Ruthenium Hydroxycarbenes as Key Intermediates in

Article pubs.acs.org/Organometallics

Ruthenium Hydroxycarbenes as Key Intermediates in Cycloisomerization and Decarbonylative Cyclization of Terminal Alkynals † ‡ ́ ́ María Batuecas,† Miguel A. Esteruelas,*,† Cristina Garcıa-Yebra, Carlos González-Rodrıguez, Enrique Oñate,† and Carlos Saá*,‡ †

Departamento de Quı ́mica Inorgánica-Instituto de Sı ́ntesis Quı ́mica y Catálisis Homogénea (ISQCH)−Centro de Innovación en Quı ́mica Avanzada (ORFEO−CINQA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain ‡ Departamento de Quı ́mica Orgánica-Centro Singular de Investigación en Quı ́mica Biológica y Materiales Moleculares (CIQUS)−Centro de Innovación en Quı ́mica Avanzada (ORFEO−CINQA), Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain S Supporting Information *

ABSTRACT: The complex [Ru(η5-C5H5)(CO)(κ1-OCMe2)(PiPr3)] BF4 (1) reacts with 3,3-bis(methoxycarbonyl)-5-hexyn-1-al to give the α,β-unsaturated cyclopentenylhydroxycarbene derivative [Ru(η5-C5H5){C(OH)CCHCH2C(CO2CH3)2CH2}(CO)(PiPr3)]BF4 (2), which undergoes deprotonation with Al2O3 to afford Ru(η5C5H5){C(O)CCHCH2C(CO2CH3)2CH2}(CO) (PiPr3) (3). In the presence of P2O5, the reaction of 1 with the alkynal leads to the alkenylvinylidene [Ru(η5-C5H5){C CCHCHC(CO2CH3)2CH2}(CO)(PiPr3)]BF4 (4), which yields the β,γ-unsaturated cyclopentenylhydroxycarbene [Ru(η5-C5H5){C(OH)CHCHCHC(CO2CH3)2CH2}(CO)(PiPr3)]BF4 (5) by means of a 1,2-addition of water. Complex 5 slowly isomerizes into 2. The deprotonation of 5 with Al2O3 gives Ru(η5-C5H5){C(O)CHCH CHC(CO2CH3)2CH2}(CO)(PiPr3) (6). Solvate Ru complex 1 and Ru hydroxycarbene 2 catalyze the cyclization of 3,3-bis(methoxycarbonyl)-5-hexyn-1-al to give mixtures of the cycloisomerized aldehyde 1,1-bis(methoxycarbonyl)-3-formylcyclopent-3-ene (7) and cycloalkene 1,1-bis(methoxycarbonyl)cyclopent-3-ene (8).

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group into the Cβ−H bond. The subsequent dehydration of the latter led to the alkynal dehydrative cyclization complex C.7 In contrast to these nucleophilic starting complexes, the electrophilic [Os(η5-C5H5)(CO)(PiPr3)]+ metal fragment stabilizes the oxetylidene derivative D, which is formed through a [2 + 2] cycloaddition reaction between the carbonyl group and the Cα−Cβ double bond of the aldehyde-substituted vinylidene intermediate A.8 We now report that the reaction of the ruthenium counterpart [Ru(η5-C5H5)(CO)(κ1-OCMe2)(PiPr3)] BF4 (1) with 3,3-bis(methoxycarbonyl)-5-hexyn-1-al affords an α,β-unsaturated cyclopentenylhydroxycarbene complex (2), which could be considered as a model key intermediate for the ruthenium-catalyzed cycloisomerization and decarbonylative cyclization of terminal 5-alkynals.

