Boronic handling

Elemental boron and the borates are not considered to be toxic, and they do not require special care in handling. However, some of the more exotic boron hydrogen compounds are definitely toxic and do require care. 

Uses. Boron triduoride is an excellent Lewis acid catalyst for numerous types of organic reactions. Its advantages are ease of handling as a gas and the absence of undesirable tarry by-products. As an electrophilic molecule, it is an excellent catalyst for Friedel-Crafts and many other types of reactions (63-65) (see Friedel-craftsreactions). 

Aluminum bromide and chloride are equally active catalysts, whereas boron trifluoride is considerably less active probably because of its limited solubiUty in aromatic hydrocarbons. The perchloryl aromatics are interesting compounds but must be handled with care because of their explosive nature and sensitivity to mechanical shock and local overheating. 

Sodium borohydride reacts with boron halides to form diborane [19287-45-7] 2 6 more conveniendy handled as the monomer... 

The choice of catalyst is based primarily on economic effects and product purity requirements. More recentiy, the handling of waste associated with the choice of catalyst has become an important factor in the economic evaluation. Catalysts that produce less waste and more easily handled waste by-products are strongly preferred by alkylphenol producers. Some commonly used catalysts are sulfuric acid, boron trifluoride, aluminum phenoxide, methanesulfonic acid, toluene—xylene sulfonic acid, cationic-exchange resin, acidic clays, and modified zeoHtes. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Boron trichlorides are highly reactive, toxic, and corrosive these ttihaUdes (BCl, BBr, BI ) react vigorously, even explosively, with water. High temperature decomposition of BX can yield toxic halogen-containing fumes. Safe handling, especially of BCl, has been reviewed (11,80). 

Boron Triiodide. There are no large-scale commercial uses of boron ttiiodide. It can cleave ethers without affecting aldehyde groups and thus finds use in the synthesis of the antibiotic fmstulosin (115,116). BI is used to prepare Snl, Sbl, and Til (117) in 99—100% yield. It is used to clean equipment for handling UE (118) and in the manufacture of lithium batteries (119). 

The nomenclature of boron hydride derivatives has been somewhat confusing and many inconsistencies exist in the Hterature. The stmctures of some reported boron hydride clusters are so compHcated that only a stmctural drawing or graph, often accompanied by explanatory text, is used to describe them. Traditional nomenclature systems often can be used to describe compounds unambiguously, but the resulting descriptions may be so long and unwieldy that they are of Htde use. The lUPAC (7) and the Chemical Abstract Service (8) have made recommendations, and nomenclature methods have now been developed that can adequately handle nearly all clusters compounds however, these methods have yet to be widely adopted. Eor the most part, nomenclature used in the original Hterature is retained herein. 

All examples mentioned so far correspond to reactions between two aromatic groups, however, couplings in which one or both partners are alkyl groups can be achieved using electron-rich boron-based nucleophiles. Fiirstner has reported the use of B-alkyl or 5-allyl methoxy-9-BBN anions for the efficient coupling with some aryl chlorides using an in situ prepared IPr HCl/Pd(OAc)j system [118], Some of the results obtained with these easy-to-handle borate-based nucleophiles are shown below (Scheme 6.34). 

Nitrogen trifluoride is a kinetically inert gas, but when sparked or heated can fluorinate substrates and release nitrogen. Sulfur, for example, is quantitatively converted to SF using NF3 at 5 atm (231). Similarly, boron is converted to BF3 (159) and CF3CN to CF4 (232). Nitrogen trifluoride has some advantages over difluorine. It is safer to handle, and double-compartment bombs to avoid self-ignition are not required. However, some self-dissociation has to be measured and corrections made. 

The alternate process, the vapor phase method, is carried out at higher pressures (450 psi) and temperatures (750—800°F), and hence, the vapor phase. Producers have been using a boron trifluoride catalyst but any trace water corrodes it unmercifully. Most have now switched to a crystalline aluminosilicate zeolyte catalyst, a more expensive but hardier catalyst. The newer catalyst is also noncorrosive and nonhazardous, cheaper to handle, and produces no waste streams to dispose of... 

Dlborane(6) was obtained from the thermal decomposition of B4H10 (2). In view of the restricted amounts of products obtained by the use of magnesium boride in boron hydride syntheses, development of their chemistry proceeded slowly even after techniques had been acquired for handling these materials. 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

Allylic boronates are more stable to atmospheric oxidation and are thns mnch easier to handle than the corresponding allylic boranes. The stability of the boronate reagents arises from the partial donation of the lone pairs of electrons on the oxygen atoms into the empty p-orbital of boron. This mesomelic effect is responsible for the npfield shift of the boron atom in NMR compared to that of allylic boranes (compare allylboronate 31 and allylborane 32). ... 

---