Highly efficient and practical hydrogenation of olefins catalyzed by in situ generated iron complex catalysts

Abstract

A new method was developed for in situ generation of active Fe catalysts for the hydrogenation of olefins from bench-stable Fe(II) complexes and easily accessible LiAlH₄. This method makes the hydrogenation very easy to handle and enables the development of several new Fe catalysts for olefin hydrogenation through practical ligand evaluation. One of the Fe catalysts derived from a Fe complex of a phosphine-bipyridine ligand exhibited unprecedented activity for the hydrogenation of olefins, with turnover numbers up to 10 000 and turnover frequencies up to 37 740 h⁻¹. The NMR studies of the active Fe catalyst showed that a Fe-hydride species stabilized by Al might be a real catalyst.

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Introduction

Transition-metal-catalyzed hydrogenation of unsaturated compounds has been widely used for the production of value-added bulk and fine chemicals.¹ Homogeneous hydrogenation is generally carried out with catalysts derived from precious metals, such as Rh, Ru, Ir, and Os, some of which are highly toxic. The high cost, toxicity, and potential depletion of precious metals have forced chemists to develop green, sustainable catalysts for hydrogenation. Because Fe is abundant, cheap, and environmentally benign, Fe-catalyzed hydrogenation has recently drawn much attention.²

Substantial progress has been made in the Fe-catalyzed hydrogenation of highly polarized carbonyl compounds.³ However, progress in the Fe-catalyzed hydrogenation of olefins has been limited,² even though Fe carbonyl complexes were used to catalyze the hydrogenation of olefins as early as 1960s.⁴ Because replacing the CO ligand of Fe carbonyl complexes with other ligands is difficult, recent work on the Fe-catalyzed hydrogenation of olefins has focused on the development of Fe complexes with phosphine⁵ or nitrogen ligands,⁶ which can be used to tune catalyst reactivity and selectivity. A significant breakthrough was achieved in 2004 by Chirik and co-workers,⁶ who reported that Fe(N₂)₂-pyridinediimine complexes are active catalysts for the hydrogenation of olefins. In some cases, the turnover frequencies (TOFs) achieved with Chirik's Fe catalysts surpass those of well-established olefin hydrogenation catalysts, such as Wilkinson's catalyst and Pd/C.^6a

However, pre-prepared Fe complexes are extremely sensitive to both oxygen and moisture, which complicates their preparation and use in most synthetic chemistry laboratories. Moreover, the difficulties in preparing these highly sensitive catalysts have limited the development of new Fe catalysts by means of ligand evaluation, which is a widely used strategy for finding efficient catalysts. Several attempts to use Grignard reagents or organolithium reagents to reduce bench-stable Fe complexes to active Fe catalysts for olefin hydrogenation have been reported;⁷ unfortunately, Fe catalysts generated in situ are much less active than pre-prepared Fe catalysts.

We report here a new method for in situ generation of active Fe catalysts for the hydrogenation of olefins by the reduction of bench-stable Fe(II) complexes with LiAlH₄, which is inexpensive and readily available. This method makes the hydrogenation very easy to handle and enables us to develop several new Fe catalysts for olefin hydrogenation through practical ligand evaluation. One of the Fe catalysts derived from a Fe complex of a phosphine-bipyridine ligand (Fe-L8) exhibited unprecedented activity for the hydrogenation of olefins, with turnover numbers (TONs = moles of product per mole of catalyst) up to 10 000 and TOFs (TONs per hour) up to 37 740 h⁻¹. The preliminary investigations of the structure of the active catalyst were also performed.

Results and discussion

Initially, the hydrogenation was performed with styrene as a substrate under 30 atm of H₂ at ambient temperature in THF with a Fe catalyst generated in situ from a FeCl₂-L1 complex⁸ and various reductants (Table 1). Among the tested reductants, NaBEt₃H,⁹ LiAlH₄,¹⁰ and activated Mg¹¹ afforded complete conversion of the substrate (entries 1, 4, and 8). LiAlH₄ and activated Mg accomplished the hydrogenation with a high average TOF (6000 h⁻¹), which surpassed the TOFs obtained with the corresponding pre-prepared Fe complexes.^6a The hydrogenation was also promoted by ⁱPrMgCl, but the conversion was only 20% (entry 6). Considering its ready availability and ease of use, LiAlH₄ was chosen as the best reductant for in situ generation of Fe catalysts for olefin hydrogenation. LiAlH₄ cannot catalyze the hydrogenation in the absence of a Fe complex under the standard reaction conditions, which implies that the Fe is the real catalyst.

