DFT Studies on the Mechanisms of the Platinum-Catalyzed Diboration

Article pubs.acs.org/Organometallics

DFT Studies on the Mechanisms of the Platinum-Catalyzed Diboration of Acyclic α,β-Unsaturated Carbonyl Compounds Bowen Liu,† Min Gao,† Li Dang,‡ Haitao Zhao,*,† Todd B. Marder,*,§,∥ and Zhenyang Lin*,‡ †

Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, People's Republic of China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People's Republic of China § Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K. ∥ Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ‡

S Supporting Information *

ABSTRACT: Detailed mechanisms of the diboration of the acyclic α,β-unsaturated carbonyl compounds acrolein, methyl acrylate, and dimethyl fumarate (DMFU) catalyzed by Pt(0) complexes were studied with the aid of density functional theory by calculating the relevant intermediates and transition states. For acrolein and methyl acrylate, the results show that the catalyzed diboration occurs via oxidative addition of the diboron reagent to the Pt(0) complex having diimine and acrolein (or methyl acrylate) as the ligands, 1,4-conjugate addition of a Pt−B bond to acrolein/methyl acrylate to give an O-bound boron enolate intermediate containing a Pt−C−CC−O−B linkage, and subsequent acrolein/methyl acrylate coordination to the Pt(II) center followed by reductive elimination to obtain the 1,4-diboration product of acrolein/methyl acrylate, i.e., the β-borylsubstituted O-bound boron enolate. For acrolein, the 1,4-diboration product is the final product, whereas for methyl acrylate, the 1,4-diboration product then isomerizes to the experimentally observed and thermodynamically favored 3,4-addition product, i.e., the β-boryl-substituted C-bound boron enolate, via a 1,3-shift of the O-bonded boryl group. Slightly different from what we have seen in the catalyzed diboration of acrolein/methyl acrylate, the catalyzed diboration of DMFU takes place through oxidative addition of the diboron reagent to the Pt(0) complex having DMFU and diimine as the ligands, 1,6-conjugate addition of both of the two Pt−B bonds to the coordinated DMFU ligand to give a 1,6-addition intermediate containing BegO−C(OMe)C−C C(OMe)−OBeg (eg = ethyleneglycolato = −OCH2CH2O−) as a ligand, and then isomerization via two consecutive 1,3-shifts of the two O-bonded boryl groups to produce the experimentally observed 3,4-diborated diastereomeric products.

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INTRODUCTION Diborations1,2 of unsaturated compounds catalyzed by transitionmetal complexes3 play an important role in the synthesis of organoboron compounds4 that are useful intermediates and reagents in organic synthesis. Recently, the catalyzed reaction of α,β-unsaturated carbonyl compounds with diboron reagents has attracted considerable experimental interest because this process introduces a boryl group at a position β to the carbonyl group. To date, Cu,5 Ni,6 Pd,7 Pt,8 and Rh9 complexes, as well as metal-free NHC10a or PR310b compounds, have been used as catalysts for these borylation reactions. In 1997, Norman, Marder, and co-workers reported the first examples of the diboration of α,β-unsaturated carbonyl compounds employing the Pt phosphine catalyst precursor [Pt(η2-C2H4)(PPh3)2].11a In 2004, the more active Pt diimine catalyst precursor Pt(BIAN)(DMFU) (BIAN = bis(phenylimino)acenapthene, DMFU = dimethyl fumarate) and B2pin2 (pin = © 2012 American Chemical Society

pinacolato = OCMe2CMe2O) as the diboron reagent were applied by the same group for the diboration of α,β-unsaturated carbonyl compounds at room temperature (Scheme 1).11b Through careful examination of the primary products, they found that both 1,4- and 3,4-addition products can be formed, depending on the nature of the substituent R3. The primary 1,4- and 3,4-addition products can then be hydrolyzed to the β-borylated products (Scheme 1). It should be noted that most researchers do not examine the primary reaction products in these borylation reactions, but those formed following hydrolysis or alcoholysis of the reaction mixtures. Thus, mechanistic information can be lost, as different primary products can give rise to the same hydrolysis product. In this theoretical study, we examine in detail the mechanism of the Received: March 16, 2012 Published: March 30, 2012 3410

dx.doi.org/10.1021/om3002153 | Organometallics 2012, 31, 3410−3425

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Scheme 1

Scheme 2

It is well-known that gas-phase calculations overestimate the entropic contribution significantly, especially for the cases where the numbers of reactant and product molecules are not equal. Corrections were applied to the calculated gas-phase free energies on the basis of the theory of free volume.21 For one-to-one or two-to-two transformations, no corrections were made. For two-to-one (or oneto-two) transformations, a correction of −2.6 (or 2.6) kcal/mol (at T = 298.15 K) was made. Similar corrections have been applied in a few earlier theoretical studies.22 In this paper, the entropy-corrected relative free energies are used to analyze the reaction mechanism. To examine the basis set dependence, we also employed a larger basis set, i.e., the triple-ζ SDD basis set with the Stuttgart-Dresden ECP for Pt23 and the cc-pVDZ basis set for all other atoms,24 to carry out geometry optimization calculations for the intermediates and transition states shown in Figures 1 and 4a (the most favorable pathway for the diboration of acrolein). The additional calculations show that the dependence of the basis set is insignificant. For instance, using the smaller basis set, the barrier heights 1 + s-trans acrolein → TSA(1-2), 2A → TSA(2-3), s-trans acrolein → TSisomer, 3 + s-cis acrolein → TSAcis(2-4), and 4Acis + s-trans acrolein → TSAcis(5-6) are 20.4, 1.0, 8.8, 18.6, and 17.9 kcal/mol, respectively, and the 1,4diboration reaction of acrolein with B2eg2 is exergonic by 38.7 kcal/mol. Using the larger basis set, the barrier heights are 18.8, 1.1, 8.6, 16.0, and 15.3 kcal/mol, respectively, and the reaction energy is exergonic by 38.5 kcal/mol.

