DFT Mechanistic Investigation into BF3-Catalyzed Alcohol Oxidation

Research Article Cite This: ACS Catal. 2019, 9, 6510−6521

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DFT Mechanistic Investigation into BF3‑Catalyzed Alcohol Oxidation by a Hypervalent Iodine(III) Compound Kaveh Farshadfar,† Antony Chipman,‡ Brian F. Yates,‡ and Alireza Ariafard*,†,‡ †

Department of Chemistry, Islamic Azad University, Central Tehran Branch, Poonak, Tehran, Iran School of Physical Science (Chemistry), University of Tasmania, Private Bag 75, Hobart, TAS 7001, Australia

‡

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S Supporting Information *

ABSTRACT: Density functional theory (DFT) at the SMD/ M06-2X/def2-TZVP//SMD/M06-2X/LANL2DZ,6-31G(d) level was employed to explore mechanistic aspects of BF3catalyzed alcohol oxidation using a hypervalent iodine(III) compound, [ArI(OAc)2], to yield aldehydes/ketones as the final products. The reaction is composed of two main processes: (i) ligand exchange and (ii) the redox reaction. Our study for 1-propanol discovered that ligand exchange is preferentially accelerated if BF3 first coordinates to the alcohol. This coordination increases the acidity of the alcohol hydroxyl proton, resulting in ligand exchange between the iodane and the alcohol proceeding via a concerted interchange associative mechanism with an activation free energy of ∼10 kcal/mol. For the redox process, the calculations rule out the feasibility of the conventional mechanism (alkoxy Cα deprotonation) and introduce a replacement for it. This alternative route commences with α-hydride elimination of the alkoxy group promoted by BF3 coordination, which yields a BF3-stabilized aldehyde/ketone product and the iodane [ArI(OAc)(H)]. The ensuing iodane is extremely reactive toward reductive elimination to give ArI + HOAc in a highly exergonic fashion (ΔG = −62.1 kcal/mol). The reductive elimination reaction is the thermodynamic driving force for the alcohol oxidation to be irreversible. Consistent with the kinetic isotope effect reported experimentally, the α-hydride elimination is calculated to be the rate-determining step with an overall activation free energy of ∼24 kcal/mol. KEYWORDS: DFT calculations, reaction mechanism, alcohol oxidation, hypervalent iodine(III), BF3 catalyst, hydride shift

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INTRODUCTION Hypervalent iodine(III) compounds have attracted much attention as oxidants in organic synthesis, as they can be a replacement for transition metals with the advantage of having environmentally benign properties.1−18 Although iodine(III) reagents are competent to conduct many organic transformations, addition of a Lewis acid such as BF3 is often a prerequisite for a process to occur. For example, it has been shown that an iodine(III) reagent in conjunction with BF3· Et2O considerably accelerates the processes that yield λ3diaryliodanes,12 olefin diacetoxylation,19 and a plethora of other products.19−35 In this context, Ochiai et al. reported that alcohols can be selectively oxidized by hypervalent iodine(III) compound 2 to aldehydes or ketones in the presence of BF3· Et2O as the catalyst (Scheme 1).36 For this catalytic reaction, the first step is proposed to be a ligand exchange between iodine(III) reagent 2 and the alcohol, followed by a redox process to give the final product. It was experimentally demonstrated by Ochiai et al. that the BF3 Lewis acid participates in accelerating both the ligand exchange and redox steps. To account for the role of the BF3 in the redox process, the formation of intermediate 6 has been postulated (Scheme 1). In this case, the BF3 coordination converts the acetate into © 2019 American Chemical Society

a strong leaving group, thereby promoting the redox step by deprotonation of the alkoxy Cα hydrogen. Using a kinetic isotope effect analysis, Ochiai et al. suggested that the alkoxy Cα deprotonation is the rate-determining step of this redox process. Although a plausible mechanism of this alcohol oxidation has already been proposed and highlighted in several important reviews,1,11,37 many questions have yet to be addressed. For example, it is not very clear how BF3 facilitates the ligand exchange process. Scheme 2 represents one possible mechanism of ligand exchange occurring through the transformation 7 → 8 → 9. According to this proposal, 7 undergoes isomerization to 8, which generates an empty site in the position cis to the phenyl ring in the λ3-iodane. The open site can then be occupied by the nucleophile Nu-H to provide suitable conditions for the proton transfer from Nu-H to the acetate trans to the phenyl ring. It is expected that the ease of the ligand exchange process depends upon the energy of intermediate 8 relative to 7: the lower the energy of Received: April 18, 2019 Revised: June 3, 2019 Published: June 6, 2019 6510

