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Insights into the Mechanisms and Chemoselectivities of Carbamates and Amides in Reactions Involving Rh(II)-Azavinylcarbene: A Computational Study Abosede Adejoke Ogunlana, Jianping Zou,* and Xiaoguang Bao* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China Downloaded via GUILFORD COLG on July 17, 2019 at 12:14:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: Reactions involving Rh(II)-azavinylcarbenes (Rh(II)-AVCs) to synthesize nitrogen-containing compounds have attracted significant research interest. Despite the importance of these reactions, controlling the chemoselectivities in the reactions involving Rh(II)-AVC remains a challenge. To understand the mechanisms and factors controlling the chemoselectivities between N−H and CO groups of carbamates and amides in reactions involving Rh(II)-AVC, computational studies were employed. The results reveal that not only the greater nucleophilicity of the N−H group than that of the carbonyl group, but also the presence of H-bonding interactions, could favor the addition of the N−H group of primary carbamates to Rh(II)-AVC. However, for secondary carbamates and amides, they could undergo either chemoselective N−H or CO addition. Secondary carbamates with less steric hindrance, such as oxazolidinone, prefer the N−H addition mode. However, a switch in chemoselectivity (preference for the CO addition) was revealed for the sterically hindered secondary carbamates/amides. In addition, a possible O−H addition pathway via the keto−enol tautomerization for isatin and isatoic anhydride was disregarded due to the energetically demanding barrier. Instead, a pathway involving the chemoselective CO addition, formal [3 + 2] cycloaddition, followed by ring opening was proposed. The origins of the chemoselectivity and the factors responsible were addressed. group12d has also by means of density functional theory (DFT) calculations accounted for the preferred chemoselective addition of the carbonyl group of salicylaldehydes to Rh(II)AVC intermediates, experimentally carried out by the groups of Li12e and Deng.12f Amides and carbamates are important functional groups commonly found in the plethora of bioactive molecules, synthetic compounds, and materials.13 Notably, the versatility of the Rh(II)-AVC intermediate, derived from Rh(II)catalyzed denitrogenation of 1-sulfonyl-1,2,3-triazole (1), is also evident in its reactions with carbamates/amides to afford various N-containing compounds.10b,13d−g For instance, Fokin and co-workers described the preferred nucleophilic addition of the N−H group of primary (2) and secondary carbamates (4) over the carbonyl group to afford substituted enamides 3 and 5, respectively (Scheme 1a,b).10b More recently, Volla’s group13d has developed a protocol involving the reaction of Rh(II)-AVC with isatoic anhydride 6 to successfully produce 2-amino-benzoxazinones 7 (Scheme 1c). Moreover, Deng13e and Volla13f independently demonstrated the transformation of isatin 8 by the Rh(II)-AVC intermediate to access indigoid 9, a core structure of indigo dyes (Scheme 1d). Furthermore, the annulation of secondary aromatic amide 10 with the Rh(II)AVC intermediate was reported to produce pyrroloindoles 11 (Scheme 1e).13g From these experimental results, different
1. INTRODUCTION Over the years, metal carbenoids derived from transitionmetal-catalyzed decomposition of diazo compounds have consistently remained at the forefront of modern synthetic organic chemistry.1 Nevertheless, the limited stability of diazo compounds restricts their broad applicability. Thus, significant research attention has been drawn to more stable, sustainable, and safe non-diazo precursors to generate metal carbenoids.2 In this regard, rhodium(II)-azavinylcarbene (Rh(II)-AVC), derived from rhodium(II)-catalyzed denitrogenation of 1sulfonyl-1,2,3-triazole has been well demonstrated to be capable of initializing diverse reactions3−9 including cycloaddition,4 cyclopropanation,5 ring expansion,6 ylide formation,7 X−H insertion8,10 (X C, N, O, etc.), and others.9 In particular, the presence of the imino moiety in the Rh(II)-AVC intermediate allows both 1,1- and 1,3-insertions into the X−H bonds of the substrates to be achieved.10 To understand the preferred chemoselective insertion of Rh(II)-AVC into the X− H bonds, Zhang and co-workers11a have recently carried out mechanistic studies on the insertion of Rh(II)-AVC into O−H and C−H bonds of 1,3-diketones, which was experimentally reported by Bi’s group.11b Meanwhile, the reactions of Rh(II)AVC intermediates with the substrates containing carbonyl group have also been well established.12 For example, the nucleophilic addition of the carbonyl groups of N,Ndisubstituted amides to the Rh(II)-AVC intermediate was independently reported by Miura et al.,12a Yoo,12b and Lee12c to generate the cis-diamino enones. More recently, our © 2019 American Chemical Society
Received: April 18, 2019 Published: May 24, 2019 8151
DOI: 10.1021/acs.joc.9b01070 J. Org. Chem. 2019, 84, 8151−8159
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amides to the Rh(II)-AVC intermediate? (3) Are there potential mechanistic differences in the reactions involving substrates 6 and 8? An in-depth understanding of the mechanisms of these reactions would be helpful to aid the development of chemoselective reactions involving Rh(II)AVC.
