Communication pubs.acs.org/Organometallics
Mesoionic Carbene−Boranes Luiza Baptista de Oliveira Freitas,†,∥ Patrick Eisenberger,‡,∥ and Cathleen M. Crudden*,‡,§ †
Departamento de Quı ́mica, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 31270-901, Brazil Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada § Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya, Japan ‡
S Supporting Information *
ABSTRACT: Mesoionic carbenes (MICs), derived from alkylation and deprotonation of triazoles, are shown to form stable complexes with BH3. The synthesis of triazoles via Huigsen cycloadditions provides considerable structural diversity. Several routes to MIC−boranes are described, and their structures have been characterized by X-ray crystallography. As predicted on the basis of the increased σ-donor capacity of MICs by comparison to NHCs, the MIC−borane adducts are more reactive reducing agents.
diminished B−H bond stability. Together, this should result in a more robust and reactive neutral hydride equivalent.32−34 Moreover, the remarkable versatility of synthetic routes to the 1,2,3-triazole core21 provides considerable opportunities for the rapid generation of diverse libraries of MIC−boranes 1. Prior to the pioneering work of Curran and co-workers describing the use of NHC·BH3 complexes 2 as reagents in organic synthesis,35−40 such complexes were sporadically described in the literature out of inherent interest in their structure,41−45 as tools to characterize the structure and/or reactivity of carbenes and carbene complexes,46−49 or as precursors to carbenes themselves.50−52 The Stephan group has also recently reported the use of NHC-stabilized boranes as precursors to catalytically active borenium salts.30,31 Despite the rich chemistry of Lewis base borane adducts, it is somewhat remarkable that no examples of MIC−borane complexes have been reported in the literature thus far.35 Difficulties in the isolation of free 1,2,3-triazol-5-ylidines may complicate the use of these species in the generation of borane adducts, in comparison with the case for NHC derivatives.19,53 In particular, less bulky Nalkyl-substituted 1,2,3-triazol-5-ylidenes are known to undergo a variety of unwanted decomposition reactions in the free state.19,53 We thus reasoned that a one-pot protocol avoiding isolated MICs would be desirable for the efficient synthesis of MIC−boranes 1. Herein we describe the facile and high-yielding synthesis of alkyltriazolium- and aryltriazolium-based carbene− borane complexes 1a−f. Structural data for compounds 1a−c,f are presented, along with preliminary reactivity data.
N-heterocyclic carbenes have become staple ligands in organometallic chemistry, transition-metal catalysis, and organocatalysis1−7 and are making inroads in bioinorganic8 and materials chemistry.9−15 The stabilization of the singlet state by flanking heteroatoms leads to exceptionally stable carbenes that can be crystallized, distilled, and stored for extended periods of time without decomposition.16−18 Recently, N-heterocyclic carbenes (NHCs) flanked by only one stabilizing heteroatom have gained attention due to their increased σ-donor properties.19−25 In particular, pioneering work by Albrecht and coworkers illustrated the facility with which 1,3,4-trisubstituted carbenes can be prepared from the 1,2,3-triazole core, which is itself prepared by a simple and modular Huisgen cycloaddition.21,26,27 The utility of these ligands in a wide range of metal-catalyzed reactions has been reported, and their unique properties relative to typical NHCs are particularly noteworthy.19,21,28,29 In the context of employing MICs and NHCs outside the realm of transition-metal chemistry, we became interested in synthesizing previously unknown MIC−borane adducts 1 (Figure 1). Related NHC−boranes 2 have been employed as precursors to stabilized borenium ions.30,31 We anticipate that the increased σ-donor ability of the MIC as an ancillary ligand should result in increased C−B bond strength and in turn
Special Issue: Applications of Electrophilic Main Group Organometallic Molecules Figure 1. Mesoionic triazolylidene boranes (1) and imidazolylidene Nheterocyclic carbene boranes (2). © 2013 American Chemical Society
Received: September 19, 2013 Published: October 29, 2013 6635
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Synthesis and Characterization of MIC−Boranes. Alkyland aryl-substituted 1,2,3-triazoles 3a−e were prepared by the well established CuI-catalyzed Huisgen cyclization54 of terminal alkynes with alkyl or aryl azides generated in situ from alkyl halides55 or synthesized from amines.56,57 Alternatively, the basecatalyzed reaction of terminal alkynes with organic azides can be employed for the preparation of 1,5-disubstituted 1,2,3-triazoles such as 3f (Scheme 1).58
Scheme 2. Synthesis of MIC−Boranes 1a−f
Scheme 1. Synthesis of 1,2,3-Triazolium Salts 4a−f
