Borylene Insertion into Cage BâH Bond: A Route to Electron-Precise B

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Borylene Insertion into Cage B-H Bond: A Route to Electron-Precise B-B Single Bond Hao Wang, Jiji Zhang, Hung Kay Lee, and Zuowei Xie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01795 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Journal of the American Chemical Society

Borylene Insertion into Cage B-H Bond: A Route to Electron-Precise B-B Single Bond Hao Wang, Jiji Zhang, Hung Kay Lee, and Zuowei Xie* Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China

Supporting Information Placeholder ABSTRACT: A new strategy to construct electron-precise B-B single bond via direct borylene insertion into B-H bond is described. Reduction of bromoborylene stabilized by carborane-fused silylenes with 2 equiv of sodium gives a new compound with a B-B single bond. Both experimental and DFT (density functional theory) results suggest that such an electron-precise B-B single bond is formed via in-situ generated borylene insertion into B-H bond.

Owing to the ubiquitous application of diborane derivatives in organic synthesis,1-2 the construction of B-B single bond has received growing interests.3,4 Over the past decades, several strategies have been developed for the formation of B-B single bonds, which includes reduction of haloboranes,5,6 reductive coupling of arylboranes,7-12 dehydrocoupling of boranes,13-23 metal-templated borylene coupling,24-26 oxidative coupling of borylenes,27 hydroboration of diborenes,28,29 reactions of boryl anions with boranes,30-33 and transition metal-catalyzed B-H borylation (Chart 1).34 It is noteworthy that the latter three methods can be used for building unsymmetrical B-B single bonds.

Chart 1. Methods for the construction of electronprecise B-B single bonds.

bonds. Though borylene insertion into B-H bond has never been reported,40 carbene insertion into B-H bond is a known direct route to access B-C single bonds.41-43 We envisaged that borylene insertion into B-H bond would serve as a new direct method for the construction of electron-precise B-B single bonds. We thought that carbene-stabilized borylenes may be too stable to react with B-H bond because of strong π-accepting property of carbenes. To enhance the nucleophilicity and reactivity of borylenes, the following two strategies may be employed: (1) to use strong σ-donating but poor π-accepting Lewis base ligands, and (2) to reduce the coordination number of boron(I) center. We carried out a proof-of-concept study using our recently reported bissilylene-stabilized monobromoborylene 1 as a model compound.44 This molecule contains a very electron-rich boron(I) center and a carborane linked bissilylenes that are very strong σ-donor and poor π-acceptor. Herein, we report a new method for the construction of B-B single bond via direct borylene insertion into B-H bond as well as the related reaction pathway. Treatment of 1 with 2 equiv of sodium naphthalene in THF at room temperature gave an unexpected borylene insertion into cage B-H product [{2}Na]2 as pale yellow crystals in 45% isolated yield (Scheme 1). The exo BH unit was observed at -28.7 ppm as a doublet, and the substituted cage B was found at 4.8 ppm as a singlet in the 1H coupled 11B NMR spectrum. Its 29Si NMR spectrum displayed a broad peak at 39.1 ppm, which was shifted down-field compared to that of 1 (18.2 ppm).44

Scheme 1. Construction of B-B single bond.

It has been well documented that borylenes can insert into C-H35-38 and C-C7,39 bonds for convenient formation of B-C

