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2H-1,2-Thiaborin: A New Boron Sulfur Heterocycle Ahleah D. Rohr, Mark M. Banaszak Holl, Jeff W. Kampf, and Arthur J. Ashe, III* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
bS Supporting Information ABSTRACT: The reaction of 2,5-dihydro-2-diisopropylamino-1,2thiaborole (5) with 2 equiv of LDA followed by CH2Cl2 gives 2-(diisopropylamino)-2H-1,2-thiaborin (2c), which has been spectroscopically and structurally characterized. DFT calculations indicate that formally aromatic 2c has a limited π-delocalized bonding in its heterocyclic ring.
The six-π-electron boron nitrogen heterocycle 1,2-dihydro1,2-azaborine (1)1 has attracted considerable recent attention, due to interest in its aromaticity2 4 and its potential uses in electronic devices5,6 and in hydrogen storage.7 The analogous boron sulfur heterocycle 2H-1,2-thiaborin (2) should be similarly interesting. Prior work on 2H-1,2-thiaborins has been confined to the ring-fused derivatives 3 reported in the 1960s.8 Although 3b was said to display an aromatic UV spectrum, little other data are available. Clearly a synthesis of a minimally substituted derivative of the parent ring system would be highly desirable. We report here on the synthesis, spectroscopic and structural characterization, and DFT calculations of 2-(diisopropylamino)-2H-1,2-thiaborin (2c).
The synthesis of 2c involves an extension of the carbenoid ring-expansion route we have previously used to prepare 1b2b,c and the analogous boron oxygen heterocycle 4b.9 The treatment of the readily available 2,5-dihydro-2-(diisopropylamino)1,2-thiaborole 510 with 2 equiv of LDA in THF followed by reaction with excess methylene chloride gave a 15% yield of 2c.11 The 2H-1,2-thiaborin 2c was distilled from the reaction mixture and isolated as a pale yellow oil, which solidified to nicely formed blocklike crystals: mp 21 °C. When the reaction was performed using methylene chloride-d2, the deuterium was exclusively at the 3-position. The reaction is consistent with the in situ formation of chlorocarbene, which adds to C(3) of 1,2-thiaborolide 6 r 2011 American Chemical Society
(path a; Scheme 1). Subsequent ring expansion followed by loss of chloride affords 2c, as illustrated in Scheme 1. It had been previously found that the C(5) position of 6 is more nucleophilic than C(3).10 We speculate that any chlorocarbene attack at C(5) (path b) does not lead to low-molecular-weight products but is responsible for the relatively modest yield of 2c. 2-(Diisopropylamino)-2H-1,2-thiaborin has been characterized by 1H, 11B, and 13C NMR spectroscopy, high-resolution mass spectroscopy, UV absorption spectroscopy, and X-ray diffraction. All spectroscopic data are consistent with the assigned structure. Slow recrystallization from the melt gave crystals of 2c suitable for X-ray diffraction.12 The molecular structure, illustrated in Figure 1, shows a planar ((0.015(2) Å) 2H-1,2-thiaborine ring. Although partial disorder between S(1) and C(1) limits the accuracy of the bond distances, the structure is consistent with diene π-bonding in the ring, in which the C C double bonds are shorter than the C C single bond. The short distance (1.401(7) Å) between boron and the sp2-hybridized nitrogen indicates exocyclic B N π bonding, which is independently shown by the rotational barrier about the B N bond. Slow rotation about the B N bond makes the i-Pr groups nonequivalent in both the 1H and 13C NMR spectra recorded at 0 °C. When 2c is heated in CDCl3 to 50 °C, the methine signals in the 13C NMR spectrum (δ 49.26, 45.76) coalesce, indicating a barrier to rotation about the B N bond of ΔG* = 14.2 ( 0.5 kcal/mol. To some extent this exocyclic πbonding is likely to diminish the endocyclic π-bonding in the ring.13 The 1H, 11B, and 13C NMR chemical shift values of 1b and 2c are compared in Figure 2. The 1H NMR spectrum of 2c in CDCl3 shows a characteristic first-order pattern which strongly resembles those shown by 1,2-dihydro-1,2-azaborines 1. The chemical Received: May 13, 2011 Published: June 21, 2011 3698
