Boron imide

Boron halides, coordination compounds with amines, 5 26 Boron imide, condensed derivatives of, from boron halide-amine coordination compounds, 6 28 Boron oxide, porous, 2 22 Bromamide, di- (NHBr2), 1 62 analysis of, 1 64... 

These two compounds are among the thermally more stable boron halide-amine compounds. Compounds derived from certain primary and secondary amines lose hydrogen halides at elevated temperatures to give condensed boron-imide or borazole (borazine) derivatives. 

Blood group A, antigenic determinant, 99 Bomeol, 99 Bomylamine, 160 Boron imide, 163... 

In the presence of a 1,3-diene this species produces [4 + 2] cycloadducts. The exact nature of this reactive intermediate has not yet been determined. It was found that a boron imide undergoes a Diels-Alder cycloaddition with cyclopentadiene under mild conditions [Eq. (25)].75... 

Synthetic and structural work has concluded that 1,3,2-dioxaborinium salts, thought to contain a three-coordinate boron center, in fact are four coordinate with a bound counterion. Evidence has been presented for a B-C double bond and Paetzold et al. have studied the chemistry of monomeric boron imides containing B-N double bonds. 

Figure 11.46 Aldol reaction of a chiral carbon-14-labeled boron imide enolate of the sultam type with TiCU pre-complexed achiral/chiral aldehydes to give anti-aldols and application to [ C]avermectin 6/a and Bji,...

It was anticipated that two of the three stereochemical relationships required for intermediate 12 could be created through reaction of the boron enolate derived from imide 21 with a-(benzyloxy)ace-taldehyde 24. After conversion of the syn aldol adduct into enone 23, a substrate-stereocontrolled 1,2-reduction of the C-5 ketone car-... 

A key step in the synthesis of the spiroketal subunit is the convergent union of intermediates 8 and 9 through an Evans asymmetric aldol reaction (see Scheme 2). Coupling of aldehyde 9 with the boron enolate derived from imide 8 through an asymmetric aldol condensation is followed by transamination with an excess of aluminum amide reagent to afford intermediate 38 in an overall yield of 85 % (see Scheme 7). During the course of the asymmetric aldol condensation... 

The synthesis of the polyol glycoside subunit 7 commences with an asymmetric aldol condensation between the boron enolate derived from imide 21 and a-(benzyloxy)acetaldehyde (24) to give syn adduct 39 in 87 % yield and in greater than 99 % diastereomeric purity (see Scheme 8a). Treatment of the Weinreb amide,20 derived in one step through transamination of 39, with 2-lithiopropene furnishes enone 23 in an overall yield of 92 %. To accomplish the formation of the syn 1,3-diol, enone 23 is reduced in a chemo- and... 

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). 

Thus, jyn-adducts arise predominantly, as expected, according to the Zimmerman-Traxier model. Provided that either boron or zirconium is the enolate-metal atom, high syn selectivity is achieved. The total amount of anti-adducts is lower than 2% in the case of amides 1 and 2, and it approaches zero when the other reagents arc used94 . The induced stereoselectivities are impressive for the amides and remarkable in the case of the imides. 

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. 

Reagent control This involves the addition of a chiral enolate or allyl metal reagent to an achiral aldehyde. Chiral enolates are most commonly formed through the incorporation of chiral auxiliaries in the form of esters, acyl amides (oxazolines), imides (oxazolidinones) or boron enolates. Chiral allyl metal reagents are also typically joined with chiral ligands. 

This radical cyclization strategy was utilized for the synthesis of the smaller fragment silyl ether 54 as well (Scheme 8). Evans aldol reaction of the boron eno-late derived from ent-32 with aldehyde 33, samarium(III)-mediated imide methyl ester conversion, and protecting group exchange led to tosylate 51. Elaboration of 51 to ketone 53 was achieved under the conditions used for construction of the second tetrahydrofuran moiety of 49 from 46. A highly diastereoselective reduc-... 

Inverting the orientation of the C4-N3 imine unit of a 2,3,1-diheterabotine gives a boron heterocycle with a markedly different chemical reactivity. In effect, the weakly basic oxime- or hydrazone-type imine nitrogen in the 2,3,1-diheteraborine is replaced by a much more basic imidate- or amidine-type imine nitrogen in the 2,4,1-diheteraborine. Likely, the Lewis acid tendency of the boron is enhanced by the ready protonation of this basic N4, and the formation of a stable borate-based zwitterion becomes thermodynamically favored. 

Boron trifluoride and boron trifluoride-diethyl ether complex can be used as a source of fluoride ions in the presence of hypobromites and hypochlorites, e.g. methyl hypobromitc, tert-butyl hypobromite, methyl hypochlorite in carbon tetrachloride at 25 C. The addition of bromine monofluoride" and chlorine monofluoride" to various alkenes is accompanied by the formation of the corresponding alkoxybromides and alkoxychlorides which hinder the isolation of the halofluorinated products.57 jV-Bromo- and A -chloro-substiluted alkyl- and arylamines. -amides, and -imides, A -chloro-A,-methylamine, A -bromo-A -methylamine, A -chloro-A, /V-dimethylamine, A-bromo-A.A-dimethylamine, ACV-dichloro-A -methylamine, V,fV-dibromo-,V-mcthylaminc, A -bromosuccinimide, -V-chlorosuccinimide, Af-bromoacct-amide, A.A -dichlorourethane, can be used in the reaction instead of the hypohalites. The reactions with various alkenes conducted in dichloromethane at room temperature in the presence of boron trifluoride-diethyl ether complex produce bromofluoro and chlorofluoro addition products in 40-80 % yield. However, the reactions are complicated by the addition of A -halo-succinimides and Af.A-dichlorourcthane to the C = C bonds.58... 

---