Synthesis and reactivity of diborane(4) with CO and tBuNC

Synthesis of the unsymmetrical diborane(4) 2, its reactions and characterization of the resulting products are summarized in Fig. 1 (see also, Supplementary Figs 1–15, Supplementary Tables 1 and 2, and Supplementary Methods). Reaction of 1 with mesitylmagnesium bromide gave 2 in 49% yield. The 1H NMR spectrum of 2 showed C 2v symmetrical pattern of signals. Two broad 11B NMR signals were observed at δ B 34 and 89 p.p.m., indicating the selective conversion of one (pinacolato)boryl group to a dimesitylboryl group. Broadening of a 13C NMR signal at δ C 144.2 p.p.m. also confirmed the connection between the mesityl groups and a quadrupolar boron nucleus. X-ray crystallographic analysis of 2 revealed twisted orientation described by the dihedral angle of O1–B1–B2–C16 in contrast to the case of 1 (Supplementary Fig. 16)62. DFT calculation showed that the vacant p-orbital of the boron atom in the Bpin moiety slightly contribute to the LUMO of 2 (Supplementary Figs 17–18), which mainly consisted of the vacant p-orbital of the boron atom in the BMes 2 moiety, in spite of the twisted structure.

Figure 1: Synthesis and reactions of unsymmetrical diborane(4) 2. (a) Synthesis and reaction of 2 with CO to form 3 and their crystal structures. (b) Reaction of 2 towards tert-butylisonitrile to form 4–6 and crystal structures of the products (isolated and 1H NMR yield in parentheses, Mes=2,4,6-(CH 3 ) 3 C 6 H 2 , pinB=[(CH 3 ) 2 CO] 2 B): Selected bond lengths (Å), bond angles (°) and dihedral angles (°); 2: B1–B2=1.722(4); 3: B1–O1=1.366(3), B1–O2=1.356(3), B1–O3=1.374(3), O3–C7=1.406(3), C7–B2=1.459(4), B2–C8=1.492(4), C8–O4=1.144(3); 4: B1–N1=1.455(3), N1–B2=1.415(3), B2–C11=1.626(4), B2–C18=1.591(4); 5: B1–N1=1.413(3), N1–B2=1.495(3), B2–C7=1.456(3), B2–C8=1.569(3), C8–N2=1.152(3), B2–C8–N2=173.6(2); 6: B1–C1=1.516(7), C1–C2=1.353(6), C2–N1=1.217(5), C1–N2=1.450(5), B2–N2=1.419(6), B1–C1–C2=114.8(4), C1–C2–N1=174.5(5), C2–N1–C3=131.7(4). Full size image

The unsymmetrical diborane(4) 2 reacted with CO or tBuNC to give a variety of products (Fig. 1). A benzene solution of 2 was exposed to CO at room temperature for 30 min to give pale yellow solids of 3 in 20% isolated yield. X-ray crystallography for 3 showed incorporation of two CO molecules (it should be noted the complete assignment of atomic order in 3, 5 and 6 would be difficult due to small difference in electron density of the second period elements; see below and Fig. 2). The O3–C7 [1.406(3) Å], C7–B2 [1.459(4) Å] and B2–C8 [1.492(4) Å] bonds are shorter than the conventional single bonds, and C8–O4 [1.144(3) Å] is slightly longer than the C≡O bond of free CO molecule (1.1283 Å)63. These data proposed resonance structures of 3 and 3' with a characteristic conjugated O–C–B–C–O linkage giving a pale yellow colour (Supplementary Fig. 19 for ultraviolet–visible spectrum and Supplementary Tables 3 and 4 for time-dependent DFT calculation). Thus, compound 3 could be described as CO-coordinated alkoxyboraalkene. Reaction of 2 with one equivalent of tBuNC gave a colourless cyclized product 4 in 50% yield through scission of the isonitrile C≡N triple bond and of a C(sp3)–H bond in one of the mesityl substituents, as the molecular structure of 4 was confirmed by X-ray crystallographic analysis. The assignment of the B2 atom in the 2-boraindane skeleton could also be supported by the relatively long B2–C11 [1.626(4) Å] and B2–C18 [1.591(4) Å] bonds. In contrast, the reaction of 2 with an excess amount of tBuNC gave a mixture of tBuNC-coordinated boraalkene 5 and borylethenylideneamine 6. The reaction with two equivalents of tBuNC in a diluted solution gave 5 as the major product, while the reaction with a large excess amount of tBuNC in a concentrated solution afforded 6 as the major product. X-ray crystallographic analysis of 5 and 6 revealed that these two compounds have similar arrangement of all atoms, except the order of the two atoms in the central B=C or C–B bond and the terminal C–N–tBu angle (5, B2–C8–N2=173.6(2)°; 6, C2–N1–C3=131.7(4)°). To form 5, the C≡N triple bond in tBuNC, one B–B bond and two B–Mes bonds were cleaved from 2, while the two mesityl groups are still attached to the boron atom in 6. In the molecular structure of 5, the boron centre has B=C double-bond character (B2–C7=1.456(3) Å) and the second equivalent of tBuNC coordinates to the boron atom in the B=C moiety. In the case of 6, the tBuNC moiety has consecutive N=C and C=C double bonds (N1=C2=1.217(5) Å, C2=C1=1.353(6) Å) with a slightly short C–B single bond (C1–B1=1.516(7) Å). All the obtained crystal structures could be reproduced by DFT calculation to support the assignment of atomic order (see below).

