Bis(amino)cyclopropenium Trifluoroborates: Synthesis, Hydrolytic

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Bis(amino)cyclopropenium Trifluoroborates: Synthesis, Hydrolytic Stability Studies, and DFT Insights Roya Mir, and Travis Dudding J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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The Journal of Organic Chemistry

Bis(amino)cyclopropenium Trifluoroborates: Synthesis, Hydrolytic Stability Studies, and DFT Insights Roya Mir and Travis Dudding* Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON L2S 3A1

ABSTRACT: A simple and direct two-step synthesis of bis(amino)cyclopropenium trifluoroborate (BAC–BF3) derivatives from readily available reagents is reported. Hydrolysis studies revealed these BAC–BF3 derivatives were remarkably stable towards defluorination. Notably, this first study of BAC–BF3 adduct hydrolytic stability establishes the compounds reported herein possess half-lives (t1/2) exceeding 0.23×106 min (~160 days). Density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) calculations exploring the basis of this high stability are described. KEYWORDS: Cyclopropenium trifluoroborates, Hydrogen bonding, Hydrolytic stability, Organotrifluoroborates, Carbene-borane adducts, Density Functional Theory calculations

Introduction Over the past two decades, organotrifluoroborates have emerged as a versatile class of compounds finding widespread use in chemical, material and biomedical applications,1 despite once being mere chemical oddities. Indeed, with over six-hundred organotrifluoroborates commercially available today2 and the number climbing, it is certain these useful reagents have reshaped the landscape of chemical synthesis. Contributing to this utility has been their functional group compatibility, low toxicity, multipurpose reactivity, chemical stability, crystallinity and general ease of synthesis2 from boronic acids RBY2 (Y = OH, OR, NR2, halide, allyl) via the protocols of either Vedejs3 or Lennox and Lloyd-Jones.4 In terms of synthetic applications, organotrifluoroborates continue to find use in transition-metal-catalyzed C-C, C-N and C-O bond forming reactions (e.g., Suzuki-Miyaura cross-couplings),5 boronate ester deprotection,6 Lewis acid catalysis7 and crystallization-induced asymmetric transformations.8 Apart from versatility as synthetic tools, fluorine-18 (18F) labeled organotrifluoroborates, such as 18 F containing aryltrifluoroborates, hold promise as positron emission tomography (PET) imaging agents.9 Furthermore, several potassium organotrifluoroborates are known to be potent reversible competitive serine protease inhibitors with activities at least tenfold higher than

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boronates.10 The activity in these cases is speculated to result from the formation of hydrogen bonds between fluorine atoms and serine protease. Critical to these bio-relevant applications, however, is the element of stability, especially towards hydrolytic defluorination which can result in free fluoride producing unwanted background signals making 18F detection difficult. Towards improving the stability of organotrifluorborates several strategies have emerged over the years. For instance, Perrin et al. have shown electron-deficient aryltrifluoroborates (ArBF3) have reduced hydrolytic defluoridation rates as attested by the half-lives (t1/2) of 43 ± 4 min vs. 2 ± 0.04 min for 2,6-difluorophenyltrifluoroborate and phenyltrifluoroborate under identical conditions.11 In extending ArBF3 stability further, Li et al. reported neighboring onium ion stabilized trifluoroborates 1 and 2 with hydrolysis rate constants of 3.4 × 10-6 min-1 and 3.9 × 105 min-1, Figure 1.12 Furthermore, ammonium trifluoroborate 3 was reported to be an effective in vivo imaging agent with a measured hydrolysis rate constant of 3.1 × 10-5 min-1.13 Meanwhile, the recent advancement of N-heterocyclic carbene boron trifluoride (NHC–BF3) adducts such as 4 marked an important turn of events in the quest for hydrolytically stable organotrifluoroborates.14

Figure 1. Examples of hydrolytically stable organotrifluorborate compounds. In reflecting upon the above-mentioned, state-of-the-art, we were intrigued by the prospect of advancing a new class of stable organotrifluoroborates sharing advantageous parallels with ArBF3−, o-(PR3+)ArBF3− and NHC‒BF3 derivatives. In this spirit, we report the synthesis and first stability studies of bis(amino)cyclopropenium trifluoroborate (BAC‒BF3) adducts 5a-c that as delineated, vide infra, have remarkable stabilities toward hydrolysis, thus, making them attractive targets for future bio-relevant, synthetic, and material science applications (Figure 2).

