Boron/zinc replacement

One new preparation method exploits a boron-zinc exchange. p-Alkylstyrenes. The nitro substituent in /3-nitrostyrenes is replaceable by an alkyl group at room temperature using dialkylzinc reagents. 

The synthesis of the amino alcohol (5S,6S)-6-amino-5-decanol begins with reaction of the 1-chloropentylboronic ester (Section 1.1.2.1.3.1.) with sodium azide under phase-transfer conditions to form the a-azido boronic ester, which yields the a-chloro- -azidoalkyl boronic ester (1) [yield 92 % 95 % de] with (dichloromethyl)lithium under the usual conditions. The reaction of 1 with butylmagnesium chloride is unusual in that it requires zinc chloride in order to accomplish the replacement of chlorine by butyl to form /J-azidoalkyl boronic ester 2 without boron-azide /1-elimination. Standard peroxidic deboronation and reduction of the azide complete the synthesis15. 

Lithium and magnesium of the metallated N-oxides 369 and 371 were readily replaced with zinc or boron. The boronic esters were very unstable undergoing deborylation. The zinc salts 373 were fairly stable and reacted with acyl chloride affording 1,2,3-triazolylketone 374 (2010UP2) (Scheme 113). 

The halogen atom in dialkylaminophosphine chlorides or bromides can be replaced by fluorine on reaction with zinc of antimony fluoride (7.100). The same product is obtained from the tris derivative and boron trifluoride (7.101). 

Aminoquinolines have been synthesized in a manner analogous to the Niementowski reaction, wherein the carboxylic acid in 1 was replaced with a nitrile (49). This transformation has been carried out under either acidic or basic conditions. Under the former conditions, a variety of Lewis acids have been employed to effect this transformation, including zinc chloride, aluminum trichloride, boron trifluoride diethyl etherate, and titanium tetrachloride. In a representative example, 49 was combined with 50 and heated at reflux in the presence of boron trifluoride diethyl etherate to furnish 51, which contains an embedded 4-aminoquinoline ring system. ... 

Alkylation of Aromatics. Aromatic hydrocarbons containing a replaceable hydrogen can be alkylated unless steric effects prevent introduction of the alkyl group (61,78-82). The reaction is called the Friedel-Crafts alkylation, first realized in the presence of aluminum chloride, which is the catalyst still the most frequently used and studied in Friedel-Crafts reactions. In addition, many other acid catalysts are effective (80,82-84). These include other Lewis acids (other aluminum halides, gallium chloride, boron trifluoride, ferric chloride, zinc chloride, stannous and stannic chloride, antimony chloride) and protic acids (hydrogen fluoride, concentrated sulfuric acid, phosphoric acid, polyphosphoric acid, trifluo-romethanesulfonic acid, and alkane- and arenesulfonic acids). 

A similar situation occurs in bis(dodecahydro-imido-decaborato)zinc, which has also been discussed earlier, in the previous chapter. Here the zinc atom can be considered to replace two hydrogen bridges in each BjoH a group, although it is also suggested that the metal can be considered to be the common point in two linked 11-point icosahedral fragments, and thus be considered fully incorporated into the boron cage structure. 

The acidic site was first removed by replacing the boron center by a tetrahedral carbon. Racemic cyclopropylmethanol was obtained in quantitative yield when cinnamyl alcohol was cyclopropanated in the presence of dimethyl tartramide 5 under the usual conditions (Eq 4). This information suggests that the boron is necessary to bring the substrate and the ligand together and that the chiral additive does not act strictly as an activator of the bis(iodomethyl)zinc reagent. 

Several types of replacement of halide by metals are known. The only one that appears to have direct utility in asymmetric synthesis is the reaction of (tributylstan-nyl)lithium with (a-chloroalkyl)boronic esters. The replacement is stereospecific and provides a route to a-lithioethers having high enantiopurity [88]. This chemistry is illustrated by the conversion of (S)-DIPED (R)-(l-chloro-2-methylpropyl)boronate (157) into the (S)-tributylstannyl derivative 158 (Scheme 8.38). The displacement is unusually sluggish and was promoted with zinc chloride. Peroxidic deboronation yielded... 

It is customary to classify the various types of enamel, based on the nature of the flux used. Thus, we can distinguish lead enamels (PbO rich enamels), boron oxide enamels, alkaline enamels, alkaline-earth enamels, zinc enamels and bismuth oxide enamels [STE 85]. Lead enamels, historically the oldest, were the most commonly used for a long time. Because of the toxicity of lead, their future hinges on the evolution of legislation relating to the leaching of this element. They terrd to be replaced by enamels containing a very small qrrantity of bismuth oxide (< 5% mass) and, especially by alkaline borosilicate products. 

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