Boron formation

An unusual difluoroboryl imidate (10) was isolated during decomposition of (9) and its stability arose from a strong intramolecular bond between the pyridine N and boron. Formation of the desired 2-amino-5-fluoropyridine followed by the use of aqueous sodium hydroxide (89S905). 

In addition, arylthiophene 70 was obtained by a one-pot Suzuki coupling of p-methoxyiodobenzene and 3-bromothiophene via an in situ boronate formation using one equivalent of the thermally stable diborane 69 [55], This method avoids the isolation of boronic acids and is advantageous when base-sensitive groups such as aldehyde, nitriles and esters are present. However, the cross-coupling yields are low when both aryl halides are electron-poor because of competitive homocoupling during the reaction. 

It seems probable that a fundamental distinction may exist between the two methods available for the synthesis of carbohydrate boronates, in that, under dehydration conditions, esterifications may be reversible, whereas boronate formation from borinates is not. In the first, therefore, products may be thermodynamically controlled, whereas kinetic control will operate in the second. It is noteworthy, however, that products obtained from several glycopyranosides (a) by use of an ethylboronate derivative, and (b) by way of the per(diethylbor-inates) had the same structures.338... 

Suzuki and Stille couplings. For coupling of aryl chlorides with arylboronic acids the catalytic system contains (CyjPjjPdClj, CsF in NMP. co-Arylalkanoic acids are prepared by coupling of aryl halides with carboxylic esters bearing a 9-BBN substituent at the other terminus, and saponification. A biaryl synthesis from two different aryl halides is accomplished with in situ boronate formation which depends on the (dppfjPdClj catalyst. For access to aryl boronates either the coupling of aryl triflates with bis(pinacolato)diboron" or that of aryl iodides with pinacolborane may be employed. 

Amino acids have likewise been converted into oxazolidinones in a rapid reaction with 1,3-dichlorotetrafluoroacetone [246,247], reminiscent of acetonide or boronate formation ... 

Snbseqnent reactions of atomic hydrogen lead to further rednction of boron chlorides as well as to the formation of boron hydrochlorides. This total boron reduction process can be performed on a hot wire or using hot walls of a pipe reactor (Carleton et al., 1970). Boron formation takes place in this approach at temperatmes of 1100-1700 K in the form of dense deposits on the walls, which is not convenient for further use. The productivity and efficiency of such a thermal process are also low. Performing the process in thermal plasma of RF-ICP discharges results in more effective production of boron in the form of fine powder (Hamblym, Reuben, Thompson, 1972 Cuelleron Crusiat, 1974). 

The hydroboration reaction is also very predictable with regard to the stereochemistry of addition. The addition occurs stereospecifically syn through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C-B and C-H bonds are therefore formed from the same side of the multiple bond. In molecular orbital terms, the addition reaction is viewed as taking place by interaction of the olefin rr-orbital with the empty p-orbital on trivalent boron. Formation of the carbon-boron bond is accompanied by concerted rupture of a B-H bond ... 

Scheme 28.7 Formation of periodic network structures (21 and 22) in solids using equilibrium boroxine and boronate formation with a monomer 23 and with monomers 23 and 24, respectively.

Figure 1. Transport Mediated by Reversible Tetrahedral Boronate Formation. pH < Boronic Acid pKa. Q" = Quaternary Ammonium Cation Such as Trioctylmethylammonium (TOMA).

Boron formate treated dropwise with a soln. of diethylamine in benzene, and the product isolated after 2 hrs. -> diethylformamide. Y ca. 100%. Also ethyl-formamide s. V. V. Gertsev and Y. Y. Makarov-Zemlyanskii, Zh. Prikl. Khim. 43, 1633 (1970) C. A. 73, 87373. 

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