Communication pubs.acs.org/Organometallics
Trisubstituted Boroles by 1,1-Carboboration Fang Ge, Gerald Kehr, Constantin G. Daniliuc, Christian Mück-Lichtenfeld, and Gerhard Erker* Organisch-Chemisches Institut, Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany S Supporting Information *
ABSTRACT: A 1,3,4-trisubstituted borole is readily obtained by a 1,1-carboboration sequence starting from a bis(ethynyl)borane and B(C6F5)3. Subsequent photolysis converts it to the 1,2,5-trisubstituted borole by means of a di-π-borane rearrangement.
T
he formally antiaromatic boroles1,2 and their derivatives are finding increasing use in materials chemistry.3,4 Calculation on the elusive parent C4BH5 attributes to it a singlet ground state with a marked HOMO/LUMO separation.3,5 Isolated monomeric boroles usually contain a set of five stabilizing substituents at carbon and boron and/or a stabilizing Lewis base attached at the Lewis acidic boron center.6−8 The closest approach to CHcontaining boroles is due to Herberich’s work on the cycloC4H4BR dianions and the transition-metal complexes derived thereof (see Scheme 1).9 The neutral C4H4BR borole generated
The starting material 6 was prepared by treatment of diisopropylamido dichloroborane with ca. 2 mol equiv of ethynylmagnesium bromide (isolated in 88% yield). It shows a typical 11B NMR signal at δ 21.1 and the 1H NMR ethynyl resonance at δ 3.00 (13C: δ 89.7, 95.5 [B]−CC−H). Compound 6 was characterized by X-ray diffraction (d(CC) = 1.183(3) Å) (see the Supporting Information for details). We then reacted 6 with B(C6F5)3 in a 1:1 molar ratio in benzene solution. The reaction was complete within a few minutes. It gave a 44:56 mixture of the 1,1-carboboration product Z-7 and the trisubstituted borole 9 (see Scheme 2). Keeping the mixture for 1 day at −36 °C precipitated compound Z-7, which was isolated in 33% yield (see the Supporting Information for its characterization). It was characterized by C, H elemental analysis and by NMR spectroscopy. The borole 9 was spectroscopically
Scheme 1
Scheme 2
in situ by treatment of the dianion with e.g. SnCl 2 instantaneously dimerized to the [4 + 2] cyclo-dimer.10 Braunschweig et al. reported the NHC-stabilized BH borole derivative 5, which was obtained by protonation of the anion 4.11 We have now prepared and isolated boroles that contain only three substituents and consequently feature two CH units at the five-membered ring using a sequence of 1,1-carboboration reactions,12,13 and we have found some interesting subsequent chemistry. © XXXX American Chemical Society
Received: August 3, 2015
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DOI: 10.1021/acs.organomet.5b00668 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
We wanted to see whether the 1,1-carboboration product Z-7, which apparently featured the wrong stereochemistry for subsequent conversion to the borole 9, could be isomerized in situ. Heating of a ca. 1:1 mixture of Z-7 and 9 in C6D6 at 40 °C for 7.5 h left compound Z-7 untouched but resulted in dimerization of the reactive borole 9 to give the [4 + 2] cycloaddition product 11 (characterized by X-ray diffraction; see the Supporting Information for further details). Since compound Z-7 was not isomerized thermally, we tried photolysis.14 This proved successful. We assume that actually the borole 9 was formed from Z-7 via E-7 and 8, but the borole 9 turned out to become cleanly isomerized to the new trisubstituted borole 13 photochemically. Consequently, photolysis of a solution of the Z-7 + 9 mixture in C6D6 for 24 h resulted in a quantitative conversion to 13. Compound 13 was not isolated but characterized directly (see below) by spectroscopy. It shows the 1H/13C NMR signals of the vicinal CH pair of the trisubstituted borole framework at δ 7.06/6.81 (3J(H,H) = 2.3 Hz) and δ 159.7/152.3, respectively. The pair of 11B NMR signals was observed at δ 58.3 (B(C6F5)2) and δ 40.2, respectively, and there are two sets of 19F NMR signals corresponding to the single C6F5 group and the B(C6F5)2 substituent. Compound 13 reacted cleanly with dimethyl acetylenedicarboxylate in the usual borole cycloaddition/rearrangement sequence16 to yield the respective borepin derivative (15, see the Supporting Information for its detailed characterization, including an X-ray crystal structure analysis). The borole 13 selectively added donor ligands to the B(C6F5)2 Lewis acid functional group. With n-butyl isocyanide it gave the adduct 16 (11B NMR δ 40.2, δ −18.3; 1H NMR δ 6.56/6.11 (3J(H,H) = 2.4 Hz), borole CH; 13C NMR δ 153.5/151.2, borole CH; δ 127.6 (NC−)). Treatment of a solution of the CH-borole 13 with pyridine in dichloromethane solution gave the B(C6F5)2 pyridine adduct 14, which was isolated in 60% yield. It was characterized by X-ray diffraction (single crystals were obtained from a toluene/pentane mixture at −36 °C). The X-ray crystal structure analysis showed the rearranged borole framework with the bulky B(C6F5)2 substituent attached at the α-position C1 and the C6F5 group at the opposite α-position C4 (see Figure 2). The borole boron atom bears the NiPr2 substituent