In tandem or alone: a remarkably selective transfer hydrogenation of alkenes catalyzed by ruthenium olefin metathesis catalysts

Abstract

A system for transfer hydrogenation of alkenes, composed of a ruthenium metathesis catalyst and HCOOH, is presented. This operationally simple system can be formed directly after a metathesis reaction to effect hydrogenation of the metathesis product in a single-pot. These hydrogenation conditions are applicable to a wide range of alkenes and offer remarkable selectivity.

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Catalytic hydrogenation olefins are one of the most important “classics” in organic chemistry.¹ Due to high complexity of targets approached by contemporary organic synthesis, there is a very high demand for chemo- and regioselective methods to hydrogenate C–C multiple bonds in the presence of many other functionalities including other olefinic moieties. A difficulty associated with catalytic hydrogenation is the use of hydrogen gas, whose physical properties complicate its safe, efficient and economical storage. This creates a need for new selective, safe and environmentally friendly hydrogenation methodologies. Recently reduction of olefins has been carried out in a homogeneous fashion using transition metals (incl. Rh,^2a Ir,^2b Ru,^2c–f Pd^2g) and gaseous H₂ or hydrogen sources such as alcohols,^2h–j hydrazine,^2k formic acid–triethylamine azeotrope,^2l ammonia–borane,^2m silanes,²ⁿ and others.^2o,p

Over the past two decades olefin metathesis has transformed the design and practice of organic synthesis.³ It should be noted that this reaction can be easily incorporated in tandem processes, where the metathesis catalyst is triggered for a second transformation.⁴ One such sequence is metathesis–hydrogenation, typically conducted under high pressure of hydrogen, which is required to convert Ru alkylidene complexes into a hydrogenation catalyst.⁵ Recently a more convenient protocol, using NaBH₄ as a hydrogen source has been reported.⁶

During our study on a new chelating ruthenium azinate complex (Az-II, Fig. 1), we have noticed its reactivity in numerous transformations.⁷ Depending on the conditions applied, the same complex promoted efficiently olefin metathesis, isomerization, cycloisomerization or reduction of a carbonyl group.⁷ Serendipitously, we have found recently that after the metathesis step is promoted by Az-II, adding sodium hydride and formic acid to the reaction mixture leads to a new catalytic system that is capable of efficient reduction of cycloolefins (Scheme 1).⁸ Intrigued by this preliminary observation we decided to study it in more detail.

Fig. 1 Selected ruthenium olefin metathesis catalysts (Cy = cyclohexyl).

Scheme 1 Observed reactivity of azinate complex Az-II.⁸ Conditions: (i) 1. Az-II (1 mol%), C₂Cl₆ (4 mol%), THF, 80 °C, 3 h; 2. Az-II (1 mol%), NaH (0.1 equiv.), HCOOH (50 equiv.), THF, 80 °C, 20 h; (ii) Az-II (2 mol%), NaH (0.1 equiv.), HCOOH (50 equiv.), 80 °C, 20 h.

Since the azinate complex Az-II is not commercially available, in the present study we decided to check if other, more standard Ru–alkylidene complexes, can also show a similar reactivity in the presence of HCOOH,^9a so can be used in transfer hydrogenation or in olefin metathesis–hydrogenation sequences. To do so, three generations of representative Grubbs’ catalysts Gru-I, Gru-II and Gru-III have been selected (Fig. 1) and tested in a model reduction of cyclopentene 3a to cyclopentane 2a (Table 1). After some initial experiments, an optimized procedure was elaborated as follows: to a solution of olefin 3a a catalyst (2 mol%), and a base (0.2 equiv.) were introduced, followed immediately by 98% formic acid (50 equiv. relative to olefin). Then the reaction mixture was heated to 80 °C for 6 h in a sealed flask.^9b After screening Ru-catalysts presented in Fig. 1, complex Gru-II, promoting hydrogenation at a comparable rate as the azinate complex Az-II, was chosen for further studies. The roles of the base (Table 1, entries 5–8) and the solvent (entries 9–11) were also investigated to find that sodium hydride can be replaced by sodium formate and the reaction can be conducted in solvents like THF, dimethyl carbonate, dimethoxyethane or dichloroethane.

