Homolytic cleavage of diboron(4) compounds using diazabutadiene derivatives

Piyush Kumar Verma , Naresh Kumar Meher and K. Geetharani *
Department of Inorganic and Physical Chemistry, Indian Institute of Science Bangalore, 560012, India. E-mail: geetharani@iisc.ac.in

Received 1st June 2021 , Accepted 14th July 2021

First published on 19th July 2021


Diazabutadiene derivatives have been identified as a distinct class of reagents, capable of cleaving B–B bonds of diboron(4). The cleavage is accompanied by the formation of a new C[double bond, length as m-dash]C bond and the product geometry is highly dependent on the substituents on the DAB units. Preliminary mechanistic investigations suggest a concerted mechanism and the absence of any radical intermediates.

Diboron compounds have become indispensable reagents for borylation of a vast spectrum of organic compounds to synthesize the corresponding boronic esters.1 The key step in these transformations is the cleavage of the B–B bond of diborons, which mostly requires transition metal catalysts or even nucleophile assisted metal free systems. Late transition metals are well known to cleave diboron(4) compounds homolytically.2 Another successful strategy established for the activation of diboron is by making adducts with nucleophiles, resulting in a weaker B(sp2)–B(sp3) bond.1,3 Such adducts serve as a source of nucleophilic boron species in the formation of metal–boryl complexes4 or as a nucleophilic reagent in the substitution or addition reactions.5 The B(sp2)–B(sp3) moiety has also been utilized to trap aryl and alkyl radicals, resulting in the homolytic cleavage of B–B bonds and the formation of the corresponding boronic esters.6 Attempts to employ neutral nucleophiles such as NHCs for the activation of B–B bonds of diboron(4) compounds revealed a distinct reactivity. Apart from the formation of NHC-diboron adducts, ring expansion products by the cleavage of B–B and C–N bonds of NHC have been reported.7 A recent remarkable development in this field is the photo catalyzed homolytic cleavage of B–B bonds in diboron(4), assisted by coordinating reagents or solvents.8 The first attempt to activate the B–B bond in diboron(4) compounds using a Lewis base by the coordination of both the boron centres was reported by Marder et al. using 4-methylpyridine, resulting in the elongation of the B–B bond of bis(catecholato)diboron (Scheme 1a).9 Later, the first homolytic cleavage using this strategy was indeed evident using pyrazine as a base by Ohmura, Suginome and co-workers (Scheme 1b).10a Their subsequent report revealed that 4,4′-bipyridines can also cleave the B–B bonds in bis(pinacolato)diborane (B2pin2) in a reductive addition fashion.10b,e Moreover, it was observed that the addition of B2pin2 to 4,4′-bipyridine and its derivatives can be reversible and boron moieties can be transferred to another pyrazine derivative catalytically. The same group extended the protocol and achieved the reduction of nitroarenes by the activation of B–B bonds in B2neop2 using 4,4′-bipyridines as a catalyst.10c,d Soon after this report, in 2016, Zhu, Li and co-workers disclosed the first evidence of a pyridine-stabilized boryl radical intermediate, formed by the homolytic cleavage of B2pin2 by 4-cyanopyridine (Scheme 1c).11 Verified by EPR and DFT studies, they established that coordination of 4-cyanopyridine molecules with both the boron atoms of B2pin2 results in homolytic cleavage of B–B bonds. This was further used for the reduction of azobenzene compounds. In subsequent studies, it has been proposed that the spin density in the 4-cyanopyridine-stabilized boryl radical is mainly localized on the C-4 carbon of the pyridine. Depending on the reaction conditions or the substrates, this boryl radical may act as a source of carbon radicals.12 A different mechanism has also been documented by Tang and co-workers in 2017, who found that two isoquinoline molecules, upon reaction with several diboron reagents (B2(OR)4), undergo asymmetric reductive coupling, without intermediacy of any boryl radical (Scheme 1d).13 The experimental evidence and DFT studies indicated that the B–B activation of diboron reagents by isoquinolines occurs via double N–B coordination followed by [3,3]-sigmatropic migration.13,14 The recent development in the field of homolytic cleavage of diborons using azabenzene derivatives thus projected a new way for the catalytic synthesis of boronic esters. Hence, we were interested in finding a different class of donor molecules which could efficiently and homolytically cleave the B–B bonds in diboron compounds. We envisaged that diazabutadiene derivatives could be a potential candidate for such reactions as the presence of conjugated π bonds across the frame may provide similar cooperative assistance as observed in the case of pyridine derivatives.15
image file: d1cc02881b-s1.tif
Scheme 1 An Overview of B–B bond cleavage in diborons by azine-heterocycles (a–d).