Figure 2. Bis(amino)cyclopropenium trifluoroborate (BAC‒BF3) adducts 5a-c. Results and discussions As outlined in Scheme 1, the synthesis of BAC–BF3 adducts 5a-c commenced with the addition of the respective N,N’-di-alkyl/aryl substituted amine (7a-c) to a solution of readily available tetrachlorocyclopropene15 6 (Scheme 1). After stirring for 4 hours, NaBF4 (1 equiv) was added resulting in counterion exchange and the reaction mixtures then treated with PPh3/H2O to afford bis(amino)cyclopropenium (BAC)–H•BF4– products 8a-c in 88-90% yield. Deprotonation of 8a-


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c to generate carbene−potassium tetrafluoroborate intermediates that were subsequently quenched in situ with BF3•2Et2O then provided the targeted BAC–BF3 products 5a-c in 75-82% yield.16 Confirming the presence of a BF3 group in 5a-c was a quartet in the 19F NMR spectrum (-137.7 ppm, JF–B = 33.3 Hz; -138.5 ppm, JF–B = 34.8 Hz; -137.5 ppm, JF–B = 33.1 Hz) and 11B NMR spectrum (-0.4, JB–F = 35.1 Hz; -0.4 ppm, JB–F = 35.8 Hz; -0.4 ppm, JB–F = 35.5 Hz) of these adducts, respectively. Notably, the latter 11B signals were in line with typical boronium species (δ = 0-15 ppm).17 The X-ray structure of 5b was obtained from a single-crystal prepared by vapor diffusion of ethyl acetate into an acetonitrile solution containing the parent compound (see Supporting Information (SI)).

Scheme 1. Synthesis of BAC–BF3 adducts 5a-c. Having 5a-c in hand, their hydrolysis was probed. Thus, to our knowledge establishing the first reported study of the hydrolytic resistance of BAC–BF3 adducts. To this end, solvolytic loss of fluoride ion from solutions of 5a-c in D2O/CD3CN or D2O/THF (8/2 vol) at pH 7.5 ([phosphate buffer] = 192 mM, [BAC–BF3] = 10 mM) was monitored by 19F NMR spectroscopy. From these NMR experiments the loss of fluoride was found to be first-order in BAC–BF3 (ν = kobs[BAC– BF3]) and most remarkably, even after six months, there was no measurable decomposition of 5a and 5b. Conversely, compound 5c was less stable towards hydrolysis as visible from trace amounts of free fluoride after only one day. Moreover, monitoring the decomposition of BAC– BF3 5c over an extended time (~5 days) revealed the slow disappearance of a 19F NMR signal at -137.5 ppm concurrent with the appearance of a peak at –151.8 ppm corresponding to free fluoride (see SI Table 1). From this data the rate of hydrolysis (kobs) of 5c was determined to be 3×10-6 min-1 based on linear regression of a plot of -ln ([BAC–BF3]/[total]) versus time (min) with a half-life (t1/2) of 0.23×106 min (Figure 3). Subsequently, the hydrolysis of BAC–BF3 derivative 5b was investigated under more forcing conditions to facilitate decomposition. To this end, a solution of 5b (10 mM) in D2O/CH3CN (8/2 vol) was heated to 100 °C and monitored by 19F NMR. Emerging from this experiment was the appearance of a new singlet peak at -130.1 ppm after ~ 6 h, which continued to grow over 48 h. The onset of a quartet signal (J = 15.75 Hz) at -143.5 ppm after ~ 12 h was also observed, though in this case the relative integrated area of this peak increased to a limit and then remained constant. Concentration of the reaction mixture after 48 h and inspection of the resulting residue by 11B NMR was additionally instructive as it revealed the presence of two boron peaks at 19 and 0.1 ppm (See SI for more details). Taken together the formation of at least two new free fluoride and boron species is inferred from these observations.



