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Iron(II) (Bis)Acetylide Complexes as Key Intermediates in the Catalytic Hydrofunctionalization of Terminal Alkynes Nikolaus Gorgas, Berthold Stöger, Luis Filipe Veiros, and Karl Kirchner ACS Catal., Just Accepted Manuscript • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018
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ACS Catalysis
Iron(II) (Bis)Acetylide Complexes as Key Intermediates in the Catalytic Hydrofunctionalization of Terminal Alkynes †
‡
§
Nikolaus Gorgas, Berthold Stöger, Luis F. Veiros, and Karl Kirchner* †
,†
‡
Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, AUSTRIA §
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa, PORTUGAL
ABSTRACT: Non-classical iron(II) polyhydride complexes supported by PNP-pincer ligands were found to be very active catalysts for the Z-selective hydroboration and the head-to-head dimerization of terminal alkynes. Iron(II) (bis)acetylide complexes are identified as catalytically active species in both transformations which, however, proceed via different reaction pathways. These species as well as intermediates relevant for the elucidation of selectivity could be trapped and fully characterized spectroscopically. X-ray structures of key intermediates are presented. Detailed DFT-calculations provide a deeper insight into the activation of the pre-catalysts and comprehensive catalytic cycles are elucidated. The proposed mechanisms provide a consistent picture of the different reaction pathways and fully explain the stereoselective steps of these transformations. KEYWORDS: Iron, pincer complexes, acetylide complexes, terminal alkynes, enynes, hydroboration, DFT calculations.
INTRODUCTION The hydrofunctionalization of alkynes provides a versatile and atom-economical tool for the construction of carbon-carbon and carbon-heteroatom bonds. However, the control of selectivity appears as an important issue as the direct addition of an H-E bond (E = C, B, Si) across the carbon-carbon triple bond may potentially generate several regio- or stereoisomeric products (Scheme 1a). In this context, we recently reported on iron(II) hydride dihydrogen complexes (1a and 1b) supported by PNP-pincer ligands, that were found to efficiently promote the catalytic hydroalkynylation or, in presence of pinacol borane, hydroboration of terminal alkynes.1 Both reactions take place under mild conditions and are highly chemo-, regio-, and stereoselective, exclusively yielding the respective anti-Markovnikov products with up to 99% Z-selectivity (Scheme 1b). This is of particular interest in the case of hydroboration reactions, as only few examples are known to convert terminal alkynes into the thermodynamically unfavored (Z)-vinyl boronic esters. Apart from early studies by Miyaura and co-workers employing Ir and Rh precursors together with monodentate phosphine ligands and in presence of triethyl amine,2 the non-classical ruthenium polyhydride [Ru(PNPtBu)(H)2(H2)] reported by the group of Leitner3 as well as the cobalt pincer complex [Co(PDICy)(CH3)] (PDI = pyridine diimine) described by Chirik et al.4 constitute the only well-defined catalysts that exhibit high (Z)-selectivity in the hydroboration of terminal alkynes. Most notably, these two examples as well as complex 1b feature pyridine derived tridentate pincer ligands as a common structural motif.
Since 1,3-enynes5 or vinyl boronates6 represent valuable synthons frequently used in the synthesis of more complex structures, the origin of selectivity constitutes an important aspect that merits further investigation. In the present paper, we report on the mechanistic background of these transformations, giving detailed insight into catalyst action and activation as well as the stereoselective steps of each reaction. Scheme 1. Iron Catalyzed (Z)-Selective Hydroalkynylation and Hydroboration of Terminal Alkynes. a) regio- and stereoisomers formed by hydrofunctionalisation of terminal alkynes H
H
H H
+
X
R
X
R
+
H
X = CCR, BR2, SiR3
X H
R
+
X
(E)-
H
R H
(Z)-
b) Z-selective hydroboration and dimerization of terminal alkynes H
1a / 1b 0.3-0.6 mol%
H
+ R
C6H6, r.t.
R
R
R'
R
3-16h
R = Ar
H PR2
N up to 99% Z-selective
N
H
Fe H
N PR2 R'
H
H O B
+ R
O
1b 0.4-4.0 mol% C6H6, r.t.