reaction had to be carried out at low temperature under rigorous exclusion of moisture to avoid rapid decomposition of the free MIC or other competing side reactions. The presence of adventitious water results in diminished product yields due to benzylic C−N bond scission in 3 via generated hydroxide ion. Moreover, the migration of backbone N-alkyl groups to the carbene carbon center in an SN2 fashion, resulting in selfcannibalization of the carbene, has been reported for similar, even more sterically hindered triazolylidenes, albeit at slightly elevated temperatures (50 °C).19 In the event, the sequential addition of base followed by borane (after 20 min) at −78 °C and then gradual warming of the reaction mixture gave the desired MIC·BH3 complexes 1 in good yield with a minimal degree of side product formation. Pure borane adducts 1a−f can be isolated after evaporating the solvent and subjecting the crude mixture to column chromatography. For those complexes that show decomposition on silica gel (for instance, 1a), filtration through a plug of Celite followed by recrystallization was effective. The slightly decreased yield of less sterically protected 1e may be rationalized by decomposition of the MIC in solution prior to coordination to BH3. The11B NMR chemical shifts of 1a−f fall in the typical range reported for four-coordinate carbene−BH3 complexes. For instance, the boron center in MIC−borane 1a resonates as a quartet at δB −36.1 with a coupling constant of 1JB−H = 85.9 Hz, comparing well to δB −35.13 (q, 1JB−H = 86.2 Hz) for the related 2-borane-l,3-diethyl-4,5-dimethylimidazole.41 The MIC·BH3 proton resonance in 1a appears as a broad quartet shifted upfield to δH 1.22 (br q, 1JB,H = 86.5 Hz) in comparison to BH3· Me2S (δH 1.50 (br q, 1JB,H = 104 Hz)).64 This observation is rationalized by increased shielding due to coordination of the electron-rich MIC to boron. A similar upfield shift is observed for the resonance of the nitrogen-bound methyl group in 1a (δH 3.97) in comparison to 4a-BF4 (δH 4.19). Notably, in all cases, the 13C resonances of the carbene carbon atom of MIC−boranes 1a−f could not be observed in the 13C NMR spectra, due to quadrupolar broadening resulting from coordination to the boron atom. All of these NMR data are consistent with the formulation of the structure as a carbene-stabilized BH3 unit (1). To unambiguously elucidate the connectivity, single crystals of 1a−c,f were obtained by slow diffusion of hexanes into a solution of the MIC−borane in CH2Cl2. Their structures, as determined by X-ray crystallography, are shown in Figure 2. The observed C(5)−B bond distances (see Figure 2) are comparable to those in reported NHC·BH3 compounds (for instance: 2-borane-l,3-dimethylimidazole, 1.600(4) Å;65 2borane-l,3-diethyl-4,5-dimethylimidazole, 1.603(3) Å;41 (2phenyl-2,5,6,7-tetrahydropyrrolo[2,1-c][1,2,4]triazol-4-ium-3yl)trihydroborate, 1.611(3) Å).17 The N(1)−C(5)−C(4) bond angle of 102.03(12)° in 1a is decreased in comparison to
Alkylation using an excess of Meerwein’s salt (MeO3+BF4−; Scheme 1, conditions A) at ambient temperature resulted in the exclusive preparation of the 3-methylated 1,2,3-triazolium tetrafluoroborate salts in 80−90% yield after flash column chromatography.59 This method proved to be general and applicable to a variety of 1,2,3-triazoles (4a-BF4, 4b-BF4, 4dBF4). However, the limited commercial availability of Meerweintype reagents restricts this method to the introduction of methyl (or potentially ethyl) substituents. Less expensive and more general alkylation protocols could also be employed that relied on readily available, bench-stable reagents such as MeOTs and alkyl halides (MeI or 2-iodopropane; Scheme 1, conditions B). In our hands, the reaction of 3a with 2.8 equiv of MeI in MeCN required slightly forcing conditions to provide high isolated yields, namely heating the reaction mixture to 80 °C in a closed ACE pressure tube, which gave the desired product in good isolated yield (84%).60 With the exception of (S)-4c-I, which gave only moderate yields, these conditions worked well for all substrates examined. Since aryl substituents cannot be installed via SN2 chemistry, the triphenyltriazolium salt 4f was prepared in 90% isolated yield following the method of You, Gao, and co-workers (Scheme 1, conditions C).61 This strategy is complementary to existing methods for the synthesis of 1,3,4-triaryl-1,2,3-triazol-5-ylidenes, reported by the laboratories of Grubbs and Bertrand,53 based on 1,3-diaryltriazenes as synthetic intermediates,62 which results in a symmetrical 1,3-substitution pattern. With a variety of triazolium salts 4 in hand, we set out to reveal the corresponding triazolylidenes by deprotonation at the C(5) position with KHMDS (potassium hexamethyldisilazide), followed by subsequent addition of a BH3 source, as shown in Scheme 2. The successful use of sterically demanding hexamethyldisilamide bases was previously demonstrated in the one-pot syntheses of NHC−boranes.63 Importantly, the 6636