0.025 -ln ([BF3]/ [F–]+[BF3])

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

kobs= 3×10-6 min-1 R² = 0.962 t1/2 = 0.23×106 min

0.02 0.015 0.01 0.005 0 0

2000

4000

6000

8000

Time (min)

Figure 3. The linear regression of a plot of -ln ([BAC–BF3]/[total]) versus time (min) of 5c. Having experimental insight, we next turned to density functional theory (DFT) B3LYP/6-311+ G(d,p) calculations to shed light on the structural and electronic facets of 5a-c. The selection of this level of theory based on the close structural resemblance of the X-ray crystallography resolved structure of 5b and in silico optimized geometry of this compound (see SI for additional details). Similar in the computed lowest energy conformers of 5a-c were boron to carbon interatomic bond distances of ~1.65 Å, deriving, in part, from σ-type bonding orbitals visible in the HOMO-6 of 5a vice versa the HOMO-2 in the case of 5b and 5c. These C–B bond metrics, notably, comparable to those of several reported NHC–BH3 adducts.18 Meanwhile, quantum theory of atoms in molecules (QTAIM) calculations revealed positive total electronic energies (Hbcp = 0.25, 0.25, 0.25) and small Laplacian values (∇ଶ ρbcp = -0.008, -0.009, -0.01) at the (3, -1) bond critical points of the C–B bonds of 5a-c consistent with weak covalent bond character. A classification further corroborated by computed Wiberg bond orders of ~0.74.

Figure 4. HOMO-6 of 5a and HOMO-2 of 5b and 5c computed at the B3LYP/6-311+G(d,p) level of theory.


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A dominant feature of 5a-c also was steric shielding of the BF3 groups by the N-alkyl/aryl substituents as clearly seen from the highlighted short H•••F contacts (distance = 2.30 – 2.49 Å) and non-covalent interaction (NCI) plots in Figures 5 and 6. Presumably, this steric shielding rendered the fluoride lone pairs less available for solvent-assisted intermolecular H-bonding leading to solvolysis and the formation of free solution-phase fluoride anion.

Figure 5. Newman projections of computed (B3LYP/6-311+G(d,p)) 5a-c (atom coloring: C = grey, N = blue, F = green, B = pink).

5a

5b

5c

Figure 6. Computed (B3LYP/6-311+G(d,p)) non-covalent interaction (NCI) plots of 5a-c. The relative orientations of the BF3 groups and cyclopropenium rings of 5a-c provided another aspect of note. Namely value θ, corresponding to the angle defined by the plane of the cyclopropenium ring (---) and B–F1 bond depicted in Figure 5. In this regard, 5c possessed a value θ distinct from that of 5a,b, viz. 91.9o versus 105.5o and 120.2o, respectively. The former



angle of 91.9o being almost ideal for delocalization of cyclopropenium ring π-density into a B–F1 anti-bonding orbital, assuming restricted C-B bond rotation; a scenario promoting borenium cation formation and therefrom decomposition. Nevertheless, C-B bond rotation is likely rapid at room temperature so to gain a deeper understanding of the hydrolytic defluorination mechanism of 5a-c, B–F bond cleavage transition states TS1-3 were computed at the B3LYP-D3/6311++G(2d,2p)/(IEFPCM, water)//B3LYP/6-31G(d)/(IEFPCM, water) level of theory (see SI for details). To provide a computationally tractable, yet probable reaction mechanism, the following simplifications and/or assumptions were made; (1) loss of the first fluorine atom was rate limiting and irreversible, (2) two explicit waters and an implicit water solvent sphere were used for these calculations, and (3) the potential role of phosphate buffer was not considered.19Key aspects of transition states TS-1-3 included bond breaking B···F and bond making O···B distances of ~2.7 – 2.9 Å, with Gibbs free energies of activation (∆G≠) of 46.3, 43.7, and 39.5 kcal mol−1, respectively (Figure 7). Present also were bifurcated H-bonding manifolds featuring two waters engaged in H-bond assisted loss of a basic fluoride anion nucleofuge from the boron atom. The F···H-O distances of these H-bond manifolds ~1.5 – 1.6 Å. Less obvious, though of ostensible importance was the impact of boron sp3 to sp2-rehybridization in transitioning from the ground state BAC‒BF3 adducts to TS-1-3. Accompanying this change from tetracoordinate to trigonal-planar configuration at boron was the elimination of several unfavorable van der Waals contacts originating from steric clashing of the trifluoroborate group and bulky N,N’-dialkyl/aryl substituents. That said, it is probable the greater flexibility of the cyclohexyl rings of 5c vice versa the N,N’-di-alkyl/aryl substituents of 5a,b is one contributor to the observed differing stabilities of these BAC–BF3 adducts.

