Dwight A. Poyne, Jr.
Tulane Universitv New Orleans, Louisiana and Ewin A. EadsL Lamar State College of Technology Beaumont, Texas
I
BO~O~-~itro~en
I
The study of boron-nitrogen heterocycles may be regarded as having an origin in 1926 when the compound borazole, B3N3H8,was first prepared by Stock and Pohland (I). Very early it was noted that the borazole molecule was isoelectronic with benzene and that borazole possessed some aromatic properties. Stock proposed a cyclic structure for borazole, and his general estimates were later confirmed when Bauer made electron diffraction measurements on borazole (2).
1
1 - 1
HBv .,BH N H 0
HB, N '
II
,BH
-
H b
I
II
HB,
,BH N/
H e
Strudure of borozole
It should be noted t,hat the extent of delocalization of the non-sigma electrons would be expected to be much more limited than that which is encountered in the benzene ring. Prior t o the formation of the boron t o nitrogen bond, there is a significant difference in the electronegativities of the boron and nitrogen atoms.2 The bond which results and any subsequent delocalization of the non-sigma electrons should definitely be affected by these electronegativity differences. The separation of the three sigma electron pairs within the borazole ring are also quite different from the analogous pairs in the benzene ring. Bond distances in both benzene and borazole have been measured and seem to verify these premises. Whereas all of the carhonto-hydrogen distances in benzene are uniform a t 1.08 A, the boron-to-hydrogen distance in borazole is 1.20 A, and the nitrogen-to-hydrogen distance is 1.02 A (3). These distances reflect a degree of electron asymmetry in the borazole ring which is not present in the benzene ring. These factors may account for some of the greater reactivity of borazole as well as some of the non-benzene-like reactions. These factors may also account for the greater degree of reactivity a t the horon positions during substitution reactions. All evidence considered, the description of the borazole ring seems t o be more nearly illustrated by a in the figure, with only limited contribution from structures shown in b and c. NSF Faculty Fellow at Tulene University, 1961, while a doctoral candidate. The authora gratefully acknowledge NSF Faculty Fellowship, 60W2,1961. 'The eonceut of eleetroneeativitvused herein is that of Senderson. For a cimplete discussion of~lectronegativity,see SANDERSON, R. T., "Chemical Periodicity," Reinhold, New York, 1960.
334 / Journal o f Chemical Education
Synthesis
I n order t o consider possible synthetic methods for the preparation of boron-nitrogen heterocycles, one would quite reasonably turn to some reaction which would provide for the utilization of unoccupied valence orbitals of the boron atoms with available valence electrons from nitrogen-containing compounds under conditions which would result in condensation. These conditions are easily attained; furthermore, cyclization of both three-coordinate and four-coordinate boron compounds is very difficult to avoid. The extension of boron coordination from three to four is often accomplished in other ways, but condensation in the presence of ammonia or amines very often leads to ring formation wherein there are alternating boron and nitrogen atoms. The classical preparation of borazole depends upon the reaction between ammonia and a boron hydride (or halide) followed by temperature elevation in order to effect the condensation and elimination. For example, Stock and Pohland (1) used ammonia and diborane in a twc-step reaction:
The yield from the diammoniate of dihorane was only 33%. Derivatives of borazole may be prepared by similar processes, i.e., from reaction between boron halides or hydrides, or substituted halides and ammonia, or amines or substituted arnines. This may best be illustrated in general as follows: 3BXs X
=
+ 3NR3
-
+ 6RX
B3XsNsRa
H, C1, Br, alkyl, or aryl, and R = H, dkyl, or sryl.
A number of investigators have utilized various specific amines and boro-halides and conditions in order to effect and perfect specific preparations, hut the general procedure is not altered (4). For example, Jones and IGnney (5) reacted aniline with boron trichloride in benzene a t room temperature for 24 hr. The reaction mixture was filtered while hot and on coolmg the substituted borazole crystallized. Wiberg and Horeld (6) reported that methylamine reacted with fluorodimethylborane a t 400°C to give a B-trifluorohorazole plus methane rather than a B-trimethylborazole and hydrogen fluoride.
