Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex

Two bench-stable Fe(II) alkyl complexes [Fe(κ3PCP-PCP-iPr)(CO)2(R)] (R = CH2CH2CH3, CH3) were obtained by the treatment of [Fe(κ3PCP-PCP-iPr)(CO)2(H)] with NaNH2 and subsequent addition of CH3CH2CH2Br and CH3I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(κ3PCP-PCP-iPr)(CO)2]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH3 and iPrOH as a hydrogen source. Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand. In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me. Accordingly, [Fe(κ3PCP-PCP-iPr)(CO)2(CH2CH2CH3)] is the more active catalyst. The reaction takes place at 25 °C with a catalyst loading of 0.5 mol%. There was no overhydrogenation, and in the case of internal alkynes, exclusively, Z-alkenes are formed. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles. Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.


■ INTRODUCTION
The preparation of alkenes from alkynes via selective semihydrogenation is an attractive process for the synthesis of pharmaceuticals and bulk and fine chemicals in the chemical industry as well as in research. 1A challenging task of this reaction is the control of the chemo-and stereoselectivity, since the reduction of C�C triple bonds to alkenes may lead to the formation of (E)-or (Z)-alkenes as well as saturated hydrocarbons. 2Consequently, the development of new and selective homogeneous semihydrogenation catalysts for reduction to either (Z)-or (E)-alkenes that prevent isomerization and over-reduction would still be of great importance. 3ithin this context, we are interested in the design of active homogeneous catalysts based on first-row transition metals.Obvious advantages such as their low price and high abundance are complemented by their intrinsic properties that may provide unprecedented reactivities and selectivities in catalytic transformations. 4Iron has evolved to a particularly promising candidate in this regard in recent years. 5In the field of alkyne reductions, only a few iron complexes were reported that operate via direct hydrogenation or transfer hydrogenation procedures.As of yet, only a few examples are currently known that can promote this transformation, employing hydrogen gas as the reductant (Scheme 1).In 1989, Bianchini et al. discovered the semihydrogenation of terminal alkynes catalyzed by the nonclassical polyhydride [Fe(PP 3 )(H)(η 2 -H 2 )] + . 6Milstein and co-workers reported on a novel acridinebased pincer type complex that bears an imino borohydride coligand that was found to reduce internal alkynes selectively to the respective (E)-olefins. 7We described that the benchstable cationic bis(σ−B-H) aminoborane complex [Fe-(PNP NMe -iPr)(H)(η 2 -H 2 B = NMe 2 )] + as well as the cationic complex [Fe(PNP NMe -iPr)(H)(η 2 -H 2 ) 2 ] + efficiently catalyzes the semihydrogenation of internal alkynes, 1,3-diynes and 1,3enynes. 8More recently, Khusnutdinova and co-workers reported on an efficient semihydrogenation of terminal alkynes with H 2 , catalyzed by a modified, tetramethylated PNP pincer Fe hydride complex. 9th respect to iron-catalyzed transfer semihydrogenations of alkynes, 1a [Fe 2 (CO) 9 ] in the presence of monodentate phosphines was able to reduce internal aryl-substituted alkynes with (EtO) 3 SiH with moderate to good Z/E selectivities. 10 The group of Plietker showed that [Fe(PPh 3 ) 2 (CO)(NO)-(H)] catalyzes the transfer semihydrogenations of diarylalkynes where the Z/E ratio was dependent on the silane used. 11Beller and co-workers reported the semihydrogenation of terminal alkynes utilizing formic acid as a hydrogen source with an in situ generated cationic Fe(II) tetraphos catalyst Scheme 1. Semihydrogenation (Left) and Transfer Semihydrogenation (Right) of Alkynes Catalyzed by Well-Defined Iron Complexes Scheme 2. Formation of a Coordinatively Unsaturated Fe(II) Hydride Species via Alkyl Migration and Deprotonation of the Entering Ligand (Scheme 1). 12Albrecht and co-workers used 1,2,3-triazolylidene and IMes iron piano-stool complexes for the catalytic semihydrogenation of alkynes by using silanes as reducing agents.Aromatic terminal alkynes were converted to styrenes without over-reduction to ethylbenzene derivatives.Internal aryl alkynes afford cis-alkenes with excellent Z-selectivity. 13he combination of boranes with amines or, alternatively, silanes such as PMHS (poly(methylhydrosiloxane)) and alcohols as hydrogen donors mediated by an Fe(II) βdiketiminate alkyl system was reported by the group of Webster. 14This system, depending on the stoichiometry, allowed for full reduction as well as semihydrogenation but limited to 50% conversion.
