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Computational Study of B(C6F5)3-Catalyzed Selective Deoxygenation of 1,2-Diols: Cyclic and Non-Cyclic Pathways Gui-Juan Cheng, Nikolaos Drosos, Bill Morandi, and Walter Thiel ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04209 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018
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ACS Catalysis
Computational Study of B(C6F5)3-Catalyzed Selective Deoxygenation of 1,2-Diols: Cyclic and Non-Cyclic Pathways Gui-Juan Cheng, Nikolaos Drosos, Bill Morandi and Walter Thiel* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany).
Supporting Information Placeholder ABSTRACT: The selective deoxygenation of polyols has emerged as an attractive approach to transform biomassderived polyols into valuable building blocks. Herein, we present a theoretical study on the boron-catalyzed selective deoxygenation of terminal 1,2-diols. The computational results explain the different product distributions obtained with different silanes and unveil the critical role of the cyclic siloxane intermediate. Compared to non-cyclic pathways, the cyclic pathway facilitates the initial deoxygenation process because the cyclic structure minimizes the steric repulsions between the reagents. It avoids overreduction because the generated bulky disiloxane moiety hinders the second deoxygenation. KEYWORDS: selective deoxygenation, boron catalyst, DFT, diols, reaction mechanism 1. INTRODUCTION Biomass conversion has attracted much attention since it provides a promising sustainable source of valuable chemical 1 feedstocks that are traditionally obtained from fossil fuels. The biomass-derived starting materials are generally overfunctionalized compared to the fine chemicals, and hence defunctionalization has become an important approach to produce useful chemicals for downstream applications. Polyols are among the most common feedstock chemicals accessible by biomass conversion. The complete deoxygenation of polyols has been well developed to access 2 simple hydrocarbons. On the contrary, the selective partial 3-5 deoxygenation of polyols has been studied less extensively even though it affords a more appealing approach to obtain valuable functionalized building blocks. This poses the challenge to defunctionalize hydroxyl groups of polyols in a controlled and predictive way. Although an especially daunting challenge, systematic efforts toward selective deoxygenation of polyols have recently begun to emerge, and some progress has been made. The 4 Gagné group developed an approach for the chemoselective partial reduction of carbohydrates by using the tris(pentafluorophenyl)borane (B(C6F5)3) catalyst and tertiary silanes (R3SiH). They found that the primary C–O reduction is preferred and that further selective partial reduction of secondary C–O bonds is also possible. They proposed that the formation of a cyclic silyl oxonium ion is the key to achieve selectivity in these reactions. With this method, they
obtained a variety of functionalized chiral synthons from biomass-derived polyols. 5
Another breakthrough was reported by the Morandi group. They realized the first selective catalytic deoxygenation of terminal 1,2-diols at the primary position. Previous approaches for catalytic deoxygenation of diols either gave alkenes, alkanes via complete deoxygenation, or 1-alkanols via selective deoxygenation of the secondary alcohols. In6 spired by the seminal work of Gevorgyan and Yamamoto, Morandi and co-workers applied B(C6F5)3 and silanes to the deoxygenation reaction (Scheme 1). Their initial attempt with a tertiary silane, Et3SiH, only led to trace amounts of the desired 2-alkanol product, and the bis-silylated intermediate was observed as the major product. When using a smaller silane, EtMe2SiH, they obtained both the bis-silylated complex and the alkane resulting from overreduction as major products. Upon sequential addition of a secondary silane (Ph2SiH2) and Et3SiH, they got the desired 2-alkanol product with high yield and selectivity. The mono-deoxygenation product is formed via a cyclic siloxane intermediate which is supposed to facilitate the first deoxygenation reaction. However, a deeper understanding of the role of the cyclic siloxane and the origin of mono-selectivity, and of the difference between cyclic and non-cyclic pathways, is still lacking. In 7 our recently published work, the B(C6F5)3-catalyzed reaction of 1,2-internal diols and silane led to rearrangement rather than deoxygenation products. The cyclic siloxane was found to be critical for the rearrangement reaction, and non-cyclic siloxane failed to generate rearrangement products. Our 7 previous computational work rationalized the divergent experimental results for internal and terminal 1,2-diols while the role of the cyclic siloxane intermediate remained unclear.