Scheme 1. Rh(II)-Catalyzed Reactions of 1 with Carbamates/Amides
2. COMPUTATIONAL METHODS All calculations were carried out by the DFT method as implemented in the Gaussian 09 program package.14 The ωB97XD hybrid density functional15 was employed for the geometry optimization of all stationary points including the reactants, products, intermediates, and transition states. The LANL2DZ basis set16 together with the LANL2DZ pseudopotential was used to describe the Rh atom, and the 6-31G(d) basis set17 was utilized for other atoms. Vibrational frequency analyses were carried out to characterize the stationary points as the minima or transition states. To verify that each transition state connects to its appropriate reactant and product, the intrinsic reaction coordinate calculations18 were employed. The solvent effect of 1,2-dichloroethane in the reaction was evaluated using the SMD solvation model developed by Truhlar and Cramer.19 This model was used for single-point energy calculations based on all of the gas-phaseoptimized geometries at a larger basis set (LANL2TZ(f) for the Rh atom and 6-311++G(d,p) for other atoms). For the purpose of discussion, the solvation Gibbs free energy was used, and it was obtained from the addition of solvation single-point energy and gasphase thermal correction to the Gibbs free energy. The threedimensional structures of the studied species are shown using CYLView software.20
nucleophilic addition modes besides the reported N−H addition for carbamates/amides are noteworthy. From a mechanistic perspective, two different pathways could stem from the nucleophilic addition of carbamates/ amides to the Rh(II)-AVC intermediate. The first involves the chemoselective addition of the N−H group of carbamates/ amides to the Rh(II)-AVC intermediate, generating the intermediate A. Subsequently, proton transfer could follow to produce C. Conversely, the carbonyl group could chemoselectively undergo nucleophilic addition to the carbenoid carbon of Rh(II)-AVC, forming the intermediate B. Thereafter, formal [3 + 2] cycloaddition could occur to produce the intermediate D, from which subsequent transformations could follow to yield the final products (Scheme 2). As a part of our
3. RESULTS AND DISCUSSION 3.1. Mechanistic Studies of the Rh(II)-Catalyzed Reactions of 1 with 2 and 4 To Produce Enamide Products. The DFT studies on the tautomerization of 1 to diazoimine and subsequent denitrogenation have been well reported to generate the key Rh(II)-AVC.11a,12d The initial complex INT1, formed by the coordination of 1 to the Rh(II) catalyst, could undergo the tautomerization of 1 to form the Rh(II)−diazoimine complex, INT2, via the transition state TS1, in which the Na···Nb bond distance is lengthened to 2.03 Å. The energy barrier required for this step is 27.9 kcal/mol relative to the separated 1 and the catalyst. The generated INT2 would then easily release N2 to afford the key Rh(II)AVC intermediate (INT3) via TS2 (Figure S1). After the formation of INT3, the two possible nucleophilic attacks of the N-atom of the N−H group and the O-atom of the carbonyl group in 2, to the carbenoid carbon (C1), were explored computationally. The located transition state (TS3a) for the N−H attack at C1 of INT3 reveals that the N−H group adopts a face-to-face attacking mode21 with the carbenoid moiety, such that a bifurcated H-bonding interaction (HBI) exists between the N−H group and INT3. The HBI distance between the N−H group and the iminic nitrogen atom (N− H1···Na) is 2.11 Å and that between the N−H group and the bridging oxygen atom (N−H2···O4) is 2.10 Å. Relative to the separated INT3 and 2, the energy barrier needed for the N−H addition to generate INT4a is 6.9 kcal/mol. On the other hand, the O atom of the carbonyl group of 2 could compete with the N−H group in attacking C1 of INT3. The inspection of the geometry for the O attack (TS4a) reveals that it also benefits from the HBI between the N−H group and the iminic nitrogen (N−H1···Na distance of 2.08 Å). Computational result showed that the CO attack would overcome an energy barrier of 11.6 kcal/mol, which is 4.7 kcal/mol higher than that for TS3a, hence suggesting that the N−H addition pathway is
Scheme 2. Proposed Mechanistic Pathways for the Reaction of Rh(II)-AVC Intermediate with Carbamates/Amides