We recently described the application of a well-defined Mn(I)-alkyl complex 15 as a catalyst for the hydrogenation of nitriles, ketones, CO 2 , alkenes, and alkynes as well as the dehydrogenative silylation of alkenes and hydroboration of alkynes. 16,17We took advantage of the fact that Mn(I)-alkyl carbonyl complexes undergo migratory insertion of the nucleophilic alkyl ligand into the polarized CO moiety, yielding a coordinatively unsaturated acyl complex, which is capable of activating weakly polar E−H bonds (E = −H, −C� C-R, −BR 2 , −SiR 3 ).The rate of alkyl migration follows the order nPr > Et > Me as already shown by Moss and co-workers some years ago. 18n this article, we apply this concept to iron alkyl PCP complexes.Accordingly, the catalytic process is initiated by  migratory insertion of a CO ligand into the Fe−alkyl bond to yield acyl intermediates, which react with silanes in the presence of alcohols to form the 16e − Fe(II) hydride catalysts (Scheme 2).We describe here an iron-catalyzed selective transfer semihydrogenation of terminal alkynes and Z-selective transfer semihydrogenation of internal alkynes utilizing a combination of phenylsilane and isopropanol as the hydrogen source under mild conditions.It has to be noted that catalytic applications of iron PCP complexes are scarce. 19Rare examples are the hydrosilylation of ketones and aldehydes, 20 Reaction conditions for terminal alkynes: alkyne (0.161 mmol, 1 equiv), PhSiH 3 (0.161 mmol, 1 equiv), iPrOH (0.201 mmol, 1.25 equiv), 3 (0.5 mol %), THF-d 8 (0.6 mL), 25 °C, and 24 h, b Reaction conditions for internal alkynes: alkyne (0.161 mmol, 1 equiv), PhSiH 3 (0.161 mmol, 1 equiv), iPrOH (0.242 mmol, 1.50 equiv), 3 (0.5 mol %), THF-d 8 (0.6 mL), 25 °C, and 24 h.c Yield determined by 1 H NMR spectroscopy using mesitylene as an internal standard.Scheme 4. Deuterium Labeling Studies of the Transfer Semihydrogenation of Phenylacetylene with PhSiH 3 and iPrOH the dehydrogenation of ammonia−borane, 21 and the dehydrogenative borylation of styrene. 22

■ RESULTS AND DISCUSSION
The new Fe(II) alkyl complexes [Fe(κ 3 PCP-PCP-iPr)-(CO) 2 (R)] (R = CH 2 CH 2 CH 3 (3), CH 3 (4)) were obtained by treatment of [Fe(κ 3 PCP-PCP-iPr)(CO) 2 (H)] (1) 23 with NaNH 2 (3 equiv) and subsequent addition of CH 3 CH 2 CH 2 Br and BrCH 3 I, respectively, in 63 and 71% isolated yields (Scheme 3).The reaction proceeds via the anionic Fe(0) intermediate [Fe(κ 3 PCP-PCP-iPr)(CO) 2 ] − (2) that could also be isolated in 91% yield.Complex 2 gives rise to ν CO vibrations at 1774 and 1708 cm −1 , which are significantly shifted to lower wavenumbers lower than the respective resonances in 1 being 1976 and 1918 cm −1 . 23These values are comparable with the anionic pyrrole-based pincer complex [Fe(PN pyrr P)(CO) 2 ] − reported by Tonzetich and co-workers. 24Both alkyl complexes are bench-stable for several weeks in the presence of air.It has to be noted that 3 and 4 form two isomers depending on the position of the N-methyl group of the pyrazole moiety in relation to the alkyl ligands (see the Supporting Information, Figure S1).Due to the large six-membered metallacycles, the rotation around the axis C ipso -Fe-CO is slow on the NMR timescale, resulting in the two conformers being detectable in NMR spectra.A similar behavior was observed with the Nmethylated congener [Fe(κ 3 PCP-PCP Me -iPr)(CO) 2 (Cl)]BF 4 reported previously. 23DFT calculations reveal that the energy difference between these isomers is merely 0.5 kcal/mol.The IR spectra of 3 and 4 exhibit two strong C−O vibrations at 1969 and 1907 and 1970 and 1908 cm −1 , respectively, being characteristic of a cis-geometry of the carbonyl ligands.They were fully characterized by 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR and IR spectroscopy and high-resolution mass spectrometry.In addition, the molecular structure of 4 was determined by X-ray crystallography.A structural view is depicted in Scheme 3 with selected bond distances and angles given in the caption.
