Direct Anti-Markovnikov Addition of Water to Olefin To Synthesize

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Direct Anti-Markovnikov Addition of Water to Olefin to Synthesize Primary Alcohols: A Theoretical Study Yavuz S. Ceylan, and Thomas R. Cundari J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10290 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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The Journal of Physical Chemistry

Direct Anti-Markovnikov Addition of Water to Olefin to Synthesize Primary Alcohols: A Theoretical Study

Yavuz S. Ceylan‡, Thomas. R. Cundari‡,* ‡Univ.

of North Texas, Dept. of Chemistry, Center for Advanced Computing and Modeling (CASCaM), 1155 Union Circle #305070, Denton, Texas 76203

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Abstract: Anti-Markovnikov addition of water to olefins has been a long-standing goal in catalysis. The [Rh(COD)(DPEphos)]+ complex was found as a general and regioselective Group 9 catalyst for intermolecular hydroamination of alkenes. The reaction mechanism was adapted for intermolecular hydration of alkenes catalyzed by a [Rh(DPEphos)]+ catalyst and studied by DFT calculations. Olefin hydration pathways were analyzed for anti-Markovnikov and Markovnikov regioselectivity. Based on the DFT results, the operating mechanism can be summarized as follows: styrene activation through nucleophilic attack by OH of water to alkene with simultaneous H transfer to the Rh; this is then followed by formation of primary alcohol via reductive elimination. The competitive formation of phenylethane was studied via a -elimination pathway followed by hydrogenation. The origin of the regioselectivity (Markovnikov vs. antiMarkovnikov) was analyzed by means of studying the molecular orbitals, plus natural atomic charges, and shown to be primarily orbital rather than charge driven.


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Introduction Primary alcohols are widely used in the cosmetics industry as well as in household and personal care products, in order to produce detergents, surfactants, lubricants, flavorings, and perfumes. For instance, some primary alcohols of industrial interest are capric alcohol (1-decanol), myristyl alcohol (1-tetradecanol) and stearyl alcohol (1-octadecanol), etc.1-3 Anti-Markovnikov selectivity for hydration of olefins is a major challenge for catalysis.4 Brown and coworkers were among the first who indirectly synthesized primary alcohols with anti-Markovnikov regiochemistry5 via multi-step hydroboration-oxidation process. However, this process requires expensive borane reagents, generates boron waste that is difficult to recycle, and is less atom-economical than a onestep direct anti-Markovnikov hydration. Ziegler and coworkers used organoaluminum compounds to produce primary alcohols;6 the process is called ALFOL, which stands for alcohol formation by oligomerization. Drawbacks of this method include cost and considerable branched alcohol production. In 1986, Trogler et al. introduced the first example of regioselective hydration of non-activated terminal alkenes to primary alcohols using a Pt catalyst (trans-PtH(Cl)(PMe3)2) with NaOH in water under mild conditions. In this research, Trogler et al. proposed a mechanism that consists of five steps: formation of a protonated aqua complex (trans-PtH(H2O)(PMe3)2)+OH-, followed by coordination of olefin to Pt2+ and subsequent formation of a Pt-alkyl complex, which then undergoes trans-cis isomerization, followed by 1◦ alcohol synthesis via C-H reductive elimination.7 The method was reevaluated by Ramprasad et al. who claimed that trans(PMe3)2Pt(H)(Cl) failed to yield alcohol products. Also, they posited that the nucleophilic attack of hydroxide ion on coordinated olefin complex was not confirmed by spectroscopic studies.8 Moreover, Grushin et al. mentioned that based on the standard procedure of preparation of 3 ACS Paragon Plus Environment


