Mechanism and Origins of Z Selectivity of the Catalytic

Article pubs.acs.org/Organometallics

Mechanism and Origins of Z Selectivity of the Catalytic Hydroalkoxylation of Alkynes via Rhodium Vinylidene Complexes To Produce Enol Ethers Yanfeng Dang,† Shuanglin Qu,† Zhi-Xiang Wang,*,† and Xiaotai Wang*,‡ †

College of Chemistry and Chemical Engineering, Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ‡ Department of Chemistry, University of Colorado Denver, Campus Box 194, P.O. Box 173364, Denver, Colorado 80217-3364, United States S Supporting Information *

ABSTRACT: We report the first theoretical study of transition-metal-catalyzed hydroalkoxylation of alkynes to produce enol ethers. The study utilizes density functional theory calculations (M06) to elucidate the mechanism and origins of Z selectivity of the anti-Markovnikov hydroalkoxylation of terminal alkynes with a Rh(I) 8-quinolinolato carbonyl chelate (1cat). The chosen system is, without any truncation, the realistic reaction of phenylacetylene and methanol with 1cat. Initiation of 1cat through phenylacetylene substitution for carbonyl generates the active catalyst, a Rh(I) η2-alkyne complex (3), which tautomerizes via an indirect 1,2hydrogen shift to the Rh(I) vinylidene complex 4. The oxygen nucleophile methanol attacks the electrophilic vinylidene Cα, forming two stereoisomeric Rh(I) vinyl complexes (15 and 16), which ultimately lead to the (Z)- and (E)-enol ether products. These complexes undergo two ligand-mediated proton transfers to yield Rh(I) Fischer carbenes, which rearrange through a 1,2β-hydrogen shift to afford complexes with π-bound product enol ethers. Final substitution of phenylacetylene gives (Z)- and (E)PhCHCHOMe and regenerates 3. The anti-Markovnikov regioselectivity stems from the Rh(I) vinylidene complex 4 with reversed Cα and Cβ polarity. The stereoselectivity arises from the turnover-limiting transition states (TSs) for the Rh(I) carbene rearrangement: the Z-product-forming TS24 is sterically less congested and hence more stable than the E-product-forming TS25. The difference in energy (1.2 kcal/mol) between TS24 and TS25 gives a theoretical Z selectivity that agrees well with the experimental value. Calculations were also performed on the key TSs of reactions involving two other alkyne substrates, and the results corroborate the proposed mechanism. The findings taken together give an insight into the roles of the rhodium− quinolinolato chelate framework in directing phenylacetylene attack by trans effect, mediating hydrogen transfers through hydrogen bonding, and differentiating the energies of key TSs by steric repulsion.

1. INTRODUCTION

catalyzed anti-Markovnikov hydroalkoxylation of terminal alkynes and aliphatic alcohols, as shown by Scheme 1.6 The Rh(I) 8-quinolinolato carbonyl complex (1cat), though known for years, has not been well explored for catalysis.6,7 This hydroalkoxylation reaction generates the C−O bond only at the terminal carbon and is not suitable for internal alkynes. Such results led Kakiuchi et al. to reason that the reaction may proceed via a rhodium vinylidene complex, followed by addition of alcohol as an O-nucleophile to the vinylidene ligand.6 However, the mechanism and origins of Z selectivity for this novel and significant catalytic reaction have not been explored and remained unclear at the outset of this work.

The concept of atom economy has motivated chemists to seek efficient ways to generate carbon−carbon and carbon− heteroatom bonds, the essential linkages in all organic molecules.1 Enol ethers contain both such linkages and act as useful intermediates in organic synthesis. Addition of alcohols to alkynes provides a straightforward entry into enol ethers, but the reaction traditionally requires the presence of strong bases as well as harsh conditions.2 Recently, there has been much interest in atom-economical syntheses of enol ethers through the hydroalkoxylation of alkynes with transition-metal catalysts under mild conditions.3 Although progress has been made on intramolecular cyclization reactions of alkynols,4 intermolecular hydroalkoxylation, particularly of terminal alkynes, for the stereoselective synthesis of enol ethers remains a difficult task.5 Recently, the Kakiuchi group made a breakthrough and developed the Rh(I)© 2013 American Chemical Society

Received: March 16, 2013 Published: April 29, 2013 2804

dx.doi.org/10.1021/om400227u | Organometallics 2013, 32, 2804−2813

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Scheme 1. Z-Selective Hydroalkoxylation of Terminal Alkynes with 1cat