Table 1 Optimization of a model hydrogenation reaction of 3a to 2a^a

Entry	Catalyst	Base	Solvent^b	NMR yield^c [%]
a Conditions: catalyst (2 mol%), base (0.2 equiv.), HCO2H (50 equiv.), solvent, 80 °C, 6 h. b DMC = dimethyl carbonate; DME = 1,2-dimethoxyethane; DCE = 1,2-dichloroethane. c Yield determined by ^1H NMR.
1	Az-II	NaH	THF	92
2	Gru-I	NaH	THF	27
3	Gru-II	NaH	THF	>99
4	Gru-III	NaH	THF	95
5	Gru-II	None	THF	68
6	Gru-II	t-BuOK	THF	90
7	Gru-II	HCOONa	THF	>99
8	Gru-II	NaOH	THF	78
9	Gru-II	NaH	DMC	>99
10	Gru-II	NaH	DME	92
11	Gru-II	NaH	DCE	>99

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To check, if this reduction can be applied one-pot together with an olefin metathesis event, we conducted the RCM reaction of diene 1a in the presence of 2 mol% of Gru-II in THF at 40 °C. After 30 minutes, TLC revealed that the olefin metathesis step was complete. At this point 0.2 equiv. of HCOONa was added to the reaction mixture, followed by immediate addition of 50 equiv. of HCOOH. The reaction tube was closed and heated to 80 °C. After 7 hours the reduction was completed, as shown by NMR. Using this procedure, RCM-transfer hydrogenation experiments were conducted with a small set of dienes, producing, after aqueous work-up, corresponding cycloalkanes in good to excellent yield (Table 2).

Table 2 Sequence of RCM-transfer hydrogenation catalysed by Gru-II^a

Entry	Substrate	Product	Time^b [h]	Yield^c [%]
a Conditions: (i) Gru-II (2 mol%), THF, 40 °C, 30 min, then (ii) HCOONa (0.2 equiv.), HCOOH (50 equiv.), 80 °C, THF. b Time of RCM (0.5 h) + time of reduction. c Yield of spectrally pure isolated products. In all cases full conversion was observed. d NaH was used instead of HCOONa. e Product isolated by extraction. Crude product was spectrally pure. f Product purified by silica gel column chromatography.
1^d			7.5	99^e
2			20.5	97^f
3			7.5	92^f
4^d			6.5	89^e

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To examine deeper the substrate scope of this transfer hydrogenation reaction, a number of unsaturated substrates were subjected to the optimised reaction conditions. A range of substrates can be hydrogenated efficiently with this system (Table 3), proving excellent compatibility with a number of functional groups.

Table 3 Hydrogenation of cyclic and acyclic unsaturated substrates^a

Entry	Substrate	Product	Time [h]	Yield^b. [%]
a Conditions: Gru-II (2 mol%), HCOONa (0.2 equiv.), HCOOH (50 equiv.), 80 °C, THF. b Yields of spectrally pure isolated products. In parenthesis are conversions determined by ^1H NMR, where not indicated full conversion was observed. c NaH was used instead of HCOONa. d Product isolated by extraction. Crude product was spectrally pure. e Product purified by silica gel column chromatography. f Product purified by bulb-to-bulb distillation. g Reaction with 3 mol% of Gru-II. h Product purified by crystallization. i Reaction with 4 mol% of Gru-II.
1			6	99^c^,^d
2			6	99^d
3			6	98^e
4			20	98^c^,^f
5			24	90^f
6^g			48	74^e (100)
7			6	83^c
8			6	91^h
9^i			336	43^f (100)
10^i			120	(50)
			336	(80)
11^i			48	96^e
12			6	99^d