Figure 2: Assignment of atomic order in 3–6 by 13C NMR experiments with 13C labelling. (a) Potential regioisomer 3(opp-B,C) derived from exchange of the positions of boron and carbon atoms in 3. (b) Reactions of 2 with 13C-labelled 13CO and tBuN13C to form the corresponding 13C-labelled 3-13C 2 , 4-13C, 5-13C 2 and 6-13C 2 . (c) Newly appeared 13C NMR signals of 3-13C 2 on 13C labelling. (d) The 4° aromatic signals of 4-13C with satellite on 13C labelling (e) enhancement of 13C NMR signal (top: 4-13C, bottom: 4). (f) The 4° aromatic signals of 5-13C 2 with satellite on 13C labelling (g) newly appeared 13C NMR signals of 5-13C 2 on 13C labelling (h). (i) Strengthened 13C NMR signals of 6-13C 2 with satellite on 13C labelling. Full size image

The NMR spectroscopic characterization of the products

The nuclear magnetic resonance (NMR) spectra of 3–5 were consistent with the crystallographically determined structures. The 1H NMR spectrum of 3 in C 6 D 6 showed two distinct Mes groups and one pinacol moiety. Two boron nuclei resonated at δ B –5 and 18 p.p.m., where the former signal could be assigned as the CO-coordinated boron atom due to the negatively charged boron atom in both the resonance structures 3 and 3′. The calculated 11B NMR chemical shift (δ B –4.1, 20 p.p.m.) of 3 by the Gauge-independent atomic orbital (GIAO) method at B3LYP/6-311++G(2d,p)//B3LYP/6-31+G(d,p) level was also in good agreement with the experimental data (Supplementary Table 5). Although the two Mes groups and the pinacol moiety could be assigned in the 13C NMR spectrum of 3, no signal corresponding to a B=C unit and a coordinating CO molecule was observed. The 1H NMR spectrum of 4 showed one benzylic methine proton (δ H 4.22) and two vicinally coupled methylene protons (δ H 2.87 and 2.93, 2J HH =21 Hz), supporting the scission of a C(sp3)–H bond in one of the two mesityl groups to give a chiral centre on C11. Reflecting the asymmetry below and above the 2-boraindane plane in 4, all the remaining five methyl groups on the mesityl substituents were separately observed and the four methyl groups on the pinacolato moiety resonated two singlet signals. Both the boron nuclei in 4 resonated at a typical region for an sp2 boron atom (δ B 27, 55 p.p.m.). The lower-field shifted signal could be assigned as the dicarbyl-substituted boron atom as supported by our GIAO calculations. Two relatively broadened 13C NMR signals at δ C 30.3 and 41.2 p.p.m., compared with other signals of 4, supported their connection to quadrupolar boron nucleus. The 1H NMR spectra of 5 and 6 similarly showed six methyl signals for the Mes groups, two methyl signals for the pinacol moiety, two tBu signals and four aromatic CH signals, because they are regioisomers with the same combination of the substituents and are close in symmetry of molecule. In the 13C NMR spectrum of 5, the number of observed signals was two short of the number of 13C nuclei expected from the symmetry of the 1H NMR spectrum. Similarly, one carbon signal was missing in the 13C NMR spectrum of 6. The missing of the 13C signals was probably due to broadening of quaternary carbon bonded to quadrupolar boron nucleus. The 11B NMR signals of 5 (δ B 13, 21 p.p.m.) and 6 (δ B 21, 63 p.p.m.) could also be assigned by our GIAO calculations.