INTRODUCTION 5-Alkynals are very useful building blocks in organic synthesis.1 Numerous metal-catalyzed cyclizations between the two functionalities of internal alkynals have been developed.2 In contrast, mainly Ni-, Pd- and Ru-catalyzed cyclizations of terminal alkynals have been established,3 with mechanistic proposals based on DFT calculations in the case of Ru-catalyzed decarbonylative cyclizations of alkynals to give cycloalkenes.4 In addition, hydroacylation has been considered as the key step in the formation of linear alkenes by intermolecular Ru-catalyzed decarbonylative addition of aldehydes to terminal alkynes,5 raising a controversial debate in the literature6 about the origin of the released CO, either from (a) the aldehyde (intermolecular coupling)5a or (b) the terminal C atom of the alkyne (intramolecular reaction) (Scheme 1).3c In all of these experiments with terminal alkynes, mechanistic hypotheses have not been corroborated by the isolation of metallic intermediates. In the search for model intermediates for transition-metalcatalyzed reactions involving terminal alkynals, we have recently investigated the reactivity of nucleophilic LnM species with 3,3-bis(methoxycarbonyl)-5-hexyn-1-al (Scheme 2). The reactions lead to vinylidene compounds with the Cβ atom incorporated into a ring. Their formation involves aldehydesubstituted vinylidenes A, which evolve to form the cyclic γ-hydroxyvinylidene species B, by formal insertion of the CO © XXXX American Chemical Society

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RESULTS AND DISCUSSION Formation of 2. The reaction of 1 with the alkynal was carried out in dichloromethane at room temperature. Under these conditions, the α,β-unsaturated cyclopentenylhydroxycarbene

derivative [Ru(η5-C5H5){C(OH)CCHCH2C(CO2CH3)2CH2}(CO)(PiPr3)] BF4 (2) was quantitatively formed after 1 h and isolated as a yellow solid in 84% yield. Received: April 11, 2014

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dx.doi.org/10.1021/om5003864 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Scheme 1

Figure 1. Molecular diagram of the cation of complex 2. Selected bond lengths (Å) and angles (deg): Ru−C(1), 1.955(10); C(2)− C(6), 1.338(11); C(1)−C(2), 1.467(11); C(2)−C(3), 1.505(12); C(1)−Ru−C(11), 93.6(4); C(1)−Ru−P, 93.1(3); C(11)−Ru−P, 90.6(3).

The formation of 2 can be rationalized according to Scheme 3. In agreement with our previous results for several rhodium, iridium, and osmium systems (Scheme 2),7,8 the reaction of 1 with 3,3-bis(methoxycarbonyl)-5-hexyn-1-al initially gives the aldehyde-substituted vinylidene ARu. This species would evolve into 2 via the oxetylidene intermediate DRu, the ruthenium counterpart of the previously isolated osmium complex D.11 The −C(sp3)H− groups adjacent to an M−C double bond are relatively acidic.12 Therefore, DRu could undergo a deprotonation−protonation process leading to 2 via the acyl derivative

Scheme 2

Ru(η 5 -C 5 H 5 ){C(O)CCHCH 2 C(CO 2 CH 3 ) 2 CH 2 }(CO)(PiPr3) (3). This oxetylidene opening is supported by our previous observation in which the oxetylidene osmium complex D underwent deprotonation with KOtBu to give the acyl osmium counterpart of 3, which subsequently reacted with HBF4 to yield the osmium analogue of 2.8 Complex 3 was isolated as a yellow solid in 74% yield from a dichloromethane solution of 2 treated with Al2O3 for 12 h at room temperature. The complex was characterized by X-ray diffraction analysis. Figure 2 gives a drawing of the molecule. The Ru−C(1), C(1)−O(1), and C(2)−C(3) bond lengths of 2.048(3), 1.224(3), and 1.329(4) Å, respectively, support the α,β-unsaturated acyl nature of the κ1-C donor ligand.13 In agreement with the presence of the acyl ligand in the complex, the IR spectrum shows a ν(CO) band at 1544 cm−1, whereas the 13 C{1H} NMR spectrum contains a doublet (JCP = 9.7 Hz) at 245.9 ppm and two singlets at 156.9 and 135.0 ppm assigned to C(1), C(2), and C(3), respectively. As expected, the addition of HBF4·OEt2 to the diethyl ether solutions of 3 regenerates 2. Complex 2 could be also formed according to the reaction sequence shown in Scheme 4. The intramolecular addition of the Cβ−H bond to the carbonyl group of ARu should lead to the cyclic hydroxyvinylidene BRu, the ruthenium counterpart of intermediates B in Scheme 2, which could dehydrate to afford the alkenylvinylidene