Scheme 1. Deoxygenation of terminal 1,2-diols using different silanes The combination of B(C6F5)3 and silanes was also recently applied to other selective deoxygenations of carbohydrates,
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such as the chemoselective deoxygenation of unsaturated 8a polyols, the diastereoselective reductive carbocyclization of 8b unsaturated carbohydrates, and the chemoselective 8c defunctionalization of alkyl tosylates. Hydrosilylation of carbonyl compounds, imines, and alkenes is another major 9 application of catalysis with B(C6F5)3. Experimental and computational work on B(C6F5)3-catalyzed hydrosilylation demonstrated that the B(C6F5)3 catalyst activates the silane 10, 11 through formation of a putative borane-silane complex. In a recent landmark study by Piers, Tuononen, and coworkers, an analogous borane-silane intermediate was structurally and spectroscopically characterized, thus clarifying the role 12 of the B(C6F5)3 catalyst. To our knowledge, no theoretical studies have yet been reported on the reaction mechanism of the boron-catalyzed deoxygenation of polyols. In the present work, we examine the deoxygenation reactions shown in Scheme 1 with a model substrate, butane-1,2-diol, to get a detailed picture of the mechanism and to understand the special role of the cyclic siloxane intermediate in promoting reactivity and selectivity, as well as the different product distributions obtained with different silanes. 2. COMPUTATIONAL METHODS All DFT calculations were performed using the Gaussian 09 13 14 program. The M06-2X functional has been proven to de11a,b scribe B(C6F5)3-catalyzed hydrosilylation reactions reliably and was therefore adopted in this work. Geometry optimiza15 tions were conducted at the M06-2X/6-311G(d, p) level, using the default SCF convergence criterion (RMS density -8 matrix converged to 1.0 × 10 ), default optimization criteria (thresholds for maximum force, RMS force, maximum dis-4 -4 placement and RMS displacement of 4.5 × 10 , 3.0 × 10 , 1.8 × -3 -3 10 and 1.2 × 10 au), and ultrafine grids for numerical integration. Frequency calculations at the optimized structures were performed at the same level of theory to verify the stationary points as being real minima (no imaginary frequency) or transition states (one imaginary frequency) and to obtain thermodynamic energy corrections. Intrinsic reaction coordinate (IRC) calculations were carried out to ensure that the transition states connect the correct reactants and products. 16 Solvent effects were included using the SMD approach with dichloromethane as solvent, by means of single-point M062X/6-311++G(d, p) calculations at optimized gas-phase geometries. Relative energies with zero-point energy corrections and free energies (at 298.15K) are given in kcal/mol. Catalytic turnover frequencies (TOFs) were evaluated by the energetic 17 18 span model. Hirshfeld charges were calculated for the analysis of electrostatic interactions. 3D structures were 19 displayed with CYLView.
(Figure S1 in Supporting Information). In our study, we use the siloxane complexes int0 as starting points for calculating the cyclic and non-cyclic reaction pathways and as reference for the computed relative energies.
Figure 1. Reactions of butane-1,2-diol and silanes A, B, and C to generate the non-cyclic siloxanes A-int0 and B-int0 and the cyclic siloxane C-int0. 3.2. Reaction Mechanism of Deoxygenation. As shown in Scheme 2, the siloxane reacts with the borane-silane complex to form a disilyl oxonium ion and borohydride H-B(C6F5)3 (silylation step: int1 → int2) followed by a direct hydride delivery from borohydride to the disilyl oxonium ion and cleavage of the C–O bond (reduction step: int3 → int4), which affords the silylated alkanol int4 and regenerates B(C6F5)3 to complete the catalytic cycle of the first deoxygenation. The generated silylated alkanol int4 may undergo a further deoxygenation to generate an alkane product. On the non-cyclic pathway (Scheme 2a), an alkyl disiloxane is released in the reduction step while the disiloxane moiety is maintained in the silylated alkanol and the intermediates of the second deoxygenation reaction on the cyclic pathway (Scheme 2b).
3. RESULTS AND DISCUSSION 3.1. Formation of Siloxane. We assume that the reaction begins with the dehydrogenative silylation of butane-1,2-diol, which affords a non-cyclic siloxane with Et3SiH (A) and EtMe2SiH (B) or a cyclic siloxane with Ph2SiH2 (C) (Figure 1). This process is exoergic by 32.3, 35.0, and 36.2 kcal/mol in Gibbs free energy for the reaction of silanes A, B, and C, respectively, due in large part to the release of dihydrogen 20 gas. This initial silylation is strongly favored over the alternative borylation, since B-O bond formation via reaction of butane-1,2-diol with B(C6F5)3 is calculated to be kinetically less favorable than Si-O bond formation by 23.5 kcal/mol
Scheme 2. Catalytic cycles of the first and second deoxygenation reaction of non-cyclic siloxane (a) and cyclic siloxane (b).