DOI: 10.1021/acscatal.9b01599 ACS Catal. 2019, 9, 6510−6521

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ACS Catalysis

Scheme 1. Reaction Scheme Showing BF3-Catalyzed Alcohol Oxidation by an Iodine(III) Reagent Which Occurs at 30 °C over 4 h Where the Alcohol Substrate Was Used in Vast Excessa

a

The proposed mechanism for such a redox process is provided below the reaction.

As discussed above (Scheme 1), once alkoxy intermediate 5 is formed, a BF3 coordinates to afford intermediate 6, which is postulated to be highly reactive toward a redox process. According to Ochiai’s mechanism (Scheme 1), for reduction from 6 to take place, deprotonation of the alkoxy Cα hydrogen by an appropriate base is a prerequisite. This is very similar to the mechanism for alcohol oxidation by 2-iodoxybenzoic acid (IBX) proposed by Goddard et al.39 and subsequently modified by Schaefer et al.40 (Scheme 3). On that basis, when ligand exchange occurs between IBX and the alcohol, alkoxy intermediate 13 is produced. This iodine(V) intermediate undergoes isomerization to 14, in which the oxo group is positioned trans to the carboxylate functional group. They reported that the oxo group serves as a base and deprotonates the alkoxy Cα hydrogen, resulting in alcohol oxidation and iodine(V) reduction to iodine(III). In the case of IBX, an oxo group acts as an internal base, but in the case of iodine(III), no such internal base is available. The question now is whether the BF3-bound acetate acts as the base or another alcohol is responsible for conducting the redox process. Alternatively, the alcohol oxidation by iodine(III) may proceed through a different mechanism that is yet to be revealed for the scientific community. In this contribution, we intend to disclose the role of BF3 in accelerating both the ligand exchange and redox steps for alcohol oxidation by using iodane 2.

Scheme 2. Plausible Mechanisms for Ligand Exchange in the Absence and Presence of BF3

intermediate 8, the more energetically accessible the ligand exchange reaction. In this context, Lledós and Shafir et al. showed that coordination of BF3 to 7 satisfies this situation and causes the isomerization to proceed much more smoothly and therefore species 11 becomes more available for Nu-H coordination.38 The question is whether a protocol similar to that proposed by Lledós and Shafir et al. operates to accelerate the ligand exchange between 1 and 2 or if the reaction proceeds through a hitherto unknown mechanism. To realize this, we have comprehensively studied such a ligand exchange in the presence of the BF3 catalyst with the aim of providing a new insight into this essential step.

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RESULTS AND DISCUSSION To explore the role of BF3 in accelerating each of the primary steps of the alcohol oxidation using iodane 2, we have divided

Scheme 3. Key Steps for Alcohol Oxidation by a Hypervalent Iodine(V) reagent (IBX) Proposed by Goddard et al

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ACS Catalysis Scheme 4. Three Possible Mechanisms Found by DFT Calculations for the Ligand Exchange Process