Complex 2 was characterized by X-ray diffraction analysis. The structure (Figure 1) proves the formation of the Fischer-type carbene ligand. The Ru−C(1) distance of 1.955(10) Å compares well with those reported for the hydroxycarbene [Ru(η5C5H5){C(OH)CHCHCPh2CH2CHCH2}(dppe)]BF4 (1.950(3) Å)9 and the alkoxycarbene [Ru(η5-C5H5){ C(OCH2CCH)CHCPh2}(CO)(PiPr3)]BF4 (1.965 (4) Å).10 The C(2)−C(6) bond length of 1.338(11) Å suggests an endocyclic Cα−Cβ double bond. The 1H and 13C{1H} NMR spectra in dichloromethane-d2 are consistent with the structure shown in Figure 1. In agreement with the presence of a hydroxy substituent at C(1), the 1H NMR spectrum contains an OH resonance at 11.14 ppm, whereas the olefinic C(6)H signal appears at 6.83 ppm. In the 13C{1H} NMR spectrum, the C(1) carbon atom displays a doublet (JCP = 9.2 Hz) at 296.2 ppm. The cycloalkenyl C(2) and C(6) resonances are observed at 156.2 and 147.9 ppm, respectively.

Ru(η5-C5H5 ){CCCHCHC(CO2CH3 )2 CH2}(CO)(PiPr3)]BF4 (4). The latter is the ruthenium analogue of the previously isolated rhodium, iridium, and osmium complexes C.7 To investigate if this pathway is also a plausible route to the formation of 2, we performed the reaction of 1 with 3,3-bis(methoxycarbonyl)-5hexyn-1-al in the presence of P2O5, a water trapping agent, in order to isolate 4. As expected, under these conditions, complex 4 was B


Organometallics

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Scheme 3

to the 1,4-addition. Treatment of its dichloromethane solutions with 1.5 equiv of water for 10 min at room temperature selectively leads to the β,γ-unsaturated cyclopentenylhydroxycarbene [Ru(η5C5H5){C(OH)CHCHCHC(CO2CH3)2CH2}(CO)(PiPr3)]BF4 (5), which was isolated as a yellow solid in 70% yield. In solution, this compound exists as a 1:1 mixture of diastereoisomers resulting from the chirality of the metal center and the cyclic C(sp3)H carbon atom. The mixture is supported for the 1H, 13 C{1H}, and 31P{1H} NMR spectra in dichloromethane-d2. The 1 H NMR spectrum shows two sets of signals at about 6.1 and 5.9 ppm for the olefinic hydrogen atoms. The OH resonances are not observed at room temperature. However, at 203 K, they appear as a broad signal centered at 14.5 ppm. In the 13C{1H} spectrum, the RuC carbon atom displays two doublets (JCP ≈ 10.5 Hz) at about 314 ppm, whereas the olefinic resonances are observed as four singlets at about 134 ppm. The 31P{1H} NMR spectrum contains two singlets at 69.4 and 69.1 ppm. As expected for the higher thermodynamic stability of the 1,4-addition product resulting from the conjugation of the C−O and C−C double bonds, complex 5 isomerizes into 2. However, the isomerization is slow at room temperature. This fact reveals that, although 2 could also be obtained through a dehydration−hydration process (Scheme 4), the contribution of the latter to the overall formation of 2, by direct reaction between 1 and 3,3-bis(methoxycarbonyl)-5-hexyn-1-al, is negligible. Stirring of a dichloromethane solution of 5 in the presence of Al2O3 produces deprotonation of the hydroxy group to give the acyl derivative Ru(η5-C5H5){C(O)CHCHCHC(CO2CH3)2CH2}(CO)(PiPr3) (6), the thermodynamically less stable isomer of 3 with a nonconjugated olefin, which was isolated as a yellow solid in 76% yield. In solution, this compound also exists as a 1:1 mixture of diastereoisomers. Thus, the 1 H NMR spectrum in benzene-d6 shows two sets of olefinic resonances between 6.5 and 6.1 ppm, whereas the 13C{1H} NMR spectrum contains two doublets at 253.0 (JCP = 8.5 Hz) and 248.9 (JCP = 10.8 Hz) for the RuC carbon atom and four singlets between 137 and 128 ppm for the olefinic atoms.