3. RESULTS AND DISCUSSION Thermodynamically, the reaction of phenylacetylene (sub1) and methanol with 1cat (Scheme 2) is favorable, with ΔG° = −15.6 kcal/mol for the Z product and ΔG° = −16.2 kcal/mol for the E product. The reaction is exothermically driven, because the net change in bond formation is breaking a weak π bond to make a strong σ bond. The reaction mechanism proposed on the basis of our DFT calculations consists of five stages: (1) the formation of a Rh(I) alkyne complex and its transformation into a Rh(I) vinylidene complex, (2) the addition of the O-nucleophile MeOH to the vinylidene α-carbon, (3) two consecutive hydrogen transfers mediated by the 8-quinolinolato ligand to give a Fischer carbene intermediate, (4) the isomerization of the Fischer carbene through a 1,2-β-hydrogen shift to afford a complex containing the π-bound product enol ether, and (5) the release of the enol ether upon attack by a substrate alkyne molecule, regenerating the Rh(I) alkyne complex, which is the active catalytic species. We have identified the most favorable pathways leading to the (Z)- and (E)-enol ethers after exploring various possibilities beginning with catalyst initiation. The unfavorable pathways that have been ruled out include concerted additions of MeO−H across the RhC and CC double bonds in the vinylidene complex, as well as hydrogen transfer to the Rh(I) center followed by reductive elimination to afford the product. 3.1. Alkyne-to-Vinylidene Transformation. Rh(I) with a d8 electron configuration tends to form square-planar 16e complexes, and such a complex can undergo a tandem substitution/tautomerization with a terminal alkyne to afford a Rh(I) vinylidene complex, on the basis of the pioneering studies by Werner et al.19 When the Rh(I) complex 1cat reacts with phenylacetylene, the latter can substitute for either CO ligand, and the resulting η2-alkyne complex can transform to the corresponding Rh(I) vinylidene (Scheme 3).

We have conducted density functional theory (DFT) calculations on the experimentally performed hydroalkoxylation of phenylacetylene (sub1) and methanol with 1cat (Scheme 2), Scheme 2. Hydromethoxylation of Three Alkynes with 1cat

which suggests an unusual mechanism that involves ligandmediated hydrogen transfer and Fischer carbene isomerization. The energetics of the most favorable pathways leading to the Z and E products give a theoretical Z selectivity that agrees with experimental results. To corroborate the proposed mechanism, we have also considered the reactions involving two other alkyne substrates (sub2 and sub3). This article gives a detailed mechanistic account of this catalytic reaction, including the full catalytic cycle and the origins of Z selectivity. To our knowledge, this represents the first theoretical work on the hydroalkoxylation of alkynes catalyzed by transition metals.

Scheme 3. Possible Routes for the Alkyne-to-Vinylidene Transformation via Substitution for CO

2. COMPUTATIONAL METHODS All calculations were performed with Gaussian 098 at the M069 level of density functional theory. The recently developed M06 functional was chosen, because it includes noncovalent interactions and can give accurate energies for organotransition-metal systems on the basis of validation and other studies.10−15 Geometry optimizations and frequency calculations were performed at the M06/BS1 level in the gas phase with default convergence criteria, BS1 designating a mixed basis set of SDD16 for the rhodium and 6-31G(d) for other atoms. Frequency outcomes were examined to confirm stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency). When necessary, intrinsic coordinate reaction (IRC) calculations were carried out to examine the connection of a transition state with its backward and forward minima.17 The M06/BS1-optimized geometries were used for solvent-corrected single-point energy calculations at the M06/BS2 level, with solvation effects modeled by SMD18 in n,n-dimethylacetamide (or DMA, the solvent used for the reaction), BS2 denoting a mixed basis set of SDD for rhodium, and 6311++G(2d,2p) for other atoms. The M06/BS1-calculated frequencies were used to obtain zero-point energy-corrected enthalpies and free energies at 298.15 K and 1 atm both in the gas phase and in solution. Natural bond orbital (NBO) analyses were performed at the M06/BS2 level on selected systems. Free energies in solution were discussed and enthalpies given for reference.

