Trifluoroborates

Cyclohexyl-l,3-dimethylbenzimidazolium trifluoroborate (433) gave 3-(methanesulfonyl)imino-1,4-dimethyl-1,2,3,4-tetrahydroquinolaline-2-spirocy-clohexane (436), via the well-established stmctures (434 and 435) [KH, THF, 20°C, 24 h then MeS02N3, 20°C, 1 h 37% after separation from a major... 

The optimal reaction conditions were applied with 59d in the addition of various aryl boronic acids and potassium trifluoroborates to several cyclic and acyclic enones (Fig. 8). Arylboronic acids added to cyclic enones in high yields (89-97%) and with good to excellent selectivities (85-98% ee). Under these conditions, the potassium trifluoroborate reagents reacted at faster rates, but with slightly lower selectivities (83-96% ee). The reactions of acyclic enones with aryl boron reagents gave also excellent yields (83-96%). 

Enantiomeric excesses of 81-91% were obtained with the boronic acids and of 73-81% with potassium trifluoroborates. 

Regarding the use of other metals for this transformation, Shirai and co-workers reported that a system constituted by palladium(II) complex [Pd(p-Cl)(r -aUyl)]2 and thioether-imidazolium chloride 19 achieved the arylation of aldehydes with boronic acids [33] and potassium trifluoroborates in good to excellent yields (Scheme 7.5) [34], More recently, Buffard and Itami showed that a NKcod) / IPr-HCl system could catalyse the reaction of arylboronate esters and inactivated aldehydes and ketones (Scheme 7.5) [35]. 

Several other organoboron polymers have been developed by various synthetic strategies and utilized to construct polymeric sensing systems for cations, dopamine, saccharides, and so on. Fabre and co-workers have reported the preparation of a conjugated trifluoroborate-substituted polythiophene system for sensing cations such as... 

Figure 16 Trifluoroborate-substituted thiophene monomer (22) used in the synthesis of a conjugated polythiophene system. (Adapted from ref. 41.)...

A major limitation of these alkylation reactions has been the regiospecificity and/or need for directing groups of the nucleophile. MacMillan has overcome this and expanded the scope of the reaction to include alkene nucleophiles by using trifluoroborate salts (Scheme 18) [87]. This approach enables alkylation of the 2-position of indoles, complimenting the 3-selective alkylation shown in Scheme 16. One equivalent of hydrogen fluoride was found to be necessary in the reaction in order to sequester the boron trifluoride generated. 

Scheme 18 Conjugate addition of trifluoroborate activated 7C-nucleophiles...

Scheme 3.2 Generation of [Cp(PPh3)2Ru=(C)4H2] by protonation of a butadiynyl complex with trifluoroboric acid and subsequent reaction with water.

Non-Sonogashira alkynylation procedures have also been employed with pyrimidines, and selective replacement of the 4- and 6-chlorine atoms of 2,4,6-trichloropyrimidine 258 was able to be achieved using potassium (1-hexyn-l-yl)-trifluoroborate 257 <2002JOC8416>. 

A novel method for the convenient synthesis of alkenyl fluorides 15, as well as diflu-oromethyl-substituted alcohols 16 and amides 17, via electrophilic fluorination with one equivalent of F-Teda BF4 (6) of alkenyl boronic acids and trifluoroborates, has been reported.87 The alkenyl fluorides 15 are obtained as Z/E mixtures when the reaction is carried out with one equivalent of F-Teda BF4 in acetonitrile at room temperature. When the reaction is performed with two equivalents of F-Teda BF4 in water or a nitrile solvent the difluoromethyl-substituted alcohols 16 or amides 17, respectively, are obtained. 

The employment of trityl trifluoroborate is particularly interesting. This reagent is able to introduce trityl groups on both primary and secondary alcohols54 and to selectively oxidize secondary trityl ethers to ketones in the presence of primary trityl ethers.55 Thus, treatment of diols with trityl trifluoroborate leads to tritylation of both alcohols followed by oxidation of the secondary trityl ether, resulting in the formation of a ketone possessing a trityl-protected primary alcohol. A work-up by mild acidic hydrolysis provides the deprotection of the primary trityl ether and formation of a hydroxy ketone.54... 

Trityl trifluoroborate produces the tritylation of both alcohols and the regioselective oxidation of the resulting secondary trityl ether. The primary trityl ether is hydrolyzed on contact with silica gel during the work-up, resulting in the formation of an 80% yield of the... 

The Suzuki Coupling, which is the palladium-catalysed cross coupling between organoboronic acid and halides. Recent catalyst and methods developments have broadened the possible applications enormously, so that the scope of the reaction partners is not restricted to aryls, but includes alkyls, alkenyls and alkynyls. Potassium trifluoroborates and organoboranes or boronate esters may be used in place of boronic acids. Some pseudohalides (for example triflates) may also be used as coupling partners. 

Lewis acids and Bu4NI catalyzed allylboration with potassium allyl- and crotyltrifluoroborates (Equation (154)).30,40 620,621 The reaction of pinacol ester derivatives was very slow even at room temperature, but Sc(OTf)3 smoothly catalyzed the addition at — 78 °C with high diastereoselectivity (Equation (155)622 and (156)623,624). A palladium pincer complex catalyzed the addition of trifluoroborate to tosylimines (Equation (157)).625... 

Pd(acac)2 efficiently catalyzes the Suzuki-type reaction of potassium alkynyl trifluoroborates 232 with vinylic tellurides (Scheme 127).303 This reaction presents the advantage of using compounds of the type 232, which can be isolated and manipulated in the air. 

A-(4-Pentenyl)iodoacetamides (i.e., 142 and 145) underwent atom transfer radical cyclization reactions in the presence of trifluoroborate etherate at room temperature, usually with an excellent 8-endo regioselectivity (Schemes 59 and 60 <2005JOC1539>). It was thus shown that 8-endo cyclization of iV-alkenyl a-carbamoyl radicals is an intrinsically favored process easily promoted by a Lewis acid, such as BF3-OEt2. 

The reactions take place under mild conditions with good to high yields. Another important advantage is the use of relatively nontoxic reagents. In the synthesis of the alkenyl fluorides, mixtures of Z/E isomers are obtained. The substrate trifluoroborates can be prepared from the corresponding boronic acids by reaction with potassium hydrogen fluoride/water. ... 

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