Figure 2. Molecular diagram of complex 3. Selected bond lengths (Å) and angles (deg): Ru−C(1), 2.048(3); C(1)−O(1), 1.224(3); C(2)− C(6), 1.516(4); C(2)−C(3), 1.329(4); C(1)−Ru−C(11), 90.70(12); C(1)−Ru−P, 91.43(8); C(11)−Ru−P, 92.14(9).

quantitatively formed and isolated as an orange solid in 91% yield. The presence of the alkenylvinylidene ligand in the complex was unequivocally confirmed by the 1H and 13C{1H} NMR spectra in dichloromethane-d2 at room temperature. Characteristic features in the 1H NMR spectrum are two doublets (JHH = 5.4 Hz) at 6.37 and 5.47 ppm, corresponding to the olefinic hydrogen atoms. In the 13 C{1H} NMR spectrum two doublets due to the vinylidene Cα (JCP = 10.5 Hz) and Cβ (JCP = 1.6 Hz) atoms are observed at 367.6 and 133.0 ppm, respectively, whereas the olefinic carbon atoms display singlets at 127.6 and 124.0 ppm. The reactivity of the conjugated alkenylvinylidene moiety of transition-metal alkenylvinylidene complexes is dominated by the electrophilicity of the C α and C γ atoms and the nucleophilicity of the Cβ and Cδ atoms. As a result, this type of ligand undergoes 1,2- or 1,4-additions of polar heteroatom−H bonds of amines, alcohols, and water to afford Fischer-type carbene derivatives.14 The 1,2-addition of water to the alkenylvinylidene ligand of 4 is kinetically favored with regard Scheme 4

C


Organometallics

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Complex 2 as a Catalytic Intermediate for the Ruthenium-Catalyzed Cycloisomerization and Decarbonylative Cyclization of 1,5-Alkynals. The cyclopentenylhydroxycarbene complex 2 is an intermediate in the reactions of cycloisomerization and decarbonylative cyclization of 3,3bis(methoxycarbonyl)-5-hexyn-1-al catalyzed by 1 (eq 1). Both processes indistinctly occur in the presence of either 1 or 2 (5 mol %) in a competitive manner in acetic acid at 90 °C. After

shift should be an acid-catalyzed process that could take place in two steps, the first of which involves the protonation of the metal center, by either acetic acid or the quinolinium cation, to afford II. In a second step, the resulting acetate or quinoline would deprotonate the hydroxy group, giving rise to III. Reductive elimination of the cycloisomerization product 7 requires previous dissociation of triisopropylphosphine.15 Indeed, we have observed the formation of [iPr3PC(OH)HCH2C(CO2Me)2CH2CCH]+ (δ31P 46.8 ppm),16 along with 7 and 8, during the NMR-monitored heating of a 1:5 mixture of hydroxycarbene 2 and 3,3-bis(methoxycarbonyl)-5-hexyn-1-al, in deuterated acetic acid at 60 °C for 12 h. Furthermore, the addition of 5 mol % of PiPr3 to a standard catalytic reaction mixture (90 °C in acetic acid) decreases the reaction rate and reverses the ratio of the organic products (vide infra). The release of aldehyde 7 from IV should lead to V, which could regenerate the catalyst by coordination of PiPr3 (cycle A). Before the reductive elimination of the cycloisomerized aldehyde, intermediate IV could undergo the deinsertion of the cycloalkenyl group to afford VI, which should yield the decarbonylative cyclization product 8 via VII, previous dissociation of carbon monoxide (cycle B). In the presence of PiPr3, cycle B should be inhibited. However, under these conditions, cycloalkene 8 is the main product (12% of 8 versus 2σ(I)) and wR2 = 0.2083, 5336/49/336 data/restraints/parameters, GOF = 1.019, largest peak and hole 1.051 and −1.483 e/Å3. Crystal data for 3: C25H37O6PRu, Mw 565.59, irregular block, yellow (0.30 × 0.14 × 0.04 mm), triclinic, space group P1̅, a = 7.7896(10) Å, b = 9.1182(12) Å, c = 18.376(3) Å, α = 82.101(2)°, β = 86.235(2)°, γ = 79.204(2)°, V = 1268.9(3) Å3, Z = 2, Z′ = 1, Dcalc = 1.480 g cm−3, F(000) = 588, T = 100(2) K, μ = 0.718 mm−1. 13524 measured reflections (2θ = 3−58°, ω scans 0.3°), 6302 unique reflections (Rint = 0.0230), minimum/maximum transmission factors 0.739/0.862, final agreement factors R1 = 0.0352 (5443 observed reflections, I > 2σ(I)) and wR2 = 0.0867, 6302/30/290 data/restraints/parameters, GOF = 1.072, largest peak and hole 1.441 and −1.458 e/Å3.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and CIF files giving NMR spectra of 2−6 and crystallographic data for 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.A.E.: [email protected]. *E-mail for C.S.: [email protected].