The 13C-labelling study to determine the atomic order

In addition to the conventional NMR spectra of 3–5, 13C-labelling experiment could confirm the structures of 3–6 including connectivity between boron and carbon atoms (Fig. 2). Complete structural characterization of 3, 5 and 6 was difficult due to the following reasons: (1) in general, X-ray crystallographic analysis has difficulty to distinguish two adjacent atoms in the same row of the periodic table. This means that positions of carbon and boron atoms in 3 versus 3(opp-B,C) (Fig. 2a) and 5 versus 6 could not be unambiguously determined by crystallography. (2) Both 10B and 11B nuclei are quadrupolar to induce significant broadening of the signal for boron-bonded nuclei, leading in difficulty for observation of quaternary carbon bonded to boron nucleus. In this context, we performed 13C-labelling experiments for 3–6 to observe the 13C–13C coupling and broadened 13C NMR signals bonded to boron atom. The unsymmetrical diborane 2 reacted with 13C-labelled 13CO (99% 13C) or tBuN13C (20% 13C) gave 13C-labelled 3-13C 2 , 4-13C, 5-13C 2 and 6-13C 2 (Fig. 2b). On 13C-labelling of 3, two broad signals at δ C 197.1 and 201.8 p.p.m. appeared without apparent coupling in the 13C NMR spectrum of 3-13C 2 (Fig. 2c), indicating that these two carbon atoms connected to a quadrupolar boron atom with the C–B=C skeleton in 3 (not C–C=B in 3(opp-B,C)). As described above, two broadened signals δ C 30.3 and 41.2 p.p.m. may be assigned to the boron nucleus in 4. The lower-field shifted signal at δ C 41.2 p.p.m. was strengthened on 13C labelling to form 4-13C (Fig. 2e), indicating this benzylic methine carbon came from tBuN13C. Concomitantly, two signals of aromatic quaternary carbons at δ C 140.7 and 144.1 p.p.m. were accompanied with satellite signal with 1J CC of 40 Hz (Fig. 2d), similar to that (43 Hz) for the C(sp2)–C(sp3) linkage in strychnine64, indicating that the two ipso carbons of the two Mes groups bonded to the sp3 methine 13C are from tBuN13C. In the case of 5-13C 2 , two broad signals appeared at δ C 132.7 and 137.1 p.p.m. on labelling (Fig. 2g), supporting the C–B=C skeleton of 5. Two split 4° aromatic signals with 1J CC of 24 Hz in 5-13C 2 also showed that the two Mes groups bonds to a carbon atom (Fig. 2f). The 13C NMR spectrum of 6-13C 2 showed two strengthened signals at δ C 89.1 and 168.0 p.p.m. (Fig. 2h,i) with a satellite (1J CC =86 Hz), supporting the C(sp2)–C(sp) coupling (107 Hz in diphenylketene-13C 2 )65. One can confirm that the carbon atom with the δ C 89.1 p.p.m. signal is bonded to a quadrupolar boron atom in the structure of 6, according to the broadening observed.

Proposed mechanism based on DFT calculation

The whole mechanisms for the formation of 3–6 from 2 were estimated by DFT calculations66,67,68,69 with full geometry optimization of all the available transition states (TSs) at the B3LYP/6-31G(d,p) level and single-point energy calculation at M06-2X/6-311+G(d,p) with solvent effect of benzene using conductor-like polarizable continuum model (Fig. 3: mechanism with curly arrows, Fig. 4: energy profiles with relative Gibbs free energies and Supplementary Table 6 for coordinates of all the structures). An initial coordination of CO or tBuNC to 2 gave the sp2–sp3 diborane(4) 7-O and 7-N, which would undergo two types of bond cleavage reactions (Fig. 3a): (1) B–Mes bond cleavage to give acyldiborane(4) 8-O or imidoyldiborane(4) 8-N, (2) B–Bpin bond cleavage to give diborylketone 9-O or diborylimine 9-N. In the reaction of 2 with CO, energy levels of TS8-O and TS9-O are comparable to each other and both TSs are higher than the TS7-O (Fig. 4a). The slightly lower TS8-O could be explained by the higher nucleophilicity of a Mes substituent than a Bpin substituent due to the electronegativity difference between carbon and boron atoms, as supported by natural bond orbital analysis (Supplementary Fig. 20). The subsequent reactions from 8-O and 9-O afforded the same product 3 (Fig. 3b). The former pathway through 8-O included a coordination of a second CO molecule to give 10-O and subsequent migration of the Bpin moiety by a nucleophilic attack of the acyl oxygen atom in 10-O with B–B bond cleavage to give 3. The large energy gain in this step may be attributed to the formation of B–O bond. The latter pathway through 9-O was initiated by a Bpin migration to form the borataalkene 11-O. TS11-O was the global TS, which lies 6.1 kcal mol−1 higher than TS8-O. Subsequently, one of the two Mes groups in 11-O migrated to the carbon atom to afford the boraalkene 12-O. Coordination of a second CO molecule to 12-O could form the same product 3. This step could be considered as a coordination of CO to electron-deficient boraalkene for a large energy gain. Formation of a possible C–O cleaved product 13-O would be suppressed due to the higher TS13-O (Fig. 4a).