continuous effort to understand the chemoselectivities in reactions involving Rh(II)-AVC, we present a detailed mechanistic study on the reactions of Rh(II)-AVC with the substrates 2, 4, 6, 8, and 10 by DFT calculations to address the following questions. (1) Why do substrates 2 and 4 undergo N−H addition while substrates 6, 8, and 10 do not? (2) What factor(s) controls the chemoselective addition of carbamates/ 8152
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computationally. The geometrical examination of the optimized TS3b for the N−H addition also reveals that the N−H group of 4 approaches INT3 in a face-to-face mode for HBI to be formed between the N−H group and the iminic nitrogen (N1−H···Na distance of 1.93 Å). The required energy barrier for this addition to generate INT4b is 6.0 kcal/mol relative to the separated 4 and INT3. From INT4b, the product 5 is expected to be formed easily via proton transfer from the N−H group to the iminic nitrogen. For the competitive CO addition of 4 to INT3, the mode of approach of the CO group to C1 in the localized TS (TS4b) is also similar to that of TS4a. The predicted energy barrier for this addition is 9.0 kcal/mol, which is 3.0 kcal/mol higher than that of TS3b (Figure S3). One may propose that the nucleophilic addition of the O−H group of 4′ (the tautomer of substrate 4) to INT3 is also a potential pathway.10b The O−H addition step via TS5b takes place simultaneously with the proton transfer from the hydroxyl group to the iminic nitrogen. However, the energy barrier for this pathway is substantially higher than those for TS3b and TS4b (Figure S3). The computational result clearly implies that the N−H addition of 4 to INT3 would preferentially take place over the addition of the carbonyl group or the tautomerized hydroxyl group, which is in agreement with the experimental report.10b Presumably, the methylene group adjacent to the N−H group has a less pronounced steric effect, thus resulting in the preferred N−H addition to INT3. When the two H atoms of the aforementioned methylene group were replaced by a bulkier group such as methyl (4a), a switch in chemoselectivity was predicted, favoring the CO addition (Figure S4). Therefore, less steric congestion together with the greater nucleophilicity of the N−H group accounted for the preferred chemoselective N−H addition in 4. 3.2. Mechanistic Study of Rh(II)-Catalyzed Reaction of 1 with Isatoic Anhydride 6 to Afford 2-Aminobenzoxazinones 7. The Rh(II)-catalyzed reaction of 1 with isatoic anhydride 6 was experimentally reported to generate 2amino-benzoxazinones 7.13d Unlike substrate 4, we hypothesize that the N−H addition of secondary carbamate (6) to INT3 might be hindered due to the presence of a fused benzene ring. As a result, different chemoselectivity might be achieved. To validate this hypothesis, the addition of the proximal CO1 group, the distal CO2 group, and the N−H group of 6 to INT3 were analyzed computationally. The located TS (TS4c) for the proximal CO1 addition adopts similar face-to-side attacking mode as TS4a and TS4b such that steric repulsion between the heterocyclic ring of 6 and the phenyl ring of INT3 was avoided. The presence of the HBI between the imino group of INT3 and N−H of 6, which would be subsequently referred to as stabilizing HBI, is evident for such an attacking mode. Relative to the separated 6 and INT3, the computed energy barrier for the proximal CO1 addition to form INT5c is 9.5 kcal/mol. The distal CO2 addition to INT3 also adopts a similar attacking mode as does TS4c, having its C1···O2 distance shortened to 1.89 Å (TS5c). Although the steric repulsion between the heterocyclic ring of 6 and the phenyl ring of INT3 was avoided, the stabilizing HBI is absent in this TS. This is reflected in the calculated energy barrier of 15.3 kcal/mol, which is 5.8 kcal/mol higher than that of TS4c. The N−H addition of 6 to INT3 was considered, revealing the shortening of the C1···N1 distance to 2.02 Å in the located TS3c. The planar geometry of 6, brought about by the fused aromatic ring, dictates the face-to-face mode of attack
kinetically and thermodynamically more favorable than the CO addition pathway (Figure 1). This result is consistent
Figure 1. Energy profiles for the nucleophilic addition of the N−H group and the carbonyl group of 2 to the carbenoid of INT3. Bond distances are given in Å.