The catalytic performance of complexes 3 and 4 was investigated for the transfer hydrogenation of alkynes with silanes and iPrOH as a hydrogen source.In order to establish the best reaction conditions, phenylacetylene was chosen as a model substrate.Selected optimization experiments are depicted in Table 1.
Various silanes were evaluated in order to optimize catalytic activity and also to investigate the influence of the silane on the product selectivity in the presence of iPrOH (1 equiv with respect to silane).In the absence of alcohol, mixtures of semihydrogenated and hydrosilylated products were obtained.Moreover, attempts to utilize hydrogen gas as a reducing agent were unsuccessful.
At 80 °C, in benzene with 1 mol % 3, PhSiH 3 and Ph 2 SiH 2 afforded the highest activity (Table 1, entries 1 and 2), while (MeO) 3 SiH and PMHS (in MeOH) gave significantly poorer conversions (Table 1, entries 3 and 4).Cheap and easy-tohandle PMHS decreased the activity by about a factor of 4. In all cases, overhydrogenation did not take place.There was also no evidence of alkyne hydrosilylation under these conditions.Further lowering of the catalyst loading to 0.25 mol % resulted in 54% conversion (  significantly lowered when switching from benzene to THF as a solvent.With a catalyst loading of 0.5 mol % and PhSiH 3 , quantitative formation of A1 was achieved at 60 °C (Table 1, entry 5), while at 20 °C, A1 was still obtained in 75% yield (Table 1, entry 11).Quantitative formation of A1 was observed at 20 °C when the amount of iPrOH was increased to 1.25 equiv.(Table 1, entry 12).Upon further increasing the amount of iPrOH to 1.5 equiv., a drop in conversion to 60% was observed (Table 1 entry 14).The catalytic activity of precatalyst 4 was also investigated, which turned out to be less efficient under similar reaction conditions (Table 1, entries 9 and 13) which is in agreement with previous findings that the rate of alkyl migration follows the order nPr > Me. 15,18 If the catalytic reaction is performed in the presence of PMe 3 , no transfer semihydrogenation reaction took place, indicating that PMe 3 coordinates to the Fe(II) center, also blocking the vacant site for incoming substrates.Moreover, in the absence of the catalyst, no reaction took place.The homogeneity of the reaction was confirmed by addition of one drop of mercury, where no decrease of reactivity and selectivity was observed.
