterolytic cleavage of a C−C σ-bond to form 12739


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Figure 3. Energy profiles for the Rh(I)-catalyzed carbonylative carbocyclization of vinyl cyclopropene (5) to form phenol derivative (6). Bond lengths are shown in angstroms.

intermediate (INT12b), which is 2.8 kcal/mol higher than the intramolecular 6π electrocyclization to form INT6b. Thus, the

direct CO attack at the carbenoid site of INT2b to form the ketene intermediate is not a favorable mechanistic route. 12740



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Scheme 6. Proposed Mechanistic Pathway for the Rh(I)Catalyzed Carbonylative Carbocyclization of Vinyl Cyclopropene (5) To Produce Phenol Derivative (6)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaoguang Bao: 0000-0001-7190-8866 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (21642004), the project of scientific and technologic infrastructure of Suzhou (SZS201708), Young Thousand Talented Program, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financial support.

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(1) For selected reviews, see: (a) Fox, J. M.; Yan, N. Metal Mediated and Catalyzed Nucleophilic Additions to Cyclopropenes. Curr. Org. Chem. 2005, 9, 719−732. (b) Rubin, M.; Rubina, M.; Gevorgyan, V. Transition Metal Chemistry of Cyclopropenes and Cyclopropanes. Chem. Rev. 2007, 107, 3117−3179. (c) Zhu, Z.-B.; Wei, Y.; Shi, M. Recent Developments of Cyclopropene Chemistry. Chem. Soc. Rev. 2011, 40, 5534−5563. (d) Miege, F.; Meyer, C.; Cossy, J. When Cyclopropenes Meet Gold Catalysts. Beilstein J. Org. Chem. 2011, 7, 717−734. (e) Song, C.; Wang, J.; Xu, Z. Recent Advances of Cyclopropene Chemistry. Huaxue Xuebao 2015, 73, 1114−1146. (f) Vicente, R. Recent Progresses towards the Strengthening of Cyclopropene Chemistry. Synthesis 2016, 48, 2343−2360. (2) For transition metal-catalyzed π-bond activation of cyclopropene derivatives, see: (a) Rubina, M.; Rubin, M.; Gevorgyan, V. Transition Metal-Catalyzed Hydro-, Sila-, and Stannastannation of Cyclopropenes: Stereo- and Regioselective Approach toward Multisubstituted Cyclopropyl Synthons. J. Am. Chem. Soc. 2002, 124, 11566− 11567. (b) Rubina, M.; Rubin, M.; Gevorgyan, V. Catalytic Enantioselective Hydrostannation of Cyclopropenes. J. Am. Chem. Soc. 2004, 126, 3688−3689. (c) Müller, D. S.; Marek, I. Asymmetric Copper-Catalyzed Carbozincation of Cyclopropenes en Route to the Formation of Diastereo- and Enantiomerically Enriched Polysubstituted Cyclopropanes. J. Am. Chem. Soc. 2015, 137, 15414−15417. (d) Teng, H.-L.; Luo, Y.; Wang, B.; Zhang, L.; Nishiura, M.; Hou, Z. Synthesis of Chiral Aminocyclopropanes by Rare-Earth-MetalCatalyzed Cyclopropene Hydroamination. Angew. Chem., Int. Ed. 2016, 55, 15406−15410. (e) Dian, L.; Müller, D. S.; Marek, I. Asymmetric Copper-Catalyzed Carbomagnesiation of Cyclopropenes. Angew. Chem., Int. Ed. 2017, 56, 6783−6787. (f) Dian, L.; Marek, I. Rhodium-Catalyzed Arylation of Cyclopropenes Based on Asymmetric Direct Functionalization of Three-Membered Carbocycles. Angew. Chem., Int. Ed. 2018, 57, 3682−3686. (3) For transition metal-catalyzed σ-bond activation of cyclopropene derivatives to form metallocyclobutene intermediates, see: (a) Semmelhack, M. F.; Ho, S.; Steigerwald, M.; Lee, M. C. Metal Carbonyl Promoted Rearrangement of Cyclopropenes to Naphthols. J. Am. Chem. Soc. 1987, 109, 4397−4399. (b) Semmelhack, M. F.; Ho, S.; Cohen, D.; Steigerwald, M.; Lee, M. C.; Lee, G.; Gilbert, A. M.; Wulff, W. D.; Ball, R. G. Metal-Catalyzed Cyclopropene Rearrangements for Benzannulation: Evaluation of an Anthraquinone Synthesis Pathway and Reevaluation of the Parallel Approach via Carbene-Chromium Complexes. J. Am. Chem. Soc. 1994, 116, 7108−7122. (c) Nakamura, I.; Bajracharya, G. B.; Yamamoto, Y. Palladium-Catalyzed Hydrocarbonation and Hydroamination of 3,3-Dihexylcyclopropene with Pronucleophiles. J. Org. Chem. 2003, 68, 2297−2299. (d) Shibata, T.; Maekawa, S.; Tamura, K. Rhodium-Catalyzed Intramolecular Cycloaddition of Cyclopropene-ynes Triggered by Carbon-Carbon Bond Cleavage. Heterocycles 2008, 76, 1261−1270. (e) Zhang, H.; Li, C.; Xie, G.; Wang, B.; Zhang, Y.; Wang, J. Zn(II)- or Rh(I)-Catalyzed

Overall, for the vinyl cyclopropene substrate 5, the sixcoordinated Rh(III) metallacycle formed via 6π electrocyclization is proposed to be responsible for the subsequent transformations to produce the final phenol derivative.

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CONCLUSIONS In summary, DFT studies were carried out to understand the mechanisms of Rh(I)-catalyzed carbonylative carbocyclization of tethered ene- and yne-cyclopropenes. Computational results suggest that it is more facile for the cyclopropene moiety to undergo the Rh(I) catalyzed heterolytic cleavage of C−C σbond to form the vinyl Rh(I) carbenoid intermediate. The homolytic cleavage of C−C σ-bond of cyclopropene moiety via oxidative addition onto Rh(I) to form the Rh(III) metallacycle intermediate in a straightforward manner, however, is not favorable. The formed vinyl Rh(I) carbenoid intermediate could undergo cyclization to generate the Rh(III) metallacyclobutene intermediate. In the presence of external CO, a six-coordinated Rh(III) metallacycle intermediate, which can be formed via the coordination of additional CO with the Rh(III) metallacycle intermediate, could be responsible for the following conversions. Subsequently, it is more facile for the six-coordinated Rh(III) metallacycle to sequentially undergo CO migratory insertion, cyclization, and reductive elimination to furnish the final cyclohexenone product in the case of the tethered enecyclopropene substrate. The cyclization step is found to be the rate-limiting step. A similar mechanistic pathway was also found for the Rh(I)-catalyzed carbonylative carbocyclization of tethered yne-cyclopropenes and vinyl cyclopropenes to produce phenol derivatives. The main difference for the vinyl cyclopropene substrate is that the conversion of Rh(I) carbenoid intermediate to the Rh(III) metallacycle proceeds via intramolecular 6π electrocyclization.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b02178. Figures S1−S8, Cartesian coordinates, and energies of all of the stationary points in the reactions (PDF) 12741


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(31) The formation of INT10b via 6π electrocyclization of a ketene intermediate, which could be generated from INT9b, could be ruled out due to the relatively higher energy barrier in comparison with the direct RE step via TS6b (Figure S7). (32) The transformation from INT10b to 6 assisted by one or two water molecules was shown in Figure S8.

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Mechanistic Insights into Cyclopropenes Involved Carbonylative

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