Table 1 Fe-catalyzed hydrogenation of styrene: evaluation of reductants^a

Entry	Reductant	Time	Conv.^b (%)
a Reaction conditions: styrene/FeCl2-L1/reductant = 5 : 0.005 : 0.025 (mmol), in 1 mL THF, 30 atm H2, rt. b Determined by GC using an interCap-1 column.
1	NaBEt3H	12 h	100
2	NaBH4	Overnight	Trace
3	NaH	Overnight	Trace
4	LiAlH4	10 min	100
5	BH3·THF	Overnight	Trace
6	^iPrMgCl	Overnight	20%
7	NH2NH2·H2O	Overnight	Trace
8	Mg	10 min	100
9	^iBu3Al	Overnight	Trace

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The above-described procedure for in situ generation of active Fe catalysts allows for ligand evaluation, and we tested various stable Fe complexes as catalyst precursors for the hydrogenation of olefins (Table 2). Fe complexes with bidentate sp² N ligands, such as bipyridine (L2), phenanthroline (L3), pyridine-imine (L4), and biimine (L5) exhibited excellent catalytic activity (entries 2–5). In contrast, N,N,N′,N′-tetramethylethylenediamine, which has sp³ N atoms as coordinating atoms, showed extremely low reactivity (entry 6). Phosphine ligands were generally less active than ligands with sp² N atoms, and Fe particles were generated after the reaction (entries 7–9). However, a tridentate phosphine ligand, L6, accomplished the reaction within 2 h (entry 10). Several pincer ligands containing pyridine and bipyridine moieties (L7–L9) were also evaluated (entries 11–13). Among these ligands, the PNN ligand (L8)¹² exhibited extremely high activity: the hydrogenation was completed within 5 min when 0.1 mol% catalyst was used (entry 12) and within 3 h when 0.01 mol% catalyst was used (entry 14). Furthermore, the Fe-L8 complex catalyzed the hydrogenation at relatively low hydrogen pressure (10 atm, entry 16). The hydrogenation reaction could also be carried out in other ether solvents, such as Et₂O, DME (1,2-dimethoxyethane), and 1,4-dioxane (entries 17–19). The TOFs at various conversion states were also determined (Table S1†). An even higher TOF (37 740) was observed at the beginning of the hydrogenation, which represents the highest TOF for Fe-catalyzed olefin hydrogenation to the best of our knowledge.

Table 2 Fe-catalyzed hydrogenation of styrene: evaluation of ligands^a

Entry	Ln	Solvent	Time	Conv. (%)
a The reaction conditions and analysis are the same as those in Table 1, entry 4 unless otherwise noted. The dppe is 1,2-bis(diphenylphosphino)ethane; the dppp is 1,3-bis(diphenylphosphino)propane. b PPh3/Fe = 2 : 1. c 0.01 mol% catalyst was used. d 0.25 mol% LiAlH4 was used. e The reaction was performed at 10 atm H2.
1	L1	THF	10 min	100
2	L2	THF	30 min	100
3	L3	THF	20 min	100
4	L4	THF	40 min	100
5	L5	THF	1 h	100
6	TMEDA	THF	12 h	3
7^b	PPh3	THF	24 h	36
8	dppe	THF	12 h	100
9	dppp	THF	12 h	83
10	L6	THF	2 h	100
11	L7	THF	1 h	100
12	L8	THF	5 min	100
13	L9	THF	1 h	100
14^c	L8	THF	3 h	100
15^d	L8	THF	10 min	100
16^e	L8	THF	10 min	100
17	L8	Et2O	5 min	100
18	L8	DME	5 min	100
19	L8	1,4-Dioxane	10 min	100

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Using the optimized reaction conditions, we investigated the substrate scope of the Fe-L8-catalyzed hydrogenation of olefins (Table 3). The steric bulk of the phenyl ring of the olefin substrate had little effect on the hydrogenation reaction (entries 2–5). However, the electronic nature of the substituents on the ring strongly influenced the reaction, with an electron-donating methoxy group giving a faster reaction rate than the electron-withdrawing Cl atom (compare entries 6 and 7). Notably, the TOF of 20 000 h⁻¹ was achieved in the hydrogenation of para-methoxy styrene (entry 6). In addition to styrenes, aliphatic terminal olefins could also be hydrogenated with high reaction rates (entries 8–11). However, substrates with a disubstituted olefin moiety or a heteroatom required higher reaction temperatures and longer reaction times, and the TOFs were lower (entries 12–17).

