J. Phys. Chem. 1994, 98, 12933-12937
12933
Benzofuroxan-o-Dinitrosobenzene Equilibrium. A Computational Study Willy Friedrichsen Institut frir Organische Chemie der Universitat, Olshausenstrasse 40/60, 0-24098 Kiel, FRG Received: June 28, 1994; In Final Form: September 20, 1994@
The structures of benzofuroxan and o-dinitrosobenzene have been studied by semiempirical (AM1, PM3) and ab-initio methods (3-21G, D95, 6-31G, 6-31G*, MP2/D95//D95, MP2/6-31G//6-31G, MP2/6-31G*//63 1G*, MP2/6-3 1+G*//6-3 1G*). For o-dinitrosobenzene there exists a minimum of three different conformers (cisltrans, transltrans, cidanti). Whereas both semiempirical methods reveal a strong preference for the o-dinitrosobenzene isomer(s), the ab-initio calculations on the MP2 level result in a qualitatively correct prediction of experimental data.
Introduction
1.500
Ever since the beginning of the benzofuroxan (benzofurazanchemistry, the 1-oxide) and furoxan (1,2,5-0xadiazole-2-oxide) structure of these compounds proved to be an extraordinary challenge to organic chemist^.'.^*^ Only in the early 1960s it was proved unequivocally-both by 'H-NMR and X-ray techniques-that benzofuroxans exist indeed as benzofurazan1-oxides and not as an o-dinitrosobenzene.8 Some of the confusion of the early days resulted from the observation that different aromatic precursors, e.g. 3-amino-4-nitrotoluene and 4-amino-3-nitrotoluene, yielded identical benzofuroxans;8-'0 isomerization reactions of different benzofuroxans can be most easily accounted for by an o-dinitrosoareneas an intermediate." Only quite recently three different groups succeeded in the generation and detection of o-dinitr~sobenzene,~~-~~ but it should be kept in mind that strong arguments in favor of a benzofuroxan-o-dinitrosobenzene equilibrium have been presented by Suschitzky and co-workers as early as 1975.17a Quite recently there appeared a hint in the literature that also in the furazano[3,4-b]quinoxaline-1-oxide series an o-dinitroso compound has been t r a p ~ e d . ~ ~ ~ . ~ From a theoretical point of view there remains the question whether the equilibrium between benzofuroxan (1) and odinitrosobenzene can be predicted correctly and whether there is more than one possible conformer of this latter compound as minimum on the potential hypersurface.18 These problems have been tackled by both semiempirical and ab-initio methods.
I
1
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(1, 1.400 0-
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* 1.200 8 4 1.200
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I
I
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,
,
I
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I
I
!
I
? I
I
o
I
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-
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.
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. , I
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r (obs) Figure 1. Calculated bond lengths (ab-initio,6-31G*) vs experimental results (X-ray) for benzofuroxan (1).
Results
a. Structures. Both semiempirial (AM1, PM3)20 and abinitio calculations2' (basis sets:223-21G, D95, 6-31G, 6-31G*) revealed that benzofuroxan (1) (Chart 1) is essentially planar. Calculated bond lengths (Table 1) on higher levels (6-31G*) are in moderate agreement with experimental (X-ray) data with one major exception (see Figure 1): The endocyclic N(1)O(2) bond is calculated far too short ( A r = 0.135 A). Such a result is not unexpected in these classes of compounds; similar disagreements have been found for furoxans, too.24-26 For o-dinitrosobenzene on both the semiempirical (AM1, PM3) and ab-initio level (3-21G, D95, 6-31G, 6-31G*) there are at least three different minima on the potential hypersurface (Tables 2 and 3) which correspond to the conformers 3 (cis/ trans), 8 (transltrans), and 9 ( c i ~ l a n t i ) Conformer .~~ 3 is not @
Abstract published in Advance ACS Abstracts, November 1, 1994.