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by 1a in the absence of additives. However, clean reduction occurred after the addition of 10 mol % of the Lewis acid Sc(OTf)3, with high conversion of the substrates obtained in 1−3 h at ambient temperature. The reactions were conducted without any special precautions in vials open to air and only required a catalytic amount of Sc(OTf)3 as Lewis acid to achieve high conversion. Similarly, the use of silica as an acidic promoter allowed the isolation of the corresponding benzylic alcohols in high yields after 1−3 h reaction time. When directly compared with 1,3dimethylimidazol-2-ylidene−borane, 1a gave the desired product in slightly higher yield (71% vs 62%) despite the significantly greater steric hindrance in the case of MIC 1a (See Supporting Information). Furthermore, unreacted 1a could be conveniently recovered from the reaction mixture under either of the reaction conditions by eluting the silica gel with dichloromethane after product isolation. Since the MIC−borane 1a itself was recovered intact and not as a complex of the reduced carbonyl (entries 1−3 and 5−7), this implies that the initially formed MIC− benzyloxyborane, resulting from the reduction of the first C− O double bond, is a more reactive reducing agent than the parent MIC−borane 1a itself. As shown in entries 4 and 8, quantitative reduction was observed when a 1:3 ratio of borane to aldehyde was employed; therefore, all three hydrides are available for reduction. To test the versatility of this species as a reducing agent in the presence of trityl salts, the hydroboration of allylbenzene was also attempted36 and proceeded smoothly at ambient temperature to give exclusively the linear product after oxidation to the corresponding alcohol (see Supporting Information). Conclusions. In conclusion, we have developed a one-pot synthesis of bench-stable crystalline MIC−boranes 1 and demonstrated their use in the stoichiometric reduction of C−O double bonds in ketones and aldehydes and as catalysts for the hydroboration of allylbenzene via the intermediacy of borenium ions. Importantly, as predicted, MIC−borane 1a has increased hydricity in comparison to NHC−boranes in the reduction of ketones and aldehydes, which translates to greater reducing power despite greater steric hindrance. The synthesis of more complex MIC−boranes and their use in reduction chemistry and catalysis will be reported in due course.
Figure 2. ORTEP-3 representations of the X-ray single-crystal structures of 1a−c,f. The structures of 1b,c were refined without addressing absolute stereochemistry, and thus the representation as the S isomer is arbitrary. Ellipsoids are plotted at 50% occupancy, and nonheteroatom-bound hydrogen atoms are omitted for clarity. Nitrogen atoms are represented in blue, boron in pink, and carbon in gray. Bond distances (Å) and bond angles (deg) are given in the Supporting Information.
106.02(18)° in 4a-PF666 with concomitant elongated bond distances for N(1)−C(5) and C(4)−C(5) in comparison to the salt 4a-PF6 (N(1)−C(5) = 1.350(2) Å, C(4)−C(5) = 1.367(2) Å). Similar values for these bond angles and distances are observed for the other three solid-state structures. These trends for the geometry of the carbene carbon are typical for NHC and related MIC coordination compounds of transition metals and related NHC−boranes.16,21,35,66 The diminished bond angles about the carbene carbon reflect an increase in s character for the lone pair orbital typically observed for other singlet carbenes.20 We next examined the activity of the new MIC−borane complex 1a in the reduction of electrophilic C−O double bonds in 4-bromoacetophenone and 4-bromobenzaldehyde as depicted in Table 1, employing a methodology inspired by Lindsay and coworkers.63 Consistent with reports on the reducing capacity of NHC−boranes,37 neither ketones nor aldehydes were reduced
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entry
R
1a:substrate
catalyst
time/h
yielda/%
recovered 1a/%
1 2 3 4 5 6 7 8
Me H H H Me H H H
1:1 1:1 1:2 1:3 1:1 1:1 1:2 1:3
Sc(OTf)3 Sc(OTf)3 Sc(OTf)3 Sc(OTf)3 silica silica silica silica
3 1 3 3 3 3 3 1
93 (89) 99 (89) 83 (80) 98 (94) 98 (85) 100 (91) 100 (91) 90 (88)
(53) (49) (33) (0) (47) (45) (26) (0)
ASSOCIATED CONTENT
S Supporting Information *
Table 1. Sc(OTf)3- and Silica-Catalyzed Reduction of 4Bromoacetophenone and 4-Bromobenzaldehyde using MIC− Borane 1a
Text, figures, a table, and CIF files giving full experimental details and spectroscopic and characterization data for new compounds and crystallographic data for 1a−c,f. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*
[email protected]. Author Contributions ∥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged in terms of operating, equipment, accelerator, and strategic grants to C.M.C. The Canada Foundation for Innovation (CFI)
a
Yield calculated by NMR using hexamethylbenzene as internal standard; isolated yields given in parentheses. 6637
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is thanked for support in terms of a Leader’s Opportunity Fund (LOF) Grant to C.M.C. L.B.d.O.F. thanks the Conselho ́ Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq) for a scholarship. Jiasheng Lu is thanked for the single-crystal studies. Eric C. Keske is thanked for helpful discussions and suggestions.
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ABBREVIATIONS MIC, mesoionic carbene; NHC, N-heterocyclic carbene REFERENCES
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