Dimethyl phenylboronate

The first authenticated borole (94) was made by the exchange reaction of phenylboron dichloride with l,l-dimethyl-2,3,4,5-tetraphenylstannole (93) (Eq. 29)w in toluene. 

Pentanediol [(R,/ )-28] has been used for the formation of chiral acetals and as a precursor for chiral alkenes (Sections D.1.5.1. and 1.6.1.5.). The original procedure for the resolution of the phenylboronic acid derivative with brucine39 was impractical, but recently a kinetic resolution by lipase-catalyzed esterification and hydrolysis has been developed40. In addition, a good method for catalytic reduction of 1,3-diketones with Raney nickel modified with sodium bromide and tartaric acid (for a procedure, see Section D.2,3.1.) allowed the production for commercial purposes41. Similarly, sterically more hindered and less water-soluble 2,6-dimethyl-3,5-heptanediol (29) has been introduced for the same purpose. It is obtained in the same way from the diketone and separated from the meio-compound by simple recrystallization42. 

Axially chiral guanidines with an external guanidine unit such as dinaphthoazepineamidine are effective catalysts for the enantioselective addition of p-oxoesters and a 1,3-diketone to di-ferf-butyl azodicarboxylates to yield cx-hydrazino-p-oxoesters and a-hydrazino-p-dike-tones in 54-99% yields and in 15-98% ee [44]. For example, stirring 2-oxocyclopenta-necarboxylate and di(tert-butyl) azodicarboxylate in THF in the presence of 0.05 mol% catalyst for four hours at —60° C provides an adduct in quantitative yield and in 97% ee (Scheme 4.15). The (R)-catalyst was prepared from (/ )-2,2 -dimethyl-3,3 -binaphthalene-diol ditriflate, 4-methoxyphenylboronic acid and 3.5-di(rert-butyl)phenylboronic acid in six steps. 

During the course of an investigation into the Suzuki-Miyaura cross-coupling of l-bromo-2,4,6-tri-tert-butylbenzene with phenylboronic acid, a, a -dimethyl-)8-phenyl dihydrostyrene by-product was isolated in excellent yield, while the desired biaryl product was not observed. This unexpected transformation likely proceeded via a pathway involving a tandem C-H activation/ Suzuki-Miyaura cross-coupling sequence (eq 40). ... 

In order to prepare the cyclohexenaldehyde 8, 3-hydroxy-2-pyrone 14 and ethyl 4-hydroxy-2-methyl-2-butenoate 15 are subjected to a Diels-Alder reaction in the presence of phenylboronic acid which arranges both reactants to the mixed boro-nate ester 19 as a template to enable a more efficient intramolecular Diels-Alder reaction with optimal control of the regiochemical course of the reaction. Refluxing in benzene affords the tricyclic boronate 20 as primary product. This liberates the intermediate cycloadduct 21 upon transesterification with 2,2-dimethylpropane-l,3-diol which, on its part, relaxes to the lactone 22. Excessive i-butyldimethyl-silyltriflate (TBSTf) in dichloromethane with 2,6-lutidine and 4-7V,A-dimethyl-aminopyridine (DMAP) as acylation catalysts protects both OH goups so that the primary alcohol 23 is obtained by subsequent reduction with lithiumaluminum-hydride in ether. 

The cross-coupling reactions of organoboronic acids and carbon monoxide with hyper-valent iodonium salts affords unsymmetrical ketones (Scheme 30). The reaction proceeds smoothly at room temperature and in most cases completes within 0.5 h. Aryl-, alkenyl-, and alkynyliodonium salts react with arylboronic adds in the presence of 0.5% of Pd(PPh3)4 and 1.2 equiv of K2CO3 in DME to provide unsymmetrical aromatic ketones in high yields (Scheme 30). Phenylboronic acid dimethyl ester can be utilized as efficiently as phenylboronic acid. In most cases, a small amount of the direct crosscoupling product (R—Ph, less than 7-8%) is produced. 

Obtained by successively adding an aqueous solution of sodium bicarbonate (6 mmol), then tetrakis(triphenyl-phosphine)palladium (0) (0.1 mmol) to a solution of 3-bromo-2-hydroxy-5-methy-lacetophenone (2 mmol) and phenylboronic acid (2 mmol) in ethylene glycol dimethyl ether (DME). After, the reaction mixture was refluxed for 10 min and then heated at 75° overnight (71%) (compound 26) [1804]. 

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