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Interestingly, carbonyl functions in ketones¹⁰ and enones (Table 3, entries 3, 4, 7 and 8) do not undergo reduction under these conditions, which makes our system different from the previously described [Ru]/NaBH₄ couple.⁶ As suggested by the experiment conducted with 3f, primary and secondary benzyl ethers and an allylic stereocenter survived the reaction untouched (Table 3, entry 5). Interestingly, compounds bearing unprotected OH group 3g (Table 3, entry 6) underwent transfer hydrogenation quantitatively and no esterification took place. The reaction of enones 3h, 3i and α,β-unsaturated ester 3l under our conditions lead to the reduction of conjugated C–C double bonds (Table 3, entries 7, 8 and 11). The second, more substituted C–C double bond present in β-damascone (3h) stayed untouched. Hydrogenation of the trisubstituted double bond in 3j proceeds much slower, requiring 336 h for completion (43% isolated yield, entry 9).¹¹ However, we were surprised when we attempted the reduction of cyclohexene derivative 3k. Hydrogenation rates of small ring cycloalkenes depend on their strain energies,^12a and in the case of reduction by diimide, it was shown that the relative rate of cyclopentene hydrogenation is 15.5 times faster than the rate of cyclohexene reduction.^12b However, in the case of 3k the reaction was not complete even after 336 h (14 days) of heating at 80 °C, while both cyclopentene (entry 11) and cycloheptene rings (entry 12) were fully reduced in 6 hours. It is noteworthy that the cyclohexene ring is easily reduced by the newly published system composed of NaBH₄ and Ru olefin metathesis catalysts.⁶

We speculated that the observed high sensitivity of the Gru-II/HCOOH catalytic system towards the ring strain and the substitution pattern can be utilised for selective reduction of one C–C double bond in the presence of others. To explore this interesting possibility, a competition experiment, shown in Scheme 2, was performed, to demonstrate the level of control offered by our transfer hydrogenation system. An equimolar mixture of 3a and 3k was subjected to the transfer hydrogenation process leading to complete reduction of the cyclopentene ring, with practically no reduction of a cyclohexene double bond under these conditions (Scheme 2).

Scheme 2 Competition study. Conditions: (i) Gru-II (2 mol%), HCOONa (0.2 equiv.), HCOOH (50 equiv.), THF, 4 h, 80 °C. Conversion determined by GC using an internal standard.

While the system studied by us shows high level of selectivity, it offers also an additional possibility of being readily coupled with olefin metathesis reactions. An example shown in Scheme 3 starts with Ru-catalysed double-RCM of tetraene 1m forming selectively product 3m bearing one five- and one six-membered unsaturated ring. Then, only one of the two seemingly very similar C–C double bonds present in 3m was selectively reduced by Gru-II/HCOOH, yielding mono-saturated product 2m in good yield (Scheme 3, route i + ii). The same two-step sequence can be easily conducted in one-pot fashion, converting directly tetraene 1m into semihydrogenated cyclohexene derivative 2m in 90% yield (Scheme 3, route iii).

Scheme 3 Tandem double RCM-selective transfer hydrogenation of 1m. Conditions: (i) Gru-II (2 mol%), DCM, 1 h, 40 °C. (ii) Gru-II (2 mol%), HCOONa (0.2 equiv.), HCOOH (50 equiv.), THF, 4 h, 80 °C. (iii) Gru-II (2 mol%), THF, 0.5 h, 40 °C, then Gru-II (2 mol%), HCOONa (0.2 equiv.), HCOOH (50 equiv.), THF, 4 h, 80 °C.

