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A Combined DFT/IM-MS Study on the Reaction Mechanism of Cationic Ru(II)-Catalyzed Hydroboration of Alkynes Li-Juan Song, Ting Wang, Xinhao Zhang, Lung Wa Chung, and Yun-Dong Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03214 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017
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A Combined DFT/IM-MS Study on the Reaction Mechanism of Cationic Ru(II)-Catalyzed Hydroboration of Alkynes Li-Juan Song,† Ting Wang,† Xinhao Zhang,*,† Lung Wa Chung*,§ and Yun-Dong Wu,*,†,‡ †
Lab of Computational Chemistry and Drug Design, Laboratory of Chemical Genomics, Peking University Shenzhen Graduate School, Shenzhen 518055, China
§
Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, China. ‡ College of Chemistry, Peking University, Beijing 100871, China
Abstract: Recently, the Fürstner group reported the first general trans-hydroboration of internal alkynes by using a cationic ruthenium(II) complex [Cp*Ru(MeCN)3]PF6 as the catalyst. Density functional theory (DFT) calculations have been carried out to elucidate the reaction mechanism and the origin of stereoselectivity. The reaction mechanism was suggested to initiate with the rate-determining oxidative hydrogen migration to stereoselectively form a metallacyclopropene intermediate (that determines the trans-selectivity), followed by a reductive boryl migration to form the unusual trans-addition alkenyl-borane product. A combined ion-mobility mass spectrometry (IM-MS) and DFT study has also been employed to investigate the unsuccessful reaction with terminal alkynes. Key oxidative-coupling intermediates have been identified. Our results suggest that [2+2+2] cycloaddition of terminal alkynes to form a very stable arene compound could be the reason for the unsuccessful hydroboration of the terminal alkynes. Moreover, unreactive catecholborane reagent attributes to strong coordination of its arene part with the catalyst. Our proposed non-classical mechanism also accounted for the other related Ru(II)-catalyzed reactions (such as hydrogenation and hydrogermylation). Our combined computational and experimental study provides in-depth mechanistic understanding and insights on the unusual trans addition catalyzed by the cationic ruthenium(II) complexes, and could help design the other trans-addition reactions. Key words: hydroboration, hydrofunctionalization, trans-selectivity, ruthenium, 1
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density functional calculations, mass spectrometry
Introduction Organoboron compounds are versatile intermediates in organic synthesis,1 in particular with the wide applications of alkenyl boranes (including the aldol reaction2 and Suzuki-Miyaura coupling).3 Consequently, development of efficient syntheses of alkenyl boranes has received considerable attention.4 Hydroboration of alkynes, especially with the assistance of transition metal catalysts, is one of the most efficient and atom-economic approaches to alkenyl boranes.3b,4d,5 However, an important issue in the hydroboration of alkyne is the control of selectivity (Scheme 1). It is well-known that transition-metal catalyzed hydroboration of terminal alkynes proceeds regio- and stereo-selectively. The reaction generally is anti-Markovnikov and
involves
cis-addition.5b,6
The
generally-accepted
mechanism
for
the
cis-hydroboration reaction involves oxidative addition of a B-H bond to the metal center, followed by alkyne insertion, generating a metal σ-vinyl intermediate, and subsequent reductive elimination (Scheme 2 (a)).7 Hydroboration of internal alkynes also involves cis-addition and forms Z-alkenyl-boronates.4d,5,8 However, the general trans-selectivity of hydroboration has remained a very challenging problem and is one of the obscure details of organoboron chemistry.
Metal-free
hydroboration
was
also reported in recent years. The selectivity was achieved by using special substrates9 or introducing directing groups.10 However, the general trans-selectivity of hydroboration has remained a very challenging problem and is one of the obscure details of organoboron chemistry.
