August, 1963
1741
COMMUNICATIONS TO THE EDITOR STRUCTURAL RIGIDITY AND INTERXAL B-N COORDINATION INFLUENCES ON THE HYDROLYSIS :RESISTANCE OF BORATE ESTERS Sir: Increased hydrolytic stability of borate esters can be achieved in moleculeu possessing nitrogen atoms stereoelectronically positioned to form partial dative bonding with the boron atom.l-* The Bll n.m.r. chemical shifts of a number of bicyclic aza,boroxanes indicate that stereochemical constraint plays a major role, in addition to the amount of B-Y interaction, in stabilizing a borate ester toward hydrolysis. These Bll n.m.r. spectra were obtained using a Varian high resolution n.m.r. spectrometer operating a t 12.83 Me. The chemical shifts were solvent insensitive and are reported relative to BF,.Etz0.6 The Bll chemical shift value may be considered to be roughly proportional to the electron shielding around a boron nucleus when closely related molecular species are involved. Higher chemical shift values indicate greater electronic shielding around the boron and, in the case of azaboroxanes, increased B-X' bonding. From Fig. 1, it is apparent that the Bll chemical shift of the bicycloborate esters is primarily determined by ring size. For a [3,3,3]-bicycloborate ester, the chemical shift value 6 occurs around -14; for a [3,3,4]bicycloborate ester, 6 = - 10; a [3,4,4]-bicycloborate ester, 6 -4; a [4,4,4],-bicycloborateester, 6 -1 to - 2 . If the amount of B-N bonding is the most important factor in accounting for the hydrolytic stability of the bicycloborate esters, then it is expected that there should be a consistent relationship between the Bll chemical shift and rate of hydrolysis. From Fig. 1, it is apparent that there is a direct relationship between B1l chemical shift and rate of hydrolysis in the four simplest structural species I, 111, V, and VIII; however, the more structurally complica,ted molecules reveal no such simple relationship. Qualitatively, it is possible to explain t,he hydrolysis rates on the basis of stereochemical constraint of the molecule. Compound IX mas found to be far more resistant to hydrolysis than VI11 which differs only by methyl groups attached to the ring system. This relationship has previously been explained as resulting from the relief of methyl-methyl interactioiis to produce a shortened B-S d i s t a n ~ e . ~ However, a shortened B-N distance would be expected to give rise t'o a significantly higher Bll cheniica,l shift value which is not observed. A more acceptable explanation involves molecular rigidity due to steric influences during proton attack. (1) T h e synthesis a n d chemical properties of the bicyclic aaaboroxanes discussed in this paper were carried o u t b y R. Swidler, U. S.P a t e n t 3,047,609, to be published. However, the general synthetic procedure4 as well as the method of determining hydrolysis rate half-times:l has already been reported. (2) M. F. Lappert, Chem. Rev., 56, 959 (1956). (3) H. C. Brown and E. A. Fletcher, J . Am. Chem. Soc., 73, 2808 (1951). (4) H. Steinberg and D. L. Hunter, dbid., 88, 853 (1960). ( 5 ) T.P. Onak, H. Landesman, R. E. Williams, and I. Bhapiro, J . P h p .
Chsm., 68, 1533 (1969).
The subsequent or nearly simultaneous attack of a water molecule on the boron atom may also influence the rate of hydrolysis.
In proceeding through such a traiisition state, the methyl groups would be forced either into interference with adjacent rings and/or into methyl group-hydrogen eclipsing on adjacent carbons. VIII, having no meth,yl groups, will be involved only in hydrogen-ring or hydrogen-hydrogen interactions. m I
CHEhUCAL SHIFT
Figure 1.
Molecular models reveal VI1 [3,3,4] to be exceptionally rigid when compared to I1 [4,4,4]. In aqueous acid, the hydrolysis half-time for VI1 was 3040 min.; in contrast only 2 min. was required for 11. Structural rigidity as a major factor in borate ester hydrolytic stability is readily apparent in this case; had the amount of B-N bonding as revealed by the Bll chemical shift been the dominant factor IT would have been more stable than VII. (6) Space-General Corporation, El Monte, California. O L I N MATHIESON CHEMICAL CORrORATlON THOMAS P. ONAK PASADENA, CALIFORXIS ROBERT E. WILLIAMS^^
STANFORD RESEARCH INSTITUTE RONALD SWIDLER SOUTHPASADENA, CALIFORNIA RECEIVED MARCH 7, 1963