0022-3654/94/2098-12933$04.50/0
Figure 2. PLUTO plot of o-dinitrosobenzene(conformer 3; ab-initio calculation with a 6-31G* basis). planar (Figure 2). Both O(9) abnd O(10) are placed slightly above (or below) the benzene ring,29 but it should be kept in mind that the variation of the total energy E(HF) with 8(l2-8-10) (5 I 8 5 40") is quite small (Figure 3) (see also AHf = AH48(3-2-8-10)) for 8 with 0 I8 I360" on the AM1 level, Figure 4). b. Energetics. As pointed out above it is well-known that benzofuroxan 1 is more stable than any open-chained odinitro~oarene.~~ AM1 and PM3 calculations yield a quite different picture: 1 is calculated to be far less stable than 3, 8, and 9. It is interesting to note that on the Hartree-Fock level even with extended basis sets again 1 comes out to be less stable than 3, 8, and 9, although the energetic differences are smaller than for the semiempirical calculations (see Tables 4 and 5). 0 1994 American Chemical Society
12934 J. Phys. Chem., Vol. 98, No. 49, 1994
CHART 1
Friedrichsen
r
8
-
n N \ O TS \ L"
TS
e 0
1
2
3
5=3
4
ii TS
00 7=1
6=2
8
9
TABLE 1: Calculated Bond Lengths for Benzofuroxan (1)
n
"p-
7
method AM1 PM3 3-21G D95 6-31G 6-31G* exptl
1-2 1.386 1.637 1.417 1.379 1.374 1.329 1.464
1-7a 1.382 1.340 1.298 1.312 1.306 1.307 1.319
1-8 1.186 1.196 1.287 1.281 1.262 1.215 1.235
2-3 1.329 1.338 1.449 1.407 1.417 1.364 1.381
3-3a 1.353 1.330 1.294 1.310 1.302 1.288 1.327
10
3a-4 1.433 1.439 1.437 1.441 1.435 1.441 1.416
3a-7a 1.474 1.461 1.425 1.433 1.425 1.417 1.418
4-5 1.363 1.354 1.340 1.354 1.347 1.342 1.347
TABLE 2: Calculated Bond Lengths for o-Dinitrosobenzene (Conformer 3); 81 = 8(2-1-7-9);
5-6 1.436 1.443 1.456 1.462 1.454 1.459 1.441
6-7 1.364 1.353 1.339 1.354 1.347 1.342 1.337
7-7a 1.422 1.437 1.422 1.431 1.425 1.430 1.426
82 = 8(1-24-10)
3
method ~ ~ 1 AMlb PM3' PM3d 3-21G' D99 6-31Gg 6-31G*h
1-2 41.417 1.418 1.403 1.404 1.388 1.401 1.395 1.396
1-6 1.404 1.406 1.398 1.399 1.381 1.393 1.386 1.384
1-7 1.460 1.461 1.466 1.465 1.452 1.447 1.434 1.442
2-3 1.406 1.405 1.398 1.397 1.380 1.390 1.384 1.383
2-8 1.468 1.467 1.467 1.467 1.446 1.445 1.432 1.436
3-4 1.393 1.392 1.390 1.390 1.382 1.394 1.387 1.386
4-5 1.394 1.394 1.392 1.193 1.384 1.395 1.387 1.385
5-6 1.390 1.391 1.388 1.389 1.382 1.394 1.387 1.386
7-9 1.157 1.157 1.177 1.177 1.220 1.214 1.205 1.179
8-10 1.153 1.153 1.173 1.173 1.219 1.213 1.203 1.178
Conformer 1 (AHf= 41.49 kcdmol; 01 = 174.3,& = -54.7). Conformer 2 (A& = 41.74 kcdmol; el = -169.0, e2= -51.3). Conformer 1 (AHf = 57.64 kcaymol; 81 = 176.4, 62 = -80.8). Conformer 2 (AHf = 57.60 kcaumol; el = -172.9, = -94.3). e 8, = 166.4, O2 = -27.0. re, = 162.0, e2 = -29.8. g el = 162.3, e2 = -29.0. h el = 158.0, e2 = -28.5. Obviously it is of importance to include electronic correlation into these calculations. The effect is striking: A single-point MP2 calculation2' using optimized geometries on the HatreeFock level (MP2hasis seuhasis set) not puts 1 below 3,8, and 9 in accordance with experimental observation^.^^ The inclusion of zero-point vibrational energies does not alter this picture considerably (Table 5).
The kinetics of the benzofuroxan isomerizations, which are assumed to proceed with an o-dinitrosoareneas an intermediate, has been studied experimentally in detail, and it seemed to be possible to calculate the structure and energy of the transition state(s) with some certainty. The variation of the bond length N(1)-O(2) for 1 within 1.40 Ir(N(1)-0(2)) 5 2.74 ( N - 0 distance in 3) gave a rough estimation of the geometry of the
Benzofuroxan-o-Dinitrosobenzene
J. Phys. Chem., Vol. 98, No. 49, 1994 12935
TABLE 3: Calculated Bond Lengths for o-Dinitrosobenzene(Conformers 8 and 9)" ~~~~~
method 8, 3-21Gb D95b 6-31Gb 6-31G*b 9, AMI' 3-2lcd D95' 6-31Gd 6-31G*f
1-2 1.383 1.398 1.392 1.390 1.418 1.393 1.408 1.401 1.401
1-6 1.382 1.395 1.387 1.389 1.406 1.378 1.388 1.382 1.379
1-7 1.452 1.451 1.438 1.444 1.463 1.452 1.450 1.438 1.442
3-4 1.380 1.390 1.384 1.380 1.392 1.385 1.397 1.391 1.391
4-5 1.387 1.398 1.390 1.391 1.393 1.379 1.390 1.383 1.379
7-9 1.220 1.215 1.205 1.179 1.155 1.217 1.212 1.201 1.177
'See footnotes b-f in Table 4. = = 180.0. = e2= 33.2. d e 1 = e2 = 29.7. e el = e2 = 30.0. re, = e2 = 32.8. -0.8225
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-0.795
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w -0.81 5
1
-0.825
/
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1.60
1.90
2.20
2.50
2.80
r"7)-0(10)) Figure 5. E (in hartree) as a function of r(N(7)-0(10)) in 3 (abinitio calculation with a D95 basis).