We established through calculations path T1 as the favored route, its details being discussed here, and excluded path T2.20a,b Ligand substitution in square-planar complexes such as 1cat generally takes place by an associative mechanism, and as shown by the free energy profile in Figure 1, the substitution of phenylacetylene for CO trans to the pyridine in 1cat occurs associatively via the transition state (TS) TS1 to form the trigonal-bipyramidal (tbp) intermediate 2, which then dissociates via TS2 to give the Rh(I) η2-alkyne π-complex 3. Of the 2805


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Figure 1. Free energy profile for the reaction of 1cat and phenylacetylene to form the Rh(I) vinylidene complex 4, including three pathways for the alkyne-to-vinylidene isomerization. Energies are relative to 3 and are mass-balanced (similarly hereafter).

intermediate 7, which may be regarded as a π-bound vinylidene complex, is unstable and converts via TS4 to the Rh(I) vinylidene complex 4.29 The extremely high barrier TS3 (41.6 kcal/mol) renders this pathway unfavorable. Path T1B (red) proceeds by an indirect 1,2-hydrogen shift via TS5 (22.0 kcal/mol),30 which connects 3 and 4, as shown by following the IRC in both directions.20c (The corresponding TS in path T2B is 25.8 kcal/mol.20b) The IRC results suggest that the negatively charged 8-quinolinolato oxygen atom facilitates the hydrogen shift, as it helps stabilize TS5 by electrostatic or hydrogen bond interactions with the migrating hydrogen atom (the O···H distance is 2.33 Å). In TS5, the phenyl ring is aligned approximately orthogonal to the rhodium−quinolinolato chelate framework to minimize steric repulsions, and such a configuration is retained in the resulting complex 4. Path T1C (blue) begins with the oxidative cleavage of the C− H bond via TS6 (20.8 kcal/mol), forming the alkynyl hydride intermediate 8. However, the transition state TS7 for the following 1,3-hydrogen shift could not be located in the geometry optimizations. The corresponding TS in path T2 was found at a high energy level (55.1 kcal/mol relative to 3).20b In addition, the Hall group calculated a 1,3-hydrogen shift TS stemming from and lying 36 kcal/mol above the experimentally relevant complex trans-[RhI(PiPr3)2(η1-PhCC)(η2-PhC CH)].29 Thus, we reason that TS7 is significantly higher than TS5, thereby making path T1C less favorable than path T1B. A σ complex intermediate with an η2-C−H bond to the metal center could sometimes be located in the 1,2-hydrogen shift and oxidative addition pathways,22,23 but it does not exist in paths T1B and T1C. (It occurs in paths T2B and T2C.20b) We conclude that the indirect 1,2-hydrogen shift is the most favorable pathway for the alkyne-to-vinylidene transformation

pyridine N and phenoxide O attachments of 8-quinolinolato to Rh(I) in 1cat, the former has a stronger trans effect21 and directs alkyne attack to form complex 3, as indicated by the energies of TS1 and TS2 being lower than those of the corresponding TSs in path T2 by 3.3 and 4.5 kcal/mol, respectively.20a In addition, 3 is thermodynamically more favorable by 3.7 kcal/mol than 5 of path T2. The precatalyst initiation from 1cat to 3 is endergonic by 16.9 kcal/mol, yet 3 should occur in a significant amount because of the equilibrium shift by the large excess of phenylacetylene and the evolution of CO as a gas. Kinetically, the largest activation free energy required for the initiation is 19.5 kcal/mol (TS1−1cat), which is attainable under the reaction conditions (70 °C). The mechanism for the terminal alkyne to vinylidene isomerization within metal complexes has been studied extensively by computational and experimental means.22−29 For d6 metal systems such as Ru(II) and Mn(I) complexes, the reaction generally proceeds via either a direct 1,2-hydrogen shift or an indirect 1,2-hydrogen shift.23−25 However, electron-rich d8 metal complexes such as Rh(I), according to early studies, tend to undergo a formal oxidative addition of the C−H bond to give an alkynyl hydride intermediate, followed by a 1,3-hydrogen shift to afford the vinylidene complex.26−28 Although both uni- and bimolecular pathways were proposed for the 1,3-hydrogen shift, a combination of crossover experiments and DFT calculations has essentially ruled out the bimolecular mechanism.27,28 We considered all three possible pathways starting from 3 and computed the transition states and intermediates, as shown in Figure 1. Path T1A (green) is the direct 1,2-hydrogen shift pathway, which begins with the hydrogen migrating via TS3 without much change in the geometry of the η2-alkyne.29 The resulting 2806


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Figure 2. Free energy profiles for the concerted addition of methanol to RhCα and ensuing reductive elimination leading to the Z and E products: [Z] = (Z)-PhCHCHOMe, [E] = (E)-PhCHCHOMe. Complex 3′ is the enantiomorph of 3.

