Relations between Structure and Coordination Stability in

3 Relations between Structure and Coordination Stability in Boroxazolidines H O W A R D K. Z I M M E R M A N

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 29, 2016 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch003

Department

of Chemistry, University of the Pacific, Stockton,

Calif.

The presence of the N-B dative bond is shown in the tricyclic boric acid esters of triethanolamines, the bicyclic boronic acid esters of diethanolamines, and the monocyclic borinic acid esters of ethanolamines. The boroxazo lidines have dipole moments in the range 8.5 to 11 debyes, showing a polar structure. The sizes of the nitrogen substituents in the b i cyclic and monocyclic series influence the rates with which they neutralize aqueous strong acid; this is reflected in strong steric influences on the entropy of activation, although the free energies of activation show little systematic variation with changes in substituent. Boroester carbon atom substituents in the tricyclic series moderately affect entropy of activation. A mech anism is suggested for the hydrolysis of triptych boroxazolidine. Τ he first reported preparation of a boroxazolidine appeared in a German patent (22) in 1933, where a preparation of triethanolamine borate was reported. This interesting new substance remained un noticed until it was rediscovered in 1951 (3) and its preparation was reduced to a convenient azeotropic process (7) in 1952. The class of related compounds was expanded by the preparation in 1955 of a few aryl boronic esters of diethanolamine (17) and in 1956 of about a dozen alkyl boronic esters of diethanolamine (10). Extension to the aryl borinic esters of ethanolamine (12, 13) occurred at about the same time with the perfection by Letsinger and his students of a reliable procedure for preparing a variety of diaryl borinic acids. Subsequent ly, the number of known aryl boronic esters and diaryl borinic esters has been greatly extended (5, 14, 20, 21, 33, 36), and over four dozen boric acid esters of variously substituted triethanolamine s have been reported by Schleppnik and Gutsche (24, 25). Against the well-known background of knowledge of the extreme sensitivity of boron esters of the ordinary alcohols toward both atmos pheric oxidation and hydrolytic cleavage (26), it was immediately observed that the foregoing amino alcohol esters possess an unusual degree of stability with respect to these influences. 23

Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

A D V A N C E S IN

24

CHEMISTRY

SERIES

Brown and Fletcher (3) were the first to advance the idea that this stability is a result of N-B dative bonding within the molecule (Figure l,a) of triethanolamine borate to produce a triptych structure,

/

C-N-C.

I

c çf

a. Triptych Boroxazolidine Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 29, 2016 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch003

C

C C

c

b. 2,2',2"-Nitrilotriethyl Borate R c

, ΝIΛC C

W

R c. Diptych Boroxazolidine T

R 2 I R -N-C 3 l Ρ R -Β-ο'

w

R d. 2,2 -Iminodiethyl Boronate f

f

1

R R N-C-C-Q-BR R 1

N

2

3

4

A4 f. 2-Aminoethyl Borinate

e. Boroxazolidine

Οχ

>

c'

R Ρ

C H,-B~N-C-C-N-B- 6 5 C

e

H

g. 1,2-Bis (i?-phenyl diptychN-boroxazolidino) ethane

Figure

o

N

or

N

c

H-N-B-C-C-B-N-H

d

b

0

>

1,2-Bis (diptych JB-boroxazolidino) ethane

1. Structures

of typical esters of amino alcohols and the boron acids in contrast to the expected noncoordinated (or cage") structure (Fig ure l,b). Support was given to this conception by evidence presented by Musgrave and Park (17) to show that, in the parasubstituted aryl boronic esters of diethanolamine, the N - H infrared frequency under goes a regular shift in wavelength which parallels the known polarity of the p -substituent on the Z?-aryl group. Thus, the boronic esters were represented as diptych structures (Figure l,c), in contrast to the open structure (Figure l,d). However, in a parallel infrared study of the diethanolamine esters of an extensive group of alkyl boronic acids (10), Lawesson was unable to find any evidence of the kind of effect reported by Musgrave and Park, and he concluded that the only inter action present was probably a hydrogen bonding involving the amine hydrogen atoms. Nevertheless, the above-mentioned stability features in these compounds have persuaded most workers in this area that the best way to represent them is in terms of the N-B coordinated struc ture, so that such a formulation has persisted, not only among the M