B R
O
O
H
1a R' = H 1b R' = Me R = iPr
3-16h R = Ar, alkyl
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RESULTS AND DISCUSSION We started our mechanistic studies focusing on the hydroalkynylation of terminal alkynes as this reaction is widely known for a large number of traditional noble metal catalysts and has been extensively investigated over the past decades.7 First experiments, in which the catalytic dimerization of phenyl acetylene in presence of 1b was monitored by NMR, showed immediate consumption the pre-catalyst and formation of paramagnetic intermediates which, however, could not successfully be separated from the reaction mixture. Nevertheless, the use of TMSacetylene allowed for the isolation of a defined complex (Scheme 2). X-ray analysis of crystals grown from a saturated pentane solution revealed formation of the 16e iron(II) (bis)acetylide complex 2.8 This new compound appears in a triplet ground state (S = 1) exhibiting solution magnetic moment of µeff = 2.8 μB. Addition of PMe3 afforded the coordinatively saturated and diamagnetic complex 3a with the acetylide ligands being in trans position to one another while crystallization of 2 from acetonitrile resulted in the formation of the analogue cis(bis)acetylide complex 3b (Scheme 3).9
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mentioned that catalysts 1a and 1b are also capable to promote cross-dimerizations employing trimethylsilyl acetylene as co-substrate (Scheme 4).10,11 Interestingly, the ethynyl trimethylsilane triple bond remains intact in the final (Z)-β-cross-dimerization product a.12 In absence of phenyl acetylene, ethynyl trimethylsilane is converted to the respective homo-coupled α-isomer (c) but this reaction proceeds significantly slower than the dimerization of aryl-substituted substrates. The competing formation of 1,4-diphenyl-1-buten-3-yne (b) can therefore be suppressed by using an excess of the trimethylsilyl cosubstrate. Despite the different behavior of TMSacetylene, trapping experiments by the addition of PMe3 also afforded a diamagnetic complex of the type trans[Fe(PNP)(C≡CPh)2(PMe3)] (4) in the case of phenyl acetylene. This strongly suggests that the respective iron(II) (bis)acetylide in fact represents the catalytically active species in the reaction. Scheme 4. Cross-dimerization Catalyzed by 1b.
Scheme 2. Formation of an Fe(II) (Bis)acetylide Complex and Structural View of 2.
Scheme 3. Trapping Experiments Affording Diamagnetic (Bis)acetylide Complexes and Structural Views of 3a and 3b.
In order to get more insight into the hydroalkynylation of terminal alkynes, DFT-calculations13 based on [Fe(PNP)(C≡CPh)2] (7) as the catalyst and employing phenylacetylene as the substrate (Az and AE) have been conducted. Catalyst initiation starting from 1b (AACT in the calculations) proceeds via a three-step sequence with an overall barrier of 16.3 kcal/mol (see Supporting Information, Figure S4). Analog to traditional rutheniumbased systems,14,15 our calculations unveiled a classical acetylene-vinylidene mechanism outlined by the simplified catalytic cycle in Scheme 5. In particular, an incoming acetylene substrate undergoes a 1,2-H-shift (II) yielding a vinylidene intermediate which suffers nucleophilic attack from the neighboring acetylide ligand (III) resulting in the formation of an alkynyl vinyl complex. Proton transfer from a second acetylene molecule leads to the liberation the final 1,3-enyne and recovery of the initial (bis)acetylide species (IV, V). Scheme 5. Proposed Catalytic Cycle for the (Z)Selective Hydroalkynylation of Terminal Alkynes.
In addition to our initial report describing the homodimerization of terminal aryl alkynes, it has to be
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ACS Catalysis As frequently suggested in such mechanisms, the preferential formation of one vinylidene rotamer in the CC bond formation step was intuitively anticipated to explain the experimentally observed (Z)-selectivity. We thus also considered the alternative formation of the pro-(E) vinyl intermediate CE in our calculations and a comparison of the free energy profiles for both isomers is presented in Scheme 6. Interestingly, no explicit vinylidene intermediate was found in case of CE as hydrogen migration and the C-C bond formation take place in a concerted manner. However, the transition state (TS-BCE) is almost equal in energy in comparison to the
Scheme 6. Free Energy Profile Calculated for the Hydroalkynylation of Terminal Alkynes Affording (Z)-1,3Enynes (blue) and (E)-1,3-Enynes (red). Free Energies (kcal/mol) are Referred to Intermediate Az ([Fe(PNP)(C≡ ≡CPh)2] + HC≡ ≡CPh).
formation of the analog (Z)-isomer (TS-ABZ), indicating that C-C coupling is not the stereoselective step in the reaction.
An important aspect of the catalytic reaction is that pro-(E) and pro-(Z) vinyl intermediates are prone to interconversion via the cumulenyl species CISO requiring an activation energy of 21.0 (E→ Z) and 23.0 (Z→ E) kcal/mol,
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respectively for their mutual interconversion (Scheme 7).16 Scheme 7. Free Energy Profile for the Interconversion of CZ and CE. Free Energies (kcal/mol) are Referred to the (Bis)acetylide Complex AZ (R = iPr).