Markovnikov fashion with respect to the alkyne.32 Some metal vinylidenes (mostly those of ruthenium) were reported to react with the O-nucleophile alcohol (ROH) to give alkoxycarbene complexes of the type [M]C(OR)R′.33 In complex 4, NBO charges on Rh, Cα, and Cβ are respectively −0.052, +0.347, and −0.440. We first considered four-center concerted additions of MeO−H across the CαCβ and RhCα double bonds, with the methoxy group attacking the electrophilic Cα. There are four possible pathways for the concerted methanol addition across CαCβ, leading to four isomeric Rh(I) carbene complexes.20d Even the lowest energy barrier (TS12-s) of any of these pathways is extremely high at 52.6 kcal/mol, suggesting that this concerted addition would be implausible.20d The concerted addition of methanol to RhCα would form five-coordinate square-pyramidal vinyl hydride complexes, which could undergo reductive elimination to yield the (Z)- and (E)enol ethers. Similar metal vinyl hydrides undergoing reductive elimination have been proposed to occur in the sequence of metal-catalyzed reactions (e.g., addition of carbamates to terminal alkynes to give vinylcarbamates) that presumably proceed via a metal vinylidene intermediate.32,34 We identified four possible pathways for the concerted addition of methanol to RhCα,20e and Figure 2 shows the free energy profiles with all

for [RhI(8-quinolinolato)(CO)(η2-PhCCH)] (3). Unusual as this result may seem for a d8 metal complex, it is not unprecedented, because the aforementioned work by the Hall group reached the same conclusion for trans-[RhI(PiPr3)2(η1PhCC)(η2-PhCCH)].29 It is worth noting that both complex 3 and trans-[RhI(PiPr3)2(η1-PhCC)(η2-PhC CH)] contain the bulky ligand PhCCH, as opposed to [RhI(PR3)2Cl(η2-CHCH)], on which the oxidative addition/ 1,3-hydrogen shift pathway was computed for Rh(I) complexes.27,28 It seems that the bulky PhCCH in place of CH CH disfavors the 1,3-hydrogen shift because of the steric hindrance of the phenyl ring. Our result is also consistent with a previous general observation that the precise mechanism for the metal-mediated alkyne-to-vinylidene conversion is highly system dependent, given the subtleties of the reaction.31 The transformation from 3 to 4 is endergonic by 8.1 kcal/mol. This is compensated by the subsequent exergonic steps, and the full catalytic cycle beginning with 3 is exergonic by approximately 16 kcal/mol for both the Z and E products (see below). 3.2. Addition of Methanol as a Nucleophile. The terminal alkyne-to-vinylidene isomerization on a metal fragment results in the reversal of Cα and Cβ polarity, so that Cα becomes electrophilic and susceptible to nucleophilic attack in an anti2807