(PMe3)2Pt(H)(Cl), there are always some impurities present. Thus, the proposed pathway was said to be irreproducible.9 Recently, Grubbs and coworkers reported the production of primary alcohols from non-activated terminal olefins, which achieved anti-Markovnikov hydration via triple relay catalysis.10 However, the triple-relay system depends on three stringent requirements: an oxidative addition step that is selective for aldehyde, an oxidative cycle that is compatible with a reductive sequence, and hydride migration must be easy from Pd to Ru. Even though these criteria caused overall yields to be low, the process received great attention because of the promising results displayed using non-activated olefins in the synthesis of primary alcohols.11 Ujaque and co-workers reported hydroamination of alkenes with amines with both Markovnikov and anti-Markovnikov selectivity. After studying several complexes, they pinpointed [Rh(DPEphos)]+ as a complex for which anti-Markovnikov regioselectivity is favored for N-H addition to terminal olefins.12 Also, Beller et al.13 reported that [Rh(COD)2]BF4/PPh3 catalyst displayed anti-Markovnikov addition. Similarly, Ujaque et al.12 reported that the catalyst [Rh(DPEphos)]+ was favored for anti-Markovnikov amination. Furthermore, Ujaque et al. mentioned that Markovnikov regioselectivity was more favorable for olefins with extreme electron-donor substituents.12 Hartwig and co-workers reported14 that rigid, electron-rich olefins bind more favorably to Rh(I). More favorable binding energies of the olefin to metal were associated with greater catalyst activity. We speculated whether the catalyst and pathway proposed by Ujaque et al.12 could accomplish anti-Markovnikov selectivity if the amine substrate was changed to water. Thus, for direct antiMarkovnikov addition of water to olefin to synthesize primary alcohols, the following reaction steps were modeled (Figure 1): nucleophilic attack by water, followed by conformational changes that facilitate proton migration to the metal center, and reductive elimination step. Also, the 4 ACS Paragon Plus Environment

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competing Markovnikov pathway was studied and compared. Herein, a plausible catalytic route is reported to produce primary alcohols with anti-Markovnikov selectivity from terminal olefins and water. Nucleophilic Addition

OH

M

H 2O

R R R

M M

OH

H 2O

R

R

Reductive Elimination

H M

OH

Proton Transfer

R

Figure 1. Proposed mechanism for RhI-catalyzed catalytic cycle for primary alcohol production from styrene and water. Adapted with permission from reference 12. Copyright 2016 Wiley. Computational Details To study the mechanism of direct anti-Markovnikov addition of water to olefins to synthesize primary

alcohols

via

a

[Rh(DPEphos)(Styrene)]+

catalyst

(DPEphos=

bis[(2-

diphenylphosphino)phenyl] ether), density functional theory (DFT) was employed. All simulations were performed using the Gaussian 0915 suite. All stationary points were acquired at the M0616/SDD17,18/6-31G(d)19,20 level of theory for Rh and main group elements, respectively; for single point energy calculations, the augmented triple--plus-polarization 6-311++G(d,p)21 basis set was used for main group elements. The M06 functional has been extensively used in modeling olefin activation and related reactions.22-25 Enthalpic and entropic corrections employ unscaled M0616/SDD17,18/6-31G(d)19,20 vibrational frequencies. The structures are optimized in a 5 ACS Paragon Plus Environment


low dielectric constant solvent (toluene) by the CPCM26 continuum model. Minima and transition states are confirmed by frequency calculations. The reported energies are Gibbs free energies (G) for optimized stationary points and assume 298.15 K and 1 atm. The functional, basis sets and solvent effects were adapted from a previously reported study of anti-Markovnikov hydroamination.12 Results and Discussion 1. Anti-Markovnikov Hydration Catalytic Cycle Initial alkene coordination and subsequent nucleophilic attack 12,27-32 by water were studied in analogy to previous work12 on the related anti-Markovnikov hydroamination process. Water exchange with olefin was first assessed. Thermodynamically, the free energy of water exchange with styrene in ([Rh(DPEphos)(styrene)]+) giving rise to [Rh(DPEphos)(aqua)]+ + styrene was predicted to be endergonic by 34.8 kcal/mol, which indicated that water does not tend to bind strongly enough to displace styrene complex ([Rh(DPEphos)(styrene)]+). Therefore, alkene coordination and subsequently three different water addition modes were modeled since water displacement of styrene was predicted to be too high in energy to be a feasible process. In these analyses, a single water molecule was selected in the calculations. Moreover, it was also intended to analyze the energy profile of the reaction by employing two more water molecules to model additional outer-sphere solvation. However, these simulations did not yield stable minima, perhaps due to the extra degrees of freedom they involved. The catalyst [Rh(DPEphos)(styrene)]+ (1) exhibited a square planar geometry in which styrene coordinated to RhI in an 2 fashion with distances of 2.21 Å for the Rh---C=C centroid. Also, Rh-P1 and P2 distances were predicted to be 2.39 and 2.30 Å. The Rh-O distance was calculated to be long at 3.53 Å, which may result from the strong trans influence of styrene, Figure 2b. The 6 ACS Paragon Plus Environment

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computed geometry is similar to that reported by Ujaque et al.12 Figure 2b shows an alternative view of the square planar Rh(I) geometry of the catalyst.