Table 3 Fe-L8-catalyzed hydrogenation of olefins^a

Entry	Substrate	Time	Conv. (%)	TOF (h^−1)
a The reaction conditions and analysis are the same as those in Table 2, entry 12 unless otherwise noted. b The data in parenthesis are the yield determined by the GC method with dodecane as an internal standard. c Reaction was performed at 50 °C. d Z/E = 1 : 4.
1		5 min	100 (>99)^b	12 000
2		10 min	100	6000
3		15 min	100	4000
4		15 min	100	4000
5		5 min	100	12 000
6		3 min	100	20 000
7		6 h	98	163
8		20 min	100	3000
9		30 min	95.8	1916
10		20 min	100	3000
11		15 min	100	4000
12^c		1 h	100	1000
13		1.5 h	100	667
14^c		2 h	78.1	391
15^c		1 h	40	400
16^c^,^d		5 h	80	160
17		6 h	100	167

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We investigated the in situ generated active Fe species by means of NMR spectroscopy (Fig. 1 and ESI†). A mixture of FeCl₂-L8 (0.005 mmol) and LiAlH₄ (0.025 mmol) in 1 mL THF-d₈ was stirred for 1 min, and the ¹H NMR spectrum of the resulting dark red solution was measured. We attribute the signal at −18.92 ppm (d, J = 20 Hz, 2H) to an Fe-hydride, on the basis of spectra in the literature.^6e,10 The other signals in the spectrum matched with those of the Fe(CO)₂-L8 complex.¹³ The hydride signal disappeared when the sample was exposed to D₂, which implies that a H–D exchange occurs. No hydride signals at < 0 ppm for LiAlH₄ were observed under identical conditions. The ³¹P NMR spectrum contained only one signal at 134.05 ppm. A broad Al signal at 99.3 ppm was also observed in ²⁷Al NMR. The NMR data clearly showed that an L8-Fe-(H)₂ species was generated when FeCl₂-L8 and LiAlH₄ were mixed and the Al may coordinate with the hydride in some way,^10c,d which stabilizes the Fe–H catalysts. The studies of the structure of the active Fe catalysts and the mechanism of the hydrogenation are still undergoing in our laboratory.

Fig. 1 The ¹H NMR studies of the in situ generated Fe-L8 catalyst.

The deuterium-labelling experiment clearly implies a significant redistribution of the hydrogen at C1 and C2 of the product during the hydrogenation (Scheme 1).

Scheme 1 A deuterium-labelling study.

Two catalytic cycles (a–b–c and a–d–e) of the hydrogenation were proposed based on the above observations and the literature (Scheme 2).^5,6 The reversible migratory insertion and β-H elimination (step a) might account for the redistribution of the hydrogen of the product.

Scheme 2 Proposed mechanism of the hydrogenation.

Conclusions

In summary, we have developed a new method for the in situ generation of an active Fe catalyst for the hydrogenation of olefins from bench-stable Fe(II) complexes and LiAlH₄, a readily available reductant. This strategy will undoubtedly be useful for developing suitable ligands for Fe-catalyzed olefin hydrogenation and other reactions. We are in the process of developing Fe-catalyzed asymmetric hydrogenation of olefins by using this new method in our laboratory.

Acknowledgements

We thank the National Natural Science Foundation of China, the National Basic Research Program of China (2011CB808600), the “111” project (B06005) of the Ministry of Education of China, and the National Program for Support of Top-notch Young Professionals for financial support. We thank Prof. Xun-Cheng Su for his help on NMR analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures; NMR characterization and GC analysis data of all hydrogenation products. See DOI: 10.1039/c5qo00064e

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