The first general synthesis of boron-substituted 1,2-dihydro-1,2-azaborines is described. The versatile 1,2-dihydro-1,2-azaborine precursor 4 is synthesized through a ring-closing metathesis-oxidation sequence. Treatment of 4 with a wide range of anionic nucleophiles furnishes the desired adducts 5 in good yields. The scope includes hydrogen- and a variety of carbon- and heteroatom-based nucleophiles. Furthermore, the boron-containing isostere (7) of the potent hypolipidemic agent, methyl 2-ethylphenoxyacetate (8), is readily prepared through our method.

Positively brilliant: The first examples of 1,2‐azaborine cations have been prepared and their structure and optoelectronic properties characterized (see picture). 1,2‐Azaborine cations exhibit solid‐state fluorescence that is distinct from their neutral all‐carbon analogues, and the 1,2‐azaborine moiety is a critical component for the optoelectronic properties. There is a potential for the utility of these complexes in materials applications.

Summary The first cationic 1,2-azaborine adducts of neutral phosphorous- and oxygen-based nucleophiles have been synthesized and characterized via spectroscopic and single crystal X-ray diffraction analysis.

1,2-Dihydro-1,2-azaborine is a six-membered aromatic heterocycle that is isoelectronic with benzene through the replacement of a C = C unit in benzene with an isoelectronic B À N unit. [1,2] Since the pioneering work by Dewar et al., [3,4] significant advances have been made in the synthesis and reactivity studies of this family of heterocycles. [5][6][7] Our continued exploration of the 1,2-azaborine motif [8][9][10][11][12][13][14][15] has led us to consider the synthesis of cationic 1,2-azaborines, for which no examples have been reported. In particular, we envisioned that substitution of 1,2-azaborine on the boron atom with pyridine derivatives would furnish cationic biaryltype structures [16] having the potential for use in materials applications (Scheme 1). Herein we report the synthesis, structural characterization, and optoelectronic properties of pyridine-substituted 1,2-azaborine cations, including a cationic heterocyclic analogue of para-terphenyl.We have previously established nucleophilic substitution of the B À Cl bond in 1,2-azaborines by anionic nucleophiles (with Cl À serving as the leaving group). [8] Less reactive neutral nucleophiles did not displace the chloride from the boron atom. We hypothesized that a better leaving group on the boron atom (e.g., OTf) could render it susceptible to nucleophilic attack by weaker neutral nucleophiles. In the course of our studies, we discovered that silver reagents facilitate the ligand exchange at the boron position in 1,2azaborines. [13] We were thus pleased to discover that treatment of 1,2-azaborine 1 with AgOTf produced the substituted 1,2-azaborine 2 in 59 % yield as an extremely moisturesensitive liquid (Scheme 2). The 1,2-azaborine 2 was characterized by 1 H, 11 B, and 13 C NMR spectroscopy as well as IR spectroscopy.Heterocycle 2 readily reacts with para-substituted pyridines to form the desired cationic 1,2-azaborines 3. As can be seen from Scheme 3, the substitution reaction is independent of the electronic nature of the nucleophile. Excellent yields have been obtained with both electron-rich and electron-poor pyridines.The 1,2-azaborine cations 3 are highly crystalline solids that fluoresce under UV light. We thus explored the solidstate fluorescence of the pyridine-substituted 1,2-azaborine cations 3. The solid-state fluorescence and quantum yields of aromatic hydrocarbons, including para-terphenyl, have been reported using an integrating sphere. [17] We have recorded the fluorescence spectra of crystalline samples of 3 a-e (Figure 1), all of which were freshly recrystallized prior to making the fluorescence measurements. The solid-state fluorescence of 3 a (R = Me) shows a peak at l em = 436 nm (F PL = 0.03) and is visibly less fluorescent than samples of 3 b and 3 c under a UV lamp (l = 365 nm; see Figure 1, right). The solid-state emission spectrum of 3 b (R = Ph) showed a relatively narrow band at l em = 448 nm (F PL = 0.86). The high quantum yield observed for 3 b is quite similar to the values obtained for Scheme 1. 1,2-Azaborine cations....

Organo-boron compounds S 0040Diversity Through Isosterism: The Case of Boron-Substituted 1,2-Dihydro--1,2-azaborines. -With a view to preparing boron-containing aromatic heterocycles that mimic benzene, ring-closing metathesis-oxidation sequence is performed to give the 2-chloroazaborine (III). Its treatment with a range of hydrogen-, carbon-, and heteroatom-based nucleophiles affords the desired title compounds. -(MARWITZ, A.