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Fig. 1. (a) Molecular structure of [{2}Na]2 with ellipsoids set at the 50% probability level, showing only one half of the molecule for clarity. Selected bond lengths [Å] and angles [°]: Si(1)-C(1) 1.793(4), Si(2)-C(2) 1.780(4), Si(1)-B(13) 1.966(5), Si(2)-B(13) 1.972(4), B(3)-B(13) 1.783(6), C(1)…C(2) 2.718(4); B(3)-B(13)-Si(1) 79.1(2), B(3)-B(13)-Si(2) 81.2(2), Si(1)-B(13)-Si(2) 115.6(2). (b) HOMO-1 for [2]-. The above structural features were supported by density functional theory (DFT) calculations at the B3LYP-D3/631G(d,p) level of theory. The highest occupied molecular orbital (HOMO) is mainly contributed by the carborane cage (see Fig. S3 in the SI), which differs significantly from that of 1.44 The HOMO-1 clearly shows the bonding orbital of B(3)-B(13) bond (Fig. 1b) that is further supported by the Wiberg bond index of 0.83. The B(13)-Si Wiberg bond indices are 0.97 in [2]- and 1.26 in 1, indicating that the B(13)-Si bond in [2]- is primarily a single bond. Thus, the anion [2]can be viewed as a zwitterionic complex in which the nidocage is formally dinegative and the exo [BH]+ unit formally bears a charge of +1.

Single-crystal X-ray analyses reveal that [{2}Na]2 is a dimer in solid-state with two [Na(THF)2]+ moieties serving as linkers via B-H…Na…H-B interactions. The exo B(13) atom is σ-bonded to one cage B(3) atom and one hydrogen atom and coordinated to two silicon atoms from silylenes in a distorted-tetrahedral geometry (Fig. 1a). The most notable structural feature is the newly formed electron-precise B(3)B(13) single bond with a distance of 1.783(6) Å. This measured value is very comparable to those of (1.75-1.88 Å) reported for B(sp3)-B(sp3) single bonds,27,28,45 but is longer than the Bcage-Bexo distances (1.673 and 1.685 Å) observed in the neutral compound 3,6-(Bpin)2-1,2-C2B10H10 (Bpin = 4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl).34 The B(13)-Si bond distances (1.966(5), 1.972(4) Å) are noticeable longer than that of 1.857(4) Å in 1,44 but is comparable to those observed in silylene-borane adducts (1.922-2.108 Å),46-48 indicating the normal donor-acceptor interaction between B(13) and Si with no obvious π bonding characters. The Ccage-Si bonds (1.793(4), 1.780(4) Å) are significantly shorter than that of 1.915(3) Å in 1,44 suggesting partial double bond (exo-π bonding) characters. The C(1)…C(2) distance of 2.718(4) Å indicates no bonding interactions between two cage C atoms, which is comparable to those found in nido-carborane dianionic salts such as 2.687 Å in [{1,3-(CH2)5-1,3C2B10H10}Na2(THF)4]n and 2.872 Å in [{1,4-(CH2)6-1,3C2B10H10}Na2(THF)4]n.49 These structural data suggest that the open cage in [{2}Na]2 is best described as nidospecies.50,51 (a)

The above reaction represents a brand new route to the construction of B-B single bond. Then, the question arises as to how such a B-B bond is formed during the reaction. To address this issue, an equimolar reaction was carried out. Treatment of 1 with 1 equiv of sodium naphthalene in THF at room temperature gave an NMR silent compound [{3}Na]2 as yellow crystals in 44% yield (Scheme 2). Its THF solution exhibited an EPR signal centered at g = 1.991 at room temperature. Further reaction of [{3}Na]2 with 2 equiv of sodium naphthalene in THF at room temperature afforded [{2}Na]2 as confirmed by 11B NMR spectra. Single-crystal X-ray analyses reveal that [{3}Na]2 is a dimer in solid-state, in which two cages are linked by two [Na(THF)2]+ cations via B-H…Na…H-B interactions. The most notable structural feature is the nido carborane with the two cage carbon separation of 2.836(4) Å, which is comparable to that of 2.718(4) Å observed in [{2}Na]2. The exo B(13) is σbonded to one bromine and coordinated to two silicon atoms in a trigonal planar geometry, suggesting a sp2 hybridization of B(13) atom (Fig. 2). The B-Si/Ccage-Si distances (1.955(5)/1.951(5) Å and 1.792(4)/1.790(4) Å) are very similar to those in [{2}Na]2 (1.966(5)/1.972(4) Å and 1.793(4)/1.780(4) Å). Compound [{3}Na]2 can be viewed as a nido carboranefused boron-centered radical cation.52

(b)

Scheme 2. Reduction of 1 with 1 equiv of sodium.

