2- Pyridylboronic ester

Under the phase-transfer catalysis conditions, 2-bromo-8-methylquino-line (67) was coupled with 2-pyridylboronic ester 68 to furnish 2-(2-pyridyl)-8-methylquinoline (69) in 56% yield (91JOC6787). At this point, it is opportune to mention that the simple 2-pyridylborane, in contrast to 3- and (4-pyridyl)boranes, is considered an unsuitable Suzuki coupling partner because it forms an unusually stable cyclic dimer resembling a dihydroanthracene. In this case, the obstacle was circumvented by using 2-pyridylboronic ester in place of 2-pyridylborane (Scheme 9). 

Platinum complexes, 39-43 Pnicogen atoms coordinated to E-E bonds, homoleptic dielement compounds with, 61-65 Polysilanes, 1-52 characterization, 34-37 mechanistic proposals, 13-18 Primary alkylsilanes, condensation, 23 Primary arylsilanes, 18-22 Primary silanes, 18, 22-24 condensation by metallocene catalysts, 19 Protic reagents, 170 2-Pyridylboronic esters, 223... 

SYNTHESIS OF 3-PYRIDYLBORONIC ACID AND ITS PINACOL ESTER. APPLICATION OF 3-PYRIDYLBORONIC ACID IN SUZUKI COUPLING TO PREPARE 3-PYRIDIN-3-YLQUINOLINE... 

B. 3-(4,4,5,5-Tetramethyl-[l,3,2]dioxaborolan-2-yl)pyridine. A 250-mL, one-necked, round-bottomed flask equipped with a magnetic stirbar and a Dean-Stark trap fitted with a condenser capped with a nitrogen inlet adaptor is charged with tris(3-pyridyl)boroxin-0.85 H20 (3.0 g, 9.1 mmol), pinacol (4.07 g, 34.4 mmol) (Note 6), and 120 mL of toluene. The solution is heated at reflux for 2.5 hr in a 120°C oil bath. The reaction is complete when the mixture changes from cloudy-white to clear. The solution is then concentrated under reduced pressure on a rotary evaporator to afford a solid residue. This solid is suspended in 15 mL of cyclohexane (Note 7) and the slurry is heated to 85°C, stirred at this temperature for 30 min, and then allowed to cool slowly to room temperature. The slurry is filtered, rinsed twice using the mother liquors, washed with 3 mL of cyclohexane, and dried under vacuum to afford 4.59 g (82%) of 3-pyridylboronic acid pinacol ester as a white solid (Note 8). 

An alternate approach to the formation of pyridylboronic acids is the cross-coupling of a halopyridine with a diboronate ester (usually bis(pinacolato)diboron, 7.7.)9 The analogous reaction of 2-chloropyridine led to pyridine formation through protodeboronation. The product of the reaction, either after hydrolysis to the boronic acid or in the ester form, can be further reacted with another aryl halide to give a biaryl. In certain cases the reaction might also be carried out in a one-pot manner.10... 

An example of transfer of the phenyl group from triphenylphosphine (e) is seen in the coupling of the phenyldiethanolamine ester of 2-pyridylboronic acid, where this side reaction represents about 20% of the product, however using tri-o-tolylphosphine circumvents the problem (4.2.7.6). 

Me (2), 4-F (1.7). For arylboronic acids sensitive to water, the reaction under nonaqueous conditions is desirable. The coupling of 2-formyl-phenylboronic acid with 2-iodotoluene at 80 °C in an aqueous solution of Na COj and DME results in a 54% yield of biaryl accompanied by benzaldehyde (39%). The yield, however, improves to 89% with the 1,3-propandiol ester of boronic acid and anhydrous K3PO4 suspended in DMF at 80 °C (Eq. 36). 2-Pyridylboronic acid is highly susceptible to... 

Conceptually, this was first demonstrated using a metallated porphyrin that was functionalized with 2 diols (zinc dicatechol porphyrin) (Fig. 5). After addition of 3-pyridylboronic acid, absorption spectroscopy indicated that the pyridyl moiety was coordinated to the Zn-porphyrin. The observed affinity constant for this interaction, however, was more than 30 times what was expected for a simple pyridyl-Zn-porphyrin complex. It was, therefore, reasoned that the diols reacted with the boronic acids to afford the esters that would result in a cyclic structure (Fig. 5). Indeed, vapor phase osmometry (VPO) confirmed the 2 2 dimeric nature of the complex. Here the ester served as the key covalent linkage, though associated coordination was required between the metal and pyridine to create the cyclic structure. 

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