Scheme 1. Transition metal-catalyzed hydroboration of alkynes with different 2
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regio- and stereo-selectivity
Scheme 2. Proposed catalytic cycles for (a) cis-hydroboration of alkynes, (b) and trans-hydroboration of terminal alkynes
Miyaura and co-workers developed the first trans-hydroboration of terminal alkynes leading to Z-alkenyl boranes, when catecholborane (HBcat) or pinacolborane (HBpin) and rhodium or iridium complexes were used.11 Isotope experiments showed that the hydrogen atom trans to the boryl group in the product was not derived from the borane reagent, but from the terminal alkyne (Scheme 2 (b)). A mechanism involving oxidative addition of the terminal C-H bond and H-migration to form a metal vinylidene intermediate was proposed (Scheme 2 (b)).7e,12 This represents a new and exceptional class of trans-hydroboration. This reaction however fails to work with internal alkynes. Recently, the first highly stereoselective trans-hydroboration of terminal alkynes was achieved by copper catalysts.13 Consequently,
selective
trans-hydroboration
of
internal
alkynes,
which
complements the conventional and widely used cis-hydroboration in organic synthesis, has long been a major challenge. Significantly, the Fürstner group successfully developed the first and general trans-selective hydroboration of internal alkynes, in the presence of a cationic ruthenium(II) complex [Cp*Ru(MeCN)3]PF6 (Scheme 3).14 Hydroboration of internal alkynes with pinacolborane was reported to form the corresponding E-alkenyl-boronates with an excellent yield of up to 95% and an E/Z selectivity of >98:2. Interestingly, a similar or same ruthenium(II) catalyst was also 3
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applied to other trans-addition reactions of alkynes,15 such as hydrosilylation pioneered by Trost,16 hydrogenation and hydrostannation by Fürstner17,18 and hydrogermylation by Murakami.19
R
R
[Cp*Ru(MeCN)3]PF6
R
H/D trans-addition
pinB
H/DBpin
R
(R=alkyl)
Scheme 3. Cationic Ru(II)-catalyzed trans-hydroboration of internal alkynes
In spite of this breakthrough to boron chemistry, the Ru(II)-catalyzed trans-hydroboration of internal alkynes has retained some questions and limitations, such as regioselectivity. The Fürstner group carried out some mechanistic studies and found, for instance that other borane such as HBcat failed to undergo hydroboration under the same reaction conditions. Moreover, the reaction of terminal alkynes failed under the same reaction conditions (Scheme 4a). The deuterated pinacolborane (DBpin) was used to shed some light on the reaction mechanism (Scheme 3). This showed that the addition of the H-B bond proceeds truly in a trans fashion, and thus the pathway via a vinylidene intermediate can be precluded. Additional experiments conducted in the dark did not affect the reaction, excluding cis-trans photoisomerization in the cis-hydroboration pathway. These results imply an inherent trans-addition (Scheme 4b).
4
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Scheme 4. Experimental observations on (a) unreactive substrates and (b) no Z/E isomerization Our earlier theoretical studies on the related trans-addition hydrosilylation20,21 suggested a new reaction pathway in which an initial oxidative hydrogen migration forms a metallacyclopropene intermediate, the migrated hydrogen pointing to the Cp (or Cp*) ring, followed by reductive silyl migration to generate the trans-addition vinylsilane product. Fürstner suggested a similar mechanism for his trans-addition hydroboration.14 We have performed computational studies in an effort to understand the mechanism of the unique trans-hydroboration of internal alkynes as well as the limitations of the substrates (HBcat or terminal alkynes). A combination of density functional theory and mass spectrometric studies suggest that oxidative coupling of terminal alkynes could compete with hydroboration of terminal alkynes.22
Methods All calculations were performed by using Gaussian 09 programs.23 Geometry optimization of all stationary points in the gas phase was conducted by B3LYP method.24 The 6-31G (d) basis set was employed for H, B, C, N, O and Si atoms, 5