Pt-catalyzed diboration reaction and, in particular, how the nature of the substituent R3 affects the regioselectivity of the insertion of the α,β-unsaturated carbonyl into the Pt−B bond (1,4- versus 3,4-insertion). Our previous theoretical studies3o,12 on Cu(I)-catalyzed borylation reactions of various substrates emphasized the nucleophilic behavior of boryl ligands.5i,k,13 The Cu(I)catalyzed borylation of α,β-unsaturated carbonyl compounds (acrolein and methyl acrylate)14 was found to occur via a nucleophilic attack of the Cu−B σ bond on the coordinated CC bond of the unsaturated substrate molecule to give a β-borylalkyl C-bound Cu(I) enolate intermediate, from which hydrolysis/alcoholysis gives the β-borylated product.

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COMPUTATIONAL DETAILS

Molecular geometries of the model complexes were optimized without constraints via DFT calculations using the Becke3LYP (B3LYP) functional,15 as implemented in the Gaussian 09 suite of programs.16 The effective core potentials (ECPs) of Hay and Wadt with a double-ζ valence basis set (LanL2DZ) were used in describing Pt,17 whereas the 6-31G* basis set was used for all other atoms. Frequency calculations at the same level of theory have also been performed to identify all of the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency) and to provide free energies at 298.15 K which include entropic contributions by taking into account the vibrational, rotational, and translational motions of the species under consideration. Transition states were located using the Berny algorithm. Intrinsic reaction coordinates (IRC) were calculated for the transition states to confirm that such structures indeed connect two relevant minima.18 All of the transition states and IRC calculations were also performed with the B3LYP functional in the Gaussian 09 package.16 Natural bond orbital (NBO) analysis was also performed at the same level of theory using the NBO 5.9 standalone package.19 To reduce computational costs, the substituents at N in the diimine ligand and the methyl groups in Bpin were replaced by CH3 and H, respectively. Thus, we used the simplest diimine ligand (CH3NCHCHNCH3) as the model and Beg (eg = ethyleneglycolato = −OCH2CH2O−) as a model for Bpin in catalytic reactions in the current studies. Acrolein, methyl acrylate, and dimethyl fumarate (DMFU) were used as the model compounds for α,β-unsaturated aldehyde and ester substrates. The reliability of the chosen models has been confirmed by using the ONIOM method,20 and the relevant details are given in the Results and Discussion.

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RESULTS AND DISCUSSION

Conformers of the Model Substrates Acrolein and Methyl Acrylate. The aim of this paper is to examine the subtle difference in mechanistic aspects between the Pt(0)catalyzed diboration reactions of α,β-unsaturated aldehydes/ ketones and esters. Acrolein exists as a mixture of s-trans and s-cis conformers, and so does methyl acrylate. Before looking into the mechanistic aspects, we first carried out calculations on the conformers, examined their relative stabilities, and determined their interconversion barriers (eqs 1 and 2 in Scheme 2). For acrolein, the s-trans conformer is 1.6 kcal/mol more stable than the s-cis conformer and the interconversion barrier between the two conformers is only 7.2 kcal/mol. For methyl acrylate, the s-trans conformer is slightly less stable by 0.6 kcal/mol than the s-cis conformer, and the interconversion barrier (7.0 kcal/mol) is also very small. Therefore, interconversion between the s-cis and s-trans conformers is rapid, and an equilibrium between the 3411


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conformers exists for both substrates, a conclusion consistent with early experimental and theoretical studies.25,26 We also calculated the relative stability of the s-cis and s-trans conformers of acrolein and methyl acrylate using MP4,27 CCSD(T),28 M06,29 and B97D.30 The calculated energy differences using these different theoretical methods are presented in Table 1. The results show that the relative stability