with the experimental report.10b After the formation of ammonium ylide INT4a, proton transfer from the ammonium nitrogen to Na would be a facile process. The predicted energy barrier for the proton transfer via TS5a is only 3.3 kcal/mol relative to INT4a, and the resulting enamide product 3 is thermodynamically very stable. It is worth noting that the nucleophilic character of the N− H group in 2 is greater than that of the CO group,22 which might be an added advantage for the nucleophilic addition of the N−H group to the electrophilic C1 of INT3. On transiting to the transition state, it was observed that bifurcated HBI exists in TS3a, while single HBI was found in TS4a. This implies that the additional HBI in favor of TS3a could further facilitate the N−H nucleophilic attack, thus promoting the chemoselective addition of the N−H group to INT3a. Collectively, these factors might be responsible for the preferred chemoselective N−H addition observed for the reaction of 1 with 2. Analogously, the Rh(II)-catalyzed reaction of 1 with oxazolidinone (4), a secondary carbamate, was also analyzed 8153
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Figure 2. Energy profiles for the various nucleophilic additions of 6 to INT3 and the subsequent formation of the product 7. Bond distances are given in Å.
INT3 by the O−H tautomer of 6, 6′, to generate an oxonium ylide intermediate.23 Relative to the separated INT3 and 6, an energy barrier of 30.1 kcal/mol is required for the O−H addition, which is significantly higher than that for TS4c by 20.6 kcal/mol, indicating that this pathway is unfeasible for the reaction of 1 and 6 (Figure 2). In light of these computational results, it is clear that there is an overwhelming preference for the proximal CO1 addition to INT3. This is due to the less pronounced steric repulsion between the heterocyclic ring of 6 and the phenyl ring of INT3, as well as the stabilizing effect of the HBI. From INT5c, the intramolecular [3 + 2] cycloaddition via TS7c would take place to afford INT7c. The energy barrier for
of the N−H group when approaching INT3. This mode via TS3c causes substantial steric repulsion between the phenyl ring of INT3 and the heterocyclic ring of 6. In spite of the existence of the stabilizing HBI in TS3c, the pronounced effect of the steric hindrance might be responsible for its higher energy barrier than that of TS4c by 8.4 kcal/mol, signifying that this nucleophilic addition is unlikely for this reaction (Figure 2). This further validates our hypothesis that when bulky groups (in this case, aromatic ring) are attached to the N−H group, the chemoselectivity disfavors the nucleophilic addition of the N−H group to INT3. Moreover, a possible mechanism for the Rh(II)-catalyzed reaction of 1 with 613d might involve the initial trapping of 8154
DOI: 10.1021/acs.joc.9b01070 J. Org. Chem. 2019, 84, 8151−8159
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Figure 3. Energy profiles for the various nucleophilic additions of 8 to INT3 and the subsequent formation of INT7d. Bond distances are given in Å.