Having established the best reaction conditions, the applicability of catalyst 3 is demonstrated in the selective hydrogenation of various terminal and internal alkynes.These results are shown in Table 2.In general, the alkyne transfer semihydrogenation is accompanied by the formation of [(1methylethoxy)silyl]benzene, [bis(1-methylethoxy)silyl]-benzene, and dihydrogen.A range of 4-substituted ethynylbenzenes were hydrogenated under the optimized reaction conditions.Both electron-donating groups such as OMe, Me, tBu, and NH 2 and electron-withdrawing substituents such as F, Cl, CN, acyl, and NO 2 are compatible with the semihydrogenation protocol (Table 2, A1−A12) and showed good selectivity to give substituted styrenes in 44−99% yields.4-Ethynylbenzonitrile yielded only 44% of A7, while 3ethynylphenol did not react to the desired alkyne A11 at all.It must be mentioned that dehalogenation or nitrile reduction was not observed.
In addition, we tested the heteroaromatic alkynes 2ethynylpyridine and 3-ethynylthiophene, which were converted to the corresponding alkenes A13 and A14, respectively, in 69 and 82% yields.Several aliphatic terminal alkynes were also tested, giving, with the exception of 3-phenylprop-1-yne (A15), good to excellent yields (A16−A18).As a challenging example, 1-ethynylcyclohexene with its conjugated double and triple bonds was selectively transformed into its diene product A18.
Aryl−aryl, aryl−alkyl, and alky−alkyl substituted alkynes were also investigated.Substrates bearing alkyl groups tend to be more challenging in selective semihydrogenations due to over-reduction and isomerization.Under the given reaction conditions, diphenylacetylene and various nonactivated alkynes bearing alkyl substituents afforded the corresponding Z-alkenes in good to excellent yields (A19−A23).Unfortunately, 1-[2-

(trimethylsilyl)ethynyl]cyclohexene was not reduced to the desired alkene 1-[(1Z)-2-(trimethylsilyl)ethenyl]cyclohexene (A24).
To gain further insights in the reaction mechanism, deuterium labeling experiments were carried out.The regioand stereoselectivity of deuterium incorporation was established via 2 H NMR spectroscopy.With phenylacetylene-d 1 as a substrate, styrene is formed with the deuterium being at the terminal cis-position with respect to the phenyl ring (Scheme 4a).This clearly shows that the syn-addition of both hydrogen atoms took place.If iPrOH-d 1 or iPrOH-d 8 was used, 75% of deuterium ended up at the terminal carbon and 25% of deuterium was incorporated at the internal carbon of styrene (Scheme 4b).Performing the reaction with both phenylacetylene-d 1 and iPrOH-d 1 or iPrOH-d 8 reveals again syn-H/D addition since the deuterium originating from phenylacetylened 1 is again located in the cis-position to the phenyl ring (Scheme 4c).
A plausible catalytic cycle based on experimental data and DFT calculations 25 with diphenylacetylene (A1) as a model substrate, iPrOH and PhSiH 3 as a hydrogen source, and [Fe(κ 3 PCP-PCP-iPr)(CO) 2 (CH 3 )] (4, A in the calculations) as a precatalyst could be established.The resulting free energy profiles for the activation of the precatalyst are represented in Figures 1 and 2. The free energy profiles for the transfer semihydrogenation are depicted in Figures 3 and 4. A simplified catalytic cycle (only key intermediates are shown) is depicted in Scheme 5.
Precatalyst activation is initiated by migration of the alkyl ligand in complex A to the carbonyl C-atom of an adjacent CO ligand (Figure 1).This occurs in an easy step with a barrier of only 8 kcal/mol, producing intermediate B, an acyl species stabilized by an agostic C−H bond.This transformation is slightly endergonic by 6.1 kcal/mol.
Upon coordination of PhSiH 3 , the first Si−H bond cleavage takes place with concomitant H-atom transfer from PhSiH 3 to the C-atom of the acyl ligand to produce silyl complex D and acetaldehyde.This step has a rather high barrier of 23.0 kcal/ mol and is slightly endergonic by 1.9 kcal/mol.In the following steps, from E to G, there will be H-transfer from the Si atom to the metal, forming a hydride and also O  It has to be mentioned that intermediate B alternatively may react first with iPrOH instead of PhSiH 3 to form an alkoxide complex as a result of the cleavage of the O−H bond with concomitant release of acetaldehyde.This process, however, is very unfavorable by 51.3 kcal/mol and rather unlikely (see Figure S2 in the Supporting Information).