Figure 7. Activation free energies in kcal mol−1 for the hydrolytic defluorination transition states TS-1, TS-2, and TS-3 arising from bis(amino)cyclopropenium trifluoroborate (BAC‒BF3) adducts 5a-c.

In summary, the first hydrolysis study of bis(amino)cyclopropenium trifluoroborate (BAC–BF3) derivatives easily prepared from readily available reagents by a one-pot synthesis was reported. These compounds were found to be remarkably stable with certain derivatives having half-lives (t1/2) in excess of 0.23×106 min (~160 days) towards hydrolytic defluorination. The basis of this


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high stability was investigated using density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) calculations. Clear from these findings is the promise of using cyclopropeniums as a structural platform for designing and synthesizing novel hydrolytically stable organotrifluoroborate compounds. Accordingly, the advancement and applications (e.g., potential biomedical applications) of cyclopropenium trifluoroborates as well as further mechanistic studies related to their hydrolysis are ongoing in our laboratory. Experimental Section General information Materials were obtained from commercial suppliers and were used without further purification. All reactions were performed under an inert atmosphere. Reactions were monitored by thin layer chromatography (TLC) using TLC silica gel 60 F254. Flash column chromatography was performed over Silicycle ultrapure silica gel (230-400 mesh). NMR spectra were obtained with a 300 MHz or 400 MHz spectrometer (1H 300 MHz/ 1H 400 MHz, 13C 75.5 MHz/13C 150.9 MHz, 19 F 376.6 MHz /19F 282.4 MHz, 11B 128.4 MHz /11B 92.3 MHz) in CDCl3 or CD3CN. The chemical shifts are reported as δ values (ppm) relative to tetramethylsilane. Mass analyzer double focusing sector used for the HRMS measurements. General procedure for synthesis of bis(amino)cyclopropenium (BAC)–H•BF4 8a-c. To a solution of tetrachlorocyclopropene (0.7g, 3.9 mmol) in CH2Cl2 (10 mL) at 0 °C was added dropwise N-benzyl-tert-butylamine (7a 7a) 7a (3.8 g, 23.4 mmol) which afterward was stirred for four hours while warming to room temperature. NaBF4 (0.4 g, 3.9 mmol) was then added and the solution stirred overnight. Subsequently, a solution of triphenylphosphine (1 g, 3.9 mmol) in CH2Cl2 (5 mL) was added followed by addition of distilled water (10 mL). The suspension was then stirred at room temperature overnight, after which time the organic layer was separated and washed with water (3 ×10 mL). The resulting solution was dried over MgSO4, concentrated using reduced pressure rotary evaporation and dried under high vacuum. The resulting solid was then used in the next step without further purification. (BAC)– (BAC)–H•BF4–(8b):20 White solid, (1.1 g, 88%), 1H NMR (300 MHz, CDCl3): δ 7.45 (s, 1 H), 4.05 (sep, 2 H), 3.87 (sep, 2 H), 1.43 (d, J = 6.8 Hz, 12 H), 1.40 (d, J = 6.8 Hz, 12 H). (BAC)– (BAC)–H•BF4–(8c):21 White solid, (1.7 g, 90%), 1H NMR (300 MHz, CDCl3): δ 7.53 (s, 1 H), 3.583.48 (m, 2 H), 3.40-3.29 (m, 2 H), 1.96-1.15 (m, 40 H);

