This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. Diverse bonding activations in the reactivity of a pentaphenylborole toward sodium phosphaethynolate : heterocycle synthesis and mechanistic studies Li, Yan; Siwatch, Rahul Kumar; Mondal, Totan; Li, Yongxin; Ganguly, Rakesh; Koley, Debasis; So, Cheuk‑Wai 2017 Li, Y., Siwatch, R. K., Mondal, T., Li, Y., Ganguly, R., Koley, D., & So, C.‑W. (2017). Diverse bonding activations in the reactivity of a pentaphenylborole toward sodium phosphaethynolate : heterocycle synthesis and mechanistic studies. Inorganic chemistry, 56(7), 4112–4120. doi:10.1021/acs.inorgchem.7b00128 https://hdl.handle.net/10356/139442 https://doi.org/10.1021/acs.inorgchem.7b00128 This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.inorgchem.7b00128 Downloaded on 28 Sep 2021 11:54:28 SGT Diverse Bonding Activations in the Reactivity of a Pentaphenylborole toward Sodium Phosphaethynolate: Heterocycle Synthesis and Mechanistic Studies Yan Li,a Rahul Kumar Siwatch,a Totan Mondal,b Yongxin Li,a Rakesh Ganguly,a Debasis Koley*band Cheuk-Wai So*a aDivision of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore bDepartment of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 246, India ABSTRACT The reaction of the pentaphenylborole [(PhC)4BPh] (1) with sodium phosphaethynolate·1,4- dioxane (NaOCP(1,4-dioxane)1.7) afforded the novel sodium salt of phosphaboraheterocycle 2. It comprises anionic fused tetracyclic P/B-heterocycles which arise from multiple bond activation between the borole backbone and [OCP]⁻anion. DFT calculations indicate that the [OCP]⁻anion prefers the form of phosphaethynolate ⁻O-CP over phosphaketenide O=C=P⁻ to interact with two molecules of 1, along with various B-C, C-P and C-C bond activations to form 2. The calculations were verified by experimental studies: (i) the reaction of 1 with NaOCP(1,4- 1 dioxane)1.7 and a Lewis base such as the N-heterocyclic carbene IAr [:C{N(Ar)CH}2] (Ar = 2,6- iPr2C6H3) and amidinato amidosilylene [{PhC(NtBu)2}(Me2N)Si:] afforded the Lewis base- pentaphenylborole adducts [(PhC)4B(Ph)(LB)] (LB = IAr (3), :Si(NMe2){(NtBu)2CPh} (4)), respectively; (ii) the reaction of 1 with the carbodiimide ArN=C=NAr afforded the seven membered B/N heterocycle [B(Ph)(CPh)4C(=NAr)N(Ar)] (5). Compounds 2-5 were fully characterized by NMR spectroscopy and X-ray crystallography. Introduction Boroles are singlet, highly Lewis acidic and antiaromatic five-membered 4π electron systems which possess rich chemistry for building boron-containing heterocycles.1Although the preparation of a borole can date back to 1969,2 the reactivity of boroles with unsaturated molecules via Diels−Alder and coordination/ring-expansion pathways was not intensively explored until recent investigation by research groups of Piers, Braunschweig and Martin.3 In cases of coordination/ring-expansion pathways, the reactions are usually driven by unfavorably antiaromatic borole rings, which proceed through the coordination of Lewis acidic boron centers with unsaturated molecules, such as alkynes,4 ketones, isocyanates,3h carbon monoxide,3b organic azides3k,5 and diazo compounds,6 and subsequent cleavage of B-C bonds, to give boron- containing six- and seven-membered heterocycles. Among these reactions, it is noteworthy that boroles reacted with triply bonded substrates resulting in a diversity of reaction mechanisms. First, carbon monoxide and acetonitrile reacted with the pentaphenylborole [(PhC)4BPh] to afford donor-acceptor adducts, which were rearranged to ring-expanded products [C(=O)B(Ph)(CPh)4] (A, Scheme 1) and [C(CH3)NB(Ph)(CPh)4]2(B) through 1,1- and 1,2- insertions in solution, respectively.3b In addition, the reaction of diphenylacetylene with [(PhC)4BPh] formed the seven-membered borepin [(PhC)6BPh] (C) via the rearrangement of a 2 4a,c bicyclic Diels-Alder intermediate. When the phenyl substituents of [(PhC)4BPh] are replaced F F F by C6F5, the resulting pentaarylborole [(Ph C)4BPh ] (Ph = C6F5) reacted with diphenylacetylene to go through a phenyl migration and ring expansion mechanism to yield the F F 4 six-membered boraheterocycle [C(=CPh2)B(Ph )(CPh )4] (D) as the major product. The F F reaction can also undergo a Diels−Alder pathway to yield the borepin [B(Ph )(CPh )6] as the minor product. Moreover, the interactions of boroles with organoazides led to various N- insertion reactions due to different reaction