Finally, despite of the fact that pro-(E) and pro-(Z) are formed with similar barriers, their interconversion has a smaller barrier than product formation (thus, they interconvert faster than product release) and pro-(Z) has a lower barrier for that part (and also is the most stable one). Thus, that is the selectivity key. Accordingly, the (Z)-selectivity originates from the product elimination step representing the highest barrier of the entire catalytic cycle. In particular, the transition state for the proton transfer TS-EFE was found to be 8 kcal/mol higher in energy than for the respective (Z)-isomer (TS-EFZ). This difference results from steric interactions between the alkyne substituents and the pyridine unit of the PNP ligand as illustrated by a comparison of the optimized structures in Figure 1.
Figure 1. Comparison of the calculated structures of transition states TS-EFZ and TS-EFE (side view) Scheme 8. Reaction of 1b with 2-Ethynyl Pyridine and X-ray Structure of 5.
The origin of stereoselectivity has further been supported by additional trapping experiments (Scheme 8). It was found that 2-ethenyl pyridine is not catalytically converted to the corresponding 1,3-enyne but its addition to a solution of 1b resulted in an immediate color change from orange to dark green. Analysis of the reaction mixture indicated the quantitative formation of a single diamagnetic complex exhibiting a sharp singlet resonance at 146.4 ppm in the 31P{1H} NMR. This complex could be isolated in pure form and was crystallized from a concentrated solution in toluene. A single crystal X-ray diffraction study revealed formation of complex 5 featuring one alkynyl and one κ2-C,N bound vinyl ligand formed by in course of the reaction. One coordination site is blocked by the pyridine group adjacent to the vinyl double bond and thus prevents catalytic turnover. The double bond of the vinyl moiety appears in a pro-(E) configuration indicating further that the origin of the (Z)-selectivity is not affiliated to the C-C bond formation event. The same experiment with 2-ethenyl pyridine was repeated in presence of pinacol borane (Scheme 9). Unlike before, a dark red reaction solution was obtained but again, clean formation of a single diamagnetic complex could be observed that exhibits a slightly shifted 31P{1H} resonance at 145.0 ppm. Isolation of this compound produced crystals identified as 6 by X-ray diffraction. Similar to 5, 6 exhibits a distorted octahedral geometry including one acetylide and the respective vinyl boronic ester, which is again bound in a κ2-C,N fashion to the iron metal center. However, the vinyl double bond of 6 appears now in the opposite pro-(Z) configuration. The reaction mechanism of the (Z)-selective hydroboration of terminal alkynes was explored in detail by means of DFT calculations, employing phenyl acetylene and pinacol borane as the substrates. Based on the similar structures of the alkynyl-vinyl complexes 5 and 6, the iron(II) (bis)acetylide complex [Fe(PNP)(C≡CPh)2] (7) was again considered as the active catalyst in the reaction resulting in the proposed catalytic cycle depicted in Scheme 10. Based on DFT calculations 7 adopts a triplet ground state. However, the singlet state is only 3 kcal/mol higher in energy and the minimum energy crossing point between the two potential energy surfaces is merely 10 kcal/mol (see Supporting Information, Figure S5). In contrast to the alkyne dimerization described above, the hydroboration reaction does not involve the formation of a vinylidene
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intermediate but rather proceeds via a σ-bond metathesis/hydrometallation reaction sequence (Scheme 10, steps I-III).17 A detailed description of the mechanism and the corresponding free energy profile is given in Scheme 11.
Scheme 10. Proposed Catalytic Cycle for the Hydroboration of Terminal Alkynes.
Scheme 9. Reaction of 1b with 2-Ethynyl Pyridine in the Presence of Pinacol Borane and X-ray Structure of 6.
In the first step, the borane closely binds to the catalyst giving intermediate B with the two alkynyl ligands being in cis-position to each other. In B, the H atom is attached to the iron metal center acting as electron donor while the boron is attached to neighboring acetylide α-carbon acting as electron acceptor (Fe-H = 1.607 Å, B-C = 1.793 Å). This initiates C-B bond formation resulting in inter mediate C. The B-H bond, which is still present in C, is cleaved and thus permits insertion of the alkyne into the iron hydride bond
(TSCD) to finally yield the iron vinyl intermediate D. This process proceeds easily with an overall barrier of 8.1 kcal/mol and is thermodynamically favorable by 14.0 kcal/mol. The elimination of the final product and recovery of the initial (bis)acetylide complex is accomplished via proton transfer from another alkyne analogously to the hydroalkynylation mechanism described above, again representing the highest barrier in the catalytic cycle requiring an activation energy of 20.1 kcal/mol. It should be noticed that protonation of the product (in D) by another borane molecule instead of acety
Scheme 11. Free Energy Profile Calculated for the (Z)-Hydroboration of Terminal Alkynes. Free Energies (kcal/mol) are Referred to Intermediate Az ([Fe(PNP)(C≡ ≡CPh)2] + HC≡ ≡CPh).