The precise details of the nature of catalytic species produced upon the action of HCOONa/HCOOH are unclear. It was reported that some Ru complexes can catalyse the decomposition of formic acid leading to the formation of CO₂ and ruthenium hydride species.¹³ This suggests that entry into the catalytic cycle starts with the conversion of Az-II or Gru-II into Ru–hydride species, which act as the actual hydrogenation catalyst.^14,15 A similar mechanism was also suggested for other Ru-catalysed transfer hydrogenation reactions.⁶ To prove the existence of Ru–H species in our system Grubbs’ second generation catalyst was placed in a dry NMR tube containing formic acid (20 equiv.) in THF-d₈. Then the tube was closed and the reaction mixture was heated at 50 °C for 4 h. Then the tube was cooled down to room temperature and the NMR spectrum was recorded. A new signal with a chemical shift of −6.86 ppm appeared. This chemical shift is in the range characteristic for ruthenium hydride species.¹⁶ In line with this observation, in hydrogenations of 3a analysed before the end of reaction was reached, we observed some amounts of product 4a formed via the alkene isomerisation process (C–C double bond migration)¹⁷ that were decreasing with time (Scheme 4). Both these observations suggest the presence of Ru hydride species during reduction with the Ru/HCOOH system described by us.

Scheme 4 Incomplete isomerisation–hydrogenation of 3a. Conditions: (i) Gru-II (2 mol%), NaH (0.2 equiv.), HCOOH (50 equiv.), THF. Conversion determined by ¹H NMR.

Conclusions

In conclusion, a mild protocol for transfer hydrogenation of alkenes has been demonstrated that exhibits a highly functional group tolerance and surprising selectivity. For a number of alkenes, the Ru/HCOOH system can be seen as a safe alternative to dangerous pressurized hydrogen gas. This method can also be coupled with olefin metathesis events, allowing for efficient one-pot sequences. Unlike other Ru-catalysed hydrogenation protocols, Ru/HCOOH allows for selective reduction of a given C–C double bond in the presence of relatively similar ones and is compatible with keto and enone functionalities that can be reduced by NaBH₄. Therefore the highly selective nature of this system provides a useful addition to the still expanding⁶ repository of hydrogenation methods.

Experimental section

Procedure A. Reduction of C–C double bonds

Olefin (1 mmol) and 5 mL of anhydrous THF were placed under argon in a reaction tube. Next, catalyst Gru-II (2 mol%) was added to the resulting solution followed by the addition of HCOONa (0.2 mmol) and 98% HCOOH (50 mmol), and the reaction tube was closed. The reaction mixture was stirred for an appropriate period of time at 80 °C, then allowed to reach room temperature and poured into saturated aqueous solution of NaHCO₃ (ca. 30 mL). The aqueous layer was extracted with an appropriate organic solvent and the combined organic phases were washed with brine, dried over MgSO₄, filtered and the solvent was evaporated in vacuo to obtain the crude product, which was purified when necessary by column chromatography or by bulb-to-bulb distillation.

Diethyl cyclopentane-1,1-dicarboxylate (2a). A spectrally pure product was synthesized according to the procedure B without further purification (212 mg, 99%) using 3a as a starting material and NaH instead of HCO₂Na. Analyses of the product were in accordance with the previously reported study.¹⁸

¹H NMR (400 MHz, CDCl₃) δ = 4.20–4.12 (m, 4H), 2.21–2.11 (m, 4H), 1.71–1.62 (m, 4H), 1.23 (t, J = 7.1 Hz, 6H); ¹³C NMR (101 MHz, CDCl₃) δ = 172.7, 61.1, 60.4, 34.4, 25.4, 14.0.

1-(2,6,6-Trimethylcyclohex-1-en-1-yl)butan-1-one (2h). A spectrally pure product was synthesized according to the procedure A using commercially available substrate 1h without further purification (161 mg, 83%). Analyses were in accordance with the previously reported study.¹⁹

¹H NMR (400 MHz, CDCl₃) δ = 2.50 (t, J = 7.4 Hz, 2H), 1.93 (t, J = 6.3 Hz, 2H), 1.69–1.59 (m, 4H), 1.53 (s, 3H), 1.45–1.39 (m, 2H), 1.04 (s, 6H), 0.93 (t, J = 7.4 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ = 212.0, 143.6, 128.7, 47.8, 39.0, 33.3, 31.2, 28.8, 20.95, 19.0, 16.7, 13.9.