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ACS Catalysis The initial deoxygenation of siloxane int0 may occur at the primary or secondary silyl ether position, which leads to a silylated 2-alkanol or 1-alkanol, respectively. The computational results for the first deoxygenation of C-int0 are presented in Figure 2. In the silylation step, the formation of the O-Si bond and the activation of the Si-H bond proceed simultaneously via a SN2-Si transition state TS1, which is similar to that found in B(C6F5)3-catalyzed carbonyl hydrosilylation 9 reactions. The silane-boron complex prefers to attack the less crowded primary silyl ether group: the corresponding activation barrier is lower by 2.3 kcal/mol than that for the silylation of the secondary silyl ether group (C-TS1’). The subsequent reduction also occurs through an SN2 mechanism in which the hydride attacks the backside of the C-O bond. The steric crowding at the tertiary carbon (C2) center hinders the hydride delivery in C-TS2’ and causes the activation barrier to be higher by 1.8 kcal/mol compared with hydride delivery in C-TS2. Additionally, the deoxygenation at the primary silyl ether group is thermodynamically favored since it affords a more stable 2-alkanol (C-int4 vs. C-int4’). Similarly, non-cyclic siloxanes A-int0 and B-int0 also preferentially afford silylated 2-alkanols (Figue S2 in SI). The calculated regioselectivity for the deoxygenation reaction is in agreement with the experimental observation that the initial deoxygenation prefers to occur at the primary position.
Figure 2. Deoxygenation of the cyclic siloxane C-int0 to generate silylated 2-alkanol (C-int4) and 1-alkanol (C-int4’). Since the formation of silylated 2-alkanol is more favorable, we focus on the initial deoxygenation of primary silyl ether (int0 → int4) and the second deoxygenation of the generated silylated 2-alkanol (int4 → butane) in the following to investigate the different reactivities of siloxanes toward deoxygenation. Table 1 lists the relative free energies for the intermediates and transition states involved in the deoxgenation reactions of A-int0, B-int0 and C-int0. Both deoxygenation processes are found to be thermodynamically favorable for all three siloxanes. For the first one, A-int0 has the highest activation barriers (compared with the other two siloxanes). The barriers for the silylation and reduction steps of A-int0 are 23.8 and 21.6 kcal/mol, respectively; A-TS2 lies 27.1 kcal/mol above A-int0. The TOF is evaluated to be 8.5 × -8 -1 10 s according to the energetic span model. Therefore, Aint0 will be reluctant to undergo deoxygenation under the 5 reported reaction conditions (room temperature), which is in line with the experimental observation that the reaction of silane Et3SiH leads to the siloxane A-int0 as major product.
Table 1. Relative free energies (in kcal/mol) for the intermediates and transition states of deoxygenation reactions of A-int0, B-int0, and C-int0. A
B
C
B
C
int0
0.0
0.0
0.0
int5
-34.0
-35.8
int1
22.9
13.1
7.0
TS3
-29.1
-22.0
TS1
23.8
15.0
9.3
int6
-39.7
-38.6
int2
8.0
10.0
-0.2
int7
-43.0
-33.8
int3
5.5
3.8
-2.1
TS4
-29.0
-22.9
TS2
27.1
23.8
14.8
butane
-87.3
-64.1
int4
-45.6
-45.0
-46.5
Siloxane B-int0 has barriers of 15.0 and 20.0 kcal/mol for the silylation and reduction steps of the first deoxygenation, respectively. The second deoxygenation of B-int0 is calculated to be more favorable, with barriers of 15.9 (B-int4 → BTS3) and 16.0 (B-int4 → B-TS4) kcal/mol for the silylation and reduction steps, respectively. The calculated TOFs for the first and second catalytic deoxygenation cycle are 2.2 × -5 -1 -1 10 s and 3.1 s , respectively. Consequently, the second deoxygenation will proceed much faster than the first one, and the initially formed silylated 2-alkanol will be quickly transformed into the alkane product. This is again consistent with the experimental observation that only traces of silylated 2-alkanol can be observed, while B-int0 and alkane are detected as major products in the reaction of silane EtMe2SiH. For cyclic siloxane C-int0, the first