dx.doi.org/10.1021/om200400j | Organometallics 2011, 30, 3698–3700
Organometallics
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Scheme 1
Table 1. NICS(1) Values and Stabilization Energy Data (kcal/mol) for Benzene, 1a, and 2a,c entry
benzenea
1aa
2ab
2cb
NICS(1) ΔH298(reacn 1)
10.4
7.3 25.7
6.8 23.9
3.6 23.3
ΔH298(reacn 2)
4.7
10.9
15.6
ΔH298(reacn 3)
24.4
26.4
23.5
ΔH298(reacn 4)
6.1
8.4
15.5
20 ( 2
16 ( 3
8 ( 0.2
RSE a
Figure 1. Solid-state structure of 2c (ORTEP). Thermal ellipsoids are set at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected mean distances from the four nonequivalent molecules in the unit cell (Å): B(1) S(1), 1.844(2); B(1) C(1), 1.601(34); C(1) C(2), 1.350(9); C(2) C(3), 1.411(3); C(3) C(4), 1.335(5); C(4) S(1), 1.707(6); B(1) N(1), 1.401(7). For comparison, selected bond distances (Å) computed using DFT (B3LYP/TZVP): B(1) S(1), 1.859; B(1) C(1), 1.541; C(1) C(2), 1.358; C(2) C(3), 1.432; C(3) C(4), 1.348; C(4) S(1), 1.728; 1.541; B(1) N(1), 1.423.
Figure 2. Comparison of the 1H NMR, 13C NMR (in parentheses), and 11 B NMR (arrows) chemical shift values of 1b and 2c in THF-d8. Note that the 1H NMR signals of 1b in chloroform overlap.
shift values of 2c are at higher field than those of 1b. However, both sets of compounds show signals in the “aromatic region” consistent with a diamagnetic ring current for both. The 11B and 13 C NMR chemical shifts of the two compounds are also quite similar, which is consistent with similar electronic structures for the two families of heterocycles.
34.1
Reference 3b. b This work.
The UV absorption spectrum of 2c in hexane displays a lowenergy band at 326 nm which shows marked vibronic fine structure. For comparison the lowest energy maximum of 1a is at 269 nm,3b while that of 1b is at 287 nm.2c Since these bands are clearly sensitive to the substituent at boron, precise comparison with 2c is tenuous. However, the significant red shift of 2c relative to derivatives of 1 is consistent with a smaller HOMO/ LUMO gap. The experimental data on 2 are supported by calculations.14 The bond distances of 2c have been calculated at the density functional theory (DFT) B3LYP/TZVP level (see Figure 1 and the Supporting Information). The calculated bond distances for corresponding atoms agree with the mean crystallographic values from the four independent molecules of 2c in the unit cell within an average value of (0.03 Å. Considering the partial disorder in the crystal, this is a good level of agreement. The 1H, 11B, and 13C NMR chemical shifts of 2c have been calculated at the B3LYP/ TZVP level. Again, agreement with the experimental values is satisfactory. The HOMO/LUMO gap for 1a has been calculated by Dixon, Liu, and co-workers to be 5.32 eV.3b We calculate the HOMO/LUMO gap to be 4.98 eV for 2a and 4.30 eV for 2c. The smaller gap is consistent with the observed red shift in the UV spectrum of 2c vs 1a. Our calculations also allow evaluation of the aromatic character of 2, for which we have no direct experimental evidence.15 Table 1 shows calculated magnetic and energy data for benzene, 1a, and 2a,c. The nucleus independent chemical shift (NICS(1)) values have become important magnetic criteria of aromaticity.16 The NICS(1) values of 1a and 2a are similar, suggesting comparable levels of aromaticity which are about 30% less than that shown by benzene. On the other hand, the NICS(1) value of 2c is significantly smaller, indicating that the B N(i-Pr)2 substituent diminishes the aromaticity of the thiaborin ring system. 3699
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Organometallics
Figure 3. Dehydrogenation reactions of 1a and 2a,c.