Procedure B. Tandem olefin metathesis and C–C double bond reduction

Diene (1 mmol) in 5 mL of dry THF were placed in a reaction tube. Catalyst Gru-II (2 mol%) was added and the ring closing metathesis reaction was carried out for 0.5 h at 40 °C. Once the RCM reaction was complete according to TLC or GC, solid HCOONa (0.2 mmol) was added immediately followed by 98% HCOOH (50 mmol) and the reaction was continued for an appropriate period of time at 80 °C in a closed tube, then allowed to reach room temperature and poured into saturated aqueous solution of NaHCO₃ (ca. 30 mL). The aqueous layer was extracted with an appropriate organic solvent and the combined organic phases were washed with brine, dried over MgSO₄, filtered and the solvent was evaporated in vacuo to obtain the crude product, which was further purified when necessary by column chromatography or by bulb-to-bulb distillation.

Diethyl cyclopentane-1,1-dicarboxylate (2a). The product was synthesized according to the procedure B using diene 1a as a starting material and NaH instead of HCO₂Na. A spectrally pure product was isolated by extraction (214 mg, 99%). Analyses were in accordance with the previously reported study.¹⁸

¹H NMR (400 MHz, CDCl₃) δ = 4.20–4.12 (m, 4H), 2.21–2.11 (m, 4H), 1.71–1.62 (m, 4H), 1.23 (t, J = 7.1 Hz, 6H); ¹³C NMR (101 MHz, CDCl₃) δ = 172.7, 61.1, 60.4, 34.4, 25.4, 14.0.

Benzoylcyclopentane (2c). The product was synthesized according to the general procedure B using diene 1c as a starting material. Purification by using column chromatography afforded a spectrally pure product 2c (160 mg, 92%). Analyses were in accordance with the previously reported study.²⁰

¹H NMR (400 MHz, CDCl₃) δ = 8.03–7.93 (m, 2H), 7.60–7.50 (m, 1H), 7.50–7.42 (m, 2H), 3.72 (quint, J = 7.88 Hz, 1H), 2.06–1.85 (m, 4H), 1.83–1.54 (m, 4H); ¹³C NMR (100 MHz, CDCl₃) δ = 202.9, 137.0, 132.8, 128.6, 128.6, 46.5, 30.1, 26.4.

Acknowledgements

The authors are grateful to the National Science Centre (Poland) for the NCN Opus grant no. UMO-2013/09/B/ST5/03535.

