Iodide-Mediated or Iodide-Catalyzed Demethylation and Friedel–Crafts C–H Borylative Cyclization Leading to Thiophene-Fused 1,2-Oxaborine Derivatives

Abstract

The first synthesis of dithieno-1,2-oxaborine derivatives was achieved via iodide-mediated or iodide-catalyzed demethylation of 3-methoxy-2,2′-bithiophene and subsequent C–H borylation. A wide variety of thiophene-fused oxaborines could be synthesized by the procedure.

Intramolecular (directed) electrophilic C–H borylation

“…Subsequently, Suga, Mitsudo and co-workers demonstrated a demethylative borylation to yield thiophene-fused 1,2-oxaborine derivatives. 80 Notably, the use of PhBCl 2 as the borylating electrophile required an iodide salt to facilitate demethylation. Iodide was proposed to be acting as the nucleophile to demethylate the less activated ArO(Me)-borane adduct (less activated due to coordination of Ar(O)Me to a less Lewis acidic borane).…”

One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

“…24,25 Compared to B,N-PAHs, analogous B,O-PAHs have been largely neglected, although a first derivative, 10-hydroxy-9,10oxaboraphenanthrene, was published by Dewar et al as early as in 1960. 26 Still today, the majority of known B,O-PAHs contains the structural motif of a 9,10oxaboraphenanthrene, [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] likely due to its convenient accessibility via B-O-bond formation/electrophilic aromatic borylation sequences (Scheme 2a; compare the analogous B,N case shown in Scheme 1a). [26][27][28][29][30][31][32][33] A second class of B,O-PAHs with a reasonably large number of members are 1,2-B,O-naphthalenes.…”

“…29,[35][36][37]43,44,47,56 On the other hand, if the β position is embedded into an aromatic benzene ring (as in the cases of the isomeric 2,1-B,Onaphthalenes shown in Scheme 2c) and the B atom is equipped with an appropriate substituent, 57 the resulting B,O-PAHs can become sufficiently inert to serve as luminescent materials for optoelectronic applications. [31][32][33]38,39,41 It is also important to note in this context that the steric demand of an O atom is even lower than that of a CH or NH fragment in analogous B-or B,N-PAHs. This promotes a coplanar conformation between the respective B,O heterocycle and, e.g., B-bonded phenyl substituents and can thus contribute to the increase of conjugation lengths and the extension of the frontier orbitals from the oxaborin moiety to the phenyl ring.…”

BN/BO-Ullazines and Bis-BO-Ullazines: Effect of BO Doping on Aromaticity and Optoelectronic Properties

“…16 Demethylation and electrophilic borylation of 4 with dichlorophenylborane (PhBCl 2 ) led to formation of only demethylation product 2-(1H-pyrrol-1-yl)phenol (method C), presumably due to the more labile nature of the BO bond (see Experimental Section for details). 17 To our delight, BO-pyrrolo[1,2-a]quinoline (5a) can be obtained in good yield by switching the phenyl group to a bulkier mesityl group attached to boron (demethylation− electrophilic borylation, methods A 18 and B 10a,b,19 ). BO-doped ullazines were synthesized following similar procedures (Scheme 1).…”