4- phenylboronic acid

Phenylboronic acid. The orientation of nitration in phenylboronic acid is very susceptible to changes in the medium (table 5.8). The high proportion of o-substitution in acetic anhydride is not attributable to a specific o-reaction, for the nt -ratios of the last tabulated pair of results are not constant. The marked change in the ratio was considered to be due to the formation in acetic anhydride of a complex, as illustrated below, which is 0 -orienting and activated as a result of the -t-1 effect. This species need only be formed in a small concentration to overwhelm... 

Substituents containing boron are of interest because of the possibility which the boron atom offers of conjugation of a vacant orbital with the 77-electrons of the benzene ring (—717). The case of phenylboronic acid has been discussed ( 5.3.4). 

Various phenols can be selectively hydroxymethylated at the ortho position by heating with paraformaldehyde and phenylboronic acid. 

Ether, water, phenylboronic acid. Cleavage occurs by transesterification. 

A/,A/ -3,5-Bis(trifluoromethyl)phenylboronic Acid Derivative, 594 A/-[Phenyl(pentacarbonylchromium- or-tungsten)]carbenyl, 595 A/-Copper or A/-Zinc Chelate, 595 18-Crown-6 Derivative, 596... 

Phenylboronic acid in particular has proved invaluable, since its complexes with cis-diols and -polyols have formed the basis... 

More recently, a double reductive ring closure of the dinitrodiphenylbenzene 139 with triethyl phosphite as the reducing agent has been reported to produce 3 in 59% yield (Scheme 18). The precursor 139 was prepared in excellent yield by Pd(0)-catalyzed coupHng of phenylboronic acid and 1,3-dibromo-4,6-dinitro-benzene (92JHC1237). 

The parent system 4 has also been jnepared employing a double reductive ring closure of a dinitrodiphenylbenzene derivative (Scheme 25). The required starting material, 195, was prepared using a palladium-catalyzed coupling of 1,4-dibromo-2,5-dinittobenzene with phenylboronic acid and after reaction with tri-ethylphosphite in hot tert-butylbenzene, 4 could be isolated in a moderate yield (92JHC1237). 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

For the synthesis of a suitable arylboron compound, usually an aryl halide is converted to an aryllithium or aryl Grignard derivative, and then reacted with a trialkoxyborane to yield an arylboronic ester, e.g. the phenylboronic acid diisopropyl ester 13 from bromobenzene 11 ... 

Reaction of methyl a-L-rhamnopyranoside with triphenylboroxole gave a syrupy boronate ester which was characterized as a crystalline phenyl-carbamate. Removal of the phenylboronic acid residue gave a product identified as methyl a-L-rhamnopyranoside 4-N-phenylcarbamate, since it was identical with that resulting from removal of the ketal group from methyl 2,3-O-isopr opylidene-a-L-rhamnopyranoside 4-N-phenylcarbamate (12). This establishes the structure of the original ester as methyl a-L-rhamnopyranoside 2,3-phenylboronate (24). 

A careful assessment of the constitution of compound 10 led to the development of a rather efficient strategy featuring the Diels-Alder reaction (see Scheme 3). Although the unassisted intermole-cular reaction between 3-hydroxy-2-pyrone (16)23 and a,/ -unsatu-rated ester 17 is unacceptable in terms of both regioselectivity and chemical yield, compounds 16 and 17 combine smoothly in refluxing benzene and in the presence of phenylboronic acid to give fused bicyclic lactone 12 (61% yield) after workup with 2,2-... 

Although there are several reports in the literature on boron-mediated amide formations, the boron reagents had to be used in stoichiometric amounts.1-4-5-6-7-8-9 Recently, Yamamoto et al. presented the first truly catalytic method allowing for a direct amide formation from free carboxylic acids and amines as the reaction partners.10-1112 Best results were obtained by using phenylboronic acids bearing electron withdrawing substituents in the meta- and/or para-positions such as 3,4,5-trifluorophenylboronic acid or 3,5-bis(trifluoromethyl)boronic acid as the catalysts. 

The Suzuki reaction has been successfully used to introduce new C - C bonds into 2-pyridones [75,83,84]. The use of microwave irradiation in transition-metal-catalyzed transformations is reported to decrease reaction times [52]. Still, there is, to our knowledge, only one example where a microwave-assisted Suzuki reaction has been performed on a quinolin-2(lH)-one or any other 2-pyridone containing heterocycle. Glasnov et al. described a Suzuki reaction of 4-chloro-quinolin-2(lff)-one with phenylboronic acid in presence of a palladium-catalyst under microwave irradiation (Scheme 13) [53]. After screening different conditions to improve the conversion and isolated yield of the desired aryl substituted quinolin-2( lff)-one 47, they found that a combination of palladium acetate and triphenylphosphine as catalyst (0.5 mol %), a 3 1 mixture of 1,2-dimethoxyethane (DME) and water as solvent, triethyl-amine as base, and irradiation for 30 min at 150 °C gave the best result. Crucial for the reaction was the temperature and the amount of water in the... 

Remarkably, one year later Leadbeater described that biaryls can be synthesized via a Suzuki-type coupling under transition-metal free conditions [51, 52]. The reaction conditions were almost identical to those reported for the ligand-free process, with the difference being that a larger amoimt of Na2C03 and arylboronic acid were used. Only one successful example of a heteroaryl haUde substrate is shown namely, the coupling of 2-bromopyridine with phenylboronic acid (Scheme 32). 3-Bromothiophene did not couple under the same reaction conditions. Unfortimately, attempts to use heteroarylboronic acids such as 3-pyridinylboronic acid, 3-thienylboronic acid, and lH-indol-5-ylboronic acid on 4-bromoacetophenone completely failed. 

Ethyl 6-methyl-4-phenyl-2-(phenylthio)-l,4-dihydropyrimidine-5-carboxyla-te was easily synthesized from ethyl 6-methyl-4-phenyl-2-thioxo- 1,2,3,4-tetra-hydropyrimidine-5-carboxylate and phenylboronic acid via microwave-assisted Cu-mediated S-phenylation (Scheme 108) [107]. The reaction involves the use of a stoichiometric amount of Cu(OAc)2 and a two-fold excess of 1,10-phenanthroline as a hgand. 

Alternatively, 3-phenyl pyrazinone was prepared via Suzuki reaction, when a polymer-bound pyrazinone was irradiated with 4 equiv of phenylboronic acid, 5 equiv of Na2C03 and 20 mol % of Pd[P(Ph)3]4 as the catalyst in DMF as the solvent (Scheme 36). Contrary to the results obtained in solution phase [29], all attempts to drive the reaction toward the formation of disub-stituted compound, using higher equivalents of reagents or longer reaction times, were unsuccessful. Apphcation of aqueous conditions afforded mixtures of 3-mono and 3,5-disubstituted pyrazinones. 

From 2,6-diacetylpyridine dioxime, ferric chloride hydrate, and phenylboronic acid as starting materials the macrocyclic dinuclear iron(ll) complexes 133 can be prepared (Fig. 36). 

The MH-type reaction of phenylboronic acids and conjugated olefins, such as butyl acrylate, acrylnitrile, and methyl vinyl ketone under Pd(0)-catalyzed... 

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