Scheme 3

Table 1. Comparison of the Gas-Phase Relative Free Energies (ΔGo) and Electronic Energies (ΔE) Calculated Using B3LYP, MP4, CCSD(T), M06, and B97D Methods (in Units of kcal/mol)a,b ΔGo

relative free energies (kcal/mol, in parentheses), and relative electronic energies (kcal/mol, in brackets) are presented, and the entropy-corrected relative free energies are used to analyze the reaction mechanism. The barrier for the oxidative addition was calculated to be 20.4 kcal/mol. In the (diimine)Pt(η2-acrolein) complex (1), the structural isomer having the acrolein CC moiety coordinated with the metal center was considered because it represents the most stable isomer.14 The complex (2A) derived from the oxidative addition adopts a trigonal-bipyramidal (TBP) geometry with an 18-electron count and can easily dissociate the acrolein ligand, leading to other structural isomers such as 2B,C shown in the energy profile (Figure 1). In the TBP structure of 2A, the acrolein ligand occupies one equatorial position and its η2-coordinated CC double bond lies horizontally on the equatorial plane to maximize the metal (d) to ligand (π*) back-bonding interactions.32 Attempts to locate the fourth structural isomer 2D failed, and the optimization starting from 2D gives 2D′, which is 0.9 kcal/mol less stable than 2A (Scheme 3). As the experimentally used ligand BIAN is much more rigid than the model diimine ligand employed in our calculations, it is considered unlikely that it dissociates one arm during catalysis. The structures calculated for these structural isomers are given in Figure 2.

ΔE

method

s-trans acrolein

s-trans methyl acrylate

s-trans acrolein

s-trans methyl acrylate

B3LYP MP4 CCSD(T) M06 B97D

−1.6 −1.3 −1.2 −1.2 −1.7

0.6 0.7 0.6 0.8 0.6

−1.7 −1.6 −1.6 −1.5 −1.8

0.7 0.6 0.6 0.8 0.8

a

All structures are fully optimized under the given functional. bThe s-cis conformers of acrolein and methyl acrylate are used as the energy reference points.

is consistently predicted by these methods. In the following calculations, we used the s-trans acrolein and the s-cis methyl acrylate as the model α,β-unsaturated carbonyl compounds. Borylation of Acrolein. It is generally assumed that the Ptcatalyzed diboration reactions of acrolein begin with oxidative addition of the diboron reagent to the Pt(0) complex having a diimine ligand to give Pt/diimine/acrolein/diboryl complexes. Figure 1 shows an energy profile calculated for the oxidative addition of B2eg2 to (diimine)Pt(η2 acrolein).31 In Figure 1, and the following figures having energy profiles, the entropycorrected relative free energies (kcal/mol), calculated gas-phase

Figure 1. Energy profiles calculated for the oxidative addition step and the interconversion among 2A−C. The entropy-corrected relative free energies, relative free energies (in parentheses), and electronic energies (in brackets) are given in kcal/mol. 3412


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Figure 2. Optimized structures of the intermediates 2A−C,D′. Selected structural parameters are given (bond lengths in Å). The hydrogen atoms have been omitted for the purpose of clarity.

In Figure 1, we failed to locate the transition states for the recoordination of acrolein to 3 leading to formation of 2B,C. Judging from the very small barrier of acrolein dissociation from 2A (1.0 kcal/mol), we believe that the transition states for the recoordination of acrolein should be similar to the structures of 2B,C (Figure 1). In our previous studies, we found that the Cu(I)-catalyzed borylation of acrolein occurs through a nucleophilic attack of the Cu−B σ bond on the coordinated olefinic CC bond of acrolein to give a β-borylalkyl C-bound Cu(I) enolate intermediate followed by a keto−enol isomerization to the Obound Cu(I) enolate intermediate with comparable stability.14 Therefore, we first considered 2A,B in our investigations. The three nucleophilic pathways shown in Scheme 4 were examined. Figure 3 gives the energy profiles calculated for paths A and B. In Figure 3, we have 3 + s-trans acrolein as the starting point for both paths because 2A,B both easily dissociate acrolein to form the square-planar complex 3. Path A is a 3,4-addition pathway in which the equatorial boryl ligand in 2A nucleophilically attacks the terminal carbon of the coordinated acrolein to give a C-bound Pt(II) enolate intermediate (4A). To our surprise, the insertion barrier (35.7 kcal/mol with 3 + s-trans acrolein as the reference point) is significantly higher than the previously calculated barrier (12.8 kcal/mol) for insertion of acrolein into a Cu−B bond in the Cu(I)-catalyzed borylation of acrolein.14

Scheme 4

Path B is a 1,2-addition pathway giving an intermediate (4B) with a remarkably high barrier of 53.3 kcal/mol. The relatively less favored 1,2-addition pathway in comparison with the 3,4addition pathway is related to the frontier molecular orbitals of acrolein. The CC double bond coordinates to a transition 3413


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Figure 3. Energy profiles calculated for paths A and B shown in Scheme 4. The entropy-corrected relative free energies, relative free energies (in parentheses), and electronic energies (in brackets) are given in kcal/mol.