and features a short B1− N1 bond (1.386(5) Å). The distal ring carbon atoms C2 and C3 each bear H substituents. The borole framework of compound 14 shows a typical carbon−carbon bond alternation (C1−C2 1.348(5) Å, C2−C3 1.465(5) Å, C3−C4 1.328(5) Å; B1−C1 1.627(6) Å, B1−C4 1.614(6) Å, angle C1−B1−C4 103.4(3)°). The free boroles 9 and 13 are both less deeply colored than many higher substituted boroles.6−8,13 Their electronic spectra and their structural features were analyzed by DFT calculations (for details see the Supporting Information). This indicated that the photochemical equilibrium upon HPK 125/Pyrex filter irradiation was lying far on the side of the 1,2,5-trisubstituted borole isomer 13. We assume a 9 → 13 isomerization pathway that is similar to a di-π-methane rearrangement.17 Inside the borole nucleus such a photochemical “di-π-borane” rearrangement would result in a series of formal 1,3-boron migration steps on the bora-housen surface (see Scheme 3). Ring opening at the step of the isomer 12C would then directly lead to the observed 1,2,5-borole isomer 13. Similar rearrangements had previously been observed in a related B(C6F5)2-substituted silole series.18 The sequential 1,1-carboboration reaction had been shown to be useful for the synthesis of a variety of interesting heterocycles.12,13,15 Our study shows that it is a powerful tool for even
characterized from the mixture, and it was positively identified by derivatization and a subsequent reaction (see below). 1,1-Carboboration reactions are regioselective but very often stereounselective.12 Therefore, we assume from the obtained Z7:9 ratio (ca. 44:56) that the reaction of 6 with B(C6F5)3 initially gives a mixture of the isomers Z-7 and E-7 in a similar ratio. The Z-7 product has the remaining [B]−CC−H acetylide functionally stereochemically shielded from a consecutive reaction with the newly introduced −B(C6F5)2 Lewis acid. Consequently, it remains untouched under our typical conditions. In contrast, the E-7 isomer is ideally set for a second 1,1-carboboration sequence, probably initiated by (reversible) alkynyl shift between the boron atoms followed by an internal 1,1-alkenylboration step15 to directly give the observed borole 9 (see Scheme 2). The CH-borole 9 is yellow. It shows two 11B NMR resonances in C6D6 at δ 58.5 (the broad B(C6F5)2 signal is overlapping with that of Z-7) and δ 38.0 (BN), 1H NMR signals of the borole CH units at δ 7.26 (1-H) and δ 6.42 (4-H), respectively, with corresponding 13C NMR signals at δ 157.3 (C1) and δ 134.7 (C4; C2, δ 167.3; C3, δ 147.5). There is one set of 19F NMR signals of the transferred C6F5 substituent and a second set (of double intensity) of the residual B(C6F5)2 moiety. We have observed two sets of isopropyl 1H/13C NMR signals of the iPr2N substituent at boron, due to the partial iPr2NB double bond character of this unit. We then treated the solution of 9, which was obtained after separation of Z-7, with pyridine. It gave immediately a colorless solution from which the crystalline pyridine adduct 10 was isolated in 20% yield (see Scheme 2). The X-ray crystal structure analysis (see Figure 1) showed that pyridine had added to the
Figure 1. View of the molecular structure of the CH-borole derivative 10. Thermal ellipsoids are shown with 30% probability.
external B(C6F5)2 Lewis acid and had the borole boron atom left untouched. In the adduct we find the B1−C1 (1.570(5) Å) and B1−C4 (1.576(6) Å) bond lengths in the B−C σ-bond range. The borole carbon framework shows bond alternation (C1−C2 1.352(4) Å, C2−C3 1.530(4) Å, C3−C4 1.341(4) Å). The internal C1−B1−C4 angle amounts to 102.8(3)°, and the N1− B1 bond is short at 1.473(7) Å. In solution compound 10 shows the typical pyridine NMR signals. The 11BN resonance is at δ 38.8, and the B(C6F5)2 11B NMR resonance was shifted to δ −2.2 due to the pyridine coordination. Compound 10 features the 1H NMR resonances of the CH-borole core at δ 5.84 (1-H) and 5.82 (4-H), respectively (13C NMR: δ 134.7 (C1) and δ 134.9 (C4)). B
DOI: 10.1021/acs.organomet.5b00668 Organometallics XXXX, XXX, XXX−XXX
Organometallics
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Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail for G.E.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Figure 2. Molecular structure of the CH-borole derivative 14. Thermal ellipsoids are shown with 30% probability.
Scheme 3
making boroles with a new substituent pattern. The 1,3,4- and 1,2,5-trisubstituted −B(C6F5)2 functionalized CH-boroles 9 and 13 are both thermally stable, although they are, of course, reactive molecules that undergo a variety of selective reactions. We expect that our new synthetic entry to boroles with less than the usual pentasubstitution pattern will prove useful in the development of borole chemistry and its subsequent utilization.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00668. Experimental and analytical details and crystallographic data (PDF) Crystallographic data (CIF) C
DOI: 10.1021/acs.organomet.5b00668 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.5b00668 Organometallics XXXX, XXX, XXX−XXX