13

C NMR (300 MHz, CDCl3): δ 134.3,

100.5, 64.8, 58.3, 31.2, 30.8, 25.7, 25.6, 24.7, 24.6. General procedure for synthesis of bis(amino)cyclopropenium (BAC)–BF3 5a-c. To (BAC)– H•BF4 (8b 8b) 8b (0.35 g, 1.1 mmol) in THF (5 mL) at -78 °C was added a 0.5 M KHMDS solution in toluene (2.2 mL,1.1 mmol). Boron trifluoride diethyl etherate (0.13 mL, 1.1 mmol) was then added dropwise. The reaction mixture was warmed to room temperature and stirred for an



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additional 30 min, thereafter it was filtered and washed with ethyl acetate to obtain 5b as a white solid (250 mg, 0.82 mmol, 75% yield). MP. 203-208 °C; 1H NMR (400 MHz, CDCl3): δ 3.89 (sep, 4 H), 1.44 (d, J = 6.8, 4 H), 1.29 (d, J = 6.8, 4 H); 138.5, 54.1, 49.3, 21.2, 20.5.

11

13

C NMR (300 MHz, CDCl3): δ

B NMR (96.3 MHz, CDCl3): δ -0.4 (qt, J = 35.8 Hz);

19

F NMR

(282.4 MHz, CDCl3): δ -138.5 (qt, J = 35.1 HZ). HRMS (FAB) ) m/z: [M + Na]+ Calcd for C15H28 N2BF3Na was 327.2189; Found: 327.2194. Comment: For synthesis of the compounds 5a and 5c a solution of n-BuLi (1.5 M in hexanes) was used instead of a solution of KHMDS (0.5 M in toluene). (BAC)– (BAC)–BF3 5a: White solid. MP. (386 mg, 82% yield). 202-206 °C; 1H NMR (300 MHz, CDCl3): δ 7.35-7.25 (m, 6 H), 6.91 (d, J = 6.72 Hz, 4 H), 4.30 (s, 4 H), 1.51 (m, 18 H);

13

C NMR (300

11

MHz, CDCl3): δ 143.3, 136.8, 129.1, 127.7, 125.1, 60.3, 53.4, 28.0. B NMR (96.3 MHz, CDCl3): 19

δ -0.3 (qt, J = 34.9 Hz);

F NMR (282.4 MHz, CDCl3): δ -137.7 (qt, J = 35.7 Hz). HRMS (EI) )

+

m/z: [M - F] Calcd for C25H32 N2BF2 was 409.2621; Found: 409.2619. (BAC)– (BAC)–BF3 5c: White solid. (398 mg, 78% yield). MP. 195-200 °C; 1H NMR (300 MHz, CDCl3): δ 3.50-3.30 (m, 4 H), 2.21-2.11 (m, 4 H), 4.30 (s, 4 H), 1.93-1.70 (m, 14 H), 1.57 (d, J = 12 Hz, 11 13

H), 1.47-1.11 (m, 11 H); 26.0, 25.7, 24.9, 24.2.

C NMR (300 MHz, CDCl3): δ 139.1, 139.0, 63.3, 57.8, 31.47, 30.1,

11

B NMR (96.3 MHz, CDCl3): δ -0.4 (qt, J = 40.0 Hz);

19

F NMR (282.4

+

MHz, CDCl3): δ -137.5 (qt, J = 31.1 HZ). HRMS (EI) m/z: [M - F] Calcd for C27H44 N2BF2 was 445.3560; Found: 445.3568. ASSOCIATED CONTENT Supporting Information Spectroscopic data for all starting compounds, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT The authors thank Sharcnet for computing resources. Financial support was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discover Grant (201404410). REFERENCES 1

Darses, S.; Genet, J.-P. Chem. Rev. 2008, 108, 288. Molander, G. A. J. Org. Chem. 2015, 80, 7837. 3 (a) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf, M. R. J. Org. Chem. 1995, 60, 3020.; (b) Vedejs, E.; Fields, S. C.; Hayashi, R.; Hitchcock, S. R.; Powell, D. R.; Schrimpf, M. 2