words, there exists the possibility that the boryl ligand behaves as an electrophile, not a nucleophile, in the diboration reactions catalyzed by Pt(0). Hence, we examined four possible electrophilic pathways for the Pt-catalyzed diboration of acrolein, as shown in Scheme 5, and the energy profiles

metal center better than the CO double bond does because in the highest occupied conjugated π orbital the CC double bond makes the major contribution and in the lowest unoccupied conjugated π orbital the terminal carbon of the CC double bond makes the highest contribution. 14 Regarding the fact that both the 1,2- and 3,4-addition pathways have significantly higher barriers than the corresponding pathways calculated for insertion of acrolein into a Cu−B bond in the Cu(I)-catalyzed borylation of acrolein, we believe that this is related to the very different natures of the Pt−B and Cu−B bonds. More discussion of this will be given below. Attempts to locate a one-step transition state for a 1,4addition (path C in Scheme 4) failed. We explored the energetics of a few likely candidate structures for the one-step 1,4-addition transition state and found that they are all highly unstable. These results suggest that, even if the transition state exists, the corresponding pathway is highly unfavorable. The transition state for the corresponding nucleophilic 1,4-addition was also not found for insertion of acrolein into a Cu−B bond in the Cu(I)-catalyzed borylation of acrolein.14 From these calculations, we can clearly see that the three nucleophilic pathways shown in Scheme 4 are energetically unfavorable. Considering the fact that the electronegativity of B is greater than that of Cu but smaller than that of Pt, we postulate a bond polarization difference between a Cu−B bond and a Pt−B bond; i.e., a Cu−B bond is polarized toward B while a Pt−B bond is polarized toward Pt. This postulate is supported by our natural bond orbital (NBO) analysis. We calculated the natural atomic charges for 2A and found that the Pt metal center indeed carries less positive charge than B by 0.2−0.3 units. In contrast, the calculations of the natural atomic charges for the corresponding Cu species (PMe3)Cu(Beg)(η2acrolein), in which the acrolein ligand coordinates to the metal through the CC bond, show that the Cu metal center carries more positive charge than B by 0.4 units. Therefore, there exist different reaction mechanisms for the Pt(0)- and Cu(I)-catalyzed borylation reactions. In other

Scheme 5

calculated for these reaction pathways are shown in Figure 4. The selected optimized structures and transition states with structural parameters for the species involved in Figure 4 are shown in Figure 5. In 2A (Figure 2), the oxygen atom of the 3414


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Figure 4. Energy profiles calculated for electrophilic diboration pathways: (a) a 1,4-addition pathway including an s-cis-coordinated acrolein ligand (path D); (b) 1,4- and 3,4-addition pathways (paths E and F) of acrolein with B2eg2. The entropy-corrected relative free energies, relative free energies (in parentheses), and electronic energies (in brackets) are given in kcal/mol.

carbonyl moiety in the s-trans-coordinated acrolein is 4.43 Å from the boron center of the axial boryl ligand, and the distance is too far to allow a boryl migration to the carbonyl oxygen. Therefore, we also considered an s-cis acrolein ligand (2Acis, path D in Scheme 5). Path D in Figure 4a shows a pathway that can eventually lead to the formation of the 1,4-diboration product. In path D, the s-trans acrolein easily isomerizes to the s-cis acrolein. Coordination of s-cis acrolein to the Pt(II) center gives the intermediate 2Acis, which also adopts a TBP geometry around the metal center. The carbonyl oxygen in the s-cis-coordinated acrolein ligand is 2.94 Å away from the boron center of the axial boryl ligand (2Acis; Figure 5). The boryl ligand in the axial position electrophilically attacks the carbonyl oxygen of the s-cis acrolein ligand to give the square-planar intermediate 4Acis with a barrier of 18.6 kcal/mol. Subsequently, another acrolein coordinates to the Pt(II) center of 4Acis with an energy increase of 9.1 kcal/mol

to give 5Acis. Then, reductive elimination in 5Acis occurs with an overall barrier of 17.9 kcal/mol to give the 1,4-diboration product 6A, regenerating catalyst 1. Early studies also showed that coordination of an additional ligand to four-coordinate Pt(II) or Ni(II) complexes can promote reductive elimination.33 The 1,4-addition reaction is exergonic by 38.7 kcal/mol and the rate-determining barrier is 20.4 kcal/mol related to the oxidative addition of B2eg2. It should be pointed out that there may exist another pathway via which the s-trans acrolein ligand in 1 first isomerizes to an s-cis conformation and then oxidative addition of the diboron reagent B2eg2 takes place. It is expected that such a pathway does not differ much energetically from that shown in Figure 4a. Path E in Figure 4b shows the energy profile calculated for another electrophilic addition pathway leading to the formation of the 1,4-diboration product from intermediate 2C. In path E, the boryl ligand in the equatorial position of 2C migrates to the 3415


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Figure 5. Optimized structures and transition states for the diboration of acrolein with B2eg2 catalyzed by platinum involved in the pathways related to Figure 4. Selected structural parameters are given (bond lengths in Å). The hydrogen atoms have been omitted for the purpose of clarity.

B−O bond length (1.36 Å) in 6A. The structural parameters show that TSC(2-4) is a very late transition state. To achieve this very late transition state structure, the Pt−Cβ and Pt−Beg bonding interactions are substantially weakened, leading to the instability of the transition state structure. The energy decomposition analysis34 given in Scheme 6 supports our argument here. The energy required to change the structure of 3 to that present in the transition state TSC(2-4) is significantly greater than that required to deform it to the structure present in the transition state TSAcis(2-4) (58.8 versus 43.4 kcal/mol).

carbonyl oxygen in the coordinated acrolein ligand to give the 1,4-addition intermediate 4C with a very large barrier of 42.7 kcal/mol. We also considered the possibility that the axial boryl ligand in 2C migrates to the carbonyl oxygen of the coordinated acrolein ligand. Our calculations lead to the same transition state structure, TSC(2-4). To understand why TSC(2-4) is so unstable in comparison with TSAcis(2-4), we carefully checked the calculated structural parameters (Figure 5). In TSAcis(2-4), the forming B−O bond length is 2.03 Å, while in TSC(2-4), the forming B−O bond length is only 1.66 Å, which is rather close to the calculated 3416


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Scheme 6

Scheme 7

Figure 6. Energy profiles calculated for the oxidative addition step for the diboration of methyl acrylate with B2eg2 and the interconversion between 2Aester and 2Bester. The entropy-corrected relative free energies, relative free energies (in parentheses), and electronic energies (in brackets) are given in kcal/mol.