conditions and substituents of boroles and organoazides, which afforded a variety of ring expanded B-N heterocycles, such as the eight- membered BN3C4 heterocycle [N(SiMe3)B(Ph){C(Ph)}4N2](E), 1,2-azaborine [N(SiMe3)B(Ph){C(Ph)}4] (F) and 1,2-azaborinine-substituted azo dye 3k,5 [N(N2Mes)B(Mes){C(Ph)}4] (Mes = 2,4,6-Me3C6H2; G). Furthermore, Martin et al. reported the synthesis of the fused P/N-heterocycle, 1-phospha-6-boratricyclohept-3-ene, by the reaction of the pentaphenylborole [(PhC)4BPh] with 1-adamantylphosphaalkyne (Ad-C≡P) (Ad = 1- adamantyl; H).7 Scheme 1. Representative ring expansion products by the reaction of boroles with triply bonded molecules (carbon monoxide: A; acetonitrile: B; alkyne: C and D; azides: E, F and G (TMS = 3 SiMe3; Mes = 2,4,6-Me3C6H2); 1-adamantylphosphaalkyne: H) Recently, sodium phosphaethynolate (NaOCP), which has an [O⎯ C≡P]- anionic structure, gained limelight due to its importance as an anionic phosphorus transfer reagent for building a variety of phosphorus-containing four-, five- and six-membered heterocycles.8 Intrigued by the chemistry of borole and sodium phosphaethynolate, we are interested in investigating whether the latter can be utilized to expand a borole ring to form an unprecedented P/B-heterocycle. Herein, we report the reactivity of the pentaphenylborole [(PhC)4BPh] toward NaOCP. In addition, DFT calculations were performed to understand the reaction mechanism. Further experimental studies: (1) the reactions of [(PhC)4BPh], NaOCP and Lewis bases (the N- heterocyclic carbene and amidinato amidosilylene), (2) the reaction of [(PhC)4BPh] with a carbodiimide, were performed to verify DFT calculations. Results and Discussion Scheme 2. Synthesis of compound 2. In the first NMR-scale experiment, the reaction of the pentaphenylborole [(PhC)4BPh] (1) and sodium phosphaethynolate·1,4-dioxane [NaOCP(1,4-dioxane)1.7] in a 1:1 molar ratio in C6D6 resulted in gradual color change from deep blue to orange. After 1 hour of the reaction, the 31P 4 NMR spectrum showed the appearance of a sharp singlet at -70 ppm, indicating the consumption of NaOCP ( −392 ppm)8b and the formation of a sole product. In the second 31 NMR-scale reaction, a 2:1 molar ratio of 1 and [NaOCP(1,4-dioxane)1.7] was employed. The P NMR spectrum showed a completed consumption of the latter. In this context, 1 mmol of compound 1 reacted with 0.5 mmol of NaOCP in toluene from -78 oC to room temperature and this conversion smoothly afforded compound 2, which was isolated as pale yellow crystals in 62 11 % yield (Scheme 1). The B NMR spectrum of 2 recorded in THF-d8 shows a broad signal at 24.1 ppm and a sharp singlet at - 2.94 ppm, indicating the presence of two boron centers with different coordinative environments. In line with the observation of abovementioned NMR-scale reactions, the 31P NMR signal of 2 ( -70.5 ppm) is significantly low field shifted relative to that of the 1-phospha-6-boratricyclohept-3-ene (H, -130.7 ppm).7 Compound 2 is stable in an argon atmosphere for at least three months but slowly decomposes to white powder when exposed in air and moisture. 5 Figure 1. X-ray crystal structure of 2: perspective view of the molecule.9 Hydrogen atoms, toluene molecules and disorder in phenyl substituents are omitted for clarity. Thermal ellipsoids are drawn at 15% probability. Selected bond lengths (Å) and angles (o): C1-B21.600(9), C1-C2 1.552(7), C2-C31.537(7), C3-C41.347(7), C4-B11.615(8), C9-B1 1.643(9), O2-C9 1.456(7), B2- O2 1.372(8), C1-P1 1.915(5), C2-P1 1.839(5), B1-P1 1.977(6), B1-C5 1.612(8), C5-C6 1.348(7), C6-C7 1.501(8), C7-C8 1.367(10), C8-C9 1.541(8), B1-C9 1.643(9), Na1-P1 2.902(3), Na1-O1 2.272(5); C1-C2-C3 112.5(4), C2-C3-C4 116.7(4), C3-C4-B1 118.0(5), C4-B1-P1 101.0(3), C4- B1-C5 113.7(5), C4-B1-C9 110.0(5), C5-B1-P1 114.2(4), C5-B1-C9, 112.2(4), C9-B1-P1 104.9(4), C1-B2-O2 122.8(6), O2-B2-C64 113.7(6), C64-B2-C1 123.5(6), C1-P1-B1 95.8(2), C2-P1-B1 94.2(2), C1-P1-C2 48.8(2). 6 Compound 2 was studied by X-ray crystallography. In the first intuition, the C-P bond in the [OCP]⁻ (O2C9P1) is cleaved. Compound 2 appears to be formed by an anionic phosphorus atom (P1) undergoing a [1+2] cycloaddition with the C=C bond (C1C2) of a fused boraheterocycle, which is afforded by the insertion of a carbon monoxide moiety (O2C9) into two borole rings (B1C5C6C7C8 and B2C1C2C3C4), along with the phenyl-migration (from B1 to C9).