Also, the addition of the distal CO2 bond to INT3 is located as TS5d. It is quite noticeable that the stabilizing HBI is absent in the geometry of this TS. This is reflected in its higher energy barrier than that of TS4d by 5.1 kcal/mol. The located TS3d for the N−H addition to INT3 on the other hand features the existence of the stabilizing HBI. However, the face-to-face mode of attack of the N−H group when approaching INT3 causes the aforementioned steric repulsion between the rings of 8 and INT3. In agreement with our hypothesis, the N−H addition to INT3 is disfavored from both kinetic and thermodynamic standpoints owing to the fused aromatic ring around the N−H group of 8 (Figure 3). In addition, the highly demanding energy barrier predicted for the O−H addition pathway24 via TS6d (32.0 kcal/mol relative to the separated 8
the cycloaddition process is 9.9 kcal/mol relative to INT5c. Subsequently, the cleavage of C···O1 would occur to afford INT8c from which tautomerization would take place to yield the final product 7. 3.3. Mechanistic Study of Rh(II)-Catalyzed Reaction of 1 with Isatins 8 To Produce Indigo Analogue 9. Similarly, the nucleophilic addition of the proximal and distal CO and the N−H groups of isatin 8 to INT3 was explored computationally. The structural examination of the optimized TS4d for the proximal CO1 nucleophilic addition showed that the CO1 bond is oriented in such a way as to avoid the aforementioned steric repulsion. Interestingly, the presence of the stabilizing HBI is evident for such an orientation. As a result, a low energy barrier of 9.2 kcal/mol relative to the separated 8 and INT3 is required for the CO1 addition. 8155
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Figure 4. Energy profiles showing the formation of the product 9 from the spirocyclic intermediate INT7d. Bond distances are given in Å.
different products 7 and 9, respectively. To provide answer to this puzzling question, the formation of the aziridine intermediate was explored from INT7c. For substrate 6, a concerted mechanism involving the simultaneous C3−O1 bond rupture and C3···C2 bond formation would occur via TS9c to produce the aziridine intermediate (INT9c). The predicted energy barrier for INT7c to generate INT9c is 25.5 kcal/mol (Figure S5), which is 3.6 kcal/mol higher than that required for it to undergo the C3−O1 bond rupture to form INT8c. To further account for the preferred formation of INT8c over INT9c, the calculation of the isodesmic reaction energy was carried out. The result revealed the exothermicity of the isodesmic reaction, which indicates that the N−H bond in 6 is more acidic than that in 8.25 As a result, INT7c would readily undergo proton transfer alongside the ring-opening process via TS8c, thereby impeding the formation of the aziridine intermediate. Similar to INT8c, it might be suggested that alkenol intermediate INT9d for substrate 8 could tautomerize to INT9d′. Unlike for substrate 6, the formed INT9d′ is thermodynamically less stable than INT7d (Figure 4). Subsequent transformation via the aziridine intermediate would occur to afford the thermodynamically more stable product 9.
and INT3) disregards its possibility in the reaction of 1 with 8 (Figure 3). Having addressed the chemoselective addition of CO1 of 8 to INT3, the resulting intermediate INT5d would readily undergo [3 + 2] cycloaddition via TS7d to afford the spirocyclic intermediate INT7d, with an energy barrier of 5.9 kcal/mol relative to INT5d. Subsequently, two major possible routes, the stepwise and concerted, could take place to afford an aziridine intermediate INT10d. The stepwise route commences with the C3−O1 bond rupture via TS8d to generate INT8d, from which proton transfer could follow readily from the N−H group to the carbonyl oxygen to produce a more stable enol tautomer INT9d. Consequently, intramolecular electrocyclization leading to the formation of C2−C3 bond coupled with proton transfer from the hydroxyl group to the N atom might occur concurrently via TS9d to afford the aziridine intermediate INT10d. Relative to INT7d, the energy barrier for this stepwise route to afford INT10d is 24.8 kcal/mol. On the other hand, the cleavage of the C3−O1 bond could simultaneously induce the nucleophilic addition of C2 to C3 via TS10d (concerted route) to produce INT10d. The energy barrier for the concerted route is also 24.8 kcal/ mol, suggesting that both the concerted and the stepwise routes are possible from INT7d to generate the aziridine complex INT10d (Figure 4). Next, the Rh(II) catalyst could act as a Lewis acid in promoting the breaking of the C3−Na bond via TS11d, thus producing INT11d. Finally, rearrangement of INT11d and dissociation of the Rh(II) catalyst would lead to the product 9. Therefore, after the formation of spirocyclic intermediate, the mechanism of the reaction would successively follow the C−O bond rupture, formation of aziridine intermediate, ring opening, and rearrangement to generate 9. One may wonder why the Rh(II)-catalyzed reaction of 1 with substrates 6 and 8, although quite similar, produces
For the Rh(II)-catalyzed reaction of 1 with 3-benzylideneindolin-2-ones 10 to produce pyrroloindoles 11, computational results suggest that the N−H addition pathway is thermodynamically and kinetically less favorable than that of the carbonyl group, which is consistent with our previous results. Alternatively, the alkene moiety in 10 could undergo cyclopropanation with INT3 via TS5e. However the energy 8156
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insights provided would further guide the chemoselective reactions involving Rh(II)-AVC with substrates containing multiple nucleophilic groups.