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while the Lanl2dz basis set (augmented with f functions) and ECP were used for Ru (denoted as basis set BSI).25 Frequency calculation was performed for each optimized structure to verify the stationary points as either minima or saddle point at the same level of theory. Solvent effect (solvent = DCM) was included by single-point energy calculation using SMD model and M06 method (with the def2-TZVP basis set for Ru and 6-311++G(3df, 3pd) basis set for the other atoms, denoted as basis set BSII).26 The reported free energies (in kcal/mol, at 298.15 K and 1 atm), which were obtained from the above-mentioned single-point SMD calculations combined with the gas-phase free-energy correction, are mainly used for our discussion unless stated otherwise. All experiments were carried out using a Synapt G2-S high definition mass spectrometer equipped with an electrospray ion source and MassLynx data processor.27 After ionization, the parent ions can be selected by a quadrupole mass filter. Mass-selected ions enter into a linear ion trap cell filled with argon, which pressure is approximately 8×10-3 mbar (Utrap). During collision, the mass of daughter ions is determined by a reflectron time-of-flight (TOF) detector. The ion source block and nitrogen desolvation gas temperatures were set to 100 °C and 200 °C. The collision induced dissociation (CID) experiments were conducted in the trap by increasing the applied voltage (Utrap). Ions were separated according to their mobility in ionmobility (IM) section, when pass a transfer cell, the mass spectra and the ion drift time are recorded. Experimental drift times can be converted to the collision cross section (CCS) values by calibration approach.28 Polylysine was used as CCS calibrant in our study. Theoretical CCSs were calculated by using the open source software program MOBCAL.29
Results and Discussion Hydroboration of Internal Alkynes. Displacement of acetonitrile ligands by alkyne (3-hexyne) and borane (HBpin) forms an intermediate (3) (Figures 1 and S1).30 Starting from intermediate 3, both oxidative hydrogen migration and oxidative boryl 6
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migration were considered. It was found that steric effect plays an essential role in this step. Oxidative hydrogen migration is lower in energy than oxidative boryl migration by 1.7 kcal/mol, as a result of steric repulsion between the borane and the alkyne (H--H: 2.12Å, Figure S2). Oxidative boryl migration can become slightly lower in free energy than oxidative hydrogen migration by 1.7 kcal/mol, when a smaller alkyne (2-butyne) was used to reduce this steric repulsion. As shown in Figure S3, an empty p orbital of the boron atom can accept electrons from the alkyne to facilitate the boryl migration when an insignificant steric effect is involved. In comparison, absence of the empty p orbital on Si and the more bulky silane strongly disfavor an oxidative silyl migration in hydrosilylation. It is 18.6 kcal/mol higher in energy than the corresponding oxidative hydrogen migration (Figure S4(a)). The oxidative hydrogen migration and oxidative boryl migration steps generate metallacyclopropene intermediates 5Htrans and 5Bcis with different conformations (Figure 1), respectively. As a result of the oxidative hydrogen migration to the alkyne, a stable metallacyclopropene intermediate 5Htrans is formed directly, in which the transferred
hydrogen
is
located
trans
to
the
boryl
group
along
the
metallacyclopropene plane (H-Cβ-Cα-Ru: 78.1°, Figure 2). Whereas, optimization of the classical metal σ-vinyl structure was characterized as a rotation transition state (6H-TStrans->cis, Figure 1), not an intermediate. 5Htrans subsequently undergoes reductive boryl migration via 6H-TStrans (B-Ru-Cα-Cβ: 116.4°, Figure 2), where the empty p orbital of the boron can interact with the π-bond of the metallacyclopropene, see Figure S5, to give the desired trans-addition product 7trans. Alternatively, 5Htrans could rotate to give the less stable cis-isomer 5Hcis via 6H-TStrans→cis. However, the energy barriers to rotation (26.3 kcal/mol) and subsequent isomerization via 6H-TSiso (28.5 kcal/mol)31 are higher than that of the reductive boryl migration (20.7 kcal/mol) by at least 5.6 kcal/mol and thus will not take place. Overall, the oxidative hydrogen migration is the rate-determining step of the more favorable pathway. It is different from hydrosilylation of internal alkynes, in which the rotation or the final reductive silyl migration can be the rate-determining step, depending on the steric effect of the system.20 7
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Figure 1. Free energy profiles of hydroboration via two different pathways in solution.
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Figure 2. Geometries for the metallacyclopropene intermediate 5Htrans and reductive boryl migration transition state 6H-TStrans.