We also considered an electrophilic 3,4-addition pathway from 2A (path F, Figure 4b). In this pathway, the axial boryl ligand attacks the α-carbon in the coordinated CC bond of the acrolein 3417


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Figure 7. Energy profiles calculated for diboration pathways: (a) a 1,4-addition pathway (path Aester); (b) two 3,4-addition pathways (paths Bester and Cester) of methyl acrylate with B2eg2. The entropy-corrected relative free energies, relative free energies (in parentheses), and electronic energies (in brackets) are given in kcal/mol.

ligand to give the 3,4-addition intermediate 4A′ through TSA′(2-4) (a distorted TBP; Figure 5) with a barrier of 33.7 kcal/mol. We can see clearly that this pathway is also not a favorable one. Another pathway (path G) includes a [2 + 2] transition state via reacting 3 and acrolein directly through an electrophilic 1,2addition in which a boryl group is added to the carbonyl oxygen and the metal fragment adds to the carbonyl carbon, as shown in Scheme 5. The barrier was calculated to be 31.2 kcal/mol.

Comparing the pathways discussed above, we can see that the electrophilic 1,4-addition via an isomerization of an s-trans acrolein ligand to an s-cis acrolein ligand leading to the formation of 6A shown in Figure 4a (path D) is the most favorable pathway both kinetically and thermodynamically. Experimentally, the major products from diboration of α,βunsaturated aldehydes were characterized as the Z-(O) isomers of 3418


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the 1,4-addition (6A) process,11 suggesting that conversion of an s-trans acrolein ligand to an s-cis acrolein ligand indeed occurs in the diboration reactions because the Z-(O) isomers can be formed conveniently through an s-cis α,β-unsaturated aldehyde. On the basis of our calculations, the catalytic cycle shown in Scheme 7 is formulated. Diboration of acrolein catalyzed by Pt(0) complexes involves oxidative addition of the diboron reagent, acrolein insertion into the Pt−B bond, acrolein coordination, and reductive elimination. Borylation of Methyl Acrylate. Up to this point, we have limited our discussion to acrolein. We expect that the results obtained should be applicable to α,β-unsaturated aldehydes or ketones. However, the mechanistic aspects for α,β-unsaturated esters are expected to be different from those of α,βunsaturated aldehydes or ketones due to the relatively inert ester group. Figures 6 and 7 show the energy profiles calculated for the reaction of B2eg2 with s-cis methyl acrylate, a model α,βunsaturated ester substrate, catalyzed by 1ester. Similar to what we have seen for the borylation of acrolein, 1ester first forms 2Aester via the oxidative addition of B2eg2 to (diimine)Pt(η2methyl acrylate) with a barrier of 21.5 kcal/mol. 2Aester can easily dissociate and reassociate the methyl acrylate ligand, leading to the structural isomer 2Bester shown in the energy profile (Figure 6). Both structural isomers 2Aester and 2Bester adopt TBP geometries at the metal center in which the methyl acrylate ligand occupies one of the three equatorial positions.32 In path Aester (Figure 7a), migration of the axial boryl ligand to the carbonyl oxygen of the coordinated s-cis methyl acrylate ligand in 2Aester takes place to give the intermediate 4Aester. The