deoxygenation is a facile process, with hydride delivery being the rate-determining step. The barriers for the silylation and reduction steps are 9.3 and 16.9 kcal/mol, respectively, which are thus lower than those for the non-cyclic siloxanes. They are also much lower than the overall barrier of 24.5 kcal/mol (C-int4 → C-TS3) computed for the second deoxygenation process. In this case, the TOFs for the first and second catalytic deoxygenation -1 -6 -1 cycle are 2.4 s and 5.6 × 10 s , respectively. Thus, the second deoxygenation step will be much slower than the initial one, and the reaction selectively leads to silylated 2-alkanol, again in agreement with the experimental observations. To summarize, the calculated reaction profiles for non-cyclic and cyclic siloxanes successfully reproduce the experimentally observed product distributions. To gain deeper insight into the origin of the different behavior of non-cyclic and cyclic siloxanes, we performed a detailed structural analysis of the stationary points in the deoxygenation reactions of A-int0, B-int0, and C-int0, addressing the following specific questions: (1) Why is the cyclic siloxane so reactive in the first deoxygenation and how does it avoid overreduction? (2) Why are non-cyclic siloxanes less reactive in the initial deoxygenation? (3) Why is the second deoxygenation faster than the first one in the case of B-int0? 3.3. The First Deoxygenation. To understand the different reactivity of the non-cyclic and cyclic siloxanes in the first deoxygenation, we analyzed the structures of the siloxane intermediates int0 and of the transition states (TSs) for the silylation and reduction steps. We found through systematic conformational studies (see Figure S3 and S4 in SI) that two siloxy groups prefer to adopt an anti conformation
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in non-cyclic siloxanes. The anti arrangement of two siloxy groups leads to a gauche arrangement between the ethyl group and 1-siloxy group, which hinders the approach of the silane reagent (Figure 3). To make space for the incoming silane and to allow its attack, the ethyl group of the substrate has to rotate away from the 1-siloxy group. In the most stable TSs of the silylation step in non-cyclic pathways (A-TS1 and B-TS1), the ethyl and 2-siloxy groups rotate to form anti and gauche conformations with respect to the 1-siloxy group, respectively (Figure 4). The corresponding distortion energies from the lowest-energy conformation are calculated to be 5.4 and 5.3 kcal/mol for A-int0 and B-int0, respectively. By contrast, the two siloxy groups in the cyclic siloxane Cint0 are constrained to be in a gauche conformation; the ethyl group is anti to the 1-siloxy group so that there is enough space for silane attack. As shown in Figures 3 and 4, C-int0 resembles the substrate structure in C-TS1, and the
distortion energy of 2.8 kcal/mol is smaller than that for the non-cyclic counterparts.
Figure 3. Lowest-energy conformers of siloxanes int0.
Figure 4. Optimized geometries of the silylation (TS1) and reduction (TS2) transition states for the first deoxygenation process. Red dashed lines indicate short H-H and H-F distances of less than 2.3 and 2.5 Å, respectively. Besides these conformational issues, steric effects and electrostatic interactions also contribute to the different reactivity to deoxygenation of non-cyclic and cyclic siloxanes. In TS1 of the non-cyclic pathways, the terminal trialkylsilyl group introduces large steric congestion around the reacting oxygen atom and causes unfavorable steric repulsion between the substrate and the incoming silane molecule. This steric repulsion is most prominent in A-TS1 which contains bulky triethylsilyl groups. The severe steric repulsions between the substrate, the silane, and the boron catalyst in A-TS1 are indicated by red dashed lines in Figure 4. On the contrary, the cyclic siloxane minimizes the steric hindrance around the oxygen atom in C-TS1 and leaves more space for the attacking silane. Thus C-TS1 suffers from less steric repulsion than A-TS1 and B-TS1.