The reaction with equimolar amounts of 2,6-diisopropylphenyl substituted diazabutadiene (DippDAB) with bis(catecholato)diboron (B2cat2) in benzene at room temperature for 15 min resulted in a pale-yellow solution of compound 3, (DippDAB)(Bcat)2 in quantitative yield (Scheme 2). Pale-yellow crystals suitable for X-ray diffraction were obtained by slow evaporation of a benzene solution of compound 3. The crystal structure reveals that cleavage of the B–B bond was accompanied by the formation of a new π bond in a cis-configuration (Fig. 1). The formation of a cis isomer is possible from the simultaneous coordination of both N atoms of diazabutadiene with the boron atoms of diboron.9b

image file: d1cc02881b-s2.tif
Scheme 2 Reaction of DippDAB with B2cat2 in benzene.

image file: d1cc02881b-f1.tif
Fig. 1 Molecular structure of compound 3, (DippDAB)(Bcat)2 with a thermal ellipsoid drawn at 50% probability level. Right one is the top view of the left one. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): B1–N1, 1.407(2); N1–C8, 1.449(3); N1–C7, 1.413(2); C7–C7, 1.329(3); B1–N1–C7, 127.1(1); N1–C7–C7, 132.3(2); N1–C7–H7, 112(1); H7–C7–C7, 116(1).

The B–B bond distance in 3 is 3.158(3) Å, compared to 1.678(3) Å in B2cat2, showing a complete cleavage of the B–B bond.16 The B1–N1 distance in 3 is 1.407(2) Å, significantly smaller than 1.659(2) Å in the [B2(cat)2(4-picoline)2] adduct.9a The bond distance is in the typical range of double bonds. Compound 3 adopts a cis geometry with a N1–C7–C7 distance of 132.3(2) Å. Moreover, the O1–H14(C14) and O2–H18(C18) bond distances were found to be 2.48(2) Å and 2.85(2) Å respectively, which fall in the range of H-bonding interactions.17

The 11B NMR spectrum of 3 shows a broad resonance at 25.6 ppm, typical of B–N bonded tricoordinate boron compounds. Interestingly, the existence of the intramolecular H-bonding was further supported by the 1H NMR spectrum of 3, which shows two sets of protons belonging to the isopropyl unit. The pair of protons (H14), attached to the diagonally opposite carbon pairs C14 (Fig. 1, right), shows a downfield shift in the spectrum and appears as a broad signal at 4.41 ppm. Another set of two protons (H18) attached to diagonally opposite carbon pairs, at C18, appears as a broad signal at 3.38 ppm. Also, in the 13C NMR spectrum of 3, the signals belonging to the carbons of isopropyl groups were suppressed (see the ESI). To further confirm this assumption, a variable temperature NMR experiment was performed.18 When the temperature was gradually reduced to −50 °C, a significant downfield shift was observed for the peak at 4.41 ppm and for the methyl protons of the isopropyl unit (Fig. S2, see the ESI). Furthermore, acylation reaction of compound 3 was carried out. The substitution of the Bcat group by the acetyl group results in the disappearance of the intramolecular H-bonding (see the ESI, compound 7). The molecular structure of compound 7 reveals inversion of the geometry to trans across the C[double bond, length as m-dash]C bond, probably to relax the steric environment due to the bulky diisopropyl substituents, during the process of acetylation. In order to investigate the reactivity of different diboron(4) compounds, reactions with equivalent amounts of DippDAB with several diboron(4) reagents were carried out. A similar reactivity was observed while using bis(dithiocatecholato)diboron; however with other diboron(4) reagents, the reaction did not proceed even after prolonged heating at elevated temperatures (Scheme 3). Furthermore, to study the effect of the substituents on diazabutadiene in the cleavage of diborons, one equivalent of mesityl substituted diazabutadiene (MesDAB) was reacted with B2cat2 in benzene (Scheme 4). Immediate colour change from dark yellow to pale-yellow was observed, suggesting the completion of the reaction. A signal at 25 ppm in the 11B NMR spectrum suggests the formation of a new N–B coupled product by cleavage of the diboron. Pale-yellow coloured crystals of 4 were obtained by slow evaporation of the solvent at room temperature. Surprisingly, the molecular structure obtained by X-ray analysis disclosed a trans conformation across the C[double bond, length as m-dash]C bond in (MesDAB)(Bcat)2 (4), which is distinct from the cis geometry observed in 3 (Fig. 2). The B–N bond distance in 4 was found to be 1.401(2) Å, indicating a typical B–N single bond. The backbone carbon–carbon bond distance of 1.332(2) Å (C4–C4) lies in the range of C[double bond, length as m-dash]C and is smaller than the C–C bond length (C1–C2 1.516(2) Å) observed in the bis-hydroboration product obtained by the addition of catecholborane to α-diimines, reported recently by Westcott and coworkers.19

image file: d1cc02881b-s3.tif
Scheme 3 Reaction of DippDAB with various diboron(4) compounds.