find pathway B collapsed to pathway C. Figure 1 compares the energy profiles assigned to pathways A and C for the ligand exchange process. From this comparison, it is inferred that the two pathways are almost isoenergetic with pathway C being 0.6 kcal/mol lower in energy than pathway A. This result is consistent with our recent study wherein the ligand exchange between a phenol and phenyliodine(III) diacetate (PIDA) was reported to preferentially proceed via pathway C.41 In pathway A, a transition structure connecting 16 to 17 is not located due to the high basicity of the acetate anion which spontaneously deprotonates the alcohol stabilized by the iodine(III) center. Now, we turn our attention to the question of how the BF3 addition accelerates the ligand exchange process. To commence, we must first determine with which substrate (alcohol or iodane) BF3·Et2O energetically prefers to interact. To this end, the two SN2-type transition structures TS20 and TS2‑21 were located (Figure 2). The calculations show that TS20 is about 2.6 kcal/mol lower in energy than TS2‑21, which suggests that BF3 should preferentially coordinate to the alcohol and not the iodane; the BF3 coordination to the alcohol was calculated to be exergonic by about −3.6 kcal/mol. Our calculations suggest that the formation of 20 is less likely to proceed via an SN1-type reaction due to the finding that TSb lies 4.1 kcal/mol higher in energy than TS20 (Figure 2). Once 20 is formed, it has the potential to initiate the ligand exchange reaction. Our results show that the ligand exchange in this case proceeds through pathway C after surmounting an energy barrier as low as 9.8 kcal/mol; efforts to find the vital transition structure of pathway B invariably collapsed to

our discussion into two distinct sections: ligand exchange and redox process. Ligand Exchange. Although various possibilities are likely, our calculations have allowed us to propose three different mechanisms for ligand exchange between the alcohol and the iodine(III) reagent which are assigned in Scheme 4 as (i) isomerization-associative mechanism (pathway A), (ii) stepwise interchange associative mechanism (pathway B), and (iii) concerted interchange associative mechanism (pathway C). Pathway A is characterized by initial isomerization of 2 to 15, then alcohol coordination, and finally deprotonation of the coordinated alcohol by the acetate ligand trans to the aryl ring in 16. Pathway B is initiated via the four-coordinate transition structure TS2‑18, in which the alcohol acts as an entering group while acetate leaves the coordination sphere. This ligand exchange leads to adduct 18, which is reactive toward forming acetic acid. The bonding of this four-coordinate transition structure can be described as four-center−six-electron (4c-6e) wherein the empty p orbital of the iodine(III) center simultaneously interacts with the lone pairs of the alcohol and the two OAc ligands. This situation is provided by the interaction of an alcohol lone pair with primarily the LUMO of structure 2. Pathway C is somehow similar to pathway B with the difference that both the substitution and the proton transfer occur simultaneously via transition structure TS2‑17. On the basis of the three pathways illustrated in Scheme 4, we commenced our investigation by calculating the ligand exchange process between hypervalent iodine(III) reagent 2 and propanol in the absence of BF3. In this case, any attempt to 6512


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Figure 1. Free energy profile for ligand exchange between iodane 2 and 1-propanol occurring via pathways A and B.42 The relative free energies are given in kcal/mol. The relative Gibbs energy of TSa is estimated on the basis of the methodology proposed by Hall and co-workers.43

Figure 2. Calculated mechanism for coordination of BF3 to either the alcohol or iodane 2. The relative free energies are given in kcal/mol. The relative free energy of TSb is estimated on the basis of the methodology proposed by Hall and co-workers.43

the BF3 coordination to the alcohol likely provides access to the most favorable pathway for ligand exchange.45 Notably, all previous reports have demonstrated that, for a particular reaction to be catalyzed by a Lewis or Brønsted acid, the acid should be added to the iodane reagent.40,46−52 In contrast, in this study, we have discovered that addition of BF3 to the alcohol (and not the iodane) best facilitates the ligand exchange. This finding shows that addition of the acid to the iodane is not always a prerequisite for a process to be accelerated but that sometimes an acid catalyst can directly activate an organic substrate to promote a given reaction in the presence of an iodane. Redox Process Based on the Conventional Mechanism. As illustrated in Figure 3, the most favorable pathway for ligand exchange affords intermediate 24, in which BF3 is coordinated directly to the oxygen atom of the alkoxy ligand. In the literature, it was proposed that, for the redox to occur, BF3 must coordinate not to the alkoxy but to the OAc ligand

pathway C (Figure 3). It follows from this result that the ligand exchange between the BF3·PrOH adduct and the iodane occurs much more quickly than the same process does between PrOH and the iodane. This acceleration can be explained on the basis of the higher acidity of the O−H proton in the BF3·PrOH adduct than in free PrOH. Indeed, coordination of the Lewis acid BF3 to the alcohol makes the hydroxyl proton more labile, thereby facilitating the ligand exchange. Although, as established above, BF3 prefers coordinating to the alcohol, we also investigated the possibility in which the ligand exchange is commenced by coordination of BF3 to the iodane. In this case, the ligand exchange was found to occur via pathway A or B (Figure 4); the reduced basicity of the ligand in BF3·OAc results in the ligand exchange taking place through pathway B and not C.44 All of the key transition structures associated with pathways A and B (Figure 4) were calculated to lie far above TS22‑23 (Figure 3). Such a result suggests that 6513