A slight rotation of the OiPr moiety about the Si−O bond, from G to G′, allows the completion of the Si−H and Fe−H bond cleavage processes, yielding the hydride intermediate H in an accessible (ΔG ‡ = 17.3 kcal/mol) and endergonic step (ΔG = 6.8 kcal/mol).Silane (H 2 SiPh(OiPr)) loss in H affords H', a 16e − complex which adopts a square pyramidal structure and undergoes a facile isomerization moving the hydride ligand from the cis to the trans position of the pyrazole moiety to yield the active catalyst I (ΔG ‡ = 6.0 kcal/mol, ΔG = −0.7 kcal/ mol).This reaction completes the initiation process involving, overall, two Si−H bond activation steps and one O−H bond activation steps.The catalyst initiation process has a global barrier of 32.9 kcal/mol (measured from A to TS CD ) and is favorable, from the thermodynamic point of view, with a free energy balance of ΔG = −9.8kcal/mol.The value calculated for the overall barrier is high, taking into account the reaction conditions.However, it corresponds simply to the initiation process that only happens once.The barrier calculated for the catalytic cycle is lower (see below).
The catalytic cycle begins with the addition of an alkyne molecule to active species I.In the first step, from J to K, there is insertion of the alkyne molecule into the iron hydride bond, in a highly exergonic process (ΔG = −31.7 kcal/mol) with an activation barrier of 3.5 kcal/mol.In J, the alkyne is only weakly bound with an average Fe−C bond distance of 4.60 Å (Figure 3), while K is a vinyl complex, and the insertion process is finished.From K, addition of iPrOH leads to intermediate L, where iPrOH is only loosely bound and, then, to the formation of the isopropanol complex M.This is an endergonic process (ΔG = 14.0 kcal/mol) with a barrier of 14.8 kcal/mol (both measured from K).The reaction proceeds with H-atom transfer from the coordinated iPrOH ligand to the C-atom of the vinyl ligand, from M to N, releasing one molecule of styrene in a fairly easy and exergonic step (ΔG = −16.8kcal/mol) with a barrier of 14.1 kcal/mol.
In the last steps, the addition of a new silane molecule to N yields O, where the silane molecule has only a weak interaction with the complex.In a concerted fashion, the silane forms one Si−O and one Fe−H bond via TS OP , leading to intermediate P (Figure 4).This process requires a free energy of activation of 14.0 kcal/mol and has free energy balance of 3.0 kcal/mol.Subsequent Si−H bond cleavage and release of H 2 SiPh-(OiPr) afford complex Q.This is the last intermediate, actually being the active catalyst I containing the alkoxysilane weakly bound to the metal center with an Fe•••O distance of 5.11 Å.From Q, exchange of the alkoxysilane SiH 2 Ph(OiPr) with a new molecule of alkyne regenerates the initial species closing the cycle with a free energy balance of ΔG = 6.5 kcal/mol.The catalytic cycle presents an overall barrier of 28.1 kcal/mol, It is important to notice that the mechanism calculated and discussed above agrees reasonably well with the deuterium labeling studies described above (Scheme 4).On the one hand, the two entering H-atoms will both add cis to each other in the resulting olefin producing a Z-alkene and, on the other hand, of those two H-atoms, one comes from the alcohol, iPrO-H with the other coming from the silane, H-SiH 2 Ph (red and green Hatoms in the simplified cycle of Scheme 5).To prove whether the initiation via an acyl species is indeed taking place, complex 3 was treated with CNtBu, which resulted in the formation of Fe(PCP-iPr)(C(�O)-CH 2 CH 2 CH 3 )(CO)(CNtBu) (5) in 91% isolated yield (Scheme 6).Complex 5 was fully characterized by 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR and IR spectroscopy and highresolution mass spectrometry.The IR spectrum contains strong stretching vibrations at 2113 (ν CNtBu ), 1897 (ν CO ), and 1670 (ν C�O ) which clearly indicates the coordination of one isocyanide, one CO, and an acyl ligand, respectively.
