Deficiency effects boron

It is thought that borate ester cross-linking of pectin is necessary for the normal growth and development of higher plants. Thus, a deficiency of boron leads to the effects described above. Application of borate fertilizers such as borax (Na2[B405(0H)4] 8H20) to crops is therefore important. A balance has to be sought, however, because an excess of boron can be toxic to plants, and cereal crops are especially sensitive. 

Due to electronegativity differences (B = 2.05, C = 2.55) and notwithstanding the electronic deficiency of boron, which is mitigated by the two electron-donating oxygen atoms (vide supra), the inductive effect of a boronate group should be that of a weak electron-donor. The NM R alpha effect of a boronate group is very small [27]. 

Coupling of Arylboronic Acids with Alkynes. The oxidative coupling of arylboronic acids with internal alkynes effectively proceeded under [Cp RhCl2]2-Cu(OAc)2 catalytic system in a 1 2 manner via C-H bond activation to produce the corresponding product (eq 24) It is noted that the electron-deficient phenyl-boronic acids were less reactive in the present reaction. 

Metal Alibis and Alkoxides. Metal alkyls (eg, aluminum boron, sine alkyls) are fairly active catalysts. Hyperconjugation with the electron-deficient metal atom, however, tends to decrease the electron deficiency. The effect is even stronger in alkoxides which are, therefore, fairly weak Lewis acids. The present discussion does not encompass catalyst systems of the Ziegler-Natta type (such as AIR. -H TiCl, although certain similarities with Friedel-Crafts systems are apparent. 

The alkylation of pyridine [110-86-1] takes place through nucleophiUc or homolytic substitution because the TT-electron-deficient pyridine nucleus does not allow electrophiUc substitution, eg, Friedel-Crafts alkylation. NucleophiUc substitution, which occurs with alkah or alkaline metal compounds, and free-radical processes are not attractive for commercial appHcations. Commercially, catalytic alkylation processes via homolytic substitution of pyridine rings are important. The catalysts effective for this reaction include boron phosphate, alumina, siHca—alurnina, and Raney nickel (122). 

Boron is a unique and exciting element. Over the years it has proved a constant challenge and stimulus not only to preparative chemists and theoreticians, but also to industrial chemists and technologists. It is the only non-metal in Group 13 of the periodic table and shows many similarities to its neighbour, carbon, and its diagonal relative, silicon. Thus, like C and Si, it shows a marked propensity to form covalent, molecular compounds, but it differs sharply from them in having one less valence electron than the number of valence orbitals, a situation sometimes referred to as electron deficiency . This has a dominant effect on its chemistry. 

It is necessary for the intermediate cation or complex to bear considerable car-bocationic character at the carbon center in order for effective hydride transfer to be possible. By carbocationic character it is meant that there must be a substantial deficiency of electron density at carbon or reduction will not occur. For example, the sesquixanthydryl cation l,26 dioxolenium ion 2,27 boron-complexed imines 3, and O-alkylated amide 4,28 are apparently all too stable to receive hydride from organosilicon hydrides and are reportedly not reduced (although the behavior of 1 is in dispute29). This lack of reactivity by very stable cations toward organosilicon hydrides can enhance selectivity in ionic reductions. 

An alternative method of producing indole-containing compounds involves a bis-Suzuki reaction of 2,3-dihaloindoles 114 with 2 equiv of boronic acids 115 with 10 mol % Pd(OAc)2 [75]. The paper describes the difference in electronic effects of the boronic acids. Electron-rich boronic acids give better yields (85-95%) whilst the electron-deficient boronic acids give poorer yields (44-55%). Scheme 28 shows the general synthesis of these compounds. 

Alkenyl- and alkynylboranes also function as dienophiles. The electron-deficient boron is responsible for the electronic effect. 

Boron also appears to be involved in redox metabolism in cell membranes. Boron deficiency was shown to inhibit membrane H -ATPase isolated from plant roots, and H -ATPase-associated proton secretion is decreased in boron-deficient cell cultures [71]. Other studies show an effect of boron on membrane electron transport reactions and the stimulation of plasma reduced nicotinamide adenine dinucleotide (NADH) oxidase upon addition of boron to cell cultures [72, 73]. NADH oxidase in plasma membrane is believed to play a role in the reduction of ascorbate free radical to ascorbate [74]. One theory proposes that, by stimulating NADH oxidase to keep ascorbate reduced at the cell wall-membrane interface, the presence of boron is important in... 

It is now clear that earlier studies indicating that very low levels of boron are toxic to fish embryos were actually documenting the effects of boron deficiency [83]. More recent studies on fish show that embryonic growth is stimulated by boron in a dose-dependent manner [80]. While very high boron levels are toxic, insufficient boron levels (<9 pmol B) were shown to result in adverse effects on rainbow trout and zebrafish development. 

Graham, R.D., R.M. Welch, D.L. Grunes, E.E. Cary, and W.A. NorveU. 1987. Effect of zinc deficiency on the accumulation of boron and other mineral nutrients in barley. Soil Sci. Soc. Amer. Jour. 51 652-657. 

Hopmans, P. and D.W. Ehnn. 1984. Boron deficiency in Pinus radiata D. Don and the effect of applied boron on height growth and nutrient uptake. Plant Soil 79 295-298. 

Smyth, D.A. and W.M. Dugger. 1980. Effects of boron deficiency on rubidium uptake and photosynthesis in the diatom Cylindrothecafusiformis. Plant Physiol. 66 692-695. 

Different works show that the nutritional status of certain nutrients, such as boron (B), calcium (Ca), nitrogen (N), phosphorus (P) and iron (Fe) can trigger changes in phenolic metabolism. Of these nutrients, B is attributed with a clear and significant effect on the metabolism of these secondary compounds. As we shall discuss below, the relationship between B metabolism and phenolics is complex and depends largely on the sensitivity of the plant to B deficiency or toxicity. 

The uniqueness of structure and properties of boron is a consequence of its electronic configuration. The small number of valence electrons (three) available for covalent bond formation leads to electron deficiency, which has a dominant effect on boron chemistry. 

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