Aryltrifluoroborate Suzuki coupling

The Pd-catalysed Miyaura-Suzuki coupling of aryltrifluoroborates with 4-tosyloxycoumarins in aqueous conditions offers an attractive route to 4-arylcoumarins <06TL1525> and 4-hydroxycoumarins have been converted into a mixture of predominantly thiopyrano[5, 4 3,4]- pyrano[5,6-c]coumarins and [6,5-c]chromones in a one-pot tandem Knoevenagel - HDA sequence with an S-prenylated 1-phenyl-lff-pyrazole-4-carbaldehyde <06TL2265>. The benzopyrano[4,3-c]benzopyranone system can be obtained from 3-aryl-4-methylcoumarins by deprotonation of the methyl function and subsequent elaboration <06TL5909>. 

Molander has published a series of papers demonstrating the utility of potassium alkyl, alkenyl-, alkynyl-, and aryltrifluoroborates in palladium coupling reactions. The crystallinity and air-stability of these trifluoroborate salts make the use of these an interesting alternative to the use of boronic acids or esters. Good yields have been obtained in several related palladium coupling processes, which are most easily classified as Suzuki couplings. The broad applicability of this process is demonstrated by the production of 58 [40], 59 [41], and 60 [42]. 

Azolecarbene ligands are shown to be favorable to Suzuki coupling involving polyfluori-nated aryltrifluoroborate salts (e.g., C6F5BF3K) catalyzed by (Ph3P)2PdCl2 under anaerobic conditions. 

One-pot Synthesis of Arylboronic Acids and Aryltrifluorobo-rates. Hartwig also reported the synthesis of arylboronic acids and aryltrifluoroborates in a one-pot sequence by Ir-catalyzed borylation of arenes followed by the oxidative cleavage of the boronate ester with NaI04 and the displacement of pinacol by KHF2, respectively (eq 4). These two-step sequences give products that are more reactive and selective in the subsequent chemistry, such as Suzuki coupling, than the initially formed pinacol-boronates. 

Aryliodoninm salts, Aral X [50], and some other hypervalent aryliodonium compounds take a part in further important alternative cross-coupling reactions with arylboronic acids [51], aryltrifluoroborates [52], triarylbismuth(V) compounds [53], and diaryltellurium dichlorides [54] to afford biaryls in good to excellent yields. For example, hypervalent iodine compoimd 466, readily produced from 2-iodobenzoic acid, reacts with arylboronic acids in palladium-catalysed process in Suzuki-Miyaura-fashion to give the 2-arylbenzoic acids in high yields [51], Table 12. 

Thiophenediazonium salts 163 can be utiUzed as electrophilic coupling partners for differently functionaUzed alkenes 164 via a Matsuda-Heck (Scheme 64, left) as well as various potassium aryltrifluoroborates via Suzuki-Miyaura (Scheme 64, right) cross-coupling reactions giving rise to 3-substituted thiophenes 165 and 166... 

Molander, G. A. Ellia, M. D. 2006. Suzuki-Miyaura cross-coupling reactions of benzyl halides with potassium aryltrifluoroborates. 7. Org. Chem. 71 9198-9202. 

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