Similar phenomena are also observed in the transition states for reduction (TS2). In C-TS2, the cyclic silane intermediate minimizes the steric hindrance between the boron catalyst and the substrate, while steric repulsions between the boron catalyst and the bulky alkyldisiloxane moiety and the ethyl group of the substrate destabilize A-TS2 and B-TS2. This leads to increased distances between the reacting atoms in ATS2 (dO-C1 = 1.95 Å and dC1-H = 1.77 Å) and B-TS2 (dO-C1 = 1.93 Å and dC1-H = 1.75 Å) compared to C-TS2 (dO-C1 = 1.86 Å and dC1-H = 1.67 Å). The electrostatic interactions between the cationic carbon (C1Hirshfeld charge = 0.1) of the substrate and the approaching hydride (HHirshfeld charge = -0.1) and the leaving group (OHirshfeld charge = -0.2) are thus stronger in cyclic C-TS2 21,7 than in non-cyclic A-TS2 and B-TS2. In summary, the initial deoxygenation process is facilitated in C-int0 because the cyclic siloxane intermediate requires less
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ACS Catalysis distortion of the substrate and has less steric repulsion in the transition states. Moreover, stronger electrostatic interactions help stabilize C-TS2. 3.4. The Second Deoxygenation. This step was further analyzed to identify the origin of the reversed reactivities for B-int0 and C-int0. The initial deoxygenation releases an alkyl disiloxane and affords a silylated 2-alkanol on noncyclic pathways. By contrast, the corresponding reaction of C-int0 leads to an acyclic intermediate C-int4 in which the generated alkyl disiloxane moiety stays in the substrate. The initial deoxygenation is exothermic for both B-int0 and Cint0, while C-int4 is more stable than B-int4 by 1.5 kcal/mol, probably due to ring strain release of the cyclic siloxane. Figure 5 shows the most stable TS structures of the silylation and reduction steps for the second deoxygenation process. For the non-cyclic pathway, the first and second silylation steps have comparable activation free energies since B-TS1 and B-TS3 have similar electrostatic interactions (as indicated by similar distances between the reacting atoms) and similar steric interactions between the substrate, silane, and boron catalyst. However, the activation barrier for the reduction step (B-TS4) is lower than that for the initial reduction step (B-TS2) by 7.8 kcal/mol. A prominent hyperconjugative effect of the alkyl substituents in B-TS4 is indicated by shorter C1-C2 and C2-C3 bonds (1.47 Å and 1.47 Å vs. 1.51 Å and 1.54 Å in B-TS2) and by natural bond orbital (NBO) analysis (Table S1); this is expected to stabilize B-TS4 and thus facilitate 21, 7 the second reduction step. For the cyclic pathway, the newly formed bulky alkyl disiloxane moiety dramatically increases the steric hindrance around the oxygen atom in C-TS3 and the C2 atom in C-TS4, which causes unfavorable steric repulsions. Additionally, the distances between the silane and substrate and boron catalyst increase (dO-Si and dSi-H are 2.26 Å and 2.03 Å in C-TS3 vs. 2.24 Å and 1.79 Å in C-TS1) which will weaken the electrostat21, 7 ic interactions. Although there is also hyperconjugative stabilization in C-TS4, the distances between the cationic C2 atom and the nucleophiles in C-TS4 are elongated (dO-C2 and dC2-H are 2.21 Å and 2.12 Å in C-TS4 vs. 1.86 Å and 1.67 Å in CTS2) which will reduce the electrostatic interactions and destabilize C-TS4. Hence, the faster reaction rate for the second deoxygenation step of B-int0 can mainly be attributed to the favorable hyperconjugative effect in B-TS4. By contrast, for the cyclic pathway the bulky alkyl disiloxane moiety introduces severe steric repulsion and weakens the electrostatic interactions, which makes the second deoxygenation step unfavorable in the case of C-int0.
Figure 5. Optimized geometries of the silylation (TS3) and reduction (TS4) transition states for the second deoxygenation process. Red dashed lines indicate short H-H and H-F distances of less than 2.3 and 2.5 Å, respectively. 4. CONCLUSION In the present study, the reaction mechanism of the boroncatalyzed deoxygenation of terminal 1,2-diols was investigated by DFT calculations. The computational results reveal the role of the cyclic siloxane intermediate and rationalize the different reactivities of non-cyclic and cyclic siloxanes toward deoxygenation. A-int0 does not undergo deoxygenation because the bulky siloxane hinders the approach of the silane reagent. Overreduction of B-int0 occurs mainly because reduced steric congestion and favorable hyperconjugative interactions facilitate the second deoxygenation. The cyclic siloxane C-int0 minimizes substrate distortion and steric repulsions, which promotes the first deoxygenation, while the bulky disiloxane moiety thus generated prevents overreduction. The unveiled electronic and steric effects help us to understand the essential role of cyclic siloxanes and 4 silyl oxonium ions in B(C6F5)3-catalyzed rearrangement and 7 reduction reactions of polyols, respectively. Importantly, our study indicates that the rigid transition state structures for the deoxygenation of cyclic siloxanes may provide a general approach to control reactivity and selectivity in partial deoxygenation reactions. Furthermore, our calculations show that the C-O bond breaking step is rate determining, a result that has strong implications for the design of stereoselective reactions proceeding through catalyst control. Overall, these mechanistic insights may provide guidance in further experimental studies on transformations of polyols, e.g. their selective partial reduction via cyclic intermediates and asymmetric deoxygenations.
AUTHOR INFORMATION Corresponding Author *Email:
[email protected] Notes The authors declare no competing financial interests.
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Supporting Information Further numerical DFT results and a separate standardized xyz file of optimized structures are available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT Generous financial support by the Max Planck Society is gratefully acknowledged.
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