The resonance stabilization energy (RSE) of 1a has been calculated by Dixon, Liu, and co-workers as 21 kcal/mol, which is approximately 13 kcal/mol less than that of benzene.3b In a similar manner using dehydogenation reactions 1 4 in Figure 3, the RSE of 2a is found to be 16 kcal/mol and 2c is found to be only 8 kcal/mol. Thus, the calculated energies suggest that aromaticity declines in the series benzene . 1a > 2a . 2c. In summary, we have prepared 2c, the first monocyclic 2H1,2-thiaborine, which has been structurally and spectroscopically characterized. However, DFT calculations indicate that the heterocyclic ring has only minimal aromatic character. The πinteraction of the exocyclic amino substituent with boron diminishes the endocyclic π-bonding expected for the parent ring. The synthesis of the parent system 2a remains an attractive goal for future work.
’ ASSOCIATED CONTENT
bS Supporting Information. Text, figures, tables, and a CIF file giving crystallographic data for 2c and a summary of the DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) For reviews of early work, see: (a) Fritsch, A. J. Chem. Heterocycl. Compd. 1977, 30, 381. (b) Ander, I. In Comprehensive Heterocyclic Chemistry; Katritzky, A. R., Rees, C. W., Eds.; Pergamon: Oxford, U.K., 1984; Vol. I, p 629. (2) (a) Ashe, A. J., III; Fang, X. D. Org. Lett. 2000, 2, 2089. (b) Ashe, A. J., III; Fang, X. D.; Fang, X. G.; Kampf, J. W. Organometallics 2001, 20, 5413. (c) Pan, J.; Kampf, J. W.; Ashe, A. J., III Organometallics 2006, 25, 197. (d) Pan, J.; Kampf, J. W.; Ashe, A. J., III Org. Lett. 2007, 9, 679. (3) (a) Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250. (b) Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. Angew. Chem., Int. Ed. 2009, 48, 973. (c) Tanjaroon, C.; Daly, A.; Marwitz, A. J. V.; Liu, S.-Y.; Kukolich, S. J. Chem. Phys. 2009, 131, 224312. (d) Daly, A. M.; Tanjaroon, C.; Marwitz, A. J. V.; Liu, S.-Y.; Kukolich, S. G. J. Am. Chem. Soc. 2010,
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132, 5501. (e) Campbell, P. G.; Abbey, E. R.; Neiner, D.; Grant, D. J.; Dixon, D. A.; Liu, S.-Y. J. Am. Chem. Soc. 2010, 132, 18048. (4) Carion, R.; Liegeois, V.; Champagne, B.; Bonifazi, D.; Pelloni, S.; Lazzeretti, P. J. Phys. Chem. Lett. 2010, 1, 1563. (5) (a) Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Angew. Chem., Int. Ed. 2007, 46, 4940. (b) Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2008, 86, 8. (6) Kwong, R. C.; Ma, B.; Tsai, J.-Y.; Beers, S.; Barron, E.; Kottas, G.; Dyatkin, A. B. PCT Int. Appl. WO 2010135519, 2010; Chem. Abstr. 2010, 153, 643680. (7) Campbell, P. G.; Zakharov, L. N.; Grant, D. J.; Dixon, D. A.; Liu, S.-Y. J. Am. Chem. Soc. 2010, 132, 3289. (8) (a) Davis, F. A.; Dewar, M. J. S. J. Am. Chem. Soc. 1968, 90, 3511. (b) Grisdale, P. J.; Williams, J. L. R. J. Org. Chem. 1969, 34, 1675. (9) Chen, J.; Bajko, Z.; Kampf, J. W.; Ashe, A. J., III Organometallics 2007, 26, 1563. (10) Ashe, A . J., III; Fang, X. D.; Kampf, J. W. Organometallics 2000, 19, 4935. (11) Experimental procedure and characterization data of 2c: a solution of 5 (1.0 g, 5.5 mmol) in THF (3.0 mL) was added