Figure 3: Possible reaction mechanism for the formation of 3–6 from 2 estimated by DFT calculations. (a) Two types of possible products 8 and 9 formed by B–Mes or B–Bpin cleavage after the coordination of CO or tBuNC. (b) Two energetically comparable pathways to 3 from 8-O and 9-O. (c) Three pathways to 4-6 from 9-N. Full size image

Figure 4: Energy profiles of possible mechanism. Energy profiles of possible mechanism for the formation of 3–6 from 2 with relative Gibbs free energies in kcal mol−1 (estimated by optimization at the B3LYP/6-31G(d,p) level and subsequent single-point energy calculation at M06-2X/6-311+G(d,p) level with consideration of entropy contribution and solvent effect of benzene (conductor-like polarizable continuum model (CPCM)), all the compound numbers are in conjunction with Fig. 3). (a) Two possible pathways for the formation of 3 by reaction of 2 with CO (red: pathway through 8-O, blue: pathway through 9-O) (b) pathway for the formation of 4-6 by reaction of 2 with tBuNC (red: main pathway to 4-6, blue: branching to each of the compounds 4-6). (Remark: before the solvation correction, TS12-O is slightly higher than 11-O and TS14-N is slightly higher than 13-N in energy.) Full size image

In the reaction of 2 with tBuNC, TS8-N was 5.8 kcal mol−1 higher than TS9-N, indicating that the pathway through 9-N would be favourable (Fig. 4a). The high energy level of TS8-N may be explained by a steric repulsion between the spectator Mes group and the tBu group (Fig. 3a and Supplementary Table 7). The intermediate 9-N would undergo Bpin migration to give 11-N followed by a Mes group migration to form the boraalkene 12-N (Fig. 3c). Coordination of tBuNC to the central carbon atom in 11-N could lead to formation of 6, but the TS to 6 was calculated to be slightly higher (by 3.8 kcal mol−1) than the TS to 12-N. Requirement of higher concentration to prepare 6 was consistent with this result. Again, the simple coordination of tBuNC to boraalkene would give a large energy gain. As a nitrogen atom in 12-N may have higher nucleophilicity than the oxygen atom in 12-O, a migration of the amino substituent (−NtBuBpin) to the Mes-bonded boron atom would take place with an assistance of electron donation from the carbon-bonded Mes group to form the amino-substituted borataalkene 13-N. This step (from 12-N to 13-N) involves a cleavage of the C–N bond originated from the C≡N triple bond in tBuNC. Electron donation from the nitrogen atom to the boron atom in 13-N induced the second Mes migration to the carbon atom to give the aminoboraalkene 14-N. The neutralization of the positively charged Mes group may contribute a large energy gain. A simple coordination of a second tBuNC molecule to 14-N affords the product 5. This result is consistent with the experimental observation that when tBuNC is in excess the product 5 was obtained. In the absence of excess tBuNC, the boron centre of 14-N would attack to one of the benzylic protons to give the cyclic hydroborate 15-N having delocalized cationic charge on the cyclized Mes ring, as a boryl anion could undergo the same deprotonation cyclization70. The energy barrier of 24.2 kcal mol−1 from 14-N to TS15-N is accessible at the room temperature (the reaction condition). Subsequent 1,2-hydride shift from 15-N and re-aromatization would form the product 4. The formation of B–N π-bond in the last step would contribute a large energy gain.