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Figure 3. Free energy profile for ligand exchange between iodane 2 and BF3·(1-propanol) occurring via pathway C. Relative free energies are given in kcal/mol. Selected bond distances in DFT-calculated structures TS22‑23, 2, and 23 are given in Å.

Figure 4. Free energy profile for ligand exchange between 1-propanol and iodane 21 occurring via pathways A and B. The relative free energies are given in kcal/mol. The relative free energies of TSc and TSd are estimated based on the methodology proposed by Hall and co-workers.43

(Scheme 1). This mode of coordination should render the OAc as a good leaving group and increase the cathodic reduction potential of the iodine(III) center to promote the redox step via deprotonation of the alkoxy Cα hydrogen. Our calculations show that 24 and 21 are almost isoenergetic (Figure 5), which probably means that the formation of 21 is

not impossible. However, our calculations show that, in contrast to the expectation, structure 21 is extremely unreactive toward the redox process. The various transition structures (all in blue) illustrated in Figure 5 confirm this claim. The Cα hydrogen is anticipated to be deprotonated by an available base such as BF3·OAc or another alcohol. Starting 6514


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Figure 5. Calculated mechanism for the redox step via Cα hydrogen deprotonation starting from intermediates 17, 21, 24, and 29.53 The relative free energies are given in kcal/mol.

Redox Process Uncovered by DFT Calculations. All of the evaluations undertaken above have brought us to the conclusion that the alcohol oxidation cannot occur through the conventional mechanism. This prompted us to seek an energetically more accessible pathway for alcohol oxidation. Interestingly, we found that alcohol oxidation in this system is most likely to proceed via alkoxy α-hydride elimination and not via Cα deprotonation. The iodine(III) center has the ability to receive a hydride from the alkoxy ligand facilitated by BF3 coordination. The energy profile for this novel mechanism is outlined in Figure 6a. Accordingly, the aldehyde formation process starts by isomerization of 24 to α-agostic iodane 32 via transition structure TS24‑32. The ensuing intermediate gains some stability through an agostic interaction and is susceptible to be converted to iodane 33 by hydride abstraction concomitant with the ejection of the BF3-stabilized aldehyde product. Once intermediate 33 is formed, it can undergo a very facile reductive elimination through the two transition structures designated in Figure 6a as TS33‑34 and TS34‑35 with an activation barrier as low as 3.1 kcal/mol. The overall activation barrier of our novel mechanism for producing the aldehyde (23.4 kcal/mol) falls far beneath those of the traditionally accepted mechanisms discussed in Scheme 1 and calculated in Figure 5. The nature of hydrogen transfer is evaluated by investigating the antibonding orbital relating to the three-center−four-

from 21, four transition structures which consider these bases as the deprotonating agent are calculated (TS21, TS′21, TS31, and TS′31). We found that these saddle points are too high in energy to be accessible under the reaction conditions; the activation energies span a range from 34.5 kcal/mol for TS31 to 40.2 kcal/mol for TS′31. Our calculations led us to conclude that, in contrast to the proposed mechanism in the literature, 21 is unreactive toward the redox process. To discover whether redox via C α deprotonation might proceed from other intermediates, species 24, 17, and 29 were considered as candidates for initiating the reaction. We found that structure 17, which does not feature any BF3 catalyst, is extremely unreactive toward the redox process; in this case, overall activation barriers through transition structures TS17 and TS′17 are calculated to be highly energy consuming (>45 kcal/mol). Such high barriers confirm the importance of BF3 in facilitating the redox process. Structure 24, which has BF3 on the alkoxy ligand, was also found to be unreactive toward a redox process via Cα deprotonation. Indeed, the presence of BF3 on the alkoxy group reduces the ability of its oxygen atom to transmit two electrons to iodine(III), thereby diminishing the potency of the alkoxy as a reductant. Finally, we found that the addition of two BF3 groups to 17 to give structure 29 makes the redox even less likely; all activation barriers were calculated to be greater than 45 kcal/mol. 6515


