14 Hydrolyses of Some Boron-Nitrogen Derivatives ROBERT J . BROTHERTON
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 9, 2017 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch014
U. S. Borax
Research
Corp.,
a n d ALLEN L. McCLOSKEY Anaheim,
Calif.
The hydrolyses of a variety of boron-nitrogen compounds have been investigated in homogeneous solvent-water solutions which contain a large excess of water. Under these conditions, hydrolysis rates followed pseudo - first - order kinetics in all cases. Hydrolyses of series of tri(amino)boranes and tetra(amino)diborons suggest that steric effects have significant influence on the observed hydrolysis rates of these materials. k variety of organoboron compounds containing covalent boron-nitrogen bonds have been described in the literature. Although qualitative references have been made to the hydrolytic stability of these materials, no comparative hydrolysis rate studies have been reported. The present paper includes the results of semiquantitative hydrolysis rate studies carried out at intervals for several years. Some of the initial results have been discussed (14). Although the experimental techniques used in these studies might not meet the strict criteria normally required for a quantitative kinetic investigation, the results a l low comparisons to be made between the boron-nitrogen derivatives investigated. It is also possible to compare the general hydrolytic stability of some of the boron-nitrogen systems with related boron-oxygen compounds which were the subject of a previous investigation in this laboratory (13) Most previous reports of the hydrolytic stability of boron-nitrogen compounds refer to hydrolytic stability in heterogeneous aqueous systems or in the presence of atmospheric moisture. In the presentwork a variety of boron-nitrogen compounds were hydrolyzed in dilute (0.02 to 0.07 mole per liter) homogeneous solvent-water solutions. Hydrolysis rates were followed by titrating the amount of free amine formed and assuming that appearance of free amine is directly related to B - N hydrolysis. The first-order plots obtained from B - N compounds derived from a variety of amines and the known chemistry of these materials tends to confirm this assumntion. The r e v e r s i b i l i t v of the hvdrolysis reaction (>B-OH > B - N O has been demonstrated in a few cases involving formation of stable B - N rings, but this reaction is extremely slow and can be ignored in the present discussion. Solvents used were acetone, tetrahydrofuran, and dioxane. No measurable differences in rates were observed when these different solvents were used in identical hydrolysis experiments. An example of this is the hydrolysis of tet131
Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.
ADVANCES
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 9, 2017 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch014
132
IN
CHEMISTRY
SERIES
rakis(dimethylamino)diboron, which hydrolyzed at essentially the same rate in acetone and tetrahydrofuran (see Table II). A large excess of water was present in all cases, and the hydrolysis reactions followed pseudo-first-order kinetics. Plots of the logarithms of the moles of unhydrolyzed starting materials vs. time approached linearity up to at least 50% hydrolysis for all reactions which were slow enough to follow accurately. The analytical procedure used involved rapid titration of the free amine formed during hydrolysis, except for derivatives of aromatic amines, which required alternative techniques. The over-all objective of this study was the determination of relative hydrolysis rates, and probable hydrolysis mechanisms are not discussed in detail. The hydrolyses of a number of tri(amino)boranes derived from both primary and secondary amines were investigated. Observed hydrolytic first-order rate constants and half lives of these materials are shown in Table I. Half lives refer to hydrolyses of 50% of all B - N bonds in the molecule. Tri(amino)boranes derived from aliphatic primary amines hydrolyzed rapidly, except for the sterically hindered teri-butyl derivative. Steric effects also appear to be significant factors in the hydrolysis of secondary amine derivatives. Tris(di-rc-butylamino)- and tris (diphenylamino)boranes, in which approach to the boron atoms and possible formation of a tetrahedral transition state are severely restricted, were comparatively stable in homogeneous solutions. Table I.
Hydrolyses of Tri(amino)Boranes
B(NRR')
3
R
R'
H H
n-C H
H
L-C H
H
n-C H
3
3
4
H
7
7
9
Half Life
90% acetone
25