In general the reactions of borazole and derivatives with chemical reagents would be expected to resemble reactions of the analogous benzene or benzene derivative with the chosen reagent. Some differences could be expected and a t least partially explained on the basis of the electron and structural differences already mentioned. We would expect substitution reactions a t the boron sites to be favored over substitution on the ring nitrogen a t o m . We also expect substitution on the borazole ring to be effected much more readily than analogous substitutions on the benzene ring. There is not presently rate data available to verify this surmise, but from qualitative results one might conclude borazole substitution under similar experimental conditions is more readily facilitated than benzene ring substitution. Although it is not the purpose of this paper to describe in detail reactions which serve to completely exemplify the expectations regarding similarities and differences of benzene ring systems and their boron-nitrogen analogs, the following discussion may be of some general value. A number of examples which would serve to substantiate these premises have been observed and are well known (7-lo), but only a few specificinstances will be pointed out. As far as we know, all carbon positions in benzene are equivalent and prior to reaction we cannot predict which of the six carbon atoms will undergo substitution. In borazole, however, substitution reactions are not known to occur a t the ring nitrogen atoms a t all. For example, in the well known substitution reaction between bromine and borazole one actually obtains a mixture of all possible B-substitution products, but no N-substituted products.
HN/
I HB,.
B \NH
I .,BH
N H
B
+
HN/
I HB,.
B
\NH
I
N H
,B-Br
+
HN< Br-B,.
I
\NH
I
.,B-Br N H
This is a common example which reflects a concentration of electron charge a t the nitrogen a t o m of the borazole ring quite unlike the smooth distribution of delocalized electrons of the benzene molecule. Further, reaction of Grignard reagents with borazoles as studied by Smalley and Stafiej (9) reflect some reluctance of continued boron substitution after one and two of the ring boron atoms have been substituted. There are several well-defined addition compounds of borazole with water and other molecules, e.g., B8NIH6.3H20and B3NaH6,3CHyOH;of course, there are also cases known where strong association exists between some of these same molecules and benzene. The importance of pi-interaction is often emphasized in the latter cases and whereas observations on structures of the borazole addition compounds are not complete, most of the borazole addition compounds are 1:3 (above examples) which precludes any significant pi contribution toward their stability. Borazoles undergo addition reactions readily in strong contrast to benzene and benzene derivatives.
One mole of borazole readily adds three moles of hydrogen chloride (or other hydrogen halides).
Such reactivity is not displayed by benzene, probably because of the stability of the delocalized electron system. This also serves to illustrate the limited ability of the ring boron a t o m to utilize the nitrogen atoms unshared pair of electrons for internal fourcoordiiation. It also points up the stability of both three and four cyclic coordinated boron. These addition products can be made to lose hydrogen a t elevated temperatures and the more electronegative atom of the added molecule invariably stays attached to the boron atoms. It should be noted that substitution reactions of borazoles are not usually well defined because addition products frequently appear in the reaction mixture. The existence of pi molecular orbitals in the borazole ring would imply that pi-bonded compounds could be observed. No definite compounds have as yet been reported, but perhaps this can be ascribed to the fact that pi molecular orbitals would not be nearly as well defined in the borazole ring as in the benzene ring. Pi-interaction, if observed, would be expected to be quite weak. Kinney and Kobezen (11) reported B,B',Bf'-trichlor+N,Nf,N"-tri-p-tolylborazole crystallized from benzene solution with one molecule of the benzene. This could be the result of pi-interaction; however not enough data are available to be certain. The asymmetry of the sigma electron distribution in the borazole ring is reflected in some chemical reactions which have been observed in this laboratory (12). Thus, a series of compounds were prepared by reaction of various carbonyl containing compounds with ether solutions of B,Bf,B"-trichloroborazole. In all cases there was observed an exothermic reaction which resulted in the formation of either a 1:1 or 1:2 adduct of the carbonyl to the B,B',Bf'-trichloroborazole. In each case only one boron site on the borazole nucleus was involved. With cyclohexanone the reaction was observed as illustrated by the equation:
-
+ C6Hto0
BIClsNaHs
+ HC1
B3C19N&OCsH~
With acetophenone, the reaction is illustrated as: B,CI,N,H,
+ CpH&OCHa
and with acetone as: 2BaClaNaHa
+ CaHsO
B,CLNsHsOCsHa
+ HC1
+ 2HC1
CzH.O.2B~ChNnHz
The infrared spectra shows a B-0-C bond in the 1:1 cyclohexanone: B,B',Bf'-trichloroborazole adduct and a B-C bond with an intact carbonyl group in the other two cases. Further work is needed in order to characterize these adducts; however, the ease of the reactions illustrates the high reactivity of the borazole nucleus as well 8s a separation of charge not usually found in an analogous benzene compound. Inactivity of the borazole nucleus following one substitution resembles the behavior of the benzene ring. Volume 41, Number 6, June 1964
/
335
Reaction of B,Bf,B"-trichloroborazole with chloral hydrate resulted in an adduct which had an odor resembling some of the highly chlorinated carbon compounds such as DDT. In this case a reaction occurred in which two moles of chloral hydrate were consumed for each mole of the B,Bf,B"-trichloroborazole. Structures have not been definitely established for these derivatives. Only very recently, Lappert and co-workers a t the University of Manchester have prepared some new boron-nitrogen ring compounds (13). Preparative reactions consisted of procedures as previously outlimed; elimination and cycliiation reactions were employed. For example, reaction of tbutylamine with boron trichloride followed by thermal agitation a t 260°C gave diazoboretane.