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Figure 6. (a) Free energy profile for aldehyde formation starting from intermediate 24 via α-hydride elimination. The relative free energies are given in kcal/mol. (b) Spatial plots of the antibonding orbitals associated with the 3c-4e bond in species 24, 33, and TS32‑33. Selected bond distances in DFT-calculated structures 24, 33, and TS32‑33 are given in Å. Intermediate 32 is impossible to locate due to the flatness of the potential energy surface in the vicinity of the transition structure TS24‑32.

ligand with the strong trans-influencing property causes the I− O bond in 33 to be weakened, leading to the acetate ligand to be more prone to change its position. This statement is supported by the longer I−O bond distances in 33 (2.419 Å) than in 2 (2.130 Å). The lower reactivity of 2 toward isomerization (Figure 1) is related to the fact that two acetate ligands with relatively weak trans-influencing properties are bonded more strongly to the iodine center. It follows that the I−OAc bond strength plays a crucial role in determining the ease of the corresponding isomerization: the weaker the I− OAc bond, the easier the isomerization. To find out the role of BF3 in stabilizing the transition structure of α-hydride elimination, TS′32 was calculated (Figure 7). This transition structure was found to be highly

electron (3c-4e) bond of species TS24‑32, TS32‑33, and 33. As shown in Figure 6b, the energy of this antibonding orbital raises from −2.6 (for TS24‑32) to −2.1 (for TS32‑33) and then to 1.6 eV (for 33). This increase in energy can be explained by the fact that the hydrogen is transferred as a hydride. Indeed, since a hydride acts as a strong σ donor ligand, the repulsive interactions in the corresponding antibonding orbital become more significant as the hydride transfer progresses. The NBO charge on the iodine center becomes less positive from TS24‑32 (1.54) to TS32‑33 (1.28) and then to 33 (1.04), further supporting the relevant hydrogen is transferred as a hydride. The isomerization from 33 proceeds with an activation barrier as low as 3.1 due to the presence of the hydride ligand positioned trans to the acetate ligand. Indeed, the hydride 6516


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Figure 7. Calculated transition structures for α-hydride elimination starting from intermediates 24, 17, and 21. The relative free energies are given in kcal/mol.

energetic (48.8 kcal/mol), which highlights the potency of BF3 as a catalyst in lowering the overall activation energy. In fact, BF3 coordination weakens the I−O (alkoxy) bond in 24, which in turn facilitates the isomerization step necessary for aldehyde formation. This assertion is further supported by calculating transition structure TS″32 in which BF3 coordinates to the acetate and not the alkoxy ligand. The energy computed for this transition structure (62.6 kcal/mol) is even higher than that of TS′32, which proves that, for the α-hydride elimination mechanism to operate, BF3 coordination to the alkoxy ligand is a prerequisite. We found that a correlation exists between I−O(alkoxy) bond distance and the activation barrier to α-hydride elimination: the shorter the I−O(alkoxy) bond, the higher the activation energy (Figure 7). This energetic trend means that the I−O(alkoxy) bond strength is the key determinant for setting the ease of the process leading to formation of aldehyde/ketone. Catalytic Cycle Proposed by DFT Calculations. Figure 8 summarizes our finding for BF3-catalyzed alcohol oxidation by a hypervalent iodine(III) reagent. As illustrated, the catalytic cycle commences with a substitution reaction between the alcohol and BF3·Et2O to form active catalyst 20. Then, the ligand exchange between 20 and iodane 2 occurs through the concerted interchange associative mechanism to afford intermediate 24 via transition structure TS22‑23. This intermediate is susceptible to undergo α-hydride elimination by crossing transition structure TS32‑33 to yield BF3-stabilized aldehyde product and iodane 33. The ensuing iodane is extremely reactive toward the reductive elimination of acetic acid in an exergonic fashion. The BF3·(alcohol) active catalyst is finally regenerated though a reaction between a free alcohol and the BF3-stabilized aldehyde.54 The rate-determining step of this process, where the alcohol is propanol, is calculated as 23.4 kcal/mol, which was assigned to transition structure TS32‑33. This result is fully consistent with the large kinetic isotope effect observed experimentally, which demonstrated that the cleavage of the alkoxy Cα−hydrogen bond occurs in the rate-determining step. A unique feature of our proposed catalytic cycle is that reduction of the iodine(III) center takes place after formation of the product and is an off-cycle process; usually, for the reactions conducted by an iodane, iodine(III) reduction coincides with product formation.55−60