1

2

(a)

(b) Figure 2. Two views of the calculated geometry for [Rh(DPEphos)(styrene)]+ (1). Bond lengths in Å. Hydrogen atoms were omitted from the figure for clarity.



Figure 3. M06/SDD+6-311++G(d,p)/CPCM-toluene calculated free energy for RhI hydration of styrene with anti-Markovnikov regioselectivity leading to the formation of primary alcohol (red pathway) and competitive phenylethane (blue pathway) products at 298.15 K. Note that * denotes a “stationary point” obtained via a constrained geometry optimization. The modeled catalytic cycle for the anti-Markovnikov hydration of styrene catalyzed by [Rh(DPEphos)]+ is shown in Figure 3. It is similar to a reported hydroamination mechanism advanced from computational and experimental studies of square planar RhI catalysts.12,33-39 The investigated pathways initiated from 1 are syn (nucleophile attacks the olefin on the same face that is coordinated to the metal) addition of water, anti (nucleophile attacks the face of the coordinated olefin away from the metal) addition of water, and simultaneous water addition with O-H activation/transfer to Rh. In order to avoid water dissociation in many of the preliminary calculations, most syn- and anti-addition stationary points required calculations with geometric constraints (such intermediates and TSs are denoted with a * in Figure 3). In general, this entailed 8 ACS Paragon Plus Environment

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freezing Oaqua---C bond distances in the geometry optimizations. The three studied pathways led to hydride intermediate 5 (H—(DPEphos)RhIII—CHPh-CH2OH), which further underwent C—H reductive elimination to give a RhI complex 6 (agostic primary alcohol adduct). Among the modeled pathways – syn, anti and simultaneous addition/OH activation – the syn addition required overcoming a barrier of 38.5 kcal/mol (TS1-4*), whereas the anti and nucleophilic attack with simultaneous H transfer transition states lie at 26.0 (TS1-2*) and 23.7 kcal/mol (TS1-5) (G‡), respectively. Based on these results, syn addition mechanism was predicted to be the least feasible among the pathways modeled. The anti-addition pathway involved a conformational change of the water to allow proton migration of water to the vacant side of the metal center yielding a RhIII-hydride, 5. This mechanism occurred in several steps. The high point on this pathway was a true transition state TS3-4 with a relative free energy (Grel-1) of 38.5 kcal/mol. TS3-4 (38.5 kcal/mol (Grel-1)) was the only true TS, while the remaining stationary points required a constrained calculation (ModRedundat in G09) to avoid water dissociation. Motivated by these difficulties, additional calculations were conducted and confirmed that catalyst-styrene-aqua adducts are very high in energy and result in outer-sphere water, which is reasonable given the hydrophobicity of styrene. In further support of these results, water exchange – [Rh(DPEphos)(styrene)]+ + H2O = [Rh(DPEphos)(H2O)]+ + styrene – was computed to be quite endergonic, with Gexch = +34.8 kcal/mol. As an alternative to the anti- and syn-addition mechanisms, a new pathway was analyzed that involved simultaneous H transfer from water to the metal center and nucleophilic attack of HO to the -carbon of styrene. The transition state for this direct proton transfer TS1-5 was located at



a free energy of 23.7 kcal/mol relative to catalyst 1. This pathway was the most favorable process found in this study that led to [H–Rh(DPEphos)(CHPhCH2OH)]+, 5 (Grel-1 = 11.3 kcal/mol). Subsequently, reductive elimination via transition state TS5-6 (14.7 kcal/mol) led to -phenethyl alcohol that is coordinated to the RhI by an agostic interaction, 6 (Grel-1 = 1.1 kcal/mol). Stationary point geometries of the active site cores of 5, TS5-6 and 6 are given in Figure 4. Finally, a second equivalent of styrene displaces the primary alcohol product to start another cycle of catalysis. This substitution was calculated to be exergonic, lowering the free energy to -12.4 kcal/mol (7), Figure 3.