calculated barrier for this migration step is 22.8 kcal/mol. Subsequently, coordination of another methyl acrylate gives 5Aester, from which reductive elimination regenerates catalyst 1ester and yields the 1,4-addition product 6Aester. The overall barrier for this coordination/reductive elimination was calculated to be 25.5 kcal/mol. Compound 6Aester then isomerizes to the more stable 3,4-diborated product 7Aester through a 1,3-shift of the O-bonded boryl group via a fourmembered-ring transition state TSAester(6-7) with a barrier of 28.7 kcal/mol. The 3,4-addition reaction is exergonic by 23.9 kcal/mol, and the rate-determining barrier is 28.7 kcal/mol related to the 1,3-shift of the O-bonded boryl group of the 1,4addition product. In our previous study of the Cu(I)-catalyzed borylation of α,β-unsaturated carbonyl compounds,14 we calculated a related 1,3-shift (CH2C(OMe)(OBeg) → CH2(Beg)COOMe) and obtained a barrier of 27.3 kcal/mol. Another possibility is that the 1,3-shift occurs in the intermediate 4Aester, followed by coordination of another methyl acrylate and then reductive elimination to give the 3,4-addition product (TS4Aester; Figure 7a). However, the barrier for this 1,3-shift was calculated to be 32.2 kcal/mol. We also considered two other pathways, paths Bester and Cester, shown in Figure 7b. Path Bester involves migration of the equatorial boryl ligand to the α-carbon of the coordinated methyl acrylate in the structural isomer 2Bester. Path Cester involves migration of the equatorial boryl ligand to the β-carbon of the coordinated methyl acrylate in the structural isomer 2Aester. The barriers of the two pathways are 41.6 and 37.1 kcal/mol, respectively. The results suggest that both of these CC insertion pathways are less favorable than path Aester shown in Figure 7a. On the basis of our calculations, the electrophilic 1,4-addition pathway shown in Figure 7a (path Aester) leading to the formation of the 1,4-addition product 6Aester, which then isomerizes via a 1,3-shift of the O-bonded boryl group to the experimentally observed 3,4-addition product 7Aester, is the most favorable both kinetically and thermodynamically. Scheme 8 summarizes the catalytic cycle for the catalyzed diboration of methyl acrylate. We can clearly see that the 3,4-diboration product for methyl acrylate is thermodynamically more stable than the 1,4diboration product by 6.5 kcal/mol (Figure 7a). However, for acrolein, the 3,4-diboration product is energetically less stable by 17.6 kcal/mol than the 1,4-diboration product (eq 3 shown in Scheme 9). The different thermochemistries of acrolein and methyl acrylate explain the different diboration products of α,β-unsaturated aldehydes/ketones and esters observed experimentally.

Scheme 8

Scheme 9

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Figure 8. Energy profiles calculated for diboration pathways of DMFU: (a) the oxidative addition step and the interconversion between 2Admfu and 2Bdmfu; (b) 1,6-conjugate addition of the two Pt−B bonds to the coordinated DMFU ligand. The entropy-corrected relative free energies, relative free energies (in parentheses), and electronic energies (in brackets) are given in kcal/mol.

3,4-diboration product of methyl acrylate is more stable than the 1,4-diboration product, because this steric repulsion should also exist in the 3,4-diboration product of methyl acrylate. In methyl acrylate, we should also consider the fact that the ester group is relatively inert because of the extensive π conjugation within the unit, giving rise to extra instability in the 1,4diboration product. Borylation of Ester Substrates Containing Two Carbonyl Groups. Marder and co-workers also investigated the diboration of ester substrates containing two carbonyl groups such as dimethyl 2-butenedioate (MeOOC−CH CH−COOMe). As shown by 1H NMR spectroscopy, it is very interesting that the diboration reaction of dimethyl 2-butenedioate yields 3,4-diborated diastereomeric products (A and B, shown in Scheme 10).11b In Scheme 10, A1 and A2 are a pair of enantiomers, whereas B is a mesomer. A and B are

To understand better why the 3,4-diboration product of acrolein is energetically less stable than the 1,4-diboration product (eq 3), we calculated thermodynamic and kinetic data for the two relevant species shown in eqs 435 and 5 (Scheme 9). All the three transformations (eqs 3−5) have comparable interconversion free energy barriers. Equations 3 and 5 have comparable reaction free energies as well. However, eq 4 is 6−7 kcal/mol more exergonic than eq 3 or 5. These results suggest that the steric effect due to the presence of a tertiary carbon (repulsive interactions among the substituents at tertiary carbon) causes extra destabilization to the 3,4-diboration product. While the boryl−boryl steric repulsion argument above can explain the relatively less stable 3,4-diboration product of acrolein in comparison with the more stable 1,4-diboration product, the same argument cannot explain the fact that the 3420


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Figure 9. Energy profiles calculated for diboration pathways of DMFU: two consecutive 1,3-shifts of the O-bonded boryl groups to give the 3,4diborated diastereomeric products (8A-1dmfu and 8A′-1dmfu). The entropy-corrected relative free energies, relative free energies (in parentheses), and electronic energies (in brackets) are given in kcal/mol.

diastereomers. A and B can be distinguished by 1H NMR spectroscopy, while A1 and A2 cannot. To examine the detailed mechanistic aspects for the formation of the diborated diastereomeric products, DFT calculations have been carried out using dimethyl (E)-2butenedioate (dimethyl fumarate, DMFU), a ligand of the

catalyst precursor (R1 = CO2Me, R2 = H, and R3 = OMe shown in Scheme 1), as the model α,β-unsaturated ester substrate having two carbonyl groups. Figures 8 and 9 show the energy profiles calculated for the reaction of B2eg2 with DMFU catalyzed by 1dmfu. Similar to what we have seen for the borylation of acrolein/methyl 3421