Complex 5 was tested as a catalyst for the semihydrogenation of phenylacetylene.However, under optimized conditions, styrene was obtained in only 23% yield.This may be due to the fact that in order to form the active species, tBuNC dissociation is required, which is apparently unfavorable.In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me.Accordingly, [Fe-(κ 3 PCP-PCP-iPr)(CO) 2 (CH 2 CH 2 CH 3 )] is the more active catalyst.The reaction proceeds at room temperature with a catalyst loading as low as 0.5 mol %.There was no overhydrogenation observed, and in the case of internal alkynes, exclusively, Z-alkenes were formed.The implemented protocol tolerates a variety of electron-donating and electronwithdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles.Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.After precatalyst activation, initiated by an incoming silane PhSiH 3 and iPrOH addition, the active 16e hydride catalyst [Fe(κ 3 PCP-PCP-iPr)(CO)(H)] is formed.This process involves the activation of two Si−H bonds and the O−H bond of the alcohol.The mechanism calculated for the catalytic cycle starts with alkyne insertion into the Fe−H bond of the hydride intermediate, forming a vinyl species that is further protonated by an incoming alcohol molecule.The cycle closes by means of product (olefin) liberation followed by addition of new silane molecule that will regenerate the hydride ligand and release the alkoxysilane byproduct.The calculated mechanism justifies the observed formation of Zalkenes that both new H atoms are cis to each other, one coming from the alcohol O−H bond and the other coming from the silane Si−H bond, in reasonable agreement with the deuterium labeling studies presented.

■ ASSOCIATED CONTENT
* sı Supporting Information
−H bond breaking in the incoming alcohol molecule, resulting in the formation of two new bonds: Si−O and a new Si−H bond.These two consecutive steps present small structural modifications and have a barrier of 10.2 kcal/mol and a free energy balance of −25.9 kcal/mol from E to G. Thus, in E, the presence of the neighbor alcohol molecule shifts the silyl ligand into an agostic Si−H--Fe interaction (d Si−H = 1.62 Å and d Fe−H = 1.70 Å).In F, the H-transfer from the Si atom to metal is almost accomplished (d Si−H = 1.71Å and d Fe−H = 1.62 Å), while the

Scheme 5 .
Scheme 5. Simplified Catalytic Cycle for the Transfer Semihydrogenation of Phenylacetylene Carbonyl ligands are known to undergo migratory insertion into the metal carbon bond of an alkyl moiety, yielding coordinatively unsaturated acyl complexes, which are capable of activating weakly polar E−H bonds (E = −H, −C�C-R, − BR 2 , −SiR 3 ).The rate of alkyl migration typically follows the order nPr > Et > Me.We successfully applied this concept to iron alkyl PCP pincer complexes.Two new bench-stable Fe(II) alkyl complexes [Fe(κ3  PCP-PCP-iPr)(CO) 2 (R)] (R = CH 2 CH 2 CH 3 , CH3 ) were obtained by treatment of [Fe-(κ 3 PCP-PCP-iPr)(CO) 2 (H)] with NaNH 2 and subsequent addition of CH 3 CH 2 CH 2 Br and CH 3 I, respectively.The reaction proceeds via anionic Fe(0) intermediate Na[Fe-(κ 3 PCP-PCP-iPr)(CO) 2 ].The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH 3 and iPrOH as a hydrogen source.Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand to form an acyl intermediate.In fact, the acyl complex Fe(PCP-iPr)(C(�O)CH 2 CH 2 CH 3 )(CO)-(CNtBu) can be readily obtained by reacting CNtBu with t h e p r o p y l c o m p l e x [ F e ( κ 3 P C P -P C P -i P r ) -(CO) 2 (CH 2 CH 2 CH 3 )].

Table 1 .
Optimization Reactions for the Catalytic Semihydrogenation of Phenylacetylene a