Figure 4. Calculated core geometries for [H–Rh(DPEphos)(CHPhCH2OH)]+ intermediate 5 (top). reductive elimination TS TS5-6 (middle). [Rh(DPEphos)(PhCH2CH2OH)]+ intermediate 6 (bottom). The DPEphos ligand was omitted for clarity in these depictions; bond lengths in Å. 10 ACS Paragon Plus Environment

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Based on the results in Figure 3, the difference in the free energy barrier between β-hydrogen elimination (to yield alkane side-product) and reductive elimination (to yield primary alcohol) indicated that the latter is more favorable (G‡) 2.9 kcal/mol. In summary, according to the present calculations the mechanism of the anti-Markovnikov hydration of olefins reaction should be updated (Figure 5a): nucleophilic attack of HO with simultaneous H transfer from water onto the coordinated styrene via TS1-5, followed by reductive elimination via TS5-6, and then styrene/alcohol exchange. The optimized structure of transition state TS1-5 is depicted in Figure 5b.

M

Simultaneous water attack H 2O

R

H M

Reductive Elimination OH

R

M R

OH

M +

R

OH +

R

R

(a)

(b) Figure 5. Proposed mechanism for the RhI-catalyzed synthesis of primary alcohols from styrene and water (a). Optimized structures of transition state TS1-5 active site atoms are depicted (b). Computed bond lengths in Å.



Figure 6. M06/SDD+6-311++G(d,p)/CPCM-toluene calculated free energy (in kcal/mol) for RhI hydration of styrene with Markovnikov (black) and anti-Markovnikov (red) regioselectivity leading to the formation of -phenethyl alcohol. T = 298.15 K.

2. Markovnikov Cycle In order to evaluate the origin of regioselectivity preferences, the Markovnikov catalytic cycle (Figure 6) was also studied. The mechanism of Markovnikov hydration was modeled in analogy to that just discussed for the anti-Markovnikov pathway. Note that free energy for the Markovnikov pathway are given in black font and anti-Markovnikov pathway in red font in Figure 6. First, RhI (1) was oxidized to an octahedral RhIII structure (5m) via the TS1-5m transition state. This RhIII structure (5m) has a relative free energy of 18.6 kcal/mol, which is 7.3 kcal/mol higher


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than the corresponding anti-Markovnikov RhIII structure 5 (Figure 6). The nucleophilic attack/proton transfer transition state was calculated to be rate determining for both Markovnikov and anti-Markovnikov pathways, 40.2 (TS1-5m) and 23.7 kcal/mol (TS1-5), respectively. Therefore, there is a clear kinetic preference for the anti-Markovnikov pathway for the RhI catalyst model in comparison to the Markovnikov pathway. The corresponding active site geometries for the transition state geometries are shown in Figure 5 and Figure 7, for anti-Markovnikov and Markovnikov pathways, respectively.

Figure 7. Calculated geometry of simultaneous transition states TS1-5m for Markovnikov pathway. Active site atoms are depicted. Computed bond lengths in Å. 3. Effect of Olefin and Nucleophile on Anti-Markovnikov Regioselectivity To gain better insight on the impact of substrate on regioselectivity, an alkene (1-hexene) and nucleophile (methanol) were investigated as well. Water and methanol attack upon the styrene complex was modeled, Figure 8a, as was water addition to the -complexes of styrene and 1hexene, Figure 8b. The computed free energy barriers of anti-Markovnikov and Markovnikov nucleophilic attack on the -complexes are given in Table 1.



(a)

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(b)

Figure 8. Core geometries of active catalysts (a) 1 [Rh(DPEphos)(styrene)]+ and (b) 1’ [Rh(DPEphos)(1-hexene)]+. Bond lengths in Å. Hydrogen atoms were omitted for clarity. In the previous sections, the most feasible reaction mechanism was indicated to be simultaneous nucleophilic attack/proton transfer to the coordinated alkene, giving rise to a RhIIIhydride intermediate. From this intermediate, a reductive elimination process to produce the alcohol product outcompeted phenylethane production, Figure 5a. Reductive elimination free energy barriers are given in Supporting Information as it was not computed to be the ratedetermining step. According to calculations on the varied substitutions and olefins, for antiMarkovnikov regioselectivity, simultaneous nucleophilic attack/ proton transfer was predicted to be the rate determining step: G‡ = 23.7, 27.1 and 35.7 kcal/mol, for entries 1 (styrene:water), 2 (1-hexene:water)and 3 (styrene:methanol), respectively, Table 1. The same step was predicted to be rate determining for Markovnikov regioselectivity: G‡ = 40.2, 30.3 and 50.0 kcal/mol for the same three entries. Hence, anti-Markovnikov regioselectivity was calculated as more favorable for all three substrate/nucleophile combinations models: G‡ = 16.5, 3.2 and 14.3, for styrene:water, 1-hexene:water and styrene:methanol, respectively Table 1. The corresponding geometries for the 14 ACS Paragon Plus Environment