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Scheme 10

Scheme 11

acrylate, the oxidative addition of B2eg2 to 1dmfu first forms 2Admfu with a barrier of 21.5 kcal/mol. 2Admfu easily dissociates and reassociates the DMFU ligand, leading to the structural isomer 2Bdmfu shown in the energy profile (Figure 8a). From 2Admfu (Figure 8b), migration of the axial boryl ligand to the carbonyl oxygen of the coordinated DMFU ligand gives the intermediate 4Admfu (with an overall barrier of 22.8 kcal/mol with respect to 3 + DMFU). Subsequently, different from what we have seen in the catalyzed diboration of acrolein/methyl acrylate, the boryl ligand in 4Admfu migrates to the remaining carbonyl oxygen of the monoborylated DMFU moiety to form 5Admfu. The calculated barrier for this migration step is only 17.5 kcal/mol, which is significantly lower than the calculated barrier (25.5 kcal/mol) for methyl acrylate coordination/ reductive elimination step (4Aester → 5Aester → 6Aester) shown in Figure 7a. In 5Admfu, the 1,6-diborated product of DMFU (6Admfu) is formed and coordinates to the metal center with one of its two CC double-bond moieties. Subsequently, ligand exchange in 5Admfu (Figure 9a) regenerates the catalyst 1dmfu and yields the intermediate 6Admfu, which contains two O-bound boron enolate moieties. 6Admfu then isomerizes to the 3,6-diboration product 7Admfu through a 1,3-shift of the O-bonded boryl group via the fourmembered-ring transition state TSAdmfu(6-7) with a barrier of 28.2 kcal/mol. From 7Admfu, the two pathways shown in Figure 9b lead to the diastereomers 8A-1dmfu and 8A′-1dmfu through two different transition states, TSAdmfu(7-8) and TSA′dmfu(7-8). We can clearly see that the energy difference between the two transition states is remarkably small (1.4 kcal/mol); therefore, it is expected that the diboration reaction of DMFU gives the 3,4diborated diastereomeric products experimentally. Experimental Evidence of Electrophilic Reactions of Boryl Ligands. Experimentally, a stoichiometric reaction of s-trans but-3-en-2-one II with the Pd boryl complex I as shown

Table 2. Relative Electronic Energies (ΔE), EntropyCorrected Relative Free Energies (ΔG), and the Energy Differences (ΔΔE and ΔΔG) Calculated Using the Model and ONIOM Calculations (in Units of kcal/mol)a,b model species TSA(1-2) 2A 3 + s-trans acrolein 3 + TSisomer 3 + s-cis acrolein 2Acis TSAcis(2-4) 4Acis 5Acis TSAcis(5-6) 1 + 6A

ONIOM ΔΔG (ΔG1 − ΔG2)

ΔΔE (ΔE1 − ΔE2)

10.6 −7.5 −3.1

−1.3 1.5 −1.7

−0.1 −0.1 −4.2

6.2 −1.5

−1.7 −1.7

−4.2 −4.1

−7.1 3.7 −8.2 −1.9 14.2 0.2 −35.7 −21.7 −33.9 −37.5 −15.0 −37.4 −26.5 −4.8 −27.6 −51.7 −40.7 −52.9

1.3 −2.0 0.7 3.1 1.7 2.0

1.1 −2.1 −1.8 −0.1 1.1 1.2

ΔG1

ΔE1

ΔG2

ΔE2

20.4 4.8 −6.4

10.5 −7.6 −7.3

21.7 3.3 −4.7

2.4 −4.8

2.0 −5.6

4.1 −3.1

5.0 12.2 −21.0 −11.9 −3.1 −38.7

a All structures are fully optimized under the given system. b1 + s-trans acrolein + B2eg2 was taken as the energy reference point.

ONIOM Calculations for the Diboration of Acrolein. As mentioned in Computational Details, we used the simplest diimine ligand (CH3NCHCHNCH3) as the model for bis(phenylimino)acenapthene (BIAN) and Beg (eg = ethyleneglycolato = −OCH2CH2O−) as the model for Bpin in all of our calculations. To validate further the use of the models, we performed two-layer ONIOM20 (B3LYP:B3LYP) calculations for the favorable pathway of the Pt-catalyzed diboration of acrolein shown in Figures 1 and 4a using the full-size chemical systems BIAN and B2pin2. The two ONIOM layer partition is illustrated in Scheme 11. In the inner layer, the LanL2DZ basis set was used to describe Pt and the 6-31G* basis set for all other atoms. In the outer layer, the LanL2DZ basis set was used to describe all of the atoms. The results of the ONIOM calculations are compared with those of the model calculations (Table 2). The ONIOM and model calculation results are remarkably consistent with each other, indicating that the models used in this study are acceptable.

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in eq 6 was reported by Tanaka and co-workers to give the 1,4addition O-bound boron enolate III having a Z configuration with respect to the CC double bond.36 The O-bound boron enolate III is an analogue of intermediate 4Acis in Figure 4a. The Z configuration in III clearly shows that s-cis but-3-en-2one should be formed before the insertion of II into the Pd−B bond, which is consistent with the favorable pathway calculated for the diboration of acrolein (path D in Figure 4a).