Notes and references

1. (a) Handbook of Homogeneous Hydrogenation, ed. J. G. de Vries and C. J. Elsevier, Wiley-VCH, Weinheim, 2007; (b) S. Siegel, Heterogeneous Catalytic Hydrogenation of C=C and C≡C, in Comprehensive Organic Synthesis, Vol. 8 Reduction, ed. B. M. Trost and I. Fleming, Pergamon Press, Oxford, 1993, pp. 417–443; (c) H. Takaya, Homogeneous Catalytic Hydrogenation of C=C and C≡C, in Comprehensive Organic Synthesis Vol. 8 Reduction, ed. B. M. Trost and I. Fleming, Pergamon Press, Oxford, 1993, pp. 443–471.
2. (a) J. A. Osborn, F. H. Jardine, J. F. Young and G. Wilkinson, J. Chem. Soc. A, 1966, 1711; (b) R. Crabtree, Acc. Chem. Res., 1979, 12, 331; (c) Q. Zhao, Y. Li, R. Liu, A. Chen, G. Zhang, F. Zhang and X. Fan, J. Mater. Chem. A, 2013, 1, 15039; (d) M. E. Vol'pin, V. P. Kukolev, V. O. Chernyshev and I. S. Kolomnikov, Tetrahedron Lett., 1971, 12, 4435; (e) I. S. Kolomnikov, V. P. Kukolev, V. O. Chernyshev and M. E. Vol'pin, Russ. Chem. Bull., Int. Ed., 1971, 21, 661; (f) I. S. Kolomnikov, Y. D. Koreshkov, V. P. Kukolev, V. A. Mosin and M. E. Vol'pin, Russ. Chem. Bull., Int. Ed., 1973, 22, 180; (g) V. Jurčík, S. P. Nolan and C. S. J. Cazin, Chem. – Eur. J., 2009, 15, 2509; (h) K. Ohkubo, T. Aoji, K. Hirata and K. Yoshinga, Inorg. Nucl. Chem. Lett., 1976, 12, 837; (i) S. Horn and M. Albrecht, Chem. Commun., 2011, 47, 8802; (j) B. Schmidt, S. Krehl and V. Sotelo-Meza, Synthesis, 2012, 1603; (k) M. Lamani, G. S. Ravikumara and K. R. Prabhu, Adv. Synth. Catal., 2012, 354, 1437; (l) W. Leitner, J. M. Brown and H. Brunner, J. Am. Chem. Soc., 1993, 115, 152; (m) C. E. Hartmann, V. Jurčík, O. Songis and C. S. J. Cazin, Chem. Commun., 2013, 49, 1005; (n) C. Menozzi, P. I. Dalko and J. Cossy, Synlett, 2005, 2449; (o) H. Imai, T. Nishiguchi and K. Fukuzumi, J. Org. Chem., 1977, 42, 431; (p) T. Nishiguchi, A. Kurooka and K. Fukuzumi, J. Org. Chem., 1974, 39, 2403.
3. (a) Handbook of Metathesis, ed. R. H. Grubbs, Wiley-VCH, Weinheim, 2003; (b) Olefin Metathesis Theory and Practice, ed. K. Grela, John Wiley & Sons, Inc., Hoboken, NY, 2014.
4. For reviews, see: (a) D. E. Fogg and E. N. dos Santos, Coord. Chem. Rev., 2004, 248, 2365; (b) B. Schmidt and S. Krehl, Domino and Other Olefin Metathesis Sequences, in Olefin Metathesis Theory and Practice, ed. K. Grela, John Wiley & Sons, Inc., Hoboken, NY, 2014, pp. 187–232.
5. Selected examples: (a) J. Louie, C. W. Bielawski and R. H. Grubbs, J. Am. Chem. Soc., 2001, 123, 11312; (b) J. Cossy, F. C. Bargigga and S. BouzBouz, Tetrahedron Lett., 2002, 43, 6715; (c) B. Schmidt and M. Pohler, Org. Biomol. Chem., 2003, 1, 2512; (d) X. Miao, C. Fischmeister, C. Bruneau, P. H. Dixneuf, J.-L. Dubois and J.-L. Couturier, ChemSusChem, 2012, 5, 1410.
6. T. Connolly, Z. Wang, M. A. Walker, I. M. McDonald and K. M. Peese, Org. Lett., 2014, 16, 4444.
7. T. Wdowik, C. Samojłowicz, M. Jawiczuk, M. Malińska, K. Woźniak and K. Grela, Chem. Commun., 2013, 49, 674.
8. T. Wdowik, C. Samojłowicz and K. Grela, unpublished results.