Figure 8. Catalytic cycle for BF3-catalyzed alcohol oxidation by iodane 2 discovered through DFT calculation.

At the closure of this section, we wish to note that, although both iodine(III) and iodine(V) hypervalent compounds are capable of oxidizing alcohols, they follow distinct oxidation mechanisms; oxidation by iodine(III) compounds occurs through the α-hydride elimination of alkoxy group (introduced in the present work), whereas that by iodine(V)39,40 occurs through Cα hydrogen deprotonation (conventional mechanism). This disparity might be related to two factors which increase the affinity of iodine(V) for the conventional mechanism: (i) an iodine(V) compound has an internal base (oxo ligand) available to serve as a deprotonating agent (Scheme 3) and (ii) an iodine(V) center has higher electron deficiency than an iodine(III), resulting in its Cα hydrogen being more acidic. Oxidation of Secondary Alcohols by Iodane 2. Ochiai et al. established that secondary alcohols are oxidized by iodane 2 faster than primary alcohols. Our calculations corroborate this claim by showing that the overall activation energy for oxidation of 2-propanol (Figure 9) is ∼2 kcal/mol lower in energy than that of 1-propanol. This consistency between theory and experiment lends further support to the validity of the α-hydride elimination mechanism. A plausible explanation for the higher reactivity of secondary alcohols is as follows: in the transition structure of the α-hydride

Figure 9. Calculated important stationary points for 2-propanol oxidation via our proposed mechanism for α-hydride elimination. The relative free energies are given in kcal/mol. 6517


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ACS Catalysis elimination, an electron deficiency is being developed at the Cα atom; this deficiency is alleviated if the Cα atom is more substituted.

criterion and ultrafine integral grid were also employed to increase the accuracy of the calculations. The free energy barriers for ligand dissociation/association via TSa, TSb, TSc, TSd, and TSe were estimated according to the protocol presented by Hall and Hartwig. In this protocol, for example, the Gibbs free energy barrier for a dissociation reaction such as A-B → A + B is estimated as ΔG⧧ ≈ ΔH = HA + HB − HA‑B. In this work, the free energy for each species in solution was calculated using the formula

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CONCLUSION DFT calculations at the SMD/M06-2X/def2-TZVP//SMD/ M06-2X/LANL2DZ,6-31G(d) level of theory were exploited to provide mechanistic insight into alcohol oxidation by an iodane catalyzed by BF3. Two main processes (ligand exchange and redox) were found to conduct the alcohol oxidation, both of which are accelerated by the involvement of BF3. The findings achieved in the course of this study may be summarized as follows. (i) In contrast to previous reports, which have shown that addition of a Brønsted/Lewis acid to iodane activates it toward a specific reaction, we found in this study that the ligand exchange between an iodane and alcohol is best facilitated if BF3 coordinates to the alcohol substrate and not iodane. (ii) Although the redox step of alcohol oxidation by an iodine(V) compound (IBX) proceeds via the conventional mechanism (alkoxy Cα deprotonation), the same mechanism does not apply to an iodine(III) reagent for alcohol oxidation. (iii) Once the ligand exchange process forms the alkoxy iodine(III) intermediate, the BF3 coordination to the alkoxy makes it prone to α-hydride elimination to give a BF3-stabilized ketone/aldehyde product and iodane [ArI(OAc)(H)]. The ensuing iodane is highly reactive toward reductive elimination to yield ArI + HOAc. (iv) In agreement with the kinetic isotope effect reported experimentally,61 the alkoxy α-hydride elimination was found to be the rate-determining step for the alcohol oxidation process. (v) Consistent with experimental observations, the αhydride elimination mechanism predicts that the oxidation of secondary alcohols is more accessible energetically than that of primary alcohols.