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lene was also studied, but, with a barrier of 5.7 kcal/mol higher, it was not found to be competitive. In fact, a comparison of reaction rates at different HBpin concentrations reveals a zero-order dependence in borane at higher concentrations which is in agreement that product elimination is the rate determining step (see Supporting Information). A similar mechanism proposed for the cobalt catalyzed (Z)-selective hydroboration of terminal alkynes has been presented by Chirik and co-workers.4 In contrast, the group of Leitner suggested a vinylidene intermediate for the hydroboration3 and hydrosilylation18 of terminal alkynes promoted by a related Ru PNP polyhydride complex. Formation of an iron(II) boryl species, as recently reported by Nishibayashi et al.,19 would account for such a mechanism but has neither been observed for 1b nor suggested by our calculations. In fact, the reaction of 1b with pinacolborane affords [Fe(PNP)(H)(η2-H2Bpin)] featuring an η2-bound H2Bpin ligand rather than an iron boryl complex with a direct Fe-B bond as reported previously.1 In accordance to our experimental results, the free energy profiles clearly show that the hydroboration is kinetically preferred over alkyne dimerization. This means that once the bis-acetylide species AZ is formed, it will react preferably with pinacolborane yielding the hydroboration product than with an acetylene to form the enyne. In fact, the overall barrier for the first process is 20.1 kcal/mol (measured from D to TS-FG, Scheme 11), while the hydroalkynylation has a barrier of 24.3 kcal/mol (measured from CZ to TS-EFZ, Scheme 6). Concerning the stereo-selectivity of the reaction, the C-B bond formation process in the mechanism practically only allows for the generation of the pro-(Z) vinyl intermediate as it was also found to be the case for the trapped complex 5. However, the reaction is not completely selec-
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tive and, depending on the substrate, a certain amount of the respective (E)-isomer of final product was detected in any case (1-29% (E)-selectivity).1 No change of the E/Z ratio could be observed after complete consumption of the substrates Scheme 12. (Z)-Selective Hydroalkynylation and Hydroboration of Phenylacetylene Employing 8 as Precatalyst Activated by Addition of a Strong Base.
excluding post-catalytic isomerization of the products. Consequently, and in analogy to the hydroalkynylation reaction, isomerization between a pro-(Z) and a pro-(E) vinyl intermediate is likely to occur with the substrate elimination step being the potential stereoselective step in the catalytic cycle. Although not considered in the DFT-calculations, rotation of the single bond in a zwitterionic carbene intermediate might provide a reasonable reaction pathway for the interconversion of pro-(Z) and a pro-(E) vinyl species.20 Based on these theoretical and experimental findings that unveil the relevance of iron(II) (bis)acetylide intermediates in the hydroalkynylation and hydroboration of ter-
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minal alkynes, an alternative catalysts activation pathway that obviates the use iron(II) polyhydride complexes has been considered. In this respect, it was found that the iron (bis)acetylide complex 2, which can be directly employed as catalyst in the reaction, might even be generated by the addition of two equivalents of a strong base to the dibromide complex 8 in presence of excess TMSacetylene (see Supporting Information). Consequently, 8 could successfully be employed as stable and easily accessible pre-catalyst. As depicted in Scheme 12, both the catalytic dimerization as well as the hydroboration of phenyl acetylene proceed smoothly using complex 8 as catalyst precursor activated in situ upon addition of potassium bis(trimethylsilyl)amide (KHMDS). The formation of the catalytically active species 7 could be verified by the addition of PMe3 affording the diamagnetic (bis)acetylide complex 4 as already shown in Scheme 3. When conducted in THF, almost the same activity and selectivity as in the case of the iron polyhydride 1b was observed while the use of benzene or toluene was much less effective resulting in incomplete conversion of the substrates presumably caused by a deficient catalyst initiation.