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Journal of the American Chemical Society 3] (n = number of vertices) framework electrons,53,54 followed by subsequent intramolecular one electron transfer from the electron-rich exo B(I) center to the carborane radical anion to form the stable radical anion [3]-.. Further one electron reduction of [3]-.by sodium to generate an active borylene B, followed by a B-H bond insertion to afford [{2}Na]2 with the formation of a new B-B single bond.

Fig. 3. Plots of SOMO and spin density of the radical anion [3]-. calculated at the B3LYP-D3/6-31G(d,p) level of theory.

Scheme 3. Proposed mechanism for the formation of [{2}Na]2 and [{3}Na]2 . Fig. 2. Molecular structure of [{3}Na]2 with ellipsoids set at the 50% probability level, showing only half of the molecule for clarity. Selected bond lengths [Å] and angles [°]: Br(1)B(13) 1.976(4), Si(1)-C(1) 1.792(4), Si(2)-C(2) 1.790(4), Si(1)B(13) 1.955(5), Si(2)-B(13) 1.951(5), C(1)…C(2) 2.836(4); Si(2)B(13)-Si(1) 118.7(2), Si(2)-B(13)-Br(1) 120.5(2), Si(1)-B(13)Br(1) 120.7(2). To get a better understanding of the electronic structure of [3]-., DFT calculations were carried out at the B3LYPD3/6-31G(d,p) level of theory (see the SI for detail). The SOMO (singly occupied molecular orbital) of [3]-. is the halffilled p orbital on the exo B(13) atom (Fig. 3), while the lowest unoccupied molecular orbital (LUMO) mainly corresponds to the π* orbitals localized on the phenyl rings (Fig. S4). The spin-population is mainly localized at the exo B(13) atom (~79% spin density). These features suggest that one electron is transferred from the exo B(13) atom in 1 to the cage, resulting in the formation of the corresponding boroncentered radical cation and nido-carborane dianion. This argument is supported by the changes of natural population analysis (NPA) charges calculated for the units of BBr and carborane cage among 1, [2]- and [3]-. (see Table S3 in the SI). The NPA charge of the BBr unit is changed from 1.56 in 1 to -1.20 in [3]-., whereas that of the cage is changed from -1.31 in 1 to -2.79 in [3]-. and to -2.75 in [2]-. These results strongly suggest that the electron flows from the BBr unit to the cage.

In summary, reduction of bromoborylene stabilized by carborane-fused bissilylenes with 2 equiv of sodium gave an unexpected compound with an electron-precise B-B single bond. Both experimental and DFT studies suggest that such a B-B single bond is formed via in-situ generated borylene insertion into B-H bond. It represents a new approach to construct B-B single bonds. This proof-of-concept investigation indicates that active borylenes (generated insitu) could serve as reagents for building B-B single bonds via B-H insertion.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Syntheses and characterization, NMR spectra and DFT (PDF) Crystallographic data for [{2}Na]2 (CIF) Crystallographic data for [{3}Na]2 (CIF)

On the basis of both experimental and DFT results, a reaction mechanism for the formation of [2]- and [3]-. was proposed (Scheme 3). Reduction of 1 by 1 equiv of sodium gives an unstable carborane radical anion A featuring [2n +



AUTHOR INFORMATION

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Corresponding Author *[email protected]

ORCID Zuowei Xie: 0000-0001-6206-004X

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by grants from the Research Grants Council of The Hong Kong Special Administration Region (Project No. 14320616) and Incentive Fund from Science Faculty (CUHK).

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Borylene Insertion into Cage BâH Bond: A Route to Electron-Precise B

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