Thienyl-boron

Aryl and heteroaryl (furyl, thienyl) boronic acids are especially suitable for the preparation of their iodonium salts, having the added advantage of better yields and lack of toxicity [108]. Tetraarylborates (sodium or potassium) reacted with (diacetoxyiodo)arenes in acetic acid to afford diaryliodonium salts in excellent yield (Scheme 37). It appears that triarylboranes formed upon reaction of the borates with acetic acid serve actually as the real arylating agents [109]. 

The solvent-free, microwave-assisted coupling of thienyl boronic acids and esters with thienyl bromides, using aluminum oxide as the solid support, served to rapidly check the reaction trends on changing times, temperature, catalyst, and base and easily optimize the experimental conditions to obtain the desired product in fair amounts. This procedure offers a novel, general, and very rapid route to the preparation of soluble thiophene oligomers. Quaterthiophene 265 was obtained in 6 min by reaction of 2-bromo-2,2 -bithiophene with bis(pinacolato)diborane(4) in 65% yield, whereas dithiophene 266 was obtained with 70% yield. The synthesis of new chiral 2,2 -bithiophenes also was reported. The detailed... 

HC(44/3)l>. The two most important methods are (i) conversion of thienyllithium to a thienyl-boronic acid by reaction with alkyl borate, followed by oxidation with H2O2, and (ii) reaction of thienylmagnesium derivatives with t-butylperbenzoate, followed by dealkylation of the /-butyl ethers with acid. The boronic acid method has been used for the synthesis of several pyridine-substituted hydroxy thiophenes <92ACS654>. 

Under certain reaction conditions, even activated arylmethylthioethers can be used as electrophiles in the SM reactions. Thus copper(l) 3-methylsalicylate (CuMeSal) does catalyse the cross-coupling reaction of 3-methylthio-l,2,4-triazine (248) with 2-thienyl boronic acid (249) to afford biaryl 250 in 70% yield, respectively [50], Scheme 3. 

To conclude, we should mention the work similar, in essence, in which thienyl-boronic acids or pinacol esters react with 1,2-dichlorohexafluorocyclopentene via the Suzuki-Miyaura coupling [111]. A doubtless advantage of the method is its applicability to various substrates bearing reactive functionalities such as cyano and ester moieties, which cannot be compatible nnder the conventional diarylethene synthesis conditions using organolithium reagents. 

Petasis has demonstrated that aryl and heteroaryl boronic adds partidpate in one step three component Mannich reactions with glyoxylic acid and diarylamines to give the corresponding a-aryl/heteroarylglycine derivatives in good yields after deprotection of the diarylamine [79]. Several examples of the reaction with para-substituted aryl boronic acids as well as 2- and 3-thienyl, -fiiryl or -benzo[b]thienyl boronic acids are described (Scheme 3.53) [80]. 

Thiazoleboronic acids are not trivial to make. Attempts to prepare 2-thiazoleboronic acid were unsuccessful [21], As a consequence, halothiazoles are chosen as the electrophilic coupling partners in the Suzuki reactions with other boronic acids. For instance, 2,5-di-(2-thienyl)thiazole (28) was installed by the union of 2,5-dibromothiazole and easily accessible 2-thiopheneboronic acid [21], Unfortunately, the yield was poor and analogous reactions of 2,5-dibromothiazole with 3-thiopheneboronic acid and 2-selenopheneboronic acid both failed. 

When 2-hydroxymethyl-5,6-methylenedioxybenzo[6]thiophene is heated in a polar solvent with a catalytic amount of mineral acid, the polycyclic compound (166) is formed in 75% yield, probably by way of the intermediate (251).190 3-Hydroxymethylbenzo[6]thiophene and di(3-benzo[6]thienyl)methane react together in the presence of boron trifluoride to give a compound with the probable structure 252.486... 

In the presence of boron trifluoride, organocuprates are added to 1-aza-1,3-butadiene derivatives to give 1,4 addition products in moderate to good yields after reaction with nucleophiles. When the nucleophile is 2-thienyl or 2-furyl, alkylation results in a simple addition on the imine double bond. The intermediate anion is also trapped by an electrophile.229... 

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