9. (a) Formic acid is considered as viable and renewable feedstock for hydrogen. Ru(II) promoted catalytic formic acid dehydrogenation to a gaseous mixture of H₂ and CO₂ was reported: I. Mellone, M. Peruzzini, L. Rosi, D. Mellmann, H. Junge, M. Beller and L. Gonsalvi, Dalton Trans., 2013, 42, 2495; (b) In an open flask, the same catalytic system (2 mol% Gru-II, 50 equiv. HCOOH and 0.2 equiv. of NaH) leads to formation of 2a in 90% yield, however, after prolonged time of 15 hours. For detailed experimental procedures see ESI.‡.
10. For representative examples of Ru-catalyzed transfer hydrogenation of ketones, see: (a) C. S. Cho, B. T. Kim, T.-J. Kim and S. C. Shim, J. Org. Chem., 2001, 66, 9020; (b) M. Babin, R. Clément, J. Gagnon and F.-G. Fontaine, New J. Chem., 2012, 36, 1548; (c) O. O. Kovalenko, H. Lundberg, D. Hübner and H. Adolfsson, Eur. J. Org. Chem., 2014, 6639; (d) ref. 13a.
11. Interestingly, trisubstituted alkenes were reported to be inert to Ru-catalysed transfer hydrogenation, cf.ref. 6.
12. (a) P. v. R. Schleyer, J. E. Williams and K. R. Blanchard, J. Am. Chem. Soc., 1970, 92, 2377; (b) The relative ratio of reduction with diimide is 15.5 for cyclopentene, 1.0 for cyclohexene, 12.1 for cycloheptene and 17.0 for cyclooctene, while for 1-methylcyclohexene is only 0.11. See: (c) E. W. Garbish Jr., S. M. Schildcrout, D. B. Patterson and C. M. Sprecher, J. Am. Chem. Soc., 1965, 87, 2932.
13. (a) N. Manashe, E. Salant and Y. Shvo, J. Organomet. Chem., 1996, 514, 97; (b) T. Koike and T. Ikariya, Adv. Synth. Catal., 2004, 346, 37.
14. (a) B. Schmidt, Eur. J. Org. Chem., 2004, 1865; (b) B. Alcaide, P. Almendros and A. Luna, Chem. Rev., 2009, 109, 3817.
15. It cannot be excluded that both, the initial ruthenium metathesis catalyst and its decomposition products after olefin metathesis step, are able to catalyse the hydrogen transfer reaction.
16. (a) T. M. Trnka, J. P. Morgan, M. S. Sanford, T. E. Wilhelm, M. Scholl, T.-L. Choi, S. Ding, M. W. Day and R. H. Grubbs, J. Am. Chem. Soc., 2003, 125, 2546; (b) M. B. Dinger and J. C. Mol, Eur. J. Inorg. Chem., 2003, 2827; (c) D. Pingen, M. Lutz and D. Vogt, Organometallics, 2014, 33, 1623.
17. For decomposition pathways of ruthenium olefin metathesis catalysts and related processes, see: (a) S. H. Hong, M. W. Day and R. H. Grubbs, J. Am. Chem. Soc., 2004, 126, 7414; (b) S. H. Hong, D. P. Sanders, C. W. Lee and R. H. Grubbs, J. Am. Chem. Soc., 2005, 127, 17160; (c) S. H. Hong, A. G. Wenzel, T. T. Salguero, M. W. Day and R. H. Grubbs, J. Am. Chem. Soc., 2007, 129, 7961; (d) B. Schmidt and S. Hauke, Org. Biomol. Chem., 2013, 11, 4194.
18. D. Domin, D. Benito-Garagorri, K. Mereiter, J. Fröhlich and K. Kirchner, Organometallics, 2005, 24, 3957.
19. E. Demole, P. Enggist, U. Säuberli and M. Stoll, Helv. Chim. Acta, 1970, 53, 56.
20. L. Li, P. Cai, Q. Guo and S. Xue, J. Org. Chem., 2008, 73, 3516.

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Footnotes

† Dedicated to Professor Mieczysław Mąkosza on the occasion of his 80^th birthday.

‡ Electronic supplementary information (ESI) available: Full experimental details and characterization data, including NMR spectra. See DOI: 10.1039/c4ob02480j

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