Photochemically- and Thermally-Generated BN- and BNP-Doped Borafluorenate Heterocycles via Intramolecular Staudinger-Type Reactions

Abstract

We report a series of BN- and BNP-borafluorenate heterocycles, bis(borafluorene-phosphinimine) (11-15), formed via intramolecular Staudinger-type reactions. The reactions were promoted by light or heat using monodentate phosphine-stabilized 9-azido-9-borafluorenes (R3P-BF-N3) (6-10) and involve the release of dinitrogen (N2), phosphine migration from boron to nitrogen, and oxidation of the phosphorus center (PIII to PV). Compounds 11-15 are blue emissive in the solution and solid states with solution phase absolute quantum yields ranging from 13-67%. Our studies were extended by comparing the reactivity of borafluorene azides supported by monodentate phosphorus ligands with those of bidentate phosphorus ligands, diphenyl-2-pyridylphosphine, trans-1,2-bis(diphenylphosphino)ethylene, and bis(diphenylphosphino)methane. A number of five-membered BNP-doped heterocycles were obtained which served as molecular precursors to access a new class of 4-membered BN-borafluorenate heterocycles via ring contraction and dimerization. All compounds discussed in this study have been fully characterized by single crystal X-ray diffraction and multinuclear NMR spectroscopy (1H, 13C, 11B, and 31P).

Mono- and Bis-Phosphine Promoted Incorporation of Boron, Nitrogen, and Phosphorus into Heterocycles via Staudinger Reactions of Borafluorene Azides

“…Reaction of Int2 with BF-N 3 readily yields 2. Int2 could potentially dimerize to Int2 dimer as was observed with R 3 P, 48 which yields a stable minima and is favorable for both R 3 P and PyPh 2 P. However, with PyPh 2 P, formation of 2 is calculated to be thermodynamically favored over dimerization by 55.3 kJ mol −1 in contrast to PMe 3 , where dimerization is favored by 21.2 kJ mol −1 (43.8 kJ mol −1 with def2-TZVPPD that was used in our previous study 48,63 ). The different reactivity of R 3 P and PyPh 2 P is a result of reaction thermodynamics, which arises due to the stabilizing Py N−B interaction with PyPh 2 P that is absent in R 3 P. The impact of Py N−B coordination is evidenced in the similarity of PyPh 2 P-BF-Br and PyPh 2 P-BF-N 3 reaction energetics for Me 3 P and PyPh 2 P where there is no Py N−B coordination, while for subsequent steps, the PyPh 2 P pathway yields a lower barrier and more stable Int2 and 2.…”

“…A PBE0-D3(BJ)/def2-TZVPP(CPCM,toluene)//PBE0-D3(BJ)/def2-SVP(CPCM,toluene) mechanistic study was carried out for the formation of 2 and 3 (Scheme 1), including a comparison to phosphines (PR 3 ) with noncoordinating R groups. 48 The calculated reaction energy pathway is illustrated in Figure 3 using both PyPh 2 P and Me 3 P. The fact that 2 is produced from either BF-N 3 or 1, together with the structure of 2, is suggestive of intramolecular Staudinger reactivity. In our previous study of R 3 P reactivity, no products analogous to 2 and 3 were isolated; here, we have investigated the different reactivity observed with a coordinating Py group in PyPh 2 P. While not shown in Figure 3, ΔG for the initial addition of PyPh 2 P to BF-Br is favorable, calculated to be −65.0 kJ mol −1 (−101.2 kJ mol −1 with PMe 3 ).…”

“…Compound 7 shows a strong IR absorption band at 2095 cm −1 corresponding to the -N 3 asymmetric stretching vibrations, which is comparable to that in compound 2 and reported boron azides. 7,48,63 Additionally, it was fully characterized by NMR spectroscopy ( 1 H, 13 C, 11 B{ 1 H}, and which then deprotonated one of the protons of the phosphine ligand to produce hydrazoic acid (HN 3 ). It is noteworthy that this reaction needs to be run with careful attention due to the presumed release of the explosive and highly toxic HN 3 (see Experimental Section for additional information).…”

“…47 More recently, we synthesized a series of mono(phosphine)coordinated borafluorene azides which, under thermal or photochemical conditions, afforded an array of compounds containing 4-membered B 2 N 2 rings with an exocyclic phosphinimine moiety (Figure 1A). 48 While organic azides participating in Staudinger ligation reactions are wellestablished, 49 examples of phosphine-promoted N 2 loss at boron are rare. 50−53 The chemistry depicted in Figure 1A represents an unusual example in which an intramolecular Staudinger reaction has been used to produce boracycles.…”

Photochemically and Thermally Generated BN-Doped Borafluorenate Heterocycles via Intramolecular Staudinger-Type Reactions

“…Transition state optimizations employed the NEB-TS approach . All transition states were confirmed to connect relevant minima with the intrinsic reaction coordinate (IRC) calculations …”