Synthesis and Catalytic Reactivity of Bis(amino)cyclopropenylidene

The first reported neutral boron(III) adducts of bis(amino)cyclopropenylidene carbenes (BAC carbenes) have been synthesized, structurally characterize...
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Synthesis and Catalytic Reactivity of Bis(amino)cyclopropenylidene Carbene−Borane Adducts Blake S. N. Huchenski,† Matt R. Adams,† Robert McDonald,‡ Michael J. Ferguson,‡ and Alexander W. H. Speed*,† †

Department of Chemistry, Dalhousie University, 6274 Coburg Road, P.O. Box 15000, Halifax, Nova Scotia, Canada B3H 4R2 X-ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2

‡

S Supporting Information *

ABSTRACT: The first reported neutral boron(III) adducts of bis(amino)cyclopropenylidene carbenes (BAC carbenes) have been synthesized, structurally characterized, and applied in homogeneous reaction chemistry. These air-stable adducts have been prepared by the combination of a BAC carbene lithium tetrafluoroborate adduct with borane dimethyl sulfide, boron trifluoride etherate, or dicyclohexylborane. A borenium cation derived in situ from the dicyclohexylborane adduct catalytically reduces unhindered benzyl ketimines, a class of substrate that cannot be reduced by prior borenium catalysts, at room temperature, employing 20 atm of hydrogen gas as the terminal reductant.

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alkylcarbene (CAAC)−boron trifluoride adduct 2a could be isolated, the corresponding borane adduct 2b was not isolable.2c Notwithstanding the above difficulties, changing the identity of the carbene has allowed development of enhanced reactivity; mesoionic carbene (MIC)−borane adducts such as 3 have been shown to be more potent reducing reagents than the corresponding NHC adducts.4 In addition to stoichiometric reactivity, carbene−borane adducts have recently been used to generate borenium cations that serve as the Lewis acid component in frustrated Lewis pair based hydrogenation catalysts.5 As an example, NHC-supported borenium cation 4 represents the most active NHC−borane-based hydrogenation catalyst, capable of reducing tert-butyl or phenyl ketimines and hindered enamines at 5 mol % loading under 102 atm of H2.6 In a further demonstration of the importance of exploring different carbene motifs, MIC-supported borenium cation 5 represents the most active borenium-based hydrogenation catalyst reported to date, capable of hydrogenating tert-butyl or phenyl imines and sterically encumbered nitrogencontaining heterocycles at pressures of hydrogen varying from 1 to 102 atm at 10 mol % loading.7 Bis(amino)cyclopropenylidene carbenes (BAC carbenes) represent an alternative and emerging carbene architecture. The first example of an isolated BAC carbene was disclosed by Bertrand and co-workers in 2006 as a highly air and moisture sensitive crystalline solid.8 Since that time, catalysts based on BAC carbenes have been used to promote Stetter and benzoin reactions9 and have been used as ligands in a nickel-catalyzed reductive vinylation.10 The aforementioned work in nickel-

dducts of carbenes and boranes have recently emerged as powerful reducing reagents. The most common type of adduct employs N-heterocyclic carbenes (NHCs): for example, adduct 1 (Figure 1).1 These neutral adducts are remarkably stable stoichiometric reductants,2 frequently being stable to silica gel chromatography,3 despite their capacity to reduce carbonyl and imine compounds and hydroborate olefins in the presence of activating reagents. Not all carbenes can form stable adducts with borane; for example, while the cyclic amino-

Figure 1. Various adducts of boron and carbenoids (DiPP = 2,6diisopropylphenyl). © XXXX American Chemical Society

Received: August 14, 2016

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DOI: 10.1021/acs.organomet.6b00654 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

In an attempt to develop a more general synthesis of BAC carbene−borane adducts, we added crystalline BAC carbene 10 to borane dimethyl sulfide or 9-BBN in ethereal solvents or toluene (Scheme 2). Each of these trials resulted in the

catalyzed reactions has shown that BAC carbenes possess reactivity characteristic of having a steric profile smaller than that of N-heterocyclic carbenes.10 We anticipated that this decreased steric demand would enhance reactivity in stoichiometric and catalytic reactions employing BAC carbene−borane adducts. Also, BAC carbene precursors are readily and inexpensively synthesized, being accessible from commercially available pentachlorocyclopropane in a single-pot operation on a multigram scale, unlike current lengthier routes to MIC precursors.4,11 This present work discloses the synthesis, stoichiometric carbonyl reduction, and catalytic imine reduction with BAC carbene−borane adducts 6 and 7. Disclosure of BAC carbene adducts with main-group elements preceded the isolation of the parent BAC carbene, and examples including tin, germanium, and lead,12 phosphorus,13 and arsenic14 adducts have been synthesized. Additional complexes of BAC carbenes and metals include copper, silver, rhodium, iridium, and palladium.15 To the best of our knowledge, BAC carbenes have not been employed in the formation of adducts with neutral boranes.16 Our initial attempt to synthesize adduct 6 involved heating BAC carbene precursor 8a at reflux in toluene for 20 h with sodium borohydride (Scheme 1). While we undertook this