CONCLUSION In summary, we presented detailed mechanistic investigations on the (Z)-selective hydroalkynylation and hydroboration of terminal alkynes catalyzed by a nonclassical PNP iron(II) polyhydride complex. Based on experimental results as well as DFT-calculations, the respective iron(II) (bis)acetylide complexes were identified as the catalytically active species in both reactions. On the one hand, the hydroalkynylation reaction proceeds either via a classical mechanism involving the formation of iron vinylidene intermediates or through a concerted pathway comprising the hydrogen migration and the nucleophilic attack in a single step. The stereoselectivity originates from product elimination of the resulting pro-(E) and pro(Z) vinyl intermediates that are in equilibrium with each other. Despite of the fact that these species form with similar barriers, their interconversion has a smaller barrier than product formation. Accordingly, they interconvert faster than product release. Moreover, the pro-(Z) vinyl intermediate has a lower barrier and also is the most stable isomer. Thus, this is the selectivity key. On the other hand, the hydroboration of terminal alkynes was found to take place through Fe-C/H-B σ-bond metathesis followed by the insertion of the resulting alkynylboronate into the intermediately formed iron hydride bond. As a formal syn-hydrometalation, this step exclusively yields the respective pro-(Z) vinyl intermediate. However, it is likely that the corresponding pro-(E) species originates through isomerization and that the product elimination again represents the stereoselective step in the catalytic cycle. Moreover, we could demonstrate that these hydrofunctionalization reactions can even be performed in presence of the simple and readily available iron(II) dibromide complex [Fe(PNP)Br2] activated upon addition of a strong base in presence of excess alkyne. Finally, we believe that our results may lead to
further progress in the field of hydrofunctionalization reactions and the rational design of selective catalytic systems.
METHODS All calculations were performed using the GAUSSIAN 09 software package21 without symmetry constraints. The optimized geometries were obtained with the PBE0 functional.24 The basis set used for the geometry optimizations consisted of the Stuttgart/Dresden ECP (SDD) basis set25 to describe the electrons of iron, and a standard 631G(d,p) basis set26 for all other atoms. The electronic energies obtained at the PBE0 level of theory were converted to free energy at 298.15 K and 1 atm by using zero-point energy and thermal energy corrections based on structural and vibration frequency data calculated at the same level. Single point energy calculations were performed on the geometries optimized at the PBE0 level, using the M06 functional and a standard 6-311++G(d,p) basis set.28,30 Solvent effects (toluene) were considered in all calculations (PBE0 geometry optimizations included) using the Polarizable Continuum Model (PCM) initially devised by Tomasi and coworkers31 with radii and non-electrostatic terms of the SMD solvation model, developed by Truhlar et al.32
ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: xxxxx X-ray crystallographic data (CIF) 1 13 1 31 Synthetic procedures, H, C{ H}, and P{H} NMR spectra of all compounds, crystallographic data and complete computational details (PDF) Optimized cartesian coordinates for DFT-optimized structures (XYZ)
AUTHOR INFORMATION Corresponding Author * E-mail (K. Kirchner):
[email protected], Tel: (+43) 1 58801 163611. Fax: (+43) 1 58801 16399.
ORCID Nikolaus Gorgas: 0000-0003-2919-1042 Luis F. Veiros: 0000-0001-5841-3519 Karl Kirchner: 0000-0003-0872-6159
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT NG and KK gratefully acknowledge the Financial support by the Austrian Science Fund (FWF) (Project No. P29584-N28). LFV thanks Fundação para a Ciência e Tecnologia, UID/QUI/00100/2013.