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TS12 and TS13. TS12 and TS13 progress to the zwitterionic Rh(I) vinyl complexes 15 and 16, respectively, where the (8quinolinolato) O···H hydrogen bonds are further strengthened and the methanol O−H bonds further weakened. In both 15 and 16, the protic H (green bold) is anti to Cβ of CαCβ and hence cannot add to Cβ. The protic H may add to the Rh(I) center via a three-center (MeO···H···Rh) TS, but optimization attempts at relevant TSs all converged to the four-center TS8 and TS10 (Figure 2), which are for the concerted additions that have been ruled out. Thus, in 15 or 16 a proton transfer cannot happen at either Rh or Cβ. In summary, the direct addition of methanol to RhCα or CαCβ, whether in a stepwise or concerted manner, would be implausible for the system in question. This conclusion prompted us to search for alternative pathways. 3.3. Pathways via Ligand-Mediated Hydrogen Transfer. The short (8-quinolinolato) O···H distances in 15 and 16 led us to envisage proton transfer to 8-quinolinolato oxygen induced by the strong hydrogen bonds. Indeed, we located the relevant TS14 and TS16, which progress to intermediates 17 and 19, respectively (Figure 4). Both reactions have a thermodynamic driving force, with ΔG° = −7.5 kcal/mol for the formation of 17 and ΔG° = −7.0 kcal/mol for the formation of 19. Kinetically, TS14 is slightly higher than 15 and TS16 is slightly higher than 16 in electronic energy in the gas phase, but in solution, the corrected free energies of TS14 and TS16 become somewhat lower, apparently due to an error with the solvation modeling. This could mean facile proton transfer reactions without much of a kinetic barrier. NBO analyses show partial negative charges on both Rh and vinylidene Cβ in 17 and 19 (Table 1), suggesting that the protonated ligand could next transfer the proton from the 8-quinolinolato oxygen to Rh or vinylidene Cβ. The Rhbound and Cβ-bound proton transfers are further justified because they bear similarities to known organometallic reactions, the former being some type of α-elimination35 and the latter being electrophilic attack by H+ at the Cβ of vinyl ligands to give carbene complexes.36 In reality, the proton transfer to Cβ would be unlikely to happen with the conformations of 17 and 19 because of the large H−Cβ separations, but the rotation of the Rh−Cα single bond could give rotamers able to facilitate such a hydrogen shift. We considered these possibilities and located the rotamers 18 and 20, each of which was found to inaugurate the most favorable pathways for proton transfer to both Rh and Cβ. Figure 5 shows the free energy profiles with all TSs and intermediates for the most favorable pathways of the Rh-bound proton transfer, which ultimately lead to the Z and E products. In path L1A (red), the Rh(I) vinyl complex 18 overcomes the proton transfer barrier TS18 to give the Rh(III) vinyl hydride complex 21, which then reductively eliminates via TS19, affording the new Rh(I) complex 22 containing the π-bound product (Z)-enol ether. Complex 22 undergoes substitution with phenylacetylene through the five-coordinate intermediate 23 to give (Z)-PhCHCHOMe and regenerate 3. Parallel to path L1A, path L2A (blue) leads to (E)-PhCHCHOMe and complex 3′ (the enantiomorph of 3). For both paths L1A and L2A, the proton transfer steps are turnover-limiting with barriers TS18 and TS20, and the energy difference (1.8 kcal/mol) between TS18 and TS20 gives a calculated Z selectivity of 95%, which is slightly higher than but qualitatively consistent with the experimental value (90%).6,37 As shown by the optimized structures of TS18 and TS20 (Figure 6), the bulky phenyl group in TS20 points closer toward the migrating hydrogen and the rhodium−quinolinolato chelate framework, thereby causing

TSs and intermediates for the most favorable pathways leading to the Z and E products. In path A1 (red), methanol attacks Rh Cα via the four-center TS8 (35.2 kcal/mol), where the methoxy and phenyl groups are on the same side of the CC bond. The four-center ring of TS8 is approximately planar and orthogonal to the rhodium−quinolinolato chelate framework. The resulting Rh(III) vinyl hydride complex 9 reductively eliminates via TS9, affording the new Rh(I) complex 10 containing the π-bound product (Z)-enol ether, which then undergoes substitution with phenylacetylene through the five-coordinate intermediate 11 to give (Z)-PhCHCHOMe and regenerate 3. Parallel to path A1, path A2 (blue) begins with methanol addition to RhCα via the four-center TS10 with the methoxy and phenyl groups in the E geometry about the CC bond. This orientation ultimately leads to the product (E)-PhCHCHOMe. Because TS10 is lower than TS8 by 3.2 kcal/mol, (E)PhCHCHOMe would be the overwhelmingly major product, which contradicts the experimentally observed 90% Z selectivity. In addition, TS8 and TS10 have high free energies at 35.2 and 32.0 kcal/mol (enthalpies at 22.7 and 19.5 kcal/mol), which is partially attributed to an unfavorable entropic decrease for the process 4 + MeOH → TS8/TS10 that requires an orderly fourcenter orientation. These two difficulties with the concerted addition of methanol to RhCα led us to rule out this mechanism. We next considered the possibilities of stepwise additions of methanol to RhCα and CαCβ, starting with the Onucleophile attacking Cα from various positions. We were able to locate two relevant TSs (TS12 and TS13), which have the methoxy and phenyl groups in Z and E orientations about the CC bond (Figure 3). Apparently, the 8-quinolinolato oxygen atom plays an important role in directing the methanol attack by exerting hydrogen bond interactions with the methanol OH group, as shown by the (8-quinolinolato) O···H distances at 1.78 and 1.84 Å in TS12 and TS13, respectively. As a result, the methanol O−H bond is lengthened from 0.96 to 0.98 Å in both

Figure 3. Free energy profiles for the nucleophilic attack of methanol to form complexes 15 and 16. Hydrogen bonds are indicated by green dashed lines. 2808


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Figure 4. Free energy profiles for hydrogen transfer to the 8-quinolinolato oxygen and ensuing Rh−Cα rotation leading to complexes 18 and 20.