Scheme 2. Synthesis and Structures of BAC Carbene−BH3 (6) and −BF3 (12) Adductsa

Scheme 1. Synthesis and Structure of BAC Carbene− Triphenylborane Adduct 9a

a

The thermal ellipsoids are scaled at the 30% probability level. Selected interatomic distances (Å): 6, C1−B1 1.608(5); 12, C1−B1 1.641(3).

formation of intractable tacky red mixtures exhibiting complex H and 11B NMR spectra. Despite these initial setbacks, our attention turned to the use of the BAC carbene−lithium tetrafluoroborate adduct 11 (the Weiss−Yoshida reagent).19 This reagent was crystallized and characterized by Bertrand and co-workers as a polymeric material consisting of a BAC carbene−lithium adduct with a 5:4 lithium tetrafluoroborate to BAC carbenenoid stoichiometry. In our hands, deprotonation of 8b with butyllithium in diethyl ether, in the presence of 0.25 equiv of lithium tetrafluoroborate, followed by concentration and pentane trituration under a dry nitrogen atmosphere gives a 93% yield of a free-flowing white powder (11) that was used in subsequent reactions without additional purification. Mixture of this reagent with borane dimethyl sulfide or boron trifluoride etherate complexes in toluene, followed by evaporation and extraction of the resulting residue with dichloromethane, resulted in clean formation of the desired adducts 6 and 12. Adducts 6 and 12 can be stored indefinitely in air without decomposition (>4 months); however, some decomposition of 6 is noted with silica gel chromatography. The structures of 6 and 12 were confirmed on the basis of NMR spectroscopy and single-crystal X-ray diffraction data. Compound 6 features a C−B interatomic distance of 1.608(5) Å, while compound 12 features a C−B interatomic distance of 1.641(3) Å. A quartet is observed at −35.1 ppm with 1JBH = 86 Hz in the 11B NMR spectrum of 6, and a quartet is also observed in the 11B NMR spectrum of 12 at −0.4 ppm, with 1 JBF = 34 Hz. These metrics are comparable to those of previously reported carbene−borane adducts.4,20 1

a

The thermal ellipsoids are scaled at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å); C1−B1 1.6394(15), C1−C3 1.3871(15), C2−C3 1.3892(14), N1−C2 1.3267(14).

operation with the hope of directly synthesizing target BAC− borane adduct 6 with concomitant loss of hydrogen and sodium tetraphenylborate, in analogy to known chemistry with imidazolium iodides,17 we instead observed clean formation of a crystalline, air-stable product exhibiting a broad singlet at −9.4 ppm in the proton-coupled 11B NMR spectrum. Integration of the phenyl and isopropyl signals in the 1H NMR spectrum suggested the formation of 9, which was verified on the basis of a single-crystal X-ray crystallography experiment (Scheme 1). Compound 9 features a boron to carbon interatomic distance of 1.6394(15) Å. This is similar to the 1.666(3) Å boron to carbon interatomic distance reported by Ong and co-workers for an NHC−triphenylboron adduct.18 The presence of an ionic additive proved necessary for the formation of 9, as 8a was stable for 24 h in refluxing toluene without any additional additive. Sodium tetrafluoroborate also promoted the formation of 9 from 8a. B

DOI: 10.1021/acs.organomet.6b00654 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics The ability of adduct 6 to serve as a source of hydride was investigated (Scheme 3). While no reaction occurred when 0.5

Scheme 4. Hydrogenations of Benzyl Imines Catalyzed by Borenium Cation 20

Scheme 3. Reduction of Carbonyl Compounds using 0.5 equiv of 6 in the Presence of Silica Gel

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Conditions: 0.5 equiv of 6, 0.2 M substrate in CH2Cl2, 15 mg of silica/mg of aldehyde. bNMR yield; all other yields are isolated yields.