REFERENCES
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(1) Gorgas, N.; Alves, L. G.; Stöger, B.; Martins, A. M.; Veiros, L. F.; Kirchner, K. Stable, Yet Highly Reactive Nonclassical Iron(II) Polyhydride Pincer Complexes: Z -Selective Dimerization and Hydroboration of Terminal Alkynes. J. Am. Chem. Soc. 2017, 139, 8130–8133. (2) Ohmura, T.; Yamamoto, Y.; Miyaura, N. Rhodium- or Iridium-Catalyzed Trans -Hydroboration of Terminal Alkynes, Giving (Z)-1-Alkenylboron Compounds. J. Am. Chem. Soc. 2000, 122, 4990–4991. (3) Gunanathan, C.; Hölscher, M.; Pan, F.; Leitner, W. Ruthenium Catalyzed Hydroboration of Terminal Alkynes to ZVinylboronates. J. Am. Chem. Soc. 2012, 134, 14349–14352. (4) Obligacion, J. V.; Neely, J. M.; Yazdani, A. N.; Pappas, I.; Chirik, P. J. Cobalt Catalyzed Z -Selective Hydroboration of Terminal Alkynes and Elucidation of the Origin of Selectivity. J. Am. Chem. Soc. 2015, 137, 5855–5858. (5) For recent examples see: (a) Zhang, W.; Zheng, S.; Liu, N.; Werness, J. B.; Guzei, I. A.; Tang, W. Enantioselective Bromolactonization of Conjugated (Z)-Enynes. J. Am. Chem. Soc. 2010, 132, 3664–3665. (b) Trost, B. M.; Masters, J. T.; Le Vaillant, F.; Lumb, J.-P. Synthesis of a 1,3-Bridged Macrobicyclic Enyne via Chemoselective Cycloisomerization Using Palladium-Catalyzed Alkyne–Alkyne Coupling. J. Org. Chem. 2016, 81, 10023–10028. (c) Wei, X.-F.; Xie, X.-W.; Shimizu, Y.; Kanai, M. Copper(I)Catalyzed Enantioselective Addition of Enynes to Ketones. J. Am. Chem. Soc. 2017, 139, 4647–4650. (d) Callingham, M.; Partridge, B. M.; Lewis, W.; Lam, H. W. Enantioselective RhodiumCatalyzed Coupling of Arylboronic Acids, 1,3-Enynes, and Imines by Alkenyl-to-Allyl 1,4-Rhodium(I) Migration. Angew. Chem., Int. Ed. 2017, 56, 16352–16356. (e) Partridge, B. M.; Callingham, M.; Lewis, W.; Lam, H. W. Arylative Intramolecular Allylation of Ketones with 1,3-Enynes Enabled by Catalytic Alkenyl-to-Allyl 1,4-Rhodium(I) Migration. Angew. Chem., Int. Ed. 2017, 56, 7227– 7232. (6) For Suzuki cross-coupling reactions see: (a) Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev. 1995, 95, 2457–2483. (b) Suzuki, A. Cross-Coupling Reactions Of Organoboranes: An Easy Way To Construct C–C Bonds (Nobel Lecture). Angew. Chem., Int. Ed. 2011, 50, 6722–6737. (c) Roscales, S.; Csákÿ, A. G. Transition-Metal-Free C–C Bond Forming Reactions of Aryl, Alkenyl and Alkynylboronic Acids and Their Derivatives. Chem. Soc. Rev. 2014, 43, 8215–8225. For other coupling reactions see: (d) Leonori, D.; Aggarwal, V. K. Stereospecific Couplings of Secondary and Tertiary Boronic Esters. Angew. Chem., Int. Ed. 2015, 54, 1082–1096. (e) Huang, Y.; Huang, R. Z.; Zhao, Y. CobaltCatalyzed Enantioselective Vinylation of Activated Ketones and Imines. J. Am. Chem. Soc. 2016, 138, 6571–6576. (7) Trost, B. M.; Masters, J. T. Transition Metal-Catalyzed Couplings of Alkynes to 1,3-Enynes: Modern Methods and Synthetic Applications. Chem. Soc. Rev. 2016, 45, 2212–2238. (8) For Examples of iron acetylide complex see: (a) Field, L. D.; George, A. V.; Malouf, E. Y.; Slip, I. H. M.; Hambley, T. W. Bis(acetylide) Complexes of Iron. Organometallics 1991, 10, 3842– 3848. (b) Field, L. D.; Turnbull, A. J.; Turner, P. AcetylideBridged Organometallic Oligomers via the Photochemical Metathesis of Methyl-iron(II) Complexes. J. Am. Chem. Soc. 2002, 124, 3692–3702. (c) Zheng, T.; Li, M.; Sun, H.; Harms, K.; Li, X. Synthesis of New Thiophenolato Hydrido Iron(II) Complexes and Their Substitution Reactions with Alkynes. Polyhedron 2009, 28, 3823–3827. (d) Wang, X.; Zhang, J.; Wang, L.; Deng, L. High-Spin Iron(II) Alkynyl Complexes with N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Reactivity Study. Organometallics 2015, 34, 2775–2782. (e) Kneebone, J. L.; Brennessel, W. W.; Neidig, M. L. Intermediates and Reactivity in Iron-Catalyzed Cross-Couplings of Alkynyl Grignards with Alkyl Halides. J. Am. Chem. Soc. 2017, 139, 6988–7003.