Fischer carbenes can undertake a 1,2-β-hydrogen shift to form thermodynamically more stable alkene complexes.39 We considered both β-hydrogen atoms (Ha and Hb) and located the favorable 1,2-β-hydrogen shift transition state TS24. TS24 is turnover-limiting and proceeds to complex 22, which contains the π-bound product (Z)-enol ether, and this species can undergo phenylacetylene substitution to give (Z)-PhCH CHOMe and regenerate 3, as discussed above (Figure 5). We verified that TS24 connects 27 and 22 by following the IRC in both directions.20h Although the 1,2-β-hydrogen shift has been characterized experimentally for Fischer carbenes without a heteroatom substituent on Cα,39 the 27 → [TS24] → 22 step is viable in the catalytic reaction on the basis of the computational results. The key factor is that TS24 (23.1 kcal/mol) is not as high on the free energy surface, which makes the activation free energy for this elementary reaction (30.4 kcal/mol) attainable under the reaction conditions (70 °C). It seems that TS24 is stabilized by the (8-quinolinolato) O···Ha hydrogen bond at 2.19 Å (Figure 8). Furthermore, the reaction has a thermodynamic driving force (22 − 27 = −11.3 kcal/mol). Path L2B (blue) is analogous to path L1B, with stationary points TS23 and 28 that are isoenergetic with TS22 and 27 (Figure 7). However, its turnover-limiting barrier TS25 is higher than the corresponding TS24 in path L1B by 1.2 kcal/mol, thereby giving a calculated Z selectivity of 89%, which agrees well with the experimental value (90%).6,37 We examined the optimized structures of TS24 and TS25 (Figure 8) in an effort to understand their difference in energy, from which the Z selectivity originates. They have similar square-planar coordination environments with comparable bond distances to rhodium. They are both stabilized by (8-quinolinolato) O···Ha hydrogen bonds. Thus, the electronic effects on the energy difference should not be significant. Sterically, the bulky phenyl group points closer toward the rhodium−quinolinolato chelate framework in TS25 than in TS24, and therefore stronger steric repulsions occur in TS25 than in TS24. This is indicated by the

Table 1. NBO Charges in Selected Complexes Rh Cα Cβ

17

19

18

20

−0.192 +0.247 −0.315

−0.223 +0.241 −0.316

−0.218 +0.321 −0.435

−0.227 +0.311 −0.405

greater steric repulsion. This is indicated by the H···H and C···H interactions at 2.49 and 2.44 Å in TS20, which are close to or less than the sums of the van der Waals radii (H, 1.20 Å; C, 1.70 Å). Thus, the steric effects destabilize TS20 relative to TS18. Although paths L1A and L2A with TS18/TS20 can explain the experimental Z selectivity for phenylacetylene (sub1), the calculated selectivities20f,g for sub2 (93% E) and sub3 (99.6% E) based on this Rh-bound proton transfer mechanism are quite opposite to the experimental results (Scheme 2), thereby casting doubt on this mechanism. As shown by the free energy profiles in Figure 7, the Cβ-bound proton transfer affords the Fischer carbene intermediates 27 and 28, which then isomerize through a 1,2-β-hydrogen shift to lead to the final Z and E products. In path L1B (red), the Cβ-bound proton transfer via TS22 is kinetically much more favorable than the Rh-bound proton transfer via TS18 in path L1A (TS18 − TS22 = 13.8 kcal/mol), and the resulting Rh(I) carbene complex 27 is also thermodynamically more stable than the Rh(III) vinyl hydride complex 21. We identified three reasons for the differences. First, in complex 18 Cβ is more electron rich than Rh, as shown by their NBO charges (Table 1). Second, in complex 18 the migrating hydrogen atom is closer to Cβ (2.33 Å) than to Rh (2.62 Å). Third, the formal oxidation state of Rh does not change in the process 18 → [TS22] → 27, but it increases from I to III in the process 18 → [TS18] → 21 despite the πacceptor ligand CO disfavoring the higher oxidation state. Complex 27 has a RhCα bond length (1.95 Å) consistent with Rh(I) Fischer carbenes,38 and it also contains two diastereomeric β-hydrogen atoms (Ha and Hb). It has been known that certain 2809


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Figure 5. Free energy profiles for the Rh-bound hydrogen transfer and subsequent reactions.