{C(OH)CCHCH2C(CO2CH3)2CH2}(CO)(PiPr3)]BF4 (2; 15 mg, 0.02 mmol), 3,3-bis(methoxycarbonyl)-5-hexyn-1-al (21.2 mg, 0.1 mmol), acetic acid-d4 (0.5 mL), and p-dichlorobenzene as internal standard (1 mg, 7 μmol) was heated at 60 °C and monitored by 1H and 31P{1H} NMR. After 68 h, the 1H NMR spectrum of the mixture showed complete conversion into 1,1-bis(methoxycarbonyl)-3-formylcyclopent-3-ene (7; 51%) and 1,1-bis(methoxycarbonyl)cyclopent-3-ene (8; 19%) as organic products (70% overall yield).3c Catalytic Reaction in the Presence of Triisopropylphosphine (5 mol %). An NMR tube containing [Ru(η5-C5H5){C(OH)-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the MINECO of Spain (projects CTQ2011-23459, CTQ2011-28258, and Consolider Ingenio 2010 (CSD2007-00006)), The Diputación General de Aragón (E35), and the European Social Fund is acknowledged. M.B. and C.G.-R. thank the Spanish MEC for a predoctoral grant and Juan de la Cierva Contract (JCI-2011-09946), respectively.

CCHCH 2 C(CO 2 CH 3 ) 2 CH 2 }(CO)(P i Pr 3 )]BF 4 (2; 3.01 mg, 0.005 mmol), 3,3-bis(methoxycarbonyl)-5-hexyn-1-al (20 mg, 0.094 mmol), PiPr3 (0.9 μL, 0.005 mmol), acetic acid-d4 (0.5 mL), and p-dichlorobenzene as internal standard (3.45 mg, 0.023 mmol), was heated at 90 °C and monitored by 1H NMR. After 7 days of reaction, NMR spectra showed formation of cycloalkane 8 (53%) as the major product versus cycloisomerized aldehyde 7 (23%). Structural Analysis of Complexes 2 and 3. Crystals were obtained by slow diffusion of pentane into saturated solutions of 2 in CH2Cl2 or from saturated solutions of 3 in pentane. X-ray data were collected on a Bruker Smart APEX DUO diffractometer equipped with a normal-focus 2.4 kW sealed-tube source (Mo radiation, λ = 0.71073 Å) operating at 50 kV and 40 mA (2) or 30 mA (3). Data were collected over the complete sphere. Each frame exposure time was 40 s (2) or 30 s (3) covering 0.3° in ω. Data were corrected for absorption by using a multiscan method applied with the SADABS program.17 The structures were solved by direct methods. Refinement of both complexes was performed by full-matrix least squares on F2 with SHELXL97,18 including isotropic and subsequently anisotropic displacement parameters (for non-hydrogen nondisordered atoms). All hydrogen atoms were calculated and refined riding to bonded atoms.

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REFERENCES

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Ruthenium Hydroxycarbenes as Key Intermediates in

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