Polyhedral Boron Hydride Derivatives

Polyhedral boron hydride derivatives with primary amino group were prepared by the reaction of cyclic oxonium derivatives with potassium phthalimide followed by removal of the phthalimide protection group with hydrazine hydrate (Figure 24.8) [31,51]. 

Polyhedral boron hydride derivatives with terminal alkyne group were prepared by oxonium ring disclosure with alkynyl alcoholates or amines (Figure 24.14) [32,56,59]. 

Polyhedral Boron Hydrides. Although relatively large quantities of polyhedral boron hydrides and carboranes have been produced under various government contracts, these materials are not currendy produced on a regular commercial basis. Pentaborane(9) nido- and decaborane(14) and carboranes, such as the ortho and meta-carboranes closo-1 1 well as various other derivatives, are... 

Polyhedral boranes, carboranes and carbametallic boron hydride derivatives... 

Polyhedral boranes, carboranes and carba-metallic boron hydride derivatives The chemistry of the polyhedral species derived from transition metals and carboranes Carboranes (Ger.)... 

In the chemistry of polyhedral boron hydrides, boron-centered cations were postulated to be key intermediates of an electrophile-induced nucleophilic substitution mechanism that is responsible for the formation of a variety of boron-substituted derivatives [14], Such boron-centered cations can be easily generated by abstraction of a hydride by the treatment of polyhedral boron hydrides with Lewis or Bronsted acids [15], Similar to the classical chelate-restrained borinium cations based on 3-coordinate boron, these species, which we called quasi-borinium cations, have an unstabilized p orbital and are strong electrophiles (Scheme 6.1). Such quasi-borinium cations are highly reactive and react with even weak nucleophiles, such as ether or nitrile solvent molecules giving the corresponding oxonium and nitrilium derivatives whose properties are close to those of similar complexes of transition metals [15-17]. 

Semioshkin, A. A., Sivaev, I. B., and Bregadze, V. I. 2008. Cyclic oxonium derivatives of polyhedral boron hydrides and their synthetic applications. Dalton Trans. 2008 977-992. 

Most known polyhedral boron hydrides are decahydro-doi o-decaborate [B,oH,o] , dodeca-hydro-c/oio-dodecaborate [B,2H,2] anions, and dicarba-cZoso-dodecaboranes [C2B,2Hi2] (see Figure 9.1). The first derivatives of [B,oH,o] and [Bi2Hi2] anions were prepared by M. F. Hawthorne, and A. R. Pitochelli [2], and the synthesis and properties of carboranes were first reported at the end of 1963 by chemists from the the Soviet Union and the United States [3,4]. 

In contrast to the previously known boron hydrides, the polyhedral boron hydrides were shown to be exceptionally stable. One of the most striking features of the carboranes is the capability of the two carbon atoms and ten boron atoms to adopt the icosahedral geometry in which the carbon and boron atoms are hexacoordinated. This feature of carboranes gives rise to the unusual properties of these molecules and their carbon and boron derivatives. Due to these features of icosahedral carboranes, their chemistry has developed extensively, and the results obtained are summarized in R. Grimes monograph and review [5]. Investigation of the properties of polyhedral boron hydrides resulted in the conclusion that these compounds have aromatic properties. It was the first example of nonplanar three-dimensional aromatic compounds and resulted in the development of the eon-cept of three-dimensional aromaticity that is generally accepted at the present time [6,7]. 

Dodecahydro-do50-dodecaborate anion [B,2H,2] , the isoelectronic and isostructural analog of carboranes, is a typical representative of anionic polyhedral boron hydrides. Sodium salts of the parent cZo o-dodecaborate and their derivatives have a good solubility in water, which is an important precondition of many medical applications. The sodium salt Na2[Bj2Hj2] was found to be practically nontoxic with the approximate lethal dose for rats >7.5 g/kg of body weight, which is roughly comparable with that of sodium chloride [12], However, functionalization of this purely inorganic system till recently was problematic due to the absence of a clear reaction center. There are two... 

The polyhedral boranes described above are available from commercial sources however, they are all very expensive due to the absence of large-scale industrial production and could find practical applications only in some exclusive areas where no alternative exists. The use of polyhedral boranes in some of the fields described below obviously has no alternative, in other fields the price of modern pharmaceuticals is comparable with price of boron hydride derivatives, and in some cases the boron hydride price is negligible in comparison with the price of drug as a whole. 

Modification of Polyhedral Boron Hydrides Using Cyclic Oxonium Derivatives.624... 

In this chapter we will try to overview the results of use of cyclic oxonium derivatives of polyhedral boron hydrides in the synthesis of various boron-containing molecules. 

The general approach to the synthesis of cyclic oxonium derivatives of polyhedral boron hydrides consists in abstraction of a hydride by treatment with Lewis or Brbnsted acids resulting in the formation of a carbocation-like center on the boron atom, which is then subjected to the attack of an ether solvent as nucleophile, resulting in the corresponding cyclic oxonium derivatives (Figure 24.1) [29]. 

At present, effective methods of synthesis of cyclic oxonium derivatives of the main types of polyhedral boron hydrides, such as doso-dodecaborate [Bi2Hi2] [31-33], doso-decaborate [BioHjo] [34,35], 7,8-dicarba-ni /o-undecaborate [7,8-C2B9Hi2] [36-38], cobalt bis(dicarbollide) [3,3 -Co(1,2-C2B<,H )2] [39-41], and iron bis(dicarbollide) [3,3 -Fe(l,2-C2B9H )2]- [42] have been elaborated. 

MODIFICATION OF POLYHEDRAL BORON HYDRIDES USING CYCLIC OXONIUM DERIVATIVES... 

There are two general strategies for the preparation of boronated biomolecules using cyclic oxonium derivatives of polyhedral boron hydrides. The first one is based on a two-step synthesis. At the first... 

The reaction of cyclic oxonium derivatives of polyhedral boron hydrides with acetamido diethyl-malonate (glycine anion equivalent for amino acid synthesis), a variation of the classical Sdrensen synthesis of amino acids, followed by acidic hydrolysis and decarboxylation was proposed as a route to boron hydride-based aminoacids (Figure 24.10) [31,51]. 

The above examples of building blocks do not exhaust the diversity of polyhedral boron hydrides prepared by disclosure of their cyclic oxonium derivatives. More examples of such compounds can be found in a comprehensive review [29] and some more recent papers [38,47,61-66]. 

Synthesis of agents for lanthanide and actinide extraction from high-level activity nuclear waste is the only one not related to the medical field where the 1,4-dioxane derivative of cobalt bis-(dicarbollide) was successfully applied. Moreover, the first described reaction of oxonium ringopening in polyhedral boron hydrides was performed specifically in this field [77]. During the last 10 years, numerous cobalt bis(dicarbollide)-based extraction agents were synthesized, including... 

In conclusion, the oxonium derivatives of polyhedral boron hydrides are very promising starting materials for the synthesis of various boronated organic and bioorganic materials. 

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