Boron acid, bifunctional

Pu reported the synthesis of axially chiral-conjugated polymer 82 bearing a chiral binaphthyl moiety in the main chain by the cross-coupling polymerization of chiral bifunctional boronic acid 80 with dibromide 81 (Equation (39)). The polymer is soluble in common organic solvents, such as THE, benzene, toluene, pyridine, chlorobenzene, dichloromethane, chloroform, and 1,2-dichloroethane. The polymer composed of racemic 80 was also synthesized, and the difference of characteristics was examined. Optically active polymer 82 was shown to enhance fluorescence quantum yield up to = 0.8 compared with the racemic 82 ( = 0.5). Morphologies of the optically active and racemic polymers were also compared with a systematic atomic-force microscopy (AEM). 

Alkynyl halides are possible monomers for the cross-coupling polymerization, in which boronic acids are used as the organometallic counterparts. For example, bifunctional boronic acid 46 is allowed to react with l,4-di(bromoethy-nyl)benzene 138 to afford the corresponding PAE 139 as shown in Equation (64). Polymerization proceeds at room temperature in toluene in the presence of silver(i) oxide as an activator of the boron reagent. The polymer 139 is obtained in 30-50% yield showing color of red-brown to deep red-brown and slight solubility in toluene (<0.1 wt.%). The molecular weight (Mr of 139 was 1700-4300 (PDI = 1.3-3.6). 

Bifunctional boronic acids have been investigated as potential catalysts in the synthesis of chiral amides by kinetic resolution of amines the synthesis of such catalysts was documented in 2008 by Whiting, and their application shown. 10 mol% of the catalyst ((pS)-2-(2-boronoferrocenyl)-N- -butylbenzimidazole) (Scheme 17.5) was still required and long reaction times of 48 hours were preferred in order to obtain reasonable conversions. However, under these conditions, the enantiomeric excess remained fairly low, reaching 41% ee with 21% conversion after 48 hours. 

Scheme 17.5 Kinetic resolution of amines with bifunctional boronic acid catalysts.

Naphthalene-based bifunctional Lewis acids that involve boron and a heavier group 13 element have also been prepared starting from the boron/tin derivative 30 (Scheme 15). Thus, the transmetalation reaction of 30 with gallium trichloride or indium trichloride in tetrahydrofuran (THF) results in high yields of l-(dichlorogallium)-8-(dimesitylboron)naphthalenediyl 35 and l-(dichloroindium)-8-(dimesitylboron)... 

Scheme 6.82 Proposed reactive complex of the Petasis reaction utilizing a-hydroxy aldehydes, amines, and organic boronic acids (A) and bifunctional mode of action of chelating thiourea catalyst 65 in the enantioselective Petasis-type 2-vinylation of N-acetylated quinolinium ions (B).

Scheme 6.141 Mechanistic proposal for the 121-catalyzed asymmetric intramolecular Michael addition exemplified for the model substrates ( )-4-hydroxy-l-phenyl-2-buten-l-one (n = 0) and ( )-5-hydroxy-l-phenyl-2-buten-l-one (n = 1) 121 functions as push/pull-type bifunctional catalyst inducing the cyclization of boronic acid hemiester (1) to form intermediate (2) release ofdiol product (3) by oxidation.

Reactions between a representative range of alkyl- and aryl-amines and of aliphatic and aromatic acids showed that the direct formation of amides from primary amines and carboxylic acids without catalyst occurs under relatively low-temperature conditions (Scheme 1). The best result obtained was a 60% yield of N-bcnzyl-4-phenylbutan-amide from benzylamine and 4-phenylbutanoic acid. For all these reactions, an anhydride intermediate was proposed. Boric and boronic acid-based catalysts improved the reaction, especially for the less reactive aromatic acids, and initial results indicated that bifunctional catalysts showed even greater potential. Again, anhydride intermediates were proposed, in these cases mixed anhydrides of carboxylic acids and arylboronic acids, e.g. (I).1... 

The rhodium-catalysed enantioselective 1,4-addition of arylboronic acids to the bifunctional Michael acceptors (192) in the presence of phosphoramidites (194) occurs regioselectively at the endocyclic C=C bond and in up to 95% ee. The presence of KOH is required to increase the reactivity so that less boronic acid and lower reaction temperatures can be used.241... 

A much higher and more efficient chiral version (>90%) of the cyclopropanation of allylic alcohols is obtained by using the amphoteric bifunctional ligand (R,R)-93 prepared from commercially available (+)-(/ ,7 )-At,TV,A, A -tetramethyltartaric acid diamide and butyl-boronic acid. The dioxyborolane chiral ligand proved to be extremely effective with several types of substituted allylic alcohols 92. The chiral ligand 93 can easily be removed and recovered (> 80%) by a simple aqueous extraction of the organic layer after the reaction. 

The determination of bifunctional compounds was reviewed by Poole and Zlatkis [267]. Only a few reagents capable of forming cyclic derivatives with bifunctional compounds have been described in the literature. Boronic acids are applicable to a wide range of bifunctional compounds. All of these reactions are highly selective, and some reagents have been developed that also have a high detection selectivity. These are of particular interest for the analysis of a few bifunctional components in a complex matrix without the need for a tedious sample clean-up [267]. 

The diborylated cobaltocenium species [(CpBR2)2Co] + (R = t-Pr) is an interesting example that illustrates the different binding modes encountered for bifunctional Lewis acids.With hydroxy counterions, an oxygen-bridged complex (178), which represents an inverse chelate structure, was confirmed in the solid state and in solution (R = t-Pr, F). Salt-like structures on the other hand were observed with PFe as the counterion and a zwitterionic 1 1 complex (179) formed upon reaction of the diborylated cobaltocene with hexachloroethane. Low-temperature NMR studies show that the chloride rapidly exchanges position between the two Lewis-acidic boron centers. 

Based on the formation of a reactive ate complex 25 (Scheme 4.14) in the Petasis reaction, as a key species playing an important role in the reactivity and diastereose-lectivity of the process, the authors designed several bifunctional thiourea catalysts having a chelating functionality and envisioning that these could activate the boronic acids and direct the stereochemical outcome, as represented in Figure 4.1. In this sense, the thiourea would play a dual role activating at the same time the electrophile and the nucleophile of this process. 

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