image file: d1cc02881b-s4.tif
Scheme 4 Reaction of MesDAB with B2cat2.

image file: d1cc02881b-f2.tif
Fig. 2 Molecular structure of compound 4, (MesDAB)(Bcat)2 showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): B1–N1, 1.401(2); C4–C4, 1.332(2); N1–C4, 1.415(2); N1–C8, 1.439(2); B1–N1–C4, 122.4(1); B1–N1–C8, 119.8(1); C4–N1–C8, 117.8(1); N1–C4–H4, 117.9; H4–C4–C4, 117.9.

Two different modes have been proposed in the literature for the cleavage of the B–B bonds in diboron(4) using azabenzene derivatives, supported well by experimental and DFT studies. Based on the Gibbs energy profile, Zhu, Li and coworkers proposed that a direct homolytic cleavage of the B–B σ-bond of B2pin2 occurred by the interaction of 4-cyanopyridine via cooperative coordination of two 4-cyanopyridine molecules with the two boron atoms of the diboron (Scheme 1c).11 In another mode of activation, DFT studies suggest the [3,3]-sigmatropic rearrangement of an intermediate, formed from the double coordination of diboron by two isoquinoline molecules.13,14 Based on the overview of the documented literature, the cleavage of B–B bonds of B2cat2 by DABs probably proceeds via the coordination of two N-atoms of DABs with B2cat2,9b followed by a concerted pathway. However, the difference in the geometry of compounds 3 and 4 put forth an obvious question about the difference in the mechanism of cleavage of B2cat2 by DippDAB and MesDAB, respectively. To obtain further insight into the B–B bond cleavage pathway, radical trapping and crossover experiments were performed. The presence of either TEMPO or 9,10-dihydroanthrecene as a radical scavenger had no observable effect on the reaction outcome (Scheme 5a). Noticeably, the reaction between equimolar amounts of MesDAB with B2cat2, and B2(dithiocat)2 did not form any crossover product (Scheme 5b). These observations suggest a concerted reaction pathway. While the formation of 3 may undergo via the cyclic transition state, as observed in the case of isoquinoline with diboron (Scheme 1d),13 compound 4 having trans-geometry may result from the isomerization of the formed cis product, as observed in the case of compound 7.

image file: d1cc02881b-s5.tif
Scheme 5 Reaction of (a) DippDAB and (b) MesDAB with B2cat2 in the presence of radical scavengers and (c) cross-over experiment.

To further assure the inference of coordination of two N-atoms of DABs with B2cat2 during the reaction, the compound bis(mesityl)acenaphthenequinonediimine 5 was chosen, as the inherent geometry limits any possibility of intramolecular rotation across the C–C bonds of the DAB unit. Indeed, the reaction of 5 with B2cat2 results in the immediate formation of a red precipitate. The 11B NMR spectrum shows a peak at 23.6 ppm, indicating the formation of a new B–N bond. The 1H NMR spectrum and the HRMS analysis verifies the formation of 6 (Scheme 6). Interestingly, three sets of methyl protons were observed in the 1H NMR spectrum as singlet; one set appeared as a broad signal which suggests the possibility of intramolecular hydrogen bonding, as in the case of 3. Despite several attempts, crystals suitable for X-ray analysis were not obtained. The formation of 6 strongly suggests that the reaction proceeds via a concerted pathway by coordination of two N- atoms of DABs with the B2cat2.

image file: d1cc02881b-s6.tif
Scheme 6 Reaction of (MesBIAN) with B2cat2.

In conclusion, diazabutadiene derivatives were recognized as a new class of reagents, which can efficiently induce B–B bond cleavage in B2cat2 by coordination of its N atoms with the B atoms. The B–B bond cleavage is accompanied by the formation of a new π bond in the DAB unit. The geometry across this newly formed double bond was found to be influenced by the substituents on the DABs. The preliminary mechanistic understanding supports a concerted reaction pathway for these transformations.

K. G. thanks the SERB (CRG/2019/002319), and MHRD-STARS (STARS/APR2019/320) for funding. P. K. V. thanks IISc Bangalore and N. K. M. thanks CSIR-JRF for a research fellowship. We also thank AllyChem Co. Ltd, China for the gift of diboron compounds.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Dedicated to Prof. Todd B. Marder on the occasion of his 65th birthday.
Electronic supplementary information (ESI) available. CCDC 2081272, 2081274, and 2086539. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1cc02881b

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