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(9) The 1H and 31P{1H} NMR spectra of 3b in acetonitrile-d3 only feature weak and poorly resolved signals presumably due to an equilibrium between the substitutionally labile 3b and the paramagnetic 16e complex 2. (10) Katayama, H.; Yari, H.; Tanaka, M.; Ozawa, F. (Z)-Selective Cross-Dimerization of Arylacetylenes with Silylacetylenes Catalyzed by Vinylidene Ruthenium Complexes Chem. Commun. 2005, 34, 4336. (11) Rivada-Wheelaghan, O.; Chakraborty, S.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Z -Selective (Cross-)Dimerization of Terminal Alkynes Catalyzed by an Iron Complex. Angew. Chem., Int. Ed. 2016, 55, 6942–6945. (12) In order to gain mechanistic insight, deuterium labeling studies using 1-d1-phenylacetylene and TMS-acetylene have been conducted. However, isotope exchange between both substrates in the course of the reaction prevented a proper interpretation of the obtained spectra. (a) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press, New York, 1989. (b) Calculations performed at the M06/(6–311++G**, PCM)//PBE0/(SDD, 6–31G**) level using the GAUSSIAN 09 package. A full account of the computational details and a complete list of references are provided as ESI. (14) (a) Jia, G.; Meek, D. W.; Gallucci, J. C. Synthesis and Reactivity of Ruthenium Hydride Complexes Containing Chelating Triphosphines. 4. Reactions of Ruthenium Hydride Complexes Containing Triphosphines with Olefins. Organometallics 1990, 9, 2549–2555. (b) Bianchini, C.; Peruzzini, M.; Zanobini, F.; Frediani, P.; Albinati, A. A Ruthenium(II) Enynyl Complex Mediates the Catalytic Dimerization of 1-Alkynes to Z-1,4-Disubstituted Enynes. J. Am. Chem. Soc. 1991, 113, 5453–5454. (c) Jia, G.; Meek, D. W. Synthesis and Reactivity of Ruthenium Hydride Complexes of Chelating Triphosphines. 5. Reactions of Acetylenes with RuHCl(Cyttp) and RuH4(Cyttp) {Cyttp = C6H5P[CH2CH2CH2P(cC6H11)2]2} Organometallics 1991, 10, 1444–1450. (d) Eaves, S. G.; Yufit, D. S.; Skelton, B. W.; Lynam, J. M.; Low, P. J. Reactions of alkynes with cis-RuCl2(dppm)2: exploring the interplay of vinylidene, alkynyl and η3-butenynyl complexes Dalton Trans. 2015, 44, 21016-21024. (15) Katayama, H.; Ozawa, F. Vinylidene Ruthenium Complexes in Catalysis Coord. Chem. Rev. 2004, 248, 1703–1715. (16) (a) Wakatsuki, Y.; Yamazaki, H.; Kumegawa, N.; Satoh, T.; Satoh, J. Y. Regio- and Stereocontrolled Dimerization of TertButylacetylene to (Z)-1,4-Di-Tert-Butylbutatriene by Ruthenium Catalysis. Reaction Mechanism Involving Alkynyl-Vinylidene Coupling and Rearrangement of the Metal-Bound C4 Unit. J. Am. Chem. Soc. 1991, 113, 9604–9610. (b) Schäfer, M.; Wolf, J.; Werner, H. C–C Coupling Reactions in the Coordination Sphere of Rhodium(I) and Rhodium(III): New Routes for the Di- and Trimerization of Terminal Alkynes Dalton Trans. 2005, 8, 1468– 1481. (17) Deuterium labeling experiments using HBPin and 1-d1-1octyne revealed approx. 95% 2H incorporation in the 2-position of the product (see Supporting Information). (18) Conifer, C.; Gunanathan, C.; Rinesch, T.; Hölscher, M.; Leitner, W. Solvent-Free Hydrosilylation of Terminal Alkynes by Reaction with a Nonclassical Ruthenium Hydride Pincer Complex. Eur. J. Inorg. Chem. 2015, 333–339. (19) Nakajima, K.; Kato, T.; Nishibayashi, Y. Hydroboration of Alkynes Catalyzed by Pyrrolide-Based PNP Pincer–Iron Complexes. Org. Lett. 2017, 19, 4323–4326. (20) (a) Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Hydrosilylation of 1-Hexyne Catalyzed by Rhodium and CobaltRhodium Mixed-Metal Complexes. Mechanism of Apparent Trans Addition Organometallics 1990, 9, 3127–3133. (b) Tanke, R. S.; Crabtree, R. H. Unusual Activity and Selectivity in Alkyne Hydrosilylation with an Iridium Catalyst Stabilized by an ODonor Ligand. J. Am. Chem. Soc. 1990, 112, 7984–7989.
