Titanium-boron system

Rogachev, A. S., Mukasyan, A. S., and Merzhanov, A. G., Structural transitions in the gasless combustion of titanium-carbon and titanium boron systems. Dokl. Phys. Chem., 1S1, 1240 (1987). 

Other than in polymer matrix composites, the chemical reaction between elements of constituents takes place in different ways. Reaction occurs to form a new compound(s) at the interface region in MMCs, particularly those manufactured by a molten metal infiltration process. Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled. Depending on the composite constituents, the atoms of the fiber surface diffuse through the reaction site, (for example, in the boron fiber-titanium matrix system, this causes a significant volume contraction due to void formation in the center of the fiber or at the fiber-compound interface (Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product. Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites. 

Results for titanium-boron and several intermetallic systems (Ni-Al, Ti-Al, Ni-Ti, Co-Ti) are presented in Fig. 49. In all cases, the measured combustion tern-... 

Taneoka, Y, Odawara, O., and Kaieda, Y, Combustion synthesis of the titanium-aluminum-boron system. J. Am. Ceram. Soc., 72, 1047 (1989). 

A variety of metal complexes have been investigated as catalysts in this procedure with boron- and titanium-based systems being the most popular. In 1991, H. Yamamoto reported that the allylation of aldehydes with allylsilanes... 

It may be noted that homogeneous cationic polymerization of ethylene oxide also generally leads to low molecular weight products typical initiators include aluminium chloride, boron trifluoride and titanium tetrachloride. Systems of this type are not utilized on a commercial scale. 

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

The pharmaceutical interest in the tricyclic structure of dibenz[6,/]oxepins with various side chains in position 10(11) stimulated a search for a convenient method for the introduction of functional groups into this position. It has been shown that nucleophilic attack at the carbonyl group in the 10-position of the dibenzoxepin structure renders the system susceptible to water elimination. Formally, the hydroxy group in the enol form is replaced by nucleophiles such as amines or thiols. The Lewis acids boron trifluoride-diethyl ether complex and titanium(IV) chloride have been used as catalysts. 

Reaction conditions that involve other enolate derivatives as nucleophiles have been developed, including boron enolates and enolates with titanium, tin, or zirconium as the metal. These systems are discussed in detail in the sections that follow, and in Section 2.1.2.5, we discuss reactions that involve covalent enolate equivalents, particularly silyl enol ethers. Scheme 2.1 illustrates some of the procedures that have been developed. A variety of carbon nucleophiles are represented in Scheme 2.1, including lithium and boron enolates, as well as titanium and tin derivatives, but in... 

These examples and those in Scheme 2.6 illustrate the key variables that determine the stereochemical outcome of aldol addition reactions using chiral auxiliaries. The first element that has to be taken into account is the configuration of the ring system that is used to establish steric differentiation. Then the nature of the TS, whether it is acyclic, cyclic, or chelated must be considered. Generally for boron enolates, reaction proceeds through a cyclic but nonchelated TS. With boron enolates, excess Lewis acid can favor an acyclic TS by coordination with the carbonyl electrophile. Titanium enolates appear to be somewhat variable but can be shifted to chelated TSs by use of excess reagent and by auxiliaries such as oxazolidine-2-thiones that enhance the tendency to chelation. Ultimately, all of the factors play a role in determining which TS is favored. 

Usually the stronger acids are also the more effective co-catalysts, but exceptions to this rule are known. Trichloroacetic acid, but not the equally strong picric acid, will co-catalyze the system isobutene-titanium tetrachloride in hexane.2 8 Some Lewis acid-olefin systems will not polymerize at all in the absence of a co-catalyst, an example being isobutene with boron trifluoride.2 4 This fact, together with the markedly slower reaction usual with carefully dried materials, has nourished the current suspicion that a co-catalyst may be necessary in every Lewis acid-olefin polymerization. It is very difficult to eliminate small traces of water which could act as a co-catalyst or generate mineral acid, and it may well be that the reactions which are slower when drier would not go at all if they could be made completely dry. 

The mechanism of initiation of cationic polymerisations by metal halides was clarified and systematized to some extent by the discovery of the phenomenon of co-catalysis or co-initiation. But, whereas there was, by the mid-1960s, good evidence that at any rate in many systems the halides of boron, titanium, and tin required a co-initiator, the position with regard to the best-known and most popular initiator, and the one which was of greatest economic significance, aluminium chloride, remained obscure. Of the vast number of published experiments on the system, aluminium chloride + isobutylene, hardly any could provide evidence concerning the initiation reaction, because they were almost exclusively concerned with measurements of yields and degree of polymerisation (DP). 

Several catalysts and initiator systems have been tested for the polymerization of GlcAnBzl3, including the following Lewis acids boron trifluoride and its etherate, phosphorus pentafluoride, titanium tetrachloride, and antimony pentachloride and pentafluoride. Several cationic initiators have also been used, including (triphenylmethyl) antimony hexachloride, 2,3,4,6-tetra-O-acetyl-D-glucopyranosyl hexa-fluorophosphate, acetyl hexafluorophosphate, pentamethylbenzyl hexa-fluorophosphate (most of which were generated in situ), and triethyl-... 

Some chirally modified metallic compounds (or combined systems) containing lithium (20d, g, o, p), boron (30), or titanium (31, 32) also... 

Although it Wits long believed that most Friedel-Crafts acids, particularly halides of boron, titanium, and tin. require an additional cation source to initiate polymerization, recent results show that in many systems Friedel-Crafts acids alone are able lo initiate cationic polymerization. 

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