Figure 6. Optimized geometries of TS18 and TS20, with selected bond distances given in Å.

O···C distance at 3.27 Å in TS25, which is close to the sum of the van der Waals radii (O, 1.52 Å; C, 1.70 Å). Such interactions,

however, were not found in TS24. Thus, it is the steric effects that destabilize TS25 relative to TS24. 2810


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Figure 7. Free energy profiles for the Cβ-bound hydrogen transfer and subsequent reactions.

Figure 8. Optimized geometries of TS24 and TS25, with selected bond distances given in Å.

ligand CO. Thus, paths L1B and L2B both lower the overall kinetic barriers by more than 5 kcal/mol in comparison with paths L1A and L2A. Furthermore, we considered the Cβ-bound proton transfer mechanism for the reactions involving sub2 and sub3; and the calculated selectivities20f,g are 33% Z for sub2 and 99.7% Z for sub3, which are consistent with the experimental results (Scheme 2). Taken together, our computational results

We compared the energies of turnover-limiting TS24/TS25 (for the Cβ-bound proton transfer paths L1B and L2B) and TS18/TS20 (for the Rh-bound proton transfer paths L1A and L2A): TS18 − TS24 = 5.6 kcal/mol and TS20 − TS25 = 6.2 kcal/mol. TS18 and TS20 are less stable because the corresponding Rh-bound proton transfer, like α-elimination, is a formal oxidation of Rh, which is disfavored by the π-acceptor 2811


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Scheme 4. Rhodium-Catalyzed Hydroalkoxylation of Terminal Alkynes

product-forming and turnover-limiting transition state TS24, which has the bulky phenyl group pointing away from the rhodium−quinolinolato chelate framework, as compared with the E-product-forming TS25, where the bulky phenyl group has a closer orientation toward the chelate framework. The results taken together lead us to an insight into the crucial roles of the robust rhodium−quinolinolato chelate framework, as it directs phenylacetylene substitution by trans effect, mediates hydrogen transfers through hydrogen bonding, and differentiates by steric repulsion the energies of key transition states that have the bulky phenyl group in different orientations toward the chelate framework.

clearly indicate that the Cβ-bound proton transfer and the subsequent Fischer carbene isomerization constitute the most plausible mechanism.

4. CONCLUSION We have presented a detailed DFT study of the mechanism of the anti-Markovnikov hydroalkoxylation of terminal alkynes catalyzed by the Rh(I) 8-quinolinolato carbonyl complex 1cat. Scheme 4 summarizes the reaction sequence and depicts the catalytic cycle. The square-planar 1cat undergoes initial substitution with phenylacetylene, affording the Rh(I) alkyne complex 3 as the active catalyst. Complex 3 tautomerizes via an indirect 1,2hydrogen shift to the Rh(I) vinylidene complex 4 with an electron-deficient Cα atom, to which methanol adds as an Onucleophile. The resulting Rh(I) vinyl complex 15 undergoes two consecutive ligand-mediated proton transfers to form the Rh(I) Fischer carbene 27. Complex 27 undertakes a 1,2-βhydrogen shift to afford the thermodynamically more stable complex 22 that contains the π-bound product (Z)-enol ether, which can undergo phenylacetylene substitution to give (Z)PhCHCHOMe and regenerate 3. Such is the major Z-selective pathway, and the minor E-selective pathway is completely analogous. The anti-Markovnikov regioselectivity stems from the Rh(I) vinylidene intermediate 4, in which the CC polarity is reversed from the CC polarity in phenylacetylene. The stereoselectivity arises from the reduced steric repulsion in the Z-

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S Supporting Information *

Text, tables, and figures giving additional computational results and the complete ref 9. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.-X.W.); xiaotai.wang@ ucdenver.edu (X.W.). Notes

The authors declare no competing financial interest. 2812


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ACKNOWLEDGMENTS We acknowledge support for this work from the Chinese Academy of Science, the National Science Foundation of China (Grant Nos. 20973197 and 21173263), and the University of Colorado Denver.

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