dropwise with stirring to a solution of LDA (11.0 mmol) in THF (20 mL) at 78 °C. The mixture was warmed slowly to 25 °C for 3 h, during which time the color darkened to red-brown. The mixture was then cooled to 78 °C, and methylene chloride (6 mL) was added dropwise. After 45 min the mixture was warmed to 25 °C for 12 h. Volatiles were removed under vacuum, and the residue was extracted with pentane (20 mL). After the extract was decanted from the insoluble material, the solvent was removed under vacuum, leaving a red-brown tar which was distilled pot-to-pot at 50 60 °C (0.05 Torr) to give 168 mg (16%) of product as an oil which solidified to pale yellow blocklike crystals: mp 21 °C. 1H NMR (500 MHz, THF-d8, T = 25 °C): δ 1.25 br d (12H); 3.75 br m (2H); 6.55 dd, J = 8.6, 6.6 Hz (1H); 6.60 d, J = 13.2 Hz (1H); 6.93 d, J = 9.4 Hz (1H); 7.14 dd, J = 13.0, 7.0 Hz (1H). 1H NMR (500 MHz, CDCl3, T = 0 °C): δ 1.25 br d (12H); 3.75 br d (2H); 6.61 dd, J = 8.9, 6.9 Hz (1H); 6.66 d, J = 13.2 Hz (1H); 6.96 d, J = 8.9 Hz (1H); 7.21 dd, J = 13.2, 6.9 Hz (1H). 11B NMR (160.4 MHz, CDCl3): δ 35.8. 13C NMR (160.4 MHz, CDCl3): T = 0 °C, δ 140.4, 127.8, 126 br, 121.5, 49.26 br, 45.76 br, 23.56 br 21.52 br; T = 50 °C, peaks >50 are the same, 47.5 br, 22.3 br. HRMS (EI, m/z): calcd for C10H1811BNS (M+) 195.1253, found 195.1251. UV (hexane; λmax, nm): 322. Anal. Calcd for C10H18BNS: C, 61.55; H, 9.30; N, 7.18. Found: C, 61.05; H, 9.81; N, 6.66. 2c-d: when the above reaction was performed using methylene chloride-d2, the isolated product had a deuterium at C(3), as shown by 1 H NMR: no signal at δ 6.60, δ 7.14 signal now br d (J = 6.6 Hz). HRMS (EI, m/z): calcd for C10H172H11BNS (M+) 196.1316, found 196.1317. (12) X-ray data for 2c: C10H18BNS, orthorhombic, Pca21, a = 26.1590(18) Å, b = 8.5982(2) Å, c = 20.5101(4) Å, V = 4613.1(3) Å3, Z = 16, Dc = 1.124 g cm 3, T = 85(2) K, λ(Cu KR) = 1.541 87 Å. Data were collected on a Rigaku AFC10K Saturn 944+ CCD-based instrument. Final R indices (I > 2σ(I)): R1 = 0.0502, wR2 = 0.1194. R indices (all data): R1 = 0.0608, wR2 = 0.1278. GOF on F2 = 1.076. (13) For other aromatic boron heterocycles see: (a) Herberich, G. E.; Schmidt, B.; Englert, U.; Wagner, T. Organometallics 1993, 12, 2891. (b) Herberich, G. E.; Schmidt, B.; Englert, U. Organometallics 1995, 14, 471. (c) Ashe, A. J., III; Kampf, J. W.; M€uller, C.; Schneider, M. Organometallics 1996, 15, 387. (d) Ashe, A. J., III; Klein, W.; Rousseau, R. Organometallics 1993, 12, 3225. (e) Ashe, A. J., III; Klein, W.; Rousseau, R. J. Organomet. Chem. 1994, 468, 21. (f) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; M€uller, C. J. Am. Chem. Soc. 1996, 118, 2291. (g) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; Kampf, J. W. Organometallics 1997, 16, 2492. (14) See the Supporting Information for computational details. (15) See: Ashe, A. J., III In Comprehensive Heterocyclic Chemistry III; Katritzky, A. R., Ramsden, C. A., Scriven, E. F. V., Taylor, R. J. K., Eds.; Elsevier: Oxford, U.K., 2008; Vol. 7, p 1040. (16) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842. 3700
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