In the oxidative boryl migration pathway, the more bulky boryl group rotates away from the Cp* ligand and thus is located cis to the hydrido ligand with respect to the metallacyclopropene plane in 5Bcis (B-Cβ-Cα-Ru: -89.9°, Figure S6). Reductive hydrogen migration from 5Bcis cannot proceed directly to form a B-C bond, as the hydride ligand is not perpendicular to the Ru-Cα-Cβ plane.32 Consequently, an additional rearrangement process via a double-bond isomerization (via 6B-TSiso) giving another intermediate 5B'cis is required to move the Ru-H bond until it is perpendicular to the Ru-Cα-Cβ plane for the subsequent reductive migration to give the undesired cis-addition product 7cis. The cis-trans isomerization barrier via 6B-TScis→trans (31.2 kcal/mol, Figure S6) is 5.0 kcal/mol, higher than that for the rate-determining oxidative hydrogen migration step of the most favorable pathway. Therefore, the initial oxidative boron migration pathway to give trans-product can be excluded. The reaction barrier via the oxidative boryl migration pathway to produce cis-product is computed to be 1.7 kcal/mol higher than that for the pathway via oxidative hydrogen migration pathway to yield trans- product (Figures 1 and S6). Therefore, the calculated ratio of trans-product to cis-product is 95:5 for 3-hexyne at room temperature. It is in agreement with the experimental ratio (97:3 for 5-decyne). Taken together, our results suggest that the lower-energy oxidative hydrogen 9
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migration pathway is preferred and forms the metallocyclopropene intermediate followed by the reductive boryl migration to afford the trans-hydroboration product. This pathway is the most likely pathway for the hydroboration reaction. The higher-energy oxidative boryl migration pathway gives the thermodynamically less stable cis-addition product. In addition, this mechanism is suggested to be general for other
hydrofunctionalizations
(including
hydrogenation,
hydrosilylation,
hydrogermylation and hydrostannation, Figures 3 and S7). Hydroboration is found to be the one with the highest barrier.
Figure 3. Free energy profiles of several hydrofunctionalizations (H-E: E=H, Bpin, SiMe3, GeMe3, SnMe3) in solution.
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Hydroboration of Terminal Alkynes. Reaction of terminal alkynes has been reported to be problematic under the same reaction conditions.14 Similar results were also found in the related Ru(II)-catalyzed hydrogenation of terminal alkynes.17 Notably, terminal alkynes are highly reactive towards the Ru(II)-catalyzed hydrosilylation and many other reactions16,19 and it would therefore be expected that a competitive side reaction may suppress the hydroboration of terminal alkynes. In a similar scenario, the Fürstner group proposed that the formation of a stable metal-vinylidene complex may account for the failure of hydrogenation of terminal alkyne.14 In an attempt to identify the side reaction that inhibits the desired hydroboration of terminal alkynes, we carried out a combined density functional theory/mass spectrometric study. The favored pathway for terminal alkynes involves oxidative boryl migration (20.3 kcal/mol, Figure S8) rather than oxidative hydrogen migration (25.8 kcal/mol), due to the diminished steric hindrance. Therefore, our calculations suggest that hydroboration of terminal alkyne may lead to the cis-product. The lower energy barrier of a terminal alkyne compared to that of an internal alkyne (26.2 kcal/mol) indicates that terminal alkyne is more reactive. Three reactions were considered
as
potential
side-reactions
(Figure
4).
The
formation
of
a
ruthenium-vinylidene intermediate by 1, 2-H migration as proposed in unconventional hydroboration (Scheme 2b)7e,12 was considered first. The calculated activation free energy (27.0 kcal/mol) for this reaction was much higher than that of hydroboration, suggesting that ruthenium-vinylidene is not involved. Second, the relative weak sp C-H bond of terminal alkyne suggest that oxidative addition should be considered.33 Oxidative addition of the C-H bond to the metal center might overcome a barrier of 24.7 kcal/mol, but is not competitive with respect to hydroboration. The less bulky terminal alkyne has been widely reported to undergo cyclotrimerization.34,35 Oxidative coupling of two alkyne substrates to generate a ruthenacyclopentene intermediate has a slightly lower energy barrier (19.9 kcal/mol). As shown in Figure S8, the oxidative boryl migration and oxidative coupling reaction pathways could compete. A very stable intermediate (6o) (Figure S8) can be formed by [2+2+2] cycloaddition of three 11
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alkynes and can generate a very stable 1,3,5-triethylbenzene through reductive elimination.36 As previously reported,35a the formation of the arene product is extremely exergonic (-102.6 kcal/mol), and our calculations suggest that the terminal alkyne may undergo oxidative coupling and a [2+2+2]-related side-reaction. The internal alkynes have a higher energy barrier to oxidative coupling (2o-TS1-in with 30.0 kcal/mol, in Figure S8) than hydroboration, which is in agreement with the experiment. The successful hydrosilylation of terminal alkynes16b,16c can be rationalized in terms of the lower barrier for hydrosilylation (17.7 kcal/mol. when using HSi(OMe)3, Figure S4b), which blocks the oxidative coupling pathway. Hydrogenation of terminal alkynes with a higher barrier (20.5 kcal/mol, see Figure S4(b)) than the oxidative coupling 2o-TS1 (19.9 kcal/mol) is qualitatively consistent with the reported unreactive terminal alkynes toward hydrogenation 17. The oxidative coupling reaction might however be suppressed by using less electron-rich Cp ligand with a higher-barrier (23.1 kcal/mol, Figure S9).