CONCLUSIONS A detailed mechanism for the Pt diimine catalyzed diboration of acyclic α,β-unsaturated carbonyl compounds acrolein, methyl acrylate and DMFU has been investigated with the aid of DFT calculations. For the Pt-catalyzed diboration of acrolein, the computational results give a mechanism which involves (1) 3422


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Notes

oxidative addition of the diboron reagent to the Pt(0) complex having acrolein and diimine as the ligands to yield a diboryl platinum intermediate having a pseudo-trigonal-bipyramidal structure with one axial and one equatorial boryl ligand, (2) 1,4-conjugate addition of the axial Pt−B bond to acrolein to give an O-bound boron enolate intermediate containing a Pt−C−CC−O−B linkage, and (3) subsequent acrolein coordination followed by reductive elimination to produce the thermodynamically stable 1,4-diboration product. The oxidative addition step was calculated to be rate-determining. In the 1,4-conjugate addition, the axial boryl ligand attacks electrophilically the carbonyl oxygen of a coordinated acrolein substrate. This is especially interesting because boryl ligands in copper complexes normally behave as nucleophiles when they interact with unsaturated organic substrates.12a−f,14 A rapid interconversion (with a small barrier of ca. 7 kcal/mol) between an s-trans acrolein ligand and an s-cis acrolein ligand makes the 1,4-conjugate addition of the axial Pt−B bond to acrolein possible. Only in its s-cis conformation can the carbonyl oxygen and the β-carbon of the coordinated acrolein substrate form bonds with the boron and platinum, respectively, in the transition state while maintaining a certain degree of interaction between Pt and B as this bond is breaking. Similar to what we have seen for acrolein, diboration of methyl acrylate catalyzed by Pt(0) complexes involves oxidative addition of diboron reagents, 1,4-conjugate addition of a Pt−B bond to methyl acrylate, methyl acrylate coordination, and reductive elimination to produce the 1,4-diboration product. However, for the ester case, the 1,4-diboration product isomerizes via a 1,3-shift of the O-bonded boryl group to the experimentally observed and thermodynamically favored 3,4addition product. Slightly different from what we have seen in the catalyzed diboration of acrolein/methyl acrylate, the Pt-catalyzed diboration of DMFU has a mechanism which involves (1) oxidative addition of the diboron reagent to the Pt(0) complex having DMFU and diimine as the ligands, (2) electrophilic 1,6conjugate addition of the two Pt−B bonds to the coordinated DMFU ligand to give a 1,6-addition intermediate containing BegO−C(OMe)C−CC(OMe)−OBeg as a ligand, and then (3) isomerization via two consecutive 1,3-shifts of the O-bonded boryl groups to produce the experimentally observed 3,4-diborated diastereomeric products. All of our discussions have thus far focused on acyclic α,βunsaturated carbonyl compounds. Detailed computational studies on the mechanisms of diboration of cyclic α,βunsaturated carbonyl compounds are presently under way, and the results will be reported in due course.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the NSFC (20872109, 20702037, and 20834002) and the Innovation Foundation of Tianjin University. Z.L. acknowledges financial support from the Hong Kong Research Grants Council (HKUST 603711P and HKU1/CRF/08). T.B.M. thanks the Royal Society (U.K.) for a Wolfson Research Merit Award, the Royal Society of Chemistry for a Journals Grant for International Authors, and the EPSRC for support via an Overseas Research Travel Grant.

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ASSOCIATED CONTENT

S Supporting Information *

Text giving the complete ref 16 and tables giving results with different theoretical methods and different basis sets and Cartesian coordinates and electronic energies for all of the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.Z.); todd.marder@ uni-wuerzburg.de (T.B.M.); [email protected] (Z.L.). 3423


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Organometallics

Article

Stringer, G.; Marder, T. B.; Baker, R. T.; Scott, A. J.; Clegg, W.; Norman, N. C. Can. J. Chem. 1996, 74, 2026. (e) Hartwig, J. F.; He, X. Organometallics 1996, 15, 400. (f) Hartwig, J. F.; Xe, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 315. (g) Dai, C.; Stringer, G.; Marder, T. B.; Scott, A. J.; Clegg, W.; Norman, N. C. Inorg. Chem. 1997, 36, 272. (h) Sakaki, S.; Kikuno, T. Inorg. Chem. 1997, 36, 226. (i) See also: Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E. G. Chem. Commun. 1997, 53. (j) Clegg, W.; Lawlor, F. J.; Lesley, G.; Marder, T. B.; Norman, N. C.; Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Scott, A. J.; Souza, F. E. S. J. Organomet. Chem. 1998, 550, 183. (k) Clegg, W.; Lawlor, F. J.; Marder, T. B.; Nguyen, P.; Norman, N. C.; Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Robins, E. G.; Scott, A. J.; Souza, F. E. S.; Stringer, G.; Whittell, G. R. J. Chem. Soc., Dalton Trans. 1998, 301. (l) Kerr, A.; Marder, T. B.; Norman, N. C.; Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Timms, P. L.; Whittell, G. R. J. Chem. Soc., Chem. Commun. 1998, 319. (32) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365. (33) (a) Komiya, S.; Abe, Y.; Yamamoto, A.; Yamamoto, T. Organometallics 1983, 2, 1466. (b) Tatsumi, K.; Nakamura, A.; Komiya, S.; Yamamoto, A.; Yamamoto, T. J. Am. Chem. Soc. 1984, 106, 8181. (c) Yamamoto, T.; Abla, M. J. Organomet. Chem. 1997, 535, 209. (d) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. J. Am. Chem. Soc. 2000, 122, 1456. (34) (a) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1. (b) Bickelhaupt, F. M. J. Comput. Chem. 1999, 20, 114. (35) The calculation methods for eq 4 are shown in Computational Details. For the reference data, please see ref 14. (36) Onozawa, S.; Tanaka, M. Organometallics 2001, 20, 2956.

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