Boron evidence

Acetamido-2-phenylethaneboronic acid, the boronic acid analog of. V-acetylphenyl-alanine and a good inhibitor of chymotrypsin, has been prepared via a stable silylated intermediate59, 62. Coordination with boron evidently facilitated the unusual chloride displacement by sterically hindered lithium hexamethyldisilazanide. 

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

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... 

Hydrides. Unlike boron and aluminum, which form many stable and useful hydrides, there is very Htde evidence for the existence of TIH and TIH (13). Lithium tetrahydridothallate(III) [82374 7-8], LiTlH, has been obtained from TlCl and LiH in ether at — 15 C (14). It decomposes rapidly... 

A series of chiral boron catalysts prepared from, e.g., N-sulfonyl a-amino acids has also been developed and used in a variety of cycloaddition reactions [18]. Corey et al. have applied the chiral (S)-tryptophan-derived oxazaborolidine-boron catalyst 11 and used it for the conversion of, e.g., benzaldehyde la to the cycloaddition product 3a by reaction with Danishefsky s diene 2a [18h]. This reaction la affords mainly the Mukaiyama aldol product 10, which, after isolation, was converted to 3a by treatment with TFA (Scheme 4.11). It was observed that no cycloaddition product was produced in the initial step, providing evidence for the two-step process. 

There are a few species in which the central atom violates the octet rule in the sense that it is surrounded by two or three electron pairs rather than four. Examples include the fluorides of beryllium and boron, BeF2 and BF3. Although one could write multiple bonded structures for these molecules in accordance with the octet rule (liable 7.2), experimental evidence suggests the structures... 

Treatment of commercially available and symmetrical 3,4,5-tri-methoxytoluene (37) with iodine, periodic acid, and acetic acid under the conditions of Suzuki19 results in the formation of symmetrical diiodide 38 in 93 % yield. Although only one of these newly introduced iodine atoms is present in intermediate 13, both play an important role in this synthesis. Selective monodemethylation of 38 with boron trichloride furnishes phenol 39 in 53% yield together with 13 % of a regioisomer. Evidently, one of the Lewis-basic iodine substituents coordinates with the Lewis-acidic boron trichloride and directs the cleavage of the adjacent methyl ether... 

Thermolysis rates are enhanced substantially by the presence of certain Lewis acids (e.g. boron and aluminum halides), and transition metal salts (e.g. Cu ", Ag1).46 There is also evidence that complexes formed between azo-compounds and Lewis acids (e.g. ethyl aluminum scsquichloridc) undergo thermolysis or photolysis to give complexed radicals which have different specificity to uncomplexed radicals.81 83... 

Mechanistically there is ample evidence that the Balz-Schiemann reaction is heterolytic. This is shown by arylation trapping experiments. The added arene substrates are found to be arylated in isomer ratios which are typical for an electrophilic aromatic substitution by the aryl cation and not for a homolytic substitution by the aryl radical (Makarova et al., 1958). Swain and Rogers (1975) showed that the reaction takes place in the ion pair with the tetrafluoroborate, and not, as one might imagine, with a fluoride ion originating from the dissociation of the tetrafluoroborate into boron trifluoride and fluoride ions. This is demonstrated by the insensitivity of the ratio of products ArF/ArCl in methylene chloride solution at 25 °C to excess BF3 concentration. 

Further evidence against the formation of a free carbonium ion in the alkylation reaction is obtained from the fact that in the presence of boron trifluoride-phosphoric acid catalyst, but-l-ene, but-2-ene, and i-butene react at different rates with alkylbenzenes, yet they would each give the same carbonium ion. In addition, only the latter alkene gave the usual activation order (in this case the hyper-... 

Evidence that the cation plays an essential role, at least in some cases, is that when the Li was effectively removed from L1A1H4 (by the addition of a crown ether), the reaction did not take place. The complex 19 must now be hydrolyzed to the alcohol. For NaBH4 the Na" " does not seem to participate in the transition state, but kinetic evidence shows that an OR group from the solvent does participate and remains attached to the boron... 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

Evolution of analytical techniques can cause data, once considered to be state of the arf to be shown to be unreliable. A good example is provided by the work of Houba et al. (1995), who demonstrated that a number of older methods for the determination of trace levels of boron in plant materials were subject to the interference by high levels of copper. This and other evidence suggest that older data, even when presented on a certificate, have to be viewed critically see also Section 3.2. The analyst must stay aware of developments and be ready to disregard certified values if the date of certification of the CRM predates the release of new developments and the certification authority concerned cannot confirm that the certified value is good in the light of the new knowledge. 

There is evidence that boron migration occurs intramolecularly.177 A TS involving an electron-deficient tt complex about 20-25 kcal above the trialkylborane that describes the migration has been located computationally.178... 

Shinkai (65) exploited the boronic acid-diol motif mentioned above to self-assemble a polymer based on pyridine-magnesium porphyrin interactions. In this case, the evidence for polymerization comes from lightscattering experiments in dilute solution which yielded an average molecular weight of 109 g mol-1 for this system. 

Because of the necessity of spatially appropriate overlap of the 2p. .-orbital on boron with the adjacent jp3-hybridized carbon orbital (12) or with the tt-MO of the double bond (13), any such ir-bonding should be most sensitive to molecular conformation. Hence, the occurrence of such 7r-bonding might be more evident in certain highly rigid cyclic structures, such as 14 and 15. 

These changes are more consistent with an /-effect, rather than an R-effect, on the B—C bond. That such diborylacetylenes have a diminished electron density at boron relative to alkyl- and vinylboranes is evident from their enhanced Lewis acidity bis(diethylboryl)acetylene (40) forms a bis(tetrahydrofuran) complex, as is evident from the1 B NMR spectrum37 (Eq. 11). Alkyl- and vinylboranes exhibit no such shifts in their11B signals in THF solution. 

Such overlap requires a planarity between the phenyl ring and the trigonal, tricoordinate boron plane. Where that planarity is significantly disrupted by ortho substituents, as is evident in the XRD structure of lithium(12-crown-4)2 cation-trimesitylboron anion,41 only 65% of the free spin is delocalized onto the rings.42... 

The macrostructure of the boron nitride obtained here is porous with pores 2 pm in diameter. There is no evidence for microporosity and the BET surface area 1s 35 m2 g-1. Transmission electron micrographs (Figure 4) show regions of well developed crystallinity. The crystalling grains are 5—10 nm on a side and 30-40 nm long. The BN (002) lattice fringes are clearly visible. 

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