Aryltrifluoroborates

Figure 3.36. Scope of Rh/(i )-binap-catalyzed asymmetric 1,4-addition of potassium aryltrifluoroborates to acyclic a,P-enamides.

The reaction which is accelerated by the presence of water can be carried out in water when a 2/3-cyclodextrin-[Rh(OH)(cod)]2,943,944 [Rh(cod)Cl]2/TPPTS,945 or amphiphilic resin-supported rhodium(i) complex was used.946 It was also reported that potassium alkenyl and aryltrifluoroborates [RBF3]K undergo similar addition to enones in the presence of an Rh(i) catalyst.935,947,948 The cationic rhodium(i) complexes such as [Rh(cod)]BF4, [Rh(cod)(CH3CN)2]BF4, or its combination with a diphosphine ligand (dppp or dppb) efficiently catalyzed the reaction. The neutral complexes, generated in situ from Rh(acac)(GH2=GH2)2, or Rh(acac)(coe)2, and dppp or dppb, were also effective. The effects of catalysts, solvents, and bases,949 and the mechanism of catalytic cycle950 have been studied in detail. 

The formation of an aryl-O bond by the reaction of a phenol with air and moisture stable potassium alkenyl and aryltrifluoroborates was mediated by a copper species. The reaction procedure involved catalytic amounts of Cu(OAc)2 with DMAP as a ligand in the presence of oxygen and molecular sieves. A variety of aliphatic primary and secondary alcohols as well as phenols were suitable reaction partners, thus displaying a broad functional group tolerance (Equation (237)).1025... 

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>. 

In a related reaction, aryltrifluoroborates PhBF3 X (12-28), are coupled to aryl halides with a palladium catalyst to give the biaryl. ... 

Molander, G. A., Biolatto, B. Efficient Ligandless Palladium-Catalyzed Suzuki Reactions of Potassium Aryltrifluoroborates. Org. Lett. 2002,... 

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]. 

Potassium aryltrifluoroborates proved to be useful candidates for microwave-promoted Suzuki reactions in aqueous environments (Scheme 1.67). ... 

Wu J, Zhang L, Luo Y (2006) Rh(I)-catalyzed cross-coupling reactions of alkenyl tosylates with potassium aryltrifluoroborates. Tetrahedron Lett 47 6747-6750... 

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. 

Alkylations. Allylation of aldehydes with the stable allyltrifluoroborate reagent proceeds at low temperatures by using BF3-OEt2 as catalyst. Crotylation is performed analogously. In the presence of (acac)Rh(CO)2 and a phosphine the alkenyl- and aryltrifluoroborates add to aldehydes and enones, the latter in a conjugate fashion. ... 

Recently, Zhang and coworkers reported a direct palladium-catalyzed C-2 arylation of indoles with potassium aryltrifluoroborate salts [204]. Remarkably, the direct arylation took place at room temperature when acetic acid was used as the solvent. 

Phenylboronic acid (17) and its derivatives are widely used. Boronic acids are sometimes difficult to purify because they undergo cyclotrimerizadon with loss of water to form boroxines. On the other hand, organotrifluoroborate salts 18 are easily prepared, purified, and handled [23]. Aryltrifluoroborate salts 18 are prepared by the reaction of arylboronic acid with HF and base [24]. Alkenyltrifluorobo-rates 19 are prepared by hydroboration of 1-alkynes, followed by treatment with KHF2 [25]. 

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. 

Table 13. Synthesis of biatyls from diaryliodonium salts and potassium aryltrifluoroborates [52]...

The use of potassium aryltrifluoroborate salts as useful bench-stable alternatives to arylboronic acids has been demonstrated in O—H bond arylations (vide supra),... 

Scheme 7.5 Cross-coupling reaction between aryidiazonium tetrafluoroborates 7 and potassium aryltrifluoroborates 8 using two different catalytic species.

SCHEME 19.4S Suzuki reactions from potassium aryltrifluoroborates (1999). 

Later, the same authors investigated the enantioselective niekel-eata-lysed addition of aryltrifluoroborates to aromatie aldehydes in the presenee of the same ligand [(i ,i )-Et-DUPHOS]. Indeed, convenient potassium... 

Scheme 7.12 Addition of aryltrifluoroborates to aromatic aldehydes in the presence of an in situ generated nickel catalyst derived from the Et-DUPHOS ligand.

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