3

ZIMMERMAN

Coordination

Stability in

Boroxazolidines

25


diethanolamine esters, but also among the monoethanolamine esters of the diaryl borinic acids (Figure l,e and f) without any actual evidence for its presence. A brief infrared study published by the author (34) early in the i n vestigations discussed here offered frequency assignments which were thought to reflect N-B coordinate bond vibrations. However, subsequent studies of the same kind on additional compounds have failed to corroborate the suggested assignments, at least in any simple way, and certain papers (2, 31) in this symposium provide clear evidence that such attempts are premature and are unreliable within the context of molecules as complex as those considered here. Therefore, no confidence can be placed in infrared data as direct proofs of N-B dative bonding in this class of substances at the present time. Thus, prior to the physicochemical investigations discussed below, the only experimental evidence supporting the N-B coordinated structure consisted of the contradictory reports concerning the infrared spectra of the diethanolamine esters, together with a few fragmentary rate studies into the acidic hydrolysis of triethanolamine and (a little later) nitrilotris(2-propand) borates (3,26,27), and even these were the subject of conflicting interpretations, since Steinberg and Hunter (26, 27) had offered the alternative suggestion that the comparatively slow interactions between these borates and strong acid are merely results of steric hindrance around the boroester constellation of atoms. As a first step, it seemed desirable to attempt to resolve the question whether the resistance of triethanolamine borate to acidic degradation is to be attributed to the steric hindrance effect or to the presence of N-B dative bonding. In a fairly detailed kinetic study (38), it was shown first that the interactions of this compound in both acid (HC1) and alkali (NaOH) in aqueous solution obey identical rate laws; in both environments the reaction is first-order and conforms to identical activation parameters. This was taken as evidence that the role of either acid or alkali is that of a "scavenger" or participant in a fast reaction which follows the rate-determining decomposition step. When rate studies were made of the reaction by which triethanolamine borate is formed in aqueous solution from the component compounds (15) y it was found that this process conforms kinetically, not to an initial N-B coordination between triethanolamine and boric acid, followed by boroester formation, but rather to a fast esterification giving the cage structure (Figure l,b), followed by a rate-determining step to give Figure l,a. Thus, the kinetic indications point toward the existence, in system-neutral aqueous solution, of an equilibrium between the structures of Figure l,aand l,b. A probable mechanism, consistent with the known facts, has been formulated as shown in Figure 2. The several rate and equilibrium constants indicated in that mechanism have been determined at 5° intervals from 10° to 30°, permitting the deduction of the appropriate activation and reaction enthalpies, entropies, and free energies which are summarized in Table I.


26

ADVANCES

IN

CHEMISTRY

N(CH CH OH) 2

c /\ ο

ik ~ ~

r

\ΐ1

^

k

h

c o \J C

d~ ΊΓ

K

c

l a

N

NB/

\ΓΒ

- „ \'

^

2. Proposed

^

N(CH CH OH) +B(OH) 2

2

3

3

,. \\ T rr

^

l b

Figure

+B(OH)~

3

c

N-»B


I \9

ç

2

SERIES

K

K

a

^

^

HN(CH CH OH) B(OH) 2

mechanism for hydrolysis

2

of triptych

3+

3

boroxazolidine

The next step led to examination of N-B coordination in a series of Ν-substituted diethanolamine esters of phenyl dihydroxyborane (20). These compounds also show a delayed basicity toward aqueous acid, but one of a higher order of rapidity than in the case of the triptych compounds. Thus it was necessary to study them in an aqueous acetone system in order to be able to reduce temperatures to a point where the reactions could be slowed to manageable limits. With the exception of the diisopropanolamine ester of the same boronic acid, all of these ester decompositions appeared to obey zeroth-order kinetics. While none of them has yet been studied completely in the sense of the case just discussed, this fact may indicate a mechanism which is not simple. As before, determination of rates over a range of temperatures permitted calculation of the activation parameters given in Table II. Table I.

Constant

0,

Thermodynamic and Activation Functions for Acid Hydrolysis of Triptych Boroxazolidine AH, K cal. /Mole

AS, Cal. /Deg.

^ 298, Kcal. /Mole F

h

11.5

-31.2

20.8

*c

20.0

0.2

20.0

d

-8.5

-31.4

0.8

*a

7.2

58.4

-10.2

K

b

-6.1

0.0

-6.1

K

d>

10.4

33.4

0.5

3.4

-31.0

12.6

6.3

-21.6

12.7

k

K

%

(6,16)

K

Relations between Structure and Coordination Stability in

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