Figure 4. Four possible reaction types for terminal alkynes Relative free energy and electronic energy (in parentheses) in solution are given in kcal/mol. 12
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To obtain experimental information concerning the reaction of terminal alkynes, an ion-mobility mass spectrometric (IM-MS) investigation was carried out to identify the critical intermediate.37 Our strategy was to compare the reactions of terminal and internal alkynes and identify those species found only in the spectrum of the terminal alkynes. A series of acetonitrile solutions of a 3:100 mixture of [Cp*Ru(MeCN)3]PF6 (0.3 mM) with alkyne substrates were subjected to ESI-MS analysis.38 Two representative spectra of terminal and internal alkynes, phenylacetylene and 6-methyl-2-heptyne, are shown in Figure S10. Common major peaks in two spectra, m/z 371, 294, 278, 253 and 237 are Ru catalyst related ions, which do not involve substrates (Figure S11). Two ions, m/z 441 and 543, observed only in the case of the terminal alkyne (Figure S10), were preliminarily assigned to [Cp*Ru, 2C8H6]+ and ([Cp*Ru, 3C8H6]+), respectively, based on the isotope patterns (Figure S12).39 These two ions suggest that ruthenium may form a complex with two or three phenylacetylene molecules (C8H6). The corresponding complexes [Cp*Ru, 2C8H14]+ (m/z 457) or [Cp*Ru, 3C8H14]+ (m/z 567) were not detected in the case of the internal alkyne, 6-methyl-2-heptyne (Figure S10(b)). To further characterize the structure of the ions of m/z 441 and 543, collision induced dissociation (CID) were conducted (Figure S13). After collision with Ar, only one dominant fragment, m/z 233, was found in both of the CID spectra of the m/z 441 and 543 ions.40 No dissociation of phenylacetylene was observed. Therefore, the neutral losses could reflect the coupling product, rather than a sequential loss of substrates. Accordingly, as predicted, the two phenylacetylenes could be coupled in a common oxidative coupling manner in ruthenium-catalyzed terminal alkynes.34,35 The high collision energy (3.8 eV) required to release phenyl-containing compound could also account for an inhibition of the Ru-catalysts as proposed in the HBCat system (see below discussion).
The collision cross section (CCS) of m/z 441 and 543 were determined by IMS to be 131 Å2 and 154 Å2, respectively (Table 1 and Figure S15).28 Theoretical CCS values were also calculated for possible structures.29 The agreement between the calculated and experimental CCS values supports our computational prediction 13
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concerning the oxidative coupling reaction of terminal alkynes. Overall, the isotope pattern and the CID and IMS results suggest that the two ions appearing only in the spectra from the terminal alkyne case are the oxidative coupling intermediates of the substrates. Similar results were obtained when using another terminal alkyne, 1-pentyne (Figure S16). Ions m/z 371 and 441, corresponding to ruthenacyclopentadiene and cyclotrimerization product complex, respectively, are also be observed. In the case of 1-hexyne, the corresponding peaks can also be found in Figure S17. A control experiment was conducted in the presence of hydroborane and the critical ions for [2 + 2 + 2] pathway were also observed (Figure S18).
Table1. Collision induced dissociation (CID) and ion-mobility spectrometric (IMS) study of [Cp*Ru, 2C8H6] + (m/z 441) and [Cp*Ru, 3C8H6] + (m/z 543).a
a. drift gas: He, gas pressure: 5.23 mbar; Utransfer: 2V
Hydroboration with HBcat. Hydroboration of catecholboranes (HBcat) containing a phenyl ring led to a very low yield (
