Diboranes hydrolysis

Diborane or aUcylboranes are used for reduaion of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

I soon discovered that simple aldehydes and ketones react rapidly with diborane at 0° C. Hydrolysis then produces the corresponding alcohol. 

The most characteristic reaction of amine boranes is their conversion into aminoboranes and, subsequently, to borazine at higher temperatures.9,17 For complexes of low stability, the transfer of H from the boron to the donor is a characteristic process,11 40 as in the utilization of diborane as an electrophilic reducing agent. The neutral complexes of boranes are fairly stable towards hydrolysis. The key step of the hydrolysis was formerly assumed to involve displacement of BH3 by a proton, whereas in recent studies ionic intermediates, containing five-coordinated boron (R3N—BH4) are also taken into consideration.41,42 The hydrolytic... 

Cleavage of B3H7 -0(CH3)2 by PF3 occurs slowly at —16° 125,126) The major product, B2H4(PF3)2, is very reactive and is potentially a very useful reagent for synthesis. This novel diborane(4) adduct melts at —114.3° and decomposes by a second-order process. It was characterized by vapor density, infra-red spectroscopy and mass spectroscopy. Acid hydrolysis produced about four moles of H2 and basic hydrolysis produced about five moles of H2 per mole of B2H4(PF3)2. In addition, the following reactions were found to proceed nearly quantitatively. 

Modification of zeolites, based on chemisorption of silane or diborane and subsequent hydrolysis of the chemisorbed hydride groups can also be applied for encapsulating gas molecules in zeolites. For example, krypton and xenon can be encapsulated in mordenite combining the modification process with a physical adsorption of the noble gases at moderate pressures and temperatures (e.g. 100 kPa, 300 K). The encapsulates are homogeneous and stable towards acids, mechanical grinding and y-irradiation. By controlling the pore size reduction however, the thermal stability can be controlled. 

Reaction of diborane with the hydroxylic species of the silica surface results in the hydrolysis of the molecule to form hydrogen gas. This reaction is expressed by the hydrolysis ratio, R, which is the ratio of hydrogen evolved in the reaction to diborane... 

Hydrolysis reactions of diborane in the presence of gaseous B2H6 are characterized by a primary hydrolysis ratio, R , while reactions occurring after excess diborane has been removed, are expressed by their secondary hydrolysis ratio, RseCondaiy The amount of hydrogen, produced in the secondary reactions, depends mainly on three parameters. 

The first studies of the modification reaction of silica with diborane were performed by Weiss and Shapiro.34,35,36,49 Their studies were focused on the nature and localization of the hydroxylic species in the silica. Reaction of untreated silica gel with at room temperature gave a hydrolysis ratio of six, indicating total hydrolysis of the B2H6 molecules. The same situation is found when reacting diborane with excess water,34 according to the reaction ... 

With the use of the experimental hydrolysis ratios found, the amount of diborane able to desorb (nD) according to reaction (L) was calculated.39 For pretreatment temperatures below 773 K a good agreement was found (within a 5 % error margin) with the experimental value (see table 10.4). At higher pretreatment temperatures the experimentally obtained results differed from the calculated amount, which the authors explained as the occurrence of yet another reaction with a hydrolysis ratio equalling 2 (reaction (I)). 

So interpretation of the B-H vibration bands confirmed the different reactions of diborane with the hydroxylic surface groups put forward by Shapiro and Weiss.35,36,49 In addition the infrared analysis of the modified surface revealed (1) the reaction of B2H6 with siloxanes, explaining the low hydrolysis ratios found under certain reaction conditions and (2) the equilibration reaction existing between B2H6 in the gas phase and the monodentate groups formed through reaction of BH3 with a surface hydroxyl. 

OH/100 C under optimal conditions of the reduction with diborane. These conditions were the hydrolysis with NaOH and the oxidation of C = C bonds in the presence of H2O2. 

When magnesium is heated with boron, the product is magnesium boride, MgB2 (sometimes written as Mg3B2). The hydrolysis of the compound produces Mg(OH)2 and a boron hydride. Instead of the expected product of BH3, borane, the product is B2H6, diborane. 

The resulting acetate 116 is formulated as the tawis-isomer, too. The carboxylic group was then converted by selective reduction with diborane to the alcohol 117. The alcohol function was converted to the nitrile 119 via the tosylate 118 and displacement of the tosylate group with sodium cyanide. Methanolysis led to the methylester 120 because the acetate moiety was not cleaved under these conditions (MeOH/HCl). Hydrolysis with base yielded the deprotected lactone acid alcohol 121, which was purified by converting it into the methylester 122. 

Another novel diborane(4) compound has previously been obtained by adventitious hydrolysis of 1, namely the cyclic reduced boric acid species B402(0H)4 (8).8 Compound 8 is produced in much higher yield from a more controlled hydrolysis of either 1 or 2 but can be prepared almost quantitatively from the reaction between 3 and aqueous ethanol as a material with co-crystallised [NH2Me2]Cl.8 Electrospray mass spectrometry revealed that the more highly condensed species 9 can also be formed indicating that a large family of condensed, reduced boric acids/borates derived from B2(OH)4 may be synthetically accessible. 

A fairly extensive metathetical chemistry of diboron compounds has been developed which provides the means for synthesis of specific derivatives from more commonly available starting materials such as tetrachloro-diborane(4) and the tetrakis(dialkylamino)diboron derivatives. Thus, reactions involving hydrolysis, aminolysis, alcoholysis, transamination, etc., are available for interconversions of diboron compounds by pathways analogous to those known in monoboron chemistry. Examples of such reactions include the following ... 

A trialkylborane. for example (3) resulting from reaction of hexene-1 (1) with diborane (2. generated from NaBH4-l- BFa). undergoes remarkably fast 1,4-addition to acrolein to produce the enol borinate (4). which on hydrolysis gives the aldehyde (5).1 The Brown group found it more convenient to add water to a solution of the... 

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