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rate determining transition states for entries 2 and 3 are shown in Figure S1 of Supporting Information. As compared to 1-hexene, styrene showed a significant kinetic advantage for hydration for anti-Markovnikov regioselectivity, with G‡AM = 3.4 kcal/mol (Table 1). As such, the styrene adduct is expected to be more reactive than the 1-hexene adduct. A similar trend was reported by Ujaque et al. for the hydroamination of terminal alkenes.12 They proposed that the inductive and resonance effect of the aromatic groups lower the rate determining step for both anti-Markovnikov and Markovnikov regioselectivity versus an alkyl substituent on the olefin. Moreover, addition of methanol to styrene, entry 3 versus entry 1 (Table 1), indicated that synthesis of ether is less favorable than hydration by G‡AM = 12.0 kcal/mol as compared to production of the primary alcohol; it is hypothesized that the steric hindrance by the methyl group causes the higher energy barrier for etheration versus hydration, Table 1. Also, non-bonded interactions between the methyl group of the nucleophile and the terminal carbon of the styrene (C were analyzed. Animating the imaginary mode for TS1-5methanol shows the distance between methyl and C shortens (cf. Figure 9a to Figure 9b) Å as nucleophilic attack occurs. This analysis supports the hypothesis that steric hindrance yields the high free energy barrier for etheration.

(a)

(b) 15 ACS Paragon Plus Environment


Figure 9. Calculated geometry snapshots when animating the imaginary mode of transition state TS1-5methanol for an anti-Markovnikov pathway. (a) Long C…O distance. (b) Short C…O distance. The DPEphos ligand was omitted for clarity in these depictions; bond lengths in Å.

Table 1. Rate determining free energy barriers and selectivity for anti-Markovnikov vs. Markovnikov regioselectivity. G‡M and G‡AM: barriers for the rate determining TSs for Markovnikov and anti-Markovnikov pathways, respectively. G‡ = G‡M - G‡AM.

4. Anti-Markovnikov versus Markovnikov Regioselectivity The anti-Markovnikov pathway is more favorable by 16.5 kcal/mol (G‡) than the Markovnikov hydration pathway, Figure 6. Thus, anti-Markovnikov addition is computed to be favorable from a kinetic viewpoint. To gain deeper insight into the origin of hydration regioselectivity, NBO40 charges of the beta and alpha carbons of styrene and also of the Rh metal were calculated for entry 1 (Table 1), which are -0.42 e-, -0.21 e- and -0.37 e-, respectively Figure 16 ACS Paragon Plus Environment

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10. Thus, we concluded that the regioselectivity is not charge-controlled given the more negatively charged C versus C.

c)

a)

b)

Figure 10. Natural bond orbital (NBO) charges (in e-) of Rh(DPEphos)(styrene)]+; a) beta carbon b) alpha carbon, and c) Rh metal. Hydrogen atoms are omitted from the figure for clarity. Styrene coordination to the RhI catalyst is nominally 2 at the C=C bond However, upon nucleophilic attack, an 2-alkene slips toward 1-coordination as addressed by Hoffmann and coworkers,41,42 Barone et al.

43

and Ujaque et al.12 The distances between rhodium and both

carbons of styrene in complex 1 [Rh(DPEphos)(styrene)]+ are 2.213 and 2.150 Å for C and C, respectively. Moreover, the Rh-C and Rh-C was elongated to 3.054 and 2.206 Å, respectively, in the simultaneous water addition/activation transition state TS 1-5. For complex 1’ [Rh(DPEphos)(1-hexene)]+ distances between rhodium and both carbons of styrene are 2.224 and 2.129 Å for C and C, respectively. Moreover, the Rh-C and Rh-C was elongated to 3.048 and 2.139 Å, respectively, in the simultaneous water addition/activation transition state for the 1hexene substrate. For both 1 and 1’ complexes, such slippage enhances the orbital character on the beta carbon of the LUMO orbital of 1 [Rh(DPEphos)(styrene)]+ and 1’ [Rh(DPEphos)(1-hexene)]+ which will enhance the interaction of this carbon with an attacking nucleophile.12,41,42 We plot the LUMO (Figure 11) of complex 1 and 1’, which shows enhanced localization on C relative to C. 17 ACS Paragon Plus Environment


Figure 11. Representation of the Kohn-Sham LUMO for the RhI catalysts 1 (top) and 1’ (bottom). IsoValue = 0.0432.