barrier for the addition is considerably higher than the carbonyl addition by 16.2 kcal/mol, implying the unfeasibility of the cyclopropanation pathway. Based on these computational results, it is obvious that the reaction of 1 with 10 would proceed through the proposed CO1 addition pathway to generate INT5e (Figure S6). After the formation of INT5e, the formal [3 + 2] cycloaddition would easily take place via TS6e to yield the thermodynamically stable spirocyclic intermediate INT6e. This cyclization process, which is accompanied by the dissociation of the Rh(II) catalyst, would require the barrier of 7.2 kcal/mol relative to INT5e. Subsequently, the C−O bond cleavage via TS7e would result in the formation of INT7e. From this intermediate, intramolecular nucleophilic attack via TS8e would take place to afford the final pyrroloindole product 11. This finding is in consonant with the experimental result.13g The delocalization effect of the lone pair of electron on the nitrogen atom with the π electron of the phenyl ring could render the N−H groups in 6 and aromatic amides (8 and 10) weaker nucleophiles when attacking INT3 compared with the aliphatic carbamates 2 and 4. This could contribute to the difference in the preferred chemoselective addition between 2/ 4 and 6/8/10. For comparison between secondary carbamate 4 (prefers the N−H addition pathway) and 6/8/10 (CO addition pathway), we have demonstrated that bulky substituents attached to the N−H group could cause a switch in chemoselectivity, favoring the CO1 addition pathway. It is presumed that the planar geometries of 6, 8, and 10 brought about by the fused aromatic ring dictate the attacking mode of the N−H group to INT3, hence causing substantial steric hindrance between the phenyl ring of the Rh(II) carbenoid and those of these substrates. Meanwhile, such a steric hindrance is absent in 4. Collectively, the reasons stated above are responsible for the different chemoselectivities observed in the primary carbamates and the secondary carbamates/amides explored.
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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.joc.9b01070.
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Figures S1−S6, Cartesian coordinates and energies of all of the stationary points in the reactions (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (J.Z.). *E-mail:
[email protected] (X.B.). ORCID
Jianping Zou: 0000-0002-8092-9527 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 and 2147213) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financial support.
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REFERENCES
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4. CONCLUSIONS The mechanistic studies of the Rh(II)-catalyzed reaction of 1sulfonyl-1,2,3-triazoles 1 with primary carbamates and secondary carbamates/amides were investigated by means of DFT calculations. For the reaction of methylcarbamate 2 with the Rh(II)-AVC intermediate, computational results suggest that the nucleophilic addition of the N−H group to Rh(II)AVC is preferred over that of the CO group. The preference can be attributed to the greater nucleophilicity of the N−H group and the presence of the bifurcated hydrogen bonding interaction (HBI) in the N−H addition as against the single HBI in the CO addition. However, for sterically hindered secondary carbamates/amides (6/8/10), the reverse is the case (preference for the CO addition). The reason for the switch in the chemoselectivity was attributed to the presence of the fused aromatic ring, which restricts the attack of the N−H group on the Rh(II)-AVC due to substantial steric repulsion between the phenyl ring of the carbenoid and the heterocyclic ring of 6/8/10. Such a steric hindrance is absent in 2/4, allowing the flexibility of the N−H group in attacking the Rh(II)-AVC. In addition, the proposed O−H addition pathway via the keto−enol isomerization for the reaction of 1 with 6 or 8 would not be operative. Instead, a pathway involving the chemoselective proximal CO addition, formal [3 + 2] cycloaddition, ring opening, and subsequent conversions was proposed. It is hoped that the mechanistic 8157
DOI: 10.1021/acs.joc.9b01070 J. Org. Chem. 2019, 84, 8151−8159
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