Boron-stabilized reactions with metal halides

Cellulose is a natural biopolymer, which is biodegradable, environmentally safe, widely abundant, inexpensive, and easy to handle [57]. Cellulose and its derivatives are widely used in chemical and bio-chemical applications and also as supports for the synthesis of organic molecules [58]. Interestingly, the cellulose fibers also act as a nanoreactor for the stabilization of metal nanoparticles [59]. However, its use as a support for catalytic applications is not well explored. Recently, Choplin and coworkers reported cellulose as the support for water soluble Pd(OAc>2/5 TPPTS system in the Trost-Tsuji allylic alkylation reaction [60]. To corroborate the above concept in the cross coupling of aryl halides and boronic acids, we reported A-arylation of imidazoles with aryl halides using a cellulose-supported Cu(0) catalyst (CELL-Cu(O) [61]. The prepared catalyst was well characterized using various instrumental techniques. For example, the X-ray diffraction pattern of CELL-Cu(O) catalyst clearly indicates the presence of Cu (111) and Cu (200) phases which are attributed to Cu(0) [46]. Further, the high resolution XPS narrow scan spectrum of the fresh CELL-Cu(O) catalyst shows a Cu 2p3/2 peak at 932.72 ev, which is attributed to Cu (0) [22]. 

One of the earliest methods for preparing aromatic boronic acids involved the reaction between diaryl mercury compounds and boron trichloride [198]. As organomer-curial compounds are to be avoided for safety and environmental reasons, this old method has remained unpopular. In this respect, trialkylaryl silanes and stannanes are more suitable and both can be transmetallated efficiently with a hard boron halide such as boron tribromide [199]. The apparent thermodynamic drive for this reaction is the higher stability of B-C and Si(Sn)-Br bonds of product compared to the respective B-Br and Si(Sn)-C bonds of substrates. Using this method, relatively simple arylboronic acids can be made following an aqueous acidic workup to hydrolyze the arylboron dibromide product [193]. For example, some boronic acids were synthesized more conveniently from the trimethylsilyl derivative than by a standard ortho-metallation procedure (entry 11, Table 1.3). 

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