Complex formation with phenylboronic

Hayashi and Miyaura pioneered the enantioselective rhodium-catalyzed conjugate addition of arylboronic acids to a variety of Michael acceptors a,P-unsaturated ketones, esters, lactones, amides, and lactams [215]. Generally, water is used as a cosolvent and plays a key role in the catalytic cycle, illustrated in Scheme 5.111 (cycle A) for the conjugate addition of phenylboronic acid to cyclohexenone that, when catalyzed by the Rh(I)-(S)-BINAP complex, leads to 3-phenylcyclohexanone in 97% ee and 93% chemical yield [205a]. The key intermediates of the catalytic cycle, the hydroxorhodium complex 433, the phenylrhodium complex 434, and -bound rhodium enolate 435 were characterized by NMR spectroscopy. The reaction of the hydrorhodium complex 433 with phenylboronic acid leads to a transmetallation to give the phenylrhodium complex 434. Then, the insertion of the carbon-carbon double bond of cyclohexenone into the phenylrhodium bond leads to the formation of the... 

Kitano S, Kataoka K, Koyama Y et al (1991) Glucose-responsive complex formation between poly (vinyl alcohol) and poly(lV-vinyl-2-pyrrolidone) with pendent phenylboronic acid moieties. Makromol Chem Rapid Commun 12 227-233... 

Sometimes smaller porphyrin assemblies could be reversibly dissolved via molecular complex formation. The protoporphyrin-bis-amide 17 with two w-phenylboronic substituents dissolves in 1 30 DMSO/water mixtures, but is heavily aggregated in this medium. The Soret band s intensity was only the half of that in pure DMSO solution and the fluorescence was almost nil. However, upon addition of 10 M fructose the carbohydrate was bound as a molecular complex and the porphyrin became more water-soluble whereby the fluo-recence increased drastically. Other monosaccharides had lesser effects. 

Liu studied the activity of cyclopalladated imines in the Suzuki reaction and found them to be extremely active (Fig. 10.13) [129]. He was able to show that reaction of his palladacycle with phenylboronic acid in the presence of K2CO3 and ethanol led to the formation of palladium nanoparticles with a diameter range of 50-60nm (TEM). Upon addition of p-bromoanisole to this solutionp-methoxybi-phenyl was formed quantitatively. Reaction of the same complex with ethanol produced acetaldehyde and also led to the formation of nanoparticles, however, in this case palladium black was soon formed. It thus would appear that aromatic boronic acids are capable of stabUising the palladium nanoparticles. 

The Suzuki-Miyaura cross-coupling reaction is a standard method for carbon-carbon bond formation between an aryl halide or triflate and a boronic acid derivative, catalyzed by a palladium-metal complex. As with the Mizoroki-Heck reaction, this cross-coupling reaction has been developed in ionic liquids in order to recycle and reuse the catalyst. In 2000, the first cross-coupling of a halide derivative with phenylboronic acid in [bmim] [BF4] was described. As expected, the reaction proceeded much faster with bromobenzene and iodobenzene, whereas almost no biphenyl 91 was obtained using the chloride derivative (Scheme 36). The ionic liquid allowed the reactivity to be increased, with a turnover number between 72 and 78. Furthermore, the catalyst could be reused repeatedly without loss of activity, even when the reaction was performed under air. Cross-coupling with chlorobenzene was later achieved - although with only a moderate yield (42%) - using ultrasound activation. 

There has been a wide-ranging review of transmetalation reactions of arenes concentrating on the use of boron-containing compounds. The reaction of arylboronate esters and related compounds with alkyl halides is catalysed by copper(I) formation of an aryl—copper intermediate followed by an 5N2-type reaction with the alkyl electrophile is likely. Palladium complexed with a diimine is an excellent catalyst in the phenylation reaction of Michael acceptors with phenylboronic acid so as to yield products such as (16). The nickel-catalysed reaction of phenylboronic acid with styryl epoxides has been shown to yield a-substituted alcohols such as (17). 

Reception of chemical and biochemical signals can be based on a physical interaction or chemical reaction between functional groups in the polymer and signaling molecules (Motomov et al. 2010). There are many examples of specific complex formation between synthetic polymer materials and ligands, e.g., glucose-responsive polymers with phenylboronic side groups (Lapeyre et al. 2008). 

The reversible formation of boronic esters by the interaction of boronic acids and polyols in water was first examined in the seminal study of Lorand and Edwards [49]. This work followed an equally important study on the elucidation of the structure of the borate ion [124]. By measuring the complexation equilibrium between phenylboronic acid and several model diols and monosaccharides using the method of pH depression, ester formation was shown to be more favorable in solutions of high pH where the boronate ion exists in high concentrations (Equation 19, Figure 1.12). This study also confirmed the Lewis acid behavior of boronic acids and the tetracoordinate structure of their conjugate base, i.e., the hydroxyboronate anion (Section 1.2.2.4). Another conclusion is that free boronic adds have lower Lewis add strengths than their neutral complexes with 1,2-diols. For example, the pJC, of PhB(OH)2 decreases... 

---