equiv of 6 was mixed with 4-cyanobenzaldehyde (13a), addition of silica gel, a precedented activator for carbonyl reduction by NHC−boranes,2 resulted in clean reduction to alcohol 14a in only 1 h. Electron-rich anisaldehyde (13b) and 2-naphthaldehyde (15) were also reduced under the same conditions. A slightly higher yield of 2-naphthylmethanol (16) was attributed to decreased product volatility. Acetophenone (13c) proved a more challenging substrate, with only 60% conversion observed after 6 h. Cinnamaldehyde (17) was cleanly reduced in a 1,2fashion, with cinnamyl alcohol (18) as the only product; no 1,4-addition product was observed. These reaction times are faster than those reported for NHC−boranes2 and are comparable to those observed for MIC−borane 3.4 Our methodology was extended to a borane bearing both hydride and alkyl groups. Addition of dicyclohexylborane to reagent 11 in diethyl ether gave adduct 7 in 84% yield as an airstable beige solid that is unstable to silica gel chromatography.21 While we could not obtain a single crystal suitable for X-ray analysis, NMR and MS data corroborate the structure of 7 depicted in Scheme 4. As a diagnostic feature, a doublet is observed at −12.1 ppm with 1JBH = 71 Hz in the 11B NMR spectrum of 7. Combination of adduct 7 with trityl tetrakis[3,5(trifluoromethyl)phenyl]borate 19 in dry CDCl3 resulted in hydride transfer and formation of putative borenium cation 20, identified by the disappearance of the doublet at −12.1 ppm and the appearance of a broad signal at 81.0 ppm in the 11B NMR spectrum (a range comparable to that of adduct 4, 82.9 ppm).6 One equivalent of triphenylmethane also appeared in the 1H spectrum of the reaction. Since borenium cations 4 and 5 are hydrogenation catalysts, we explored the application of 7 to catalytic hydrogenation. A 10 mol % combination of 7 and 19 in trifluorotoluene was able to effect the hydrogenation of benzyl imines 21 and 23 and p-methoxybenzyl (PMB) imine 25 to the corresponding amines with high conversion under 20 atm of H2 at ambient temperature. The hydrogenation was conducted in a Parr bomb, with 99.999% purity grade hydrogen, which was otherwise used as received.22 Importantly, benzyl imines have not been substrates for previously reported borenium-based hydrogenation catalysts.23 Catalysts 4 and 5 cannot hydrogenate substrate 21 (0% conversion reported at

102 atm H2 pressure for 4, and “trace” conversion reported at 102 atm H2 for 5).6,7 Since the benzyl and PMB groups are some of the most commonly used protecting groups for amines, the work disclosed herein represents an advance in borenium-catalyzed hydrogenation reactions, enabled by the use of BAC carbene−borane complexes. In conclusion, we have developed a reliable and high-yielding synthesis of crystallographically characterized BAC carbene− borane adducts, where success was dependent on the use of lithium tetrafluoroborate adducts of BAC carbenes rather than the free carbene. We have shown BAC carbene−borane adducts are capable of stoichiometric reduction of carbonyl compounds and catalytic reductions of benzyl imines, a challenging class of substrate for catalytic reduction by borenium catalysts. Preparation of additional BAC carbene− borane adducts to increase reaction efficiency, expand substrate scope, and effect asymmetric imine hydrogenations is underway and will be reported in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00654. General considerations, solvents, reagents, crystallographic solution and refinement details, ORTEP drawings for compounds 6, 9, and 12, synthetic procedures, references, and NMR spectra of boron adducts and reduction products (PDF) Crystallographic data for 6 (CIF) Crystallographic data for 9 (CIF) Crystallographic data for 12 (CIF)

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AUTHOR INFORMATION

Corresponding Author

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

DOI: 10.1021/acs.organomet.6b00654 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Notes

was unstable to concentration or isolation and was not otherwise characterized. (22) The equipment we had access to was limited to a maximum pressure of 20 atm. Higher pressures of hydrogen gas will be explored in due course. Trifluorotoluene was an optimal solvent in ref 7. Dichloromethane provides inferior conversions in our chemistry (34% for 21). (23) A single example of hydrogenation of the benzyl imine of benzophenone, employing a MIC carbene supported borenium cation, with 99% conversion at 102 atm was reported in the Supporting Information of ref 7. Presumably the steric bulk of this substrate prevents product inhibition, as the same catalyst provided only trace hydrogenation of less bulky 21 under the same conditions.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support in the form of start-up funding from Dalhousie University is gratefully acknowledged. Dr. Mike Lumsden and Mr. Xiao Feng are thanked for assistance with NMR spectroscopy and mass spectrometry, respectively.

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00654 Organometallics XXXX, XXX, XXX−XXX