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R. J. Am. Chem. Soc. 1999, 121 ,2460.; (c) Darses, S.; Michaud, G.; Genet, J.-P. Eur. J. Org. Chem. 1999, 1875. 4 (a) Molander, G. A.; Beaumard, F. Sci. Synth., Knowl. Updates 2013, 3, 101. (b) Lennox, A. J. J.; Lloyd-Jones, G. C. Angew. Chem., Int. Ed. 2012, 51, 9385. 5 (a) Han, F.-S., Chem, Soc, Rev., 2013, 42, 5270. (b) Molander, G. A.; Bernardi, C. R. J. Org. Chem. 2002, 67, 8424. 6 Matteson, D. S.; Kim, G. Y. Org. Lett. 2002, 4, 2153 7 (a) Coghlan, S. W.; Giles, R. L.; Howard, J. A. K; Patrick, L. G. F.; Probert, M. R.; Smith, G. E.; Whiting, A. J. Organomet. Chem. 2005, 690, 4784. (b) Giles, R. L.; Howard, J. A. K.; Patrick, L. G. F.; Probert, M. R.; Smith, G. E.; Whiting, A. J. Organomet. Chem. 2003, 680, 257. 8 (a) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf, M. R. J. Org. Chem. 1995, 60, 3020. (b) Vedejs, E.; Fields, S. C.; Hayashi, R.; Hitchcock, S. R.; Powell, D. R.; Schrimpf, M. R. J. Am. Chem. Soc. 1999, 121, 2460. (c) Vedejs, E.; Fields, S. C.; Schrimpf, M. R. J. Am. Chem. Soc. 1993, 115, 11612. 9 a) Li, Y.; Guo, J.; Tang, S.; Lang, L.; Chen, X.; Perrin, D. M. Am. J. Nucl. Med. Mol. Imaging 2013, 3, 44. b) Ting, R.; Harwig, C. W.; auf dem Keller, U.; McCormick, S.; Austin, P.; Overall, C. M.; Adam, M. J.; Ruth, T. J.; Perrin, D. M. J. Am. Chem. Soc. 2008, 130, 12045. 10 Smoum, R.; Rubinstein, A.; Srebnik, M. Org. Biomol. Chem. 2005, 3, 941. 11 Ting, R.; Harwig, C. W.; Lo, J.; Li, Y.; Adam, M. J.; Ruth T. J.; Perrin, D. M. J. Org. Chem., 2008, 73, 4662. 12 Li, Z.; Chansaenpak, K.; Liu, S.; Wade, C. R.; Zhao, H.; Conti P. S.; Gabbaї, F. P. Med. Chem. Commun., 2012, 3, 1305. 13 Liu, Z.; Pourghiasian, M.; Radtke, M. A.; Lau, J.; Pan, J.; Dias, G. M.; Yapp, D.; Lin, K.-S.; Bénard, F.; Perrin, D. M. Angew. Chem., Int. Ed., 2014, 53, 11876. 14 Chansaenpak, K.; Wang, M.; Wu, Z.; Zaman, R.; Li, Z.; Gabbaї, F. P. Chem. Commun., 2015, 51, 12439. 15 a) Tobey, S. W.; West, R. J. Am. Chem. Soc. 1966, 88, 2481. b) Tobey, S. W.; West, R. J. Am. Chem. Soc. 1966, 88, 2478. 16 During the preparation of this manuscript, Speed et al. reported the preparation of adduct 5b in two steps starting from (BAC)–H•BF4–. Huchenski, B. S. N.; Adams, M. R.; McDonald, R.; Ferguson, M. J.; Speed, A. W. H. Organometallics 2016, 35, 3101. 17 Piers, W. E.; Bourke, S. C.; Conroy, K. D. Angew. Chem. Int. Ed. 2005, 44, 5016. 18 de Oliveira Freitas, L. B.; Eisenberger, P.; Crudden, C. M. Organometallics 2013, 32, 6635. 19 Studies are underway in our lab exploring the impact of these variables on the hydrolysis of BAC‒BF3 adducts. 20 Lavallo, V.; Ishida, Y.; Donnadieu, B.; Bertrand, G. Angew. Chem. Int. Ed. 2006, 45, 6652. 21 Kuchenbeiser, G.; Soleilhavoup, M.; Donnadieu, B.; Bertrand, G. Chem. Asian J. 2009, 4, 1745.


Bis(amino)cyclopropenium Trifluoroborates: Synthesis, Hydrolytic

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