Vinylboronic acid, reaction

The utility of the Suzuki reaction in the challenging arena of natural product total synthesis has been explored. The constitution of bombykol (106) (see Scheme 26), a well-known pheromone, lends itself to a Suzuki coupling. Indeed, in a short stereospecific synthesis of 106, Suginome et al. demonstrated that ( )-vinylboronic acid ( )-104 can be smoothly cross-coupled with (Z)-l-pentenyl bromide [(Z)-105] 44 the configurations of both coupling partners are preserved in the C-C bond forming process. 

A series of remarkable stoichiometric CH functionalization reactions have been employed, including transmetalla-tion with a vinylboronic acid moiety, to synthesize the core of teleocidin B-4 (Equation (200)).1... 

The Mannich reaction of secondary amines with paraformaldehyde and vinylboronic acids gives excellent yields of pure (E)-allylamines, e.g. equation 63181. 

When vinylboronic acids are used as reactants, bases, especially Tl(OH)3, can accelerate the reaction.143 Scheme 8.11 gives a number of examples of coupling using organoborane reagents. 

The Heck reactions of thiophenes, particularly in the 2-position are well documented. In a recent example 2-bromothiophene was converted into a thienyl-vinylboronic acid derivative using a conventional palladium-triphenylphosphine catalyst and tributylamine as base (6.59.),... 

With the TES protected alcohol at C-19 and the free alcohol at C-15 the subsequent reactions could be performed. Hydroboration of the triple bond and aqueous work-up creates vinylboronic acid 10, which is the other coupling partner with 6. 

Cross-coupling reactions with vinylboronic acids can yield either the normal product (ipso-substitution of boron) or a regioisomer formed via a Heck-type reaction (cine-substitution Scheme 8.17) [135]. Formation of the normal product (1-phenylhexene in Scheme 8.17) requires a base capable of binding to the boronic acid, thereby increasing the nucleophilicity of the boron-bound carbon atom (typically ROM, MOH, M2C03, M3P04, where M= alkali metal [136]). Products of cine-substitution result when tertiary amines are used as bases, i.e. under Heck-type reaction conditions. 

Scheme 8.17. Regioselectivity of cross-coupling reactions with vinylboronic acids and vinylstannanes [9, 144].

Silver salts are also common activators in numerous Suzuki coupling reactions. The earliest example could probably be found in the Kishi s palytoxin synthesis. Silver oxide as well as thallium hydroxide provided dramatic rate enhancements in the couplings of vinylboronic acids (Scheme 10.41).69 Both thallium and silver ions are most probably abstracting halide in palladium intermediates, but silver is clearly the most efficient. Moreover, with the right counterion, the silver salt also acts as a mild base and activator. 

Radioiodination of vinylboronic acid derivatives has been applied443-445 for the synthesis of 17 tf-iodovinylestradiol derivatives443 (equations 206) and (E)-1 -[123I]iododecene, 306 (equation 207). In the iodination of a vinylboronic acid, chloramine-T has been found to be the most efficient oxidant under both carrier-added and NCA conditions and has been applied in reaction 207. 

Trialkylboranes were converted into alkyl phenyl selenides and alkyl phenyl tellurides by reactions with PhSeSePh and PhTeTePh in the presence of stoichiometric amounts of air.550 Vinyl selenides and tellurides were synthesized by treating vinylboronic acids or esters with phenylselenyl chloride in ionic liquids (Equation (114))551 or by palladium-catalyzed coupling reaction of diorgano ditellurides (Equation (115)).552... 

Multi-component procedures based upon the Mannich reaction and boron reagents were reported in 1993 as a reaction between vinylboronic acids and the adducts of secondary amines and paraformaldehyde (Table 5, entries 1 and 2).558... 

Based on our experience in using A1203 as base in the iodination15 and bromination16 of vinylboronic acids, we hoped that this solid would likewise function as base in the Suzuki reaction. To simplify the reaction, we hoped to use ligandless palladium black [Pd(0)] as catalyst, thus alleviating the need for expensive or custom-made phosphine ligands. 

The cross-coupling of allyl alcohols with aryl- and vinylboronic acids has been accomplished in [C4Ciim][BF4] and [C4Ciim][PF6] with rhodium catalysts.1"31 While no reaction takes place in polar solvents such as water or DMF, good results were obtained in [C4Ciim][PF6], see Scheme 6.12, and rhodium(I) as well as rhodium(III) compounds worked well in the latter solvent. The presence of copper salts as well as acids led to an increase of the reaction rate. 

A convincing reaction from organometallic chemistry was a long time coming. Petasis and Zavialov now report [7a-b] that vinylboronic acids 18 [7c-d], which are readily accessible and easy to handle, can participate in a three-component reaction with primary and secondary amines 19 and with a-keto acids 20. The process, which proceeds at room temperature, leads to )ff,y-unsaturated a-amino acids 21 in good yields. The reaction of vinylboronic acid 22 with the chiral amino alcohol 23 and glyoxylic... 

Monoalkylthallium compounds are usually prepared either by the reaction of dialkylthallium carboxylates and mercury(II) carboxylates (equation 22), or by reacting dialkylthallium halides with Br2 or I2 (equation 23). Aryl- and vinylthallium dichlorides are prepared conveniently by the reaction of aryl- and vinylboronic acid, respectively, with excess TICI3 in aqueous solution (equation 24). > ... 

Vinylboronic acids undergo analogous reactions to give the mono- and divinyl-thallium derivatives in high yields " with retention of configuration, e.g., heating a 1 1 cis-propenylboronic acid and Tl(III) chloride in aq ether for 8.5 h produces 30% cis-propenylthallium dichloride and 30% bis(cis-propenyl)thallium chloride. The same reaction with trans-propenylboronic acid produces 28% trans-propenylthallium dichloride and 67 % bis(trans-propenyl)thallium chloride. 

A variety of 2-substituted 2//-chromenes can be obtained from the facile reaction of 2-hydroxybenzaldehydes with vinylboronic acids in ionic liquid solvents <04SL2194>. In a one-pot sequence also in an ionic liquid, a Knoevenagel condensation between O-prenylated salicylaldehydes and 4-hydroxycoumarins is followed by an intramolecular hDA reaction to yield ci5-fused chromano[4, 3 4,5]pyrano[3,2-c]coumarins e.g. 20 small amounts of the corresponding chromone are also formed <04S1783>. In like manner, cii-fused furopyranopyran derivatives have been obtained from sugar aldehydes <04TL3493>. 

Allylamines. Alkeneboronic acids are converted to allylamines of defined configuration on treatment with conventional Mannich reaction components (R2NH, paraformaldehyde). The required vinylboronic acids are available from 1-alkynes by reaction with catecholborane followed by hydrolysis. 

An electronically and sterically diverse array of arylboronic acids serve as useful reaction partners (Table 2, entries 1-4). In addition, vinylboronic acids can be cross-coupled in good yield (entry 5), although reactions of alkylboronic acids proceed with somewhat lower efficiency (entry 6). The catalyst tolerates a range of functional groups, including esters, thioethers, and cyanides. 

The direct coupling of aryl- and vinylboronic acids has been achieved by simple Rh(i) and Rh(iii) compounds in [G4GiIm]PF6 (Scheme 44), giving higher yields than the reactions performed in toluene of dichloromethane. ... 

Suzuki-Miyaura coupling that is the reaction of an aryl- or vinylboronic acid with an aryl or vinyl halide catalyzed by a palladium(O) complex (Figure 1.1) [63]. 

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