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Medical Management Guidelines for Diborane
Synonyms include boroethane, boron hydride, diboron hexahydride.
Persons exposed only to diborane pose little risk of secondary contamination to personnel outside the Hot Zone.
Diborane is a colorless highly flammable gas with a repulsive, sickly sweet odor. At high concentrations, it ignites spontaneously in moist air at room temperature. It reacts with water to form hydrogen and boric acid. Diborane vapors are heavier than air and may collect in low-lying areas.
Diborane is highly irritant when it contacts moist tissues such as the eyes, skin, and upper respiratory tract and can cause thermal burns. Burns are caused by the exothermic reaction of hydrolysis. Ingestion of diborane is unlikely since it is a gas at ambient temperatures.
Description
At room temperature, diborane is a colorless gas with a repulsive, sickly sweet odor. It is generally shipped in pressurized cylinders diluted with hydrogen, argon, nitrogen, or helium. It reacts with water to form hydrogen and boric acid. It mixes well with air and explosive mixtures are easily formed. At high concentrations, it will ignite spontaneously in moist air at room temperature. The main toxic effect of exposure to diborane is irritation of the respiratory airway, skin, and eyes.
Routes of Exposure
Inhalation
Inhalation is the major route of exposure to diborane. An odor threshold between 2 and 4 ppm has been reported for diborane, which is higher than the OSHA permissible exposure limit (PEL) of 0.1 ppm. Prolonged, low-level exposures, such as those that occur in the workplace, can lead to olfactory fatigue and tolerance of diborane's irritant effects. Odor does not provide adequate warning of hazardous concentrations. Diborane is heavier than air; exposure to concentrations exceeding the PEL may result in skin, respiratory, and eye irritation in poorly ventilated, enclosed, or low-lying areas.
Children exposed to the same levels of diborane as adults may receive larger dose because they have a greater lung surface area:body weight ratios and higher minute volume:weight ratios. In addition, they may be exposed to higher levels than adults in the same location because of their short stature and the higher levels of diborane found nearer to the ground.
Skin/Eye Contact
Direct contact with concentrated diborane vapors may cause severe eye or skin burns, leading to cell death and ulceration.
Ingestion
Ingestion is unlikely to occur because diborane is a gas at room temperature.
Sources/Uses
Diborane is produced by the reaction of lithium hydride with boron trifluoride catalyzed by ether at 25oC.
Diborane is used in rocket propellants and as a reducing agent, as a rubber vulcanizer, as a catalyst for olefin polymerization, as a flame-speed accelerator, and as a doping agent in the manufacture of semiconductor devices.
Standards and Guidelines
OSHA PEL (permissible exposure limit) = 0.1 ppm
NIOSH REL (recommended exposure limit) = 0.1 ppm
NIOSH IDLH (immediately dangerous to life or health) = 15 ppm
AIHA ERPG-2 (maximum airborne concentration below which it is believed that nearly all persons could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair their abilities to take protective action) = 1 ppm.
Boronic acid
Boron containing compounds have not been widely studied in Medicinal Chemistry, mainly due to the idea that this group could confer some toxicity. Nowadays, this concept has been demystified and, especially after the discovery of the drug bortezomib, the interest for these compounds, mainly boronic acids, has been growing. In this review, several activities of boronic acids, such as anticancer, antibacterial, antiviral activity, and even their application as sensors and delivery systems are addressed. The synthetic processes used to obtain these active compounds are also referred. Noteworthy, the molecular modification by the introduction of boronic acid group to bioactive molecules has shown to modify selectivity, physicochemical, and pharmacokinetic characteristics, with the improvement of the already existing activities. Besides, the preparation of compounds with this chemical group is relatively simple and well known. Taking into consideration these findings, this review reinforces the relevance of extending the studies with boronic acids in Medicinal Chemistry, in order to obtain new promising drugs shortly.
The use of boron in the design of drugs is fairly recent and most biological activities of these compounds have been reported over the last decade. For a long time, compounds with boron were put aside in studies of Medicinal Chemistry because they were thought to be toxic, mainly because of their use in ant poisoning. Nowadays, this believe is demystified, and boron-containing compounds are usually considered as non-toxic. Boron compounds are found in nature in high concentrations, mainly in vegetables, fruits, and nuts, and the boron natural derivative boric acid is even used as a preservative in eyewash and as a buffer in biological assays. There are many organoboron compounds, but, in organic chemistry, boronic acid is the most commonly studied boron compound.