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(21) Gaussian 09, Revision A.02, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009. (22) Hehre, W. J.; Radom, L.; Schleyer, P. v.R. & Pople, J. A. Ab Initio Molecular Orbital Theory, John Wiley & Sons, NY, (1986). (23) Parr, R. G. & Yang, W. in Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, (1989). (24) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 77, 3865-3868; (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1997, 78, 1396-1396. (c) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas Phys. Rev. B 1986, 33, 8822-8824. (25) (a) Haeusermann, U.; Dolg, M.; Stoll, H.; Preuss, H.; Schwerdtfeger, P.; Pitzer, R. M. Accuracy of energy-adjusted quasirelativistic ab initio pseudopotentials Mol. Phys. 1993, 78, 1211-1224. (b) Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H. Energyadjusted pseudopotentials for the actinides. Parameter sets and test calculations for thorium and thorium monoxide J. Chem. Phys. 1994, 100, 7535-7542. (c) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg, M.; Schwerdtfeger, P. The accuracy of the pseudopotential approximation. II. A comparison of various core sizes for indium pseudopotentials in calculations for spectroscopic constants of InH, InF, and InCl J. Chem. Phys. 1996, 105, 1052-1059. (26) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules J. Chem. Phys. 1971, 54, 724-728. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. 12. Further extensions of Gaussian-type basis sets for use in molecular-orbital studies of organic-molecules J. Chem. Phys. 1972, 56, 2257-2261. (c) Hariharan, P. C.; Pople, J. A. Accuracy of AH equilibrium geometries by single determinant molecular-orbital theory Mol. Phys. 1974, 27, 209-214. (d) Gordon, M. S. The isomers of silacyclopropane Chem. Phys. Lett. 1980, 76, 163-168. (e) Hariharan, P. C.; Pople, J. A. Influence of polarization functions on molecularorbital hydrogenation energies Theor. Chim. Acta 1973, 28, 213222. (27) (a) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. Using redundant internal coordinates to optimize equilibrium geometries and transition states J. Comp. Chem. 1996, 17, 49-56. (b) Peng, C.; Schlegel, H. B. Combining Synchronous Transit and
Quasi-Newton Methods for Finding Transition States Israel J. Chem. 1993, 33, 449-454. (28) (a) McClean, A. D.; Chandler, G. S. Contracted Gaussian basis sets for molecular calculations. I. Second row atoms, Z=1118 J. Chem. Phys. 1980, 72, 5639-5648. (b) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions J. Chem. Phys. 1980, 72, 650-654. (c) Wachters, A. J. H. Gaussian Basis Set for Molecular Wavefunctions Containing Third-Row Atoms J. Chem. Phys. 1970, 52, 1033-1036. (d) Hay, P. J. Gaussian basis sets for molecular calculations - representation of 3D orbitals in transition-metal atoms J. Chem. Phys. 1977, 66, 4377-4384. (e) Raghavachari, K.; Trucks, G. W. Highly correlated systems: Excitation energies of first row transition metals Sc-Cu J. Chem. Phys. 1989, 91, 1062-1065. (f) Binning Jr., R. C.; Curtiss, L. A. Compact contracted basis-sets for 3rd-row atoms - Ga-Kr J. Comp. Chem. 1990, 11, 1206-1216. (g) McGrath, M. P.; Radom, L. Extension of Gaussian-1 (G1) theory to bromine-containing molecules J. Chem. Phys. 1991, 94, 511-516. (h) Curtiss, L. A.; McGrath, M. P.; Blaudeau, J.-P.; Davis, N. E.; Binning Jr., R. C.; Radom, L. Extension of Gaussian-2 theory to molecules containing third-row atoms GaKr J. Chem. Phys., 1995, 103, 6104-6113. (i) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. Efficient diffuse function-augmented basis-sets for anion calculations. 3. The 3-21+G basis set for 1st-row elements, Li-F J. Comp. Chem. 1983, 4, 294301. (j) Frisch, M. J.; Pople, J. A.; Binkley, J. S. Self-Consistent Molecular Orbital Methods. 25. Supplementary Functions for Gaussian Basis Sets J. Chem. Phys. 1984, 80, 3265-3269. (29) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06class functionals and 12 other functionals Theor. Chem. Acc., 2008, 120, 215-241. (30) (a) Zhao, Y.; Truhlar, D. G. Density Functionals with Broad Applicability in Chemistry Acc. Chem. Res. 2008, 41, 157167. (b) Zhao, Y.; Truhlar, D. G. Applications and validations of the Minnesota density functionals Chem. Phys. Lett. 2011, 502, 113. (31) (a) Cancès, M. T.; Mennucci, B.; Tomasi, J. A new integral equation formalism for the polarizable continuum model: Theoretical background and applications to isotropic and anistropic dielectrics J. Chem. Phys. 1997, 107, 3032-3041. (b) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Ab initio study of ionic solutions by a polarizable continuum dielectric model Chem. Phys. Lett. 1998, 286, 253-260. (c) Mennucci, B.; Tomasi, J. Continuum solvation models: A new approach to the problem of solute's charge distribution and cavity boundaries J. Chem. Phys. 1997, 106, 5151-5158. (d) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models Chem. Rev. 2005, 105, 2999-3094. (32) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal solvation model based on solute electron density and a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions J. Phys. Chem. B, 2009, 113, 63786396.
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