Arylboron fluorides, preparation

Reactions of Organic Fluorine Compounds Table 29. Preparation of Alkyl and Arylboron Fluorides [112 ... 

The aryl bromide 40, prepared from cross-coupling between 1,2-dibromobenzene and (trimethylsilyl)acetylene, was converted to the corresponding arylzinc halide 41a and arylboronic acid 41b for subsequent coupling with the haloallenes 42 to produce the benzannulated enyne-allene 43 in -40% yield (Scheme 20.10) [38]. Desilylation with tetrabutylammonium fluoride (TBAF) then afforded 44 in 67% yield. 

Aryl fluorides and 19 are prepared by ipso substilution of arylboronic acid.s or... 

Aryl fluorides 1635 -37 and 1936 are prepared by ipso substitution of arylboronic acids or esters with cesium fluoroxysulfate (Tables 6 and 7).35-37... 

The two key catalytic intermediates have been observed by electrospray mass spectrometry [394]. Although the exact role and influence of the base remains unclear [395], the transmetallation is thought to be facilitated by base-mediated formation of the tetracoordinate boronate anion [396], which is more electrophilic than the free boronic acid (Sections 1.5.1 and 1.5.2). A useful carbonylative variant has also been developed to access benzophenones (Equation 70) [397], which can also be produced from the coupling of acid chlorides [398] or anhydrides [399], A variant of this method allows the preparation of a, 3-unsaturated esters from alkenylboronic esters [243]. In all of these reactions, one dreaded limitation with some ortho-substituted and electron-poor arylboronic acids is the possible occurrence of a competitive protolytic de-boronation, which is exacerbated by the basic conditions and the use of a transition metal catalyst (Section 1.5.1). Methods to minimize this side reaction were developed in particular the use of milder alternative bases [400] such as fluoride salts [401], and... 

A ligandless Suzuki coupling reaction was described between arylboronic acids and aryl iodides in the presence of palladium powder and potassium fluoride to provide coupled products in excellent yields. Importantly, the palladium metal can be recovered and recycled by a simple decantation of the reaction solution. In one series of experiments, Kabalka et al. carried out eight consecutive preparations of 4-methyl-biphenyl with no significant loss in product yields. In each case, the palladium powder was recovered by decantation, washed with methanol, and the experiment repeated (Equation 88) [126]. 

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