Boron hydrides hydrogen substitution

At low temperatures in a variety of solvents, hexaborane(lO) undergoes exchange of a maximum of five hydrogens with deuterodiborane(6) 209>. The product is shown by boron-11 and proton nmr to be substituted only at basal terminal sites. There is no precedent for an exchange of this type at low temperature for a boron hydride without BH2 groups and it has been proposed 209> that the unique short boron-boron bond in the base of the pentagonal pyramidal framework of hexaborane(lO) is involved in the reaction intermediate. 

As a result of the hydridic nature of the hydrogen attached to boron, amine-boranes are interesting and useful reducing agents and have been employed in the reduction of numerous organic carbonyl compounds. They have been utilized in studying the kinetics and mechanism of hydride reactions and are precursors for the synthesis of substituted boranes, borazines, boronium ions, higher boron hydrides, and carboranes. 

IR-6.2.4.1 Heteronuclear acyclic parent hydrides in general IR-6.2.4.2 Hydrides consisting of chains of alternating skeletal atoms IR-6.2.4.3 Heteronuclear monocyclic parent hydrides Hantzsch-Widman nomenclature IR-6.2.4.4 Skeletal replacement in boron hydrides IR-6.2.4.5 Heteronuclear polycyclic parent hydrides IR-6.3 Substitutive names of derivatives of parent hydrides IR-6.3.1 Use of suffixes and prefixes IR-6.3.2 Hydrogen substitution in boron hydrides IR-6.4 Names of ions and radicals derived from parent hydrides... 

Well-authenticated boron hydrides are listed in Table I. Characterization of these hydrides does not preclude the isolation of others (33). Properties known at present to be common to all boranes include thermal instability susceptibility to hydrolysis, to alcoholysis, and to oxidation toxicity ability to react with ammonia and with many electron-pair donor molecules [for example, (CH3)3N] and a tendency to undergo substitution of their hydrogen atoms by halogen or alkyl groups. 

The B-H bonds in the c/oso-dodecaborate anion have a hydridic character and amenable to attack by electrophilic reagents resulting in substitution of hydrogen atoms. There are two distinct mechanisms for substitution in polyhedral boron hydrides, ordinary aromatic electrophilic substitution and electrophile-induced nucleophilic substitution (EEMS). The last one includes abstraction of a hydride anion from a BH vertex under electrophilic attack (or by Lewis acid) followed by attack of the boronium ylide formed with nucleophile [32],... 

A bidentate bridging mode (Porchia et al. 1987b, Zanella et al. 1987) of a boron hydride ligand to a 5f actinide metal has been achieved, by alkyl substitution of two hydrogens... 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

Fig. 10.24. Asymmetric carbonyl group reduction with Alpine-Borane (preparation Figure 3.27 for the "parachute-like" notation of the 9-BBN part of this reagent see Figure 3.21). The hydrogen atom that is in the cis-position to the boron atom (which applies to both ft- and /T-H) and that after removal of the reducing agent leaves behind a tri- instead of a disubstituted C=C double bond (which applies to ft-, but not / -H) is transferred as a hydride equivalent. In regard to the reduction product depicted in the top row, the designation S of the configuration relates to the aryl-substituted and R to the Rtert-substituted propargylic alcohol.

Dihydroxylation of the stilbene double bond in the trans isomers of Combretastatin A-1 and A-4 produced diols which by treatment with boron trifluoride in ethyl ether [44] or with trifluoroacetic acid [17] resulted in pinacolic rearrangement to produce an aldehyde. The aldehyde was converted in a variety of derivatives, as illustrated in the Scheme 20, via the following reaction sequence reduction with sodium borohydride to primary alcohol which was derivatized to the corresponding mesylate or tosylate, substitution with sodium azide and final reduction to amine with lithium aluminum hydride. Alternatively the aldehyde was converted to oxime which was catalitically hydrogenated to amine [17]. 

The cyclization can be carried out with halogenated amines, and substitution products of the new ring systems can also be obtained in the conventional way, by nitration, etc. Similar compounds can be prepared directly by using arylboron dichlorides in place of boron trichloride in the procedure indicated above. The parent borazarene derivatives, with hydrogen attached to boron, can be made from the B-hydroxy compounds with lithium aluminum hydride in the presence of aluminum chloride. N-Alkyl derivatives can be made either by using N-alkyl derivatives of the aminobiphenyls as starting materials, or by N-alkylation of the unsubstituted compounds via their N-lithio derivatives. Apart from their inherent interest, compounds of this type can serve as intermediates in various syntheses. Thus benzocinnolines and 2,2 -dihydroxybiphenyls can be obtained from derivatives of 2-aminobiphenyl. ... 

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