G = E(BS2) + G(BS1) − E(BS1) + ΔG1atm → 1M

(1)

where ΔG1atm→1M = 1.89 kcal/mol is the free-energy change for compression of 1 mol of an ideal gas from 1 atm to the 1 M solution phase standard state. An additional correction to Gibbs free energies was made to consider solvent (alcohol) concentration where an (alcohol)n is directly involved in transformations. In such a case, the free energy of (alcohol)n is described as G(alcohol)n = E(BS2) + G(BS1) − E(BS1) + ΔG1atm → 1M + RT ln([alcohol] /n)

(2)

where the last term corresponds to the free energy required to change the standard state of solvent from [alcohol]/n M to 1 M.74,75 These numerical correction values for monomeric and dimeric forms of 1-propanol are 1.7 and 1.3 kcal/mol, respectively, and that for the monomeric form of 2-propanol is 1.5 kcal/mol.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site. Figures S1−S2 and Cartesian coordinates of all calculated species (PDF) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01599. Free energy profiles comparing ligand exchange between iodane 2 and 10 and that between 21 and 20, scan of potential energy surface associated with the alcoholassisted proton transfer to acetate starting from 24′ by optimizing the geometries with various O−H distances, and Cartesian coordinates and total energies for all of the calculated structures (PDF)

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COMPUTATIONAL DETAILS Gaussian 0962 was used to fully optimize all the structures reported in this paper at the M06-2X level of theory.63−65 For all of the calculations, solvent effects were considered using the SMD solvation model with 1-propanol as the solvent, except for the calculated energy profile shown in Figure 9, for which 2-propanol was used as the solvent.66 The effective core potential of Hay and Wadt with a double-ξ valence basis set (LANL2DZ) was chosen to describe iodine.67,68 The [631G(d)] basis set was used for other atoms.69 A polarization function was also added for I (ξd = 0.289).70 This basis set combination will be referred to as BS1. Frequency calculations were carried out at the same level of theory as those for the structural optimization. Transition structures were located using the Berny algorithm. Intrinsic reaction coordinate (IRC) calculations were used to confirm the connectivity between transition structures and minima.71,72 To further refine the energies obtained from the SMD/M06-2X/LANL2DZ,631G(d) calculations, we carried out single-point energy calculations using the M06-2X functional method for all of the structures with a larger basis set (BS2). BS2 utilizes the def2-TZVP basis set73 on all atoms. A tight convergence

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.A.: [email protected]. ORCID

Kaveh Farshadfar: 0000-0002-0863-1136 Brian F. Yates: 0000-0001-9663-3301 Alireza Ariafard: 0000-0003-2383-6380 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Research Council (ARC) for project funding (DP18000904) and the Australian National Computational Infrastructure and the University of Tasmania for the generous allocation of computing time. 6518