Chirality and Symmetry
All objects may be classified with respect to a property we call chirality (from the Greek cheir meaning hand). A chiral object is not identical in all respects (i.e. superimposable) with its mirror image. An achiral object is identical with (superimposable on) its mirror image. Chiral objects have a "handedness", for example, golf clubs, scissors, shoes and a corkscrew. Thus, one can buy right or left-handed golf clubs and scissors. Likewise, gloves and shoes come in pairs, a right and a left. Achiral objects do not have a handedness, for example, a baseball bat (no writing or logos on it), a plain round ball, a pencil, a T-shirt and a nail. The chirality of an object is related to its symmetry, and to this end it is useful to recognize certain symmetry elements that may be associated with a given object. A symmetry element is a plane, a line or a point in or through an object, about which a rotation or reflection leaves the object in an orientation indistinguishable from the original. Some examples of symmetry elements are shown below.The face playing card provides an example of a center or point of symmetry. Starting from such a point, a line drawn in any direction encounters the same structural features as the opposite (180o) line. Four random lines of this kind are shown in green. An example of a molecular configuration having a point of symmetry is (E)-1,2-dichloroethene. Another way of describing a point of symmetry is to note that any point in the object is reproduced by reflection through the center onto the other side. In these two cases the point of symmetry is colored magenta.
The boat conformation of cyclohexane shows an axis of symmetry (labeled C[sub]2[/sub] here) and two intersecting planes of symmetry (labeled σ). The notation for a symmetry axis is C[sub]n[/sub], where n is an integer chosen so that rotation about the axis by 360/no returns the object to a position indistinguishable from where it started. In this case the rotation is by 180o, so n=2. A plane of symmetry divides the object in such a way that the points on one side of the plane are equivalent to the points on the other side by reflection through the plane. In addition to the point of symmetry noted earlier, (E)-1,2-dichloroethene also has a plane of symmetry (the plane defined by the six atoms), and a C[sub]2[/sub] axis, passing through the center perpendicular to the plane.
The existence of a reflective symmetry element (a point or plane of symmetry) is sufficient to assure that the object having that element is achiral.
Chiral objects, therefore, do not have any reflective symmetry elements, but may have rotational symmetry axes, since these elements do not require reflection to operate. In addition to the chiral vs achiral distinction, there are two other terms often used to refer to the symmetry of an object.
Solubility of Phenylboronic Acid and its Cyclic Esters in Organic Solvents
The solubilities of phenylboronic acid, its pinacol ester and azaester in organic solvents (chloroform, 3-pentanone, acetone, dipropyl ether and methylcyclohexane) have been determined experimentally by a dynamic method, in which the disappearance of turbidity was determined by measuring of light intensity using a luminance probe. Phenylboronic acid has high solubility in ether and ketones, moderate in chloroform and very low in hydrocarbon. Pinacol ester and azaester show better solubility than the parent acid in all tested solvents. For pinacol ester differences between particular solvents are small, while for azaester the differences are significant. For both esters the highest solubility is observed in chloroform and the lowest in the hydrocarbon. The results have been correlated by the Wilson, NRTL and Redlich–Kister equations. For the phenylboronic acid better correlation of the data is obtained by polynomials in comparison with the above equations. It is connected with additional acid-anhydride equilibrium in the system. The influence of polarity of the solvents on the solubility is discussed.
Arylboronic acids and their derivatives are an important group of compounds due to their broad applications in organic synthesis, catalysis, supramolecular chemistry, and materials engineering. The compounds have been known for about 150 years, but increasing interest is observed after the discovery of new areas of their use. The most important fields are the synthesis of biaryl compounds in the Suzuki–Miyaura reaction (Nobel 2010), the molecular receptors of sugars, covalent organic frameworks, and their use as biologically active compounds. Such wide applications require recognition of the physicochemical properties of these compounds. An important issue is the description of phase equilibria, including solubility in water and organic solvents. Knowledge of these data allows correct selection of the solvent for a particular reaction or for purification of the products by crystallization. They are also important for biological tests and formulation of biologically active compounds.
Organoboron compounds
One of the most important strategies for the formation of C–C bonds uses organometallic or metalloid
compounds as nucleophilic reagents. Organoboron compounds occupy a privileged position among
these reagents owing to their ease of synthesis, stability, and increasingly, their commercial availability
and synthetic versatility. Our initial interest in using organoboron compounds focused on borontethered cycloaddition and free-radical cyclization reactions. The discovery of new reactions utilizing boronic esters or other boron compounds at this oxidation state would allow for the design of novel
synthetic strategies. Moreover, the emergence of combinatorial chemistry over the past decade has provided additional impetus for the discovery of new reactions that are operationally simple, use air- and
water-stable reagents, and give products that require minimal purification. One of the objectives of our
laboratory, therefore, is the development of new practical methods using organoboron compounds as an
efficient means for the discovery of novel biologically active molecules. This article summarizes some
of the research in this field emanating from our laboratory over the past three years.
- Created: 06-07-22
- Last Login: 06-07-22