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Preparation of Pyranylidene Complexes of Ruthenium

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Preparation of Pyranylidene Complexes of Ruthenium Dalton Transactions View Article Online PAPER View Journal | View Issue Preparation of pyranylidene complexes of ruthenium† Cite this: Dalton Trans., 2015, 44, 7411 Gabriele Albertin,*a Stefano Antoniutti,a Marco Bortoluzzi,a Alessandra Bottera and Jesús Castrob 5 The reaction of the chloro-complex RuCl(η -C5H5)(PPh3)[P(OMe)3] with alkylpropiolates HCuCCOOR1 in 5 alcohol R2OH affords pyranylidene derivatives [Ru(η -C5H5){vC(COOR1)C(H)C(H)C(OR1)O}(PPh3)- 5 {P(OMe)3}]BPh4 (1, 3) and alkoxycarbene complexes [Ru(η -C5H5){vC(OR2)(CH2COOR1)}(PPh3){P(OMe)3}]- Received 29th January 2015, BPh4 (2, 4). A reaction path for the formation of compounds 1–4, involving reactions on a vinylidene Accepted 12th March 2015 intermediate complex, is also discussed. The complexes were characterized spectroscopically DOI: 10.1039/c5dt00418g 1 13 31 5 (IR and H, C, P NMR) and by X-ray crystal structure determination of [Ru(η -C5H5)- www.rsc.org/dalton {vC(COOMe)C(H)C(H)C(OMe)O}(PPh3){P(OMe)3}]BPh4 (1). Creative Commons Attribution 3.0 Unported Licence. Introduction Despite the large number of Fischer-type transition metal carbene complexes reported so far,1 six-membered pyranyl- idene carbene derivatives (Chart 1) are relatively few and 2–5 involve mainly Cr, Mo and W central metals. This fact is Chart 1 somewhat surprising, given the increasing interest in their use in ring-opening reactions,6 Diels–Alder reactions4b,7 and 1,6 This article is licensed under a addition.4a Several methods for the synthesis of pyranylidene have been developed, mainly including the reaction of pre- the synthesis and characterization of pyranylidene and alkoxy- formed carbene complexes with pyridinium ylides,2b,c enol carbene complexes of Ru(II), are reported here. 8 3 9 Open Access Article. Published on 24 March 2015. Downloaded 10/15/2018 3:05:37 AM. ethers and 1,3-diketones, or multicomponent species such as alkynyl esters and N-methylmorpholine-N-oxide. Alterna- tively, pyranylidene complexes can be prepared from the reac- β Results and discussion tion of M(CO)5L (M = Cr, Mo, W; L = THF, NEt3) with -alkynyl α β 4 5 , -unsaturated carbonyl compounds and from the reaction The half-sandwich chloro-complex RuCl(η -C5H5)(PPh3)- 5 of M(CO)6 with 1-lithio-1,3-dienes. However, in only one case [P(OMe)3] reacts with an excess of alkylpropiolate the simplest method for preparing a pyranylidene complex, HCuCCOOR1 in alcohol R2OH to give a mixture of pyranyl- u 5 involving dimerization of alkylpropiolate HC CCOOR on a idene [Ru(η -C5H5){vC(COOR1)C(H)C(H)C(OR1)O}(PPh3)- 9 5 metal fragment, has been reported. {P(OMe)3}]BPh4 (1, 3) and alkoxycarbene [Ru(η -C5H5)- v 2 4 We were interested in the reactivity of half-sandwich com- { C(OR2)(CH2COOR1)}(PPh3){P(OMe)3}]BPh4 ( , ) in about plexes containing phosphite ligands10 and, in the course of 1 : 4 ratio, which were separated by fractional crystallization our studies, found that the reaction of the complex RuCl(η5- and characterized (Scheme 1). C5H5)(PPh3)[P(OMe)3] with alkylpropiolate leads to the first Crucial for successful syntheses was the use of alcohol con- pyranylidene complexes of ruthenium. Our results, including taining the salt NaBPh4 as a solvent, which probably favors substitution of the chloride ligand in the starting complex, yielding the two types of complexes 1, 3 and 2, 4. aDipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, The formation of both pyranylidene and alkoxycarbene 5 Dorsoduro 2137, 30123 Venezia, Italy. E-mail: [email protected] derivatives in the reaction of RuCl(η -C5H5)(PPh3)[P(OMe)3] b Departamento de Química Inorgánica, Universidade de Vigo, Facultade de Química, with alkylpropiolate is somewhat surprising, but may be Edificio de Ciencias Experimentais, 36310 Vigo, Galicia, Spain explained according to the reaction path shown in Scheme 2, †Electronic supplementary information (ESI) available: xyz file for DFT-opti- mized structures; NMR spectra (pdf). CCDC 1046060. For ESI and crystallo- which involves the initial formation of a vinylidene intermedi- graphic data in CIF or other electronic format see DOI: 10.1039/c5dt00418g ate [A]. Reaction of this intermediate with alkylpropiolate gives This journal is © The Royal Society of Chemistry 2015 Dalton Trans.,2015,44,7411–7418 | 7411 View Article Online Paper Dalton Transactions Scheme 3 Scheme 1 R1 = Me (1, 2), Et (3, 4); R2 = Me (a), Et (b). 