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ACS Catalysis (41) Ganji, B.; Ariafard, A. DFT Mechanistic Investigation into Phenol Dearomatization Mediated by an Iodine(III) Reagent. Org. Biomol. Chem. 2019, 17, 3521−3528. (42) Our calculations demonstrated that the free energy change for the transformation HOAc → 1/2(HOAc)2 is only 0.04 kcal/mol, suggesting that the monomeric and dimeric forms are likely in equilibrium. Therefore, it does not make any difference which form of acetic acid is used in the DFT calculations. (43) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan, Y.; Webster, C. E.; Hall, M. B. Rhodium Boryl Complexes in the Catalytic, Terminal Functionalization of Alkanes. J. Am. Chem. Soc. 2005, 127, 2538−2552. (44) Our calculations demonstrated that [OAc]− dissociation from 2 is endergonic by 23.7 kcal/mol with ΔG⧧ = 34.2 kcal/mol and [F3B· OAc]− dissociation from 21 is endergonic by 9.2 with ΔG⧧ = 18.3 kcal/mol (the dissociation activation free energies are computed based on Hall and Hartwig’s protocol). These results suggest that, for 2 and 21 to undergo isomerization, they should preferentially proceed via a concerted pathway (i.e., via transition structures TS2‑15 and TS21‑25, respectively) and not a dissociation pathway. (45) Following the suggestion of a reviewer, we also investigated the ligand exchange between PrOH·BF3 (20) and (iodane)·BF3 (21) and found that this pathway is less likely to occur due to the fact that its transition structure (TS27‑24) lies 2.8 kcal/mol higher in energy than TS22‑23 (for details see Figure S1). (46) Shu, S.; Li, Y.; Jiang, J.; Ke, Z.; Liu, Y. Mechanism of Hypervalent Iodine Promoted Fluorocyclization of Unsaturated Alcohols: Metathesis via Double Acids Activation. J. Org. Chem. 2019, 84, 458−462. (47) Zhang, J.; Szabó, K. l. n. J.; Himo, F. Metathesis Mechanism of Zinc-Catalyzed Fluorination of Alkenes with Hypervalent Fluoroiodine. ACS Catal. 2017, 7, 1093−1100. (48) Zhou, B.; Yan, T.; Xue, X.-S.; Cheng, J.-P. Mechanism of SilverMediated Geminal Difluorination of Styrenes with a Fluoroiodane Reagent: Insights into Lewis-Acid-Activation Model. Org. Lett. 2016, 18, 6128−6131. (49) Jobin-Des Lauriers, A.; Legault, C. Metathetical Redox Reaction of (Diacetoxyiodo) Arenes and Iodoarenes. Molecules 2015, 20, 22635−22644. (50) Zhou, B.; Haj, M. K.; Jacobsen, E. N.; Houk, K. N.; Xue, X.-S. Mechanism and Origins of Chemo-and Stereoselectivities of Aryl Iodide-Catalyzed Asymmetric Difluorinations of β-Substituted Styrenes. J. Am. Chem. Soc. 2018, 140, 15206−15218. (51) Ling, L.; Liu, K.; Li, X.; Li, Y. General Reaction Mode of Hypervalent Iodine Trifluoromethylation Reagent: A Density Functional Theory Study. ACS Catal. 2015, 5, 2458−2468. (52) Dohi, T.; Kita, Y. Hypervalent Iodine Reagents as a New Entrance to Organocatalysts. Chem. Commun. 2009, No. 16, 2073− 2085. (53) Another alternative for the Cα deprotonation is the alcoholassisted proton transfer to acetate starting from 24′. Although we are unable to locate a transition structure for this process, the energy scan calculations indicate that this possibility is unlikely to take place, as evidenced by the fact that it is very energy consuming with ΔE⧧ > 50 kcal/mol (Figure S2). (54) Although under the reaction conditions mentioned in ref 36 an amount of BF3·Et2O equivalent to that of iodane was used, BF3 still acts as a catalyst (not a promoter) for the alcohol oxidation process. The reaction PrOH + (propanal)·BF3 → PrOH·BF3 + propanal is calculated to be exergonic by −4.3 kcal/mol and occurs with ΔG⧧ = 13.4 kcal/mol. This result supports the facile regeneration of the active catalyst (PrOH·BF3) and confirms that BF3 is not consumed during the reaction. (55) Zhu, C.; Liang, Y.; Hong, X.; Sun, H.; Sun, W.-Y.; Houk, K.; Shi, Z. Iodoarene-Catalyzed Stereospecific Intramolecular sp3 C−H Amination: Reaction Development and Mechanistic Insights. J. Am. Chem. Soc. 2015, 137, 7564−7567. (56) Funes-Ardoiz, I.; Sameera, W.; Romero, R. M.; Martínez, C.; Souto, J. A.; Sampedro, D.; Muñiz, K.; Maseras, F. DFT Ration-

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DFT Mechanistic Investigation into BF3-Catalyzed Alcohol Oxidation

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