Boron compounds amides

Bonm-alabilized earbanions. Ordinarily bases coordinate with the boron atom of organoboranes. However, Rathke and Row report that a highly hindered lithium amide such as lithio-2,2,6,6-tetramcthylpiperidine or lithio-t-butylneopentylamine can remove the a-proton from an organoborane to generate earbanions. Thus treatment of the boron compound B-tnethyl-9-borabicyclononane (1) in benzene with the former base for 12 hr. at room temperature followed by quenching with deuterium oxide results in deuterium incorporation of 50% (equation I). 

Silyl enol ethers react with aldehydes in the presence of chiral boranes or other additives " to give aldols with good asymmetric induction (see the Mukaiyama aldol reaction in 16-35). Chiral boron enolates have been used. Since both new stereogenic centers are formed enantioselectively, this kind of process is called double asymmetric synthesis Where both the enolate derivative and substrate were achiral, carrying out the reaction in the presence of an optically active boron compound ° or a diamine coordinated with a tin compound ° gives the aldol product with excellent enantioselectivity for one stereoisomer. Formation of the magnesium enolate anion of a chiral amide, adds to aldehydes to give the alcohol enantioselectively. 

The stereochemical results of the reduction by lithium amides (X = NR, M = Li) and Grignard reagents are included in this overview the related boron compounds are discussed in Section D.2.3.5.123. 

The cubic 2inc blende form of boron nitride is usually prepared from the hexagonal or rhombohedral form at high (4—6 GPa (40—60 kbar)) pressures and temperatures (1400—1700°C). The reaction is accelerated by lithium or alkaline-earth nitrides or amides, which are the best catalysts, and form intermediate Hquid compounds with BN, which are molten under synthesis conditions (11,16). Many other substances can aid the transformation. At higher pressures (6—13 GPa) the cubic or wurt2itic forms are obtained without catalysts (17). 

A -( 1-Chloro- or bromoalkyl)amides are generally moisture-sensitive, unstable compounds, which are often directly used without further purification. Standard Lewis acids such as boron trifluoride-diethyl ether, aluminum(lll) chloride, zinc(II) chloride, tin(IV) chloride and titani-um(IV) chloride are used to generate the /V-acyliminium ion, although sometimes a catalyst is not necessary. 

The enolates of other carbonyl compounds can be used in mixed aldol reactions. Extensive use has been made of the enolates of esters, thiol esters, amides, and imides, including several that serve as chiral auxiliaries. The methods for formation of these enolates are similar to those for ketones. Lithium, boron, titanium, and tin derivatives have all been widely used. The silyl ethers of ester enolates, which are called silyl ketene acetals, show reactivity that is analogous to silyl enol ethers and are covalent equivalents of ester enolates. The silyl thioketene acetal derivatives of thiol esters are also useful. The reactions of these enolate equivalents are discussed in Section 2.1.4. 

Enantiospecific syntheses of amino derivatives of benzo[ ]quinolizidine and indolo[2,3- ]quinolizidine compounds have also been achieved via A-acyliminium ion cyclization reactions, as an alternative to the more traditional Bischler-Napieralski chemistry (see Section 12.01.9.2.2). One interesting example involves the use of L-pyroglutamic acid as a chiral starting material to construct intermediates 240 via reaction with arylethylamine derivatives. Diisobutylaluminium hydride (DIBAL-H) reduction of the amide function in 240 and subsequent cyclization and further reduction afforded piperidine derivatives 241, which stereoselectively cyclized to benzo[ ]quinolizidine 242 upon treatment with boron trifluoride (Scheme 47) <1999JOC9729>. 

The same regiochemistry is observed when nitroimidazole (48-2, 48-3) acts as a nucleophile in unionized form. Thus, the reaction of a compound with benzoyl-aziridine (49-1) in the presence of boron trifluoride probably involves an initial salt formation with an amide attack by the imidazole results in a ring opening and the formation of the alkylated product (49-2) the free primary amine (49-3) is obtained on basic hydrolysis. Acylation of the primary amine with methyl thiochloroformate gives the corresponding thiourethane, carnidazole (49-4) [52]. 

Methyl Ethers. Methylation of sucrose is generally conducted under basic conditions. Etherification occurs initially at the most acidic hydroxyl groups, HO-2, HO-T, and HO-3f, followed by the least hindered groups, HO-6 and HO-6. Several reagents have found use in the methylation of sucrose, including dimethyl sulfate—sodium hydroxide (18,19), methyl iodide—silver oxide—acetone, methyl iodide—sodium hydride in N, N- dimethyl form amide (DMF), and diazomethane—boron trifluoride etherate (20). The last reagent is particularly useful for compounds where mild conditions are necessary to prevent acyl migration (20). 

---