15.9, JCP = 11.6 Hz), which might be attributed to the Cα carbon atom of a vinylidene species [A]. However, this value is at lower frequency than those observed for the Cα of several known vinylidene derivatives.11 As suggested by a reviewer, this resonance may be attributed to the Cα of the vinyl η5 v species [Ru( -C5H5){C(OTf) C(H)COOMe}(PPh3)[P(OMe)3] [C], formed by nucleophilic attack of the triflate ion on the carbene carbon atom of the vCvC(H)COOMe ligand. In an HMBC experiment, the 13C signal at 199.7 ppm is correlated 1 Creative Commons Attribution 3.0 Unported Licence. with the multiplet at 5.47 ppm of the H spectrum, attribu- table to the vC(H)COOMe of the vinyl ligand, fitting the pro- posed formulation for [C]. The triflate ligand is labile in the 1 5 complex Ru(κ -OTf)(η -C5H5)(PPh3)[P(OMe)3] and can be sub- stituted by alkyne, which then tautomerizes11 on the metal center to give vinylidene intermediate [A]. Reaction with the − triflate ion OTf can give the vinyl intermediate [C], which may be in equilibrium with [A] (Scheme 3). DFT calculations on model systems, where P-donor ligands are replaced by PH3 This article is licensed under a and PF3, support this hypothesis, the ΔH difference between 5 5 [Ru(η -C5H5){C(OTf)vC(H)COOMe}(PH3)(PF3)] and [Ru(η - v v Scheme 2 R1 = Me, Et; R2 = Et. C5H5){ C C(H)COOMe}(PH3)(PF3)]OTf being only about −1 A Open Access Article. Published on 24 March 2015. Downloaded 10/15/2018 3:05:37 AM. 1.2 kcal mol in favor of the latter. The fact that vinylidene [ ] and/or vinyl [C] complexes are really intermediates of the reac- rise to the dimerization of HCuCCOOR1, affording pyranyl- tion path proposed in Scheme 2 was confirmed by treatment idene derivatives 1, 3. Parallel nucleophilic attack by the with ethanol, which gave ethoxycarbene 2b as the final oxygen atom of R2OH on the Cα of the vinylidene, followed by product, the addition of methanol affording methoxycarbene proton-transfer, yields the final alkoxycarbene derivatives 2, 4. 2a. Instead, addition of alkylpropiolate to vinylidene inter- In order to verify this reaction path, we attempted to isolate mediate [A] only yielded traces of the pyranylidene complex, the vinylidene intermediate [A] or, at least, to identify it in the and it was only with the addition of alcohol to the solution of reaction mixture. At first, we treated the compound RuCl(η5- [A] that the reaction started, affording a mixture of pyranyli- 1 2 C5H5)(PPh3)[P(OMe)3] with HCuCCOOR1, in the presence of dene and alkoxycarbene complexes. However, the greater 2 NaBPh4, in a solvent other than alcohol such as dichloro- amount of carbene with respect to pyranylidene (4 : 1 ratio) methane, but no reaction was observed, probably owing to the suggests a faster reaction of vinylidene [A] with alcohol than insolubility of NaBPh4 in this solvent. We therefore used a that with propiolate. different strategy, involving a reaction of the starting chloro- The need for alcohol as a solvent is explained by the fact complex first with silver triflate, to form the triflate intermedi- that the formation of pyranylidene from the reaction of vinyli- 1 5 A ate Ru(κ -OTf)(η -C5H5)(PPh3)[P(OMe)3], and then with an dene [ ] with alkylpropiolate must involve a hydrogen shift, excess of methylpropiolate in dichloromethane as a solvent which may be a proton transfer strongly favored by protic (Scheme 3). solvents such as alcohols (Scheme 4). The triflate complex quickly reacted with methylpropiolate In fact, plausible mechanisms for the formation of the pyra- by changing color in the solution, from which we were not nylidene probably involve cyclization reactions between an able to isolate a solid, but only an oily product. Its 13C NMR alkylpropiolate molecule and coordinated vinylidene or η2- ff B1 B2 spectrum showed a doublet of doublets at 199.71 ppm (JCP = alkyne, a ording intermediates [ ] (path 1) or [ ] (path 2), 7412 | Dalton Trans.,2015,44,7411–7418 This journal is © The Royal Society of Chemistry 2015 View Article Online Dalton Transactions Paper 5 + Scheme 4 [Ru] = [Ru(η -C5H5)(PPh3){P(OMe)3}] ; R1 = Me, Et. Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 24 March 2015. Downloaded 10/15/2018 3:05:37 AM. 2 5 + Scheme 5 Computed reaction pathway for the formation of [B2] from {[Ru]-η -HCuCH + methylpropiolate}. [Ru] = [Ru(η -C5H5)(PH3)(PF3)] . respectively. Subsequent hydrogen transfers in these inter- the greater stability of vinylidene may partially explain the mediates gave the final pyranylidene complexes 1, 3. DFT cal- competitive reaction with alcohols and the formation of the η5 culations on a simplified system, in which [Ru] = [Ru( -C5H5) corresponding carbenes as prevailing species. + (PH3)(PF3)] and R1 = Me (see ESI†), indicated the formation of The reaction path involving [B2] was more deeply investi- − intermediate [B2], which is about 20.7 kcal mol 1 more stable gated from a computational point of view.
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