/
\
B-N
I
N-B
I
+ 6HC1
available. As is well known, the limitation of the effectiveness of some organic medicinals is the toxicity or inconvenience of the presence of the benzene ring. If the six-membered boron-nitrogen ring is not as undesirable, some interesting physiological experiments may be forthcoming. Conclusion
The fascination of discovery of new boron-nitrogen ring systems is only now being realized. There are apparently no practical applications in vogue, yet both basic and applied research should rapidly expand in the next decade. The electron average of an alternatiig boron-nitrogen system analogous to some corresponding carbon system, combined with the innate differences of the boron and nitrogen atoms, should provide a large number of interesting and perhaps useful derivatives. Investigation of boronnitrogen heterocycles may also lead to a method for the avoidance of uncontrolled polymerization of boron-nitrogen compounds and render some method for the preparation of more desirable polymers containing predominantly boron and nitrogen atoms. Literature Cited
This product represents a compound wherein the boron is three-coordinated and is analogous to a tetrasubstituted cyclobutane. It is not likely there is any significant contribution of internally four-coordinated boron in this ring system. Preliminary Toxicity Studies
Derivatives of boron-nitrogen heterocycles may well prove of some future interest since the results of one experiment performed on some of our carbonyl derivatives indicate that perhaps the borazole ring in these kinds of molecules has a low order of toxicity (14). As a precautionary measure a sample of three derivatives, which resulted from reaction between B,B1,B"-trichloroborazole and a carbonyl compound, were sent to an office of Health, Education and Welfare in Washington, D. C. for screening as possible anticancer agents. The reports were negative insofar as anticancer activity, but the compounds were apparently harmless to the general well-being of the warm-blooded test animals. Of course, much more detailed information would be required before any quantitative knowledge of toxicity level would be
336 / Journol of Chernicol Education
STOCK,A,, AND POHLAND, E., Chem. Ber., 59,2215 (1926). BAITER,S., J. Am. Chem. Sac., 59,1804 (1937). T., "Inorganic Chemistly," John Wiley and MOELLER, Sons, Inc.,NewYork, 1952,p.801. J. C., AND SMITH. B. C., Quad. Rm., 2, XIV, 201 SHELDON, (1960). JONES,R. G., AND KINNEY,C. R., J. Am. Chem. Sac., 61, 1378 (1939). WmERQ,VONEQON,AND HOREW,G., 2.Nalurforsch., 6B, 338 (1951). RITTER.D. M.. AND BURG.A. B.. J. Am. C h a . Soe... 60,. 1296 kl938). ' BROWN, C. A., AND LAWENGAYER, A. W., J. Am. Chem. Soc., 77, 168 (1955). SM~~LE , AND STAPIEJ, J.YH., S. F.,J. Am. Chem. Soc., 81, 582 (1959). J. W., J. Am. C h a . Soc., 81,3561 (1959). DAWBON, M. J., J . Am. C h a . Sac., KINNEY, C. R.,AND KOLREZEN, 64,1584 (1942). EADS,EWINA., Dissertation, "A Study of the Syntheses of Boraeole Derivatives from B-trichloroborazole and Selected Cmbonyl Compounds," Tulaue University (19621, to be published. "New Boron-Nitrogen R i g Systems Prepared and Characterized," Chem. Eng. News, No. 18, 41, 40 (1963). Health, Education and Welfare report on anti-cancer activity (not published). Available from E. A. Ems, Deoartmeut of Chemistrv, Lamar State College of Technoiagy, Beaumont, Texas.