Conclusions It is speculated whether the catalyst and pathway proposed by Ujaque et al.12 could accomplish anti-Markovnikov selectivity if the amine substrate was changed to water. A competing Markovnikov pathway was also modeled. A plausible catalytic route is reported to produce primary alcohols with anti-Markovnikov selectivity from styrene and water. The DPEphos (bis[(2diphenylphosphino)phenyl]ether) ligand was used to support a cationic rhodium(I) catalyst to produce primary alcohol from water and styrene. For the purpose of better understanding of the origin of regioselectivity, the impact of an alkene chain (1-hexene) and (methanol) were investigated as well. The catalytic cycle was studied by DFT calculations. Previous research on 18 ACS Paragon Plus Environment

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RhI catalyst12 with the same DPEphos ligand showed great success in producing hydroaminated alkenes with anti-Markovnikov selectivity. To yield primary alcohol, RhI catalytic cycles were initiated by three different pathways involving water attack upon styrene, which gave rise to a RhIII-hydride intermediate, which then underwent reductive elimination. The following points are highlighted. (1)

The free energy of water exchange with styrene in ([Rh(DPEphos)(styrene)]+)

giving rise to [Rh(DPEphos)(aqua)]+ + styrene was predicted to be 34.8 kcal/mol endergonic. Therefore, styrene coordination followed by three different water addition modes were modeled since water displacement of styrene was predicted to be too exergonic to be feasible. (2)

The present calculations on RhI showed an important difference between formation

of primary alcohol and phenylethane. It was calculated that C—H reductive elimination (G‡ = 3.4 kcal/mol) from the RhIII—H intermediate, leading to alcohol product, is energetically more favorable as compared to -hydrogen elimination (G‡ = 6.3 kcal/mol), which leads to phenylethane. (3)

In comparing anti-Markovnikov to Markovnikov regioselectivity with RhI catalyst

1, calculations of the free energy profile indicated exergonic products, -12.4 and -13.7 kcal/mol relative to 1, respectively. Therefore, one may posit that the regioselectivity is not governed by thermodynamic, but rather kinetic considerations. (4)

For both regiochemistries, the rate determining steps were predicted to involve

simultaneous water addition/activation/proton transfer. The rate determining step in antiMarkovnikov pathway was predicted to have G‡= 23.7 kcal/mol, while the rate determining step had G‡ = 40.2 kcal/mol for the Markovnikov regiochemistry. As such, one would expect the anti-



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Markovnikov pathway to be more favorable (G‡= 16.5 kcal/mol) and also to be able to operate at low temperatures. (5)

According to calculations for different nucleophiles (water and methanol) and

olefins (styrene and 1-hexene), anti-Markovnikov regioselectivity was maintained to be more favorable as compared to Markovnikov regioselectivity and simultaneous nucleophilic attack step was predicted as the rate determining step for all cases studies. For the case of phenyl ring bonded to the alkene, styrene showed a sizeable preference via G‡AM = 3.4 kcal/mol (Table 1). Also, results showed that synthesis of an ether with methanol as the nucleophile is less favorable by 12.0 kcal/mol as compared to production of the primary alcohol via hydration. (6)

The origin of the regioselectivity was also examined by means of studying the

molecular orbitals and charges. Based on NBO charges, the beta C atom of styrene was the most negatively charged atom of the substrate. Therefore, it was concluded that the reaction is not charge-controlled.

However,

orbital

analysis

indicated

that

styrene

coordination

to

[Rh(DPEphos)]+ yields a LUMO with substantial contribution of a p orbital of the beta carbon atom of styrene mixed with a d orbital of rhodium. The orbital analysis showed that the LUMO of catalyst 1 has increased localization on the beta carbon (C) of styrene. Thus, these observations suggested that the anti-Markovnikov selectivity arose from an orbitally-controlled nucleophilic attack directed by the LUMO of the [Rh(DPEphos)(styrene)]+ catalyst. Supporting Information Full computational details, including Cartesian coordinates for all stationary points for catalyst models. This material is available free of charge via the Internet. Corresponding Author *E-mail: [email protected] 20 ACS Paragon Plus Environment

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Acknowledgements This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (Chemical Sciences, Geosciences and Biosciences Division) via grant number DE-FG0203ER15387. The authors acknowledge the National Science Foundation for their support of the UNT Chemistry CASCaM high performance computing facility through grant CHE-1531468. References (1)

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