0
6 -0.8240 00 d
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Figure 6. Geometry of the transition state 2 (ab-initio calculation with
1
40
a D95 basis).
6(1-24-10)
Figure 3. Variation of E (in hartree) with 19 (1-2-8-10)
for 3 (basis
set: D95). 43.0 42.8 42.6
1
Figure 7. Geometry of the transition state 4 (ab-initio calculation with a 6-31G* basis). The transition state 4 (isomerization of cidtrans-o-dinitrosobenzene) has been calculated by a similar procedure. Expectedly, E(4,HF) and E(4, MP2) do not differ as much as in the former case (see Table 5 ) . The geometry of 4 is shown in Figure 7 is found to be nearly (see also Table 6 ) . The plane 1-2-8-10 orthogonal to the plane 2-1-7-9. The results of AM1 and PM3 are also given in Table 6.27
41.2 41.0
1
Appendix In the early days of benzofuroxan chemistry a benzodioxa-
diazine (10) structure was raised as a possibility for this class 1 1 0 60 120 180 240 300 360 of compounds, but has been abandoned quite soon because no 19( 3- 2 -8- 1 0)
Figure 4. Rotational barrier of the N(8)-O(10) bond in 8 (AM1 calculation). transition state (see Figure 5 ) ; reoptimization on the HartreeFock level and single-point calculation with inclusion of electron correlation gave E(2, HF) = 21.5 k c d m o l and E(2, MP2) = 5.2 k c d m o l , respectively (with E(1)= 0.0). Both values are not in agreement with experimental data (AG* = 14.0 kcaY mol).gb The geometry of the transition state31 2 is shown in Figure 6 (see also Table 6); the bond distance N(7)-O(10) (numbering as in o-dinitrosobenzene) is obtained as 1.714 8,.
peroxide type of reaction has ever been observed. It was of interest to investigate computationally whether 10 is a true minimum on the potential hypersurface. On the PM3 level no minimum was found. AM1 calculations reveal a very shallow minimum in the vicinity of r(0(1)-0(2)) x 1.42 8, (Figure 8). On reoptimization (keywords ef, precise) a true minimum was found (see Figure 9);27the molecule is found to be planar. It is interesting to note that, in agreement with calculations in the furoxan series on the MP2/6-31G* level, no minimum exists on the Hartree-Fock level (basis sets: D/95, 6-31G, 6-31G*); the structures obtained by these methods were characterized as transition states.27
12936 J. Phys. Chem., Vol. 98, No. 49, 1994
Friedrichsen
TABLE 4: Calculated Heats of Formation ( A H r in kcaYmol; AM1, PM3) and Total Energies (in hartree, ab-Initio Results) for Benzofuroxan (1). o-Dinitrosobenzene (3, 8, 9). and Transition States (2, 4) structure method AM1 PM3 3-21G D95 MP2/D95//D95 6-31G MP2/6-3 1G//6-31G 6-31G* MP2/6-3 1G*//6-31G* MP2/6-3l+G*//6-31G*
1 118.28 90.43 -485.195 -487.796 -488.780 -487.717 -488.740 -487.954 -489.412 -489.444
2 119.35 91.43 17 22 32 21 31 32 92 04
-487.761 89 -488.772 01
4
3" 41.49; 41.74 5 7 ~ 5 457.606 ;~ -485.205 728 -487.824 70 -488.775 90 -487.747 41 -488.737 35 -487.974 59 -489.393 99 -489.425 44
42.45 58.39 -487.820 -488.769 -487.742 -488.730 -487.970 -489.387 -489.419
17 68 17 91 17 41 54
8 [41.55Ib.' [58.61Id -485.210 928 -487.827 83 -488.776 49 -487.751 25 -488.739 60 -488.977 80 -489.395 50 -489.425 33
9 42.26e [59.53Ibf -485.198 89 -487.819 58 -488.771 15 -487.741 82 -488.732 47 -487.969 86 -489.389 04 -489.428 29
On the AM1 and PM3 level two different conformers are observed (see Table 2). It is not clear whether this conformer is a true minimum. < 0. * V I ,v2 < 0. e V I = 81.04 cm-'. f v 1 < 0. 8 These values (obtained by complete geometry optimization) differ slightly from those reportedI9 (obtained by partial geometry optimization). a
VI
TABLE 5: Calculated Energies (in kcaYmol) (E(Benzofuroxan [l]= 0.0); ZPE, Zero-Point Vibrational Energy structure method 3-21G D95 MP2/D95//D95 ZPE" 6-31G MP2/6-31G//6-31G ZPEb 6-31G* MP2/6-31G*//6-31G* ZPE' MP2/6-31+G*//6-31G* a
2
1
0.0 0.0 0.0 65.4 0.0 0.0 65.7 0.0 0.0 66.3 0.0
+21.5 +5.2 63.8
3 -6.6 -17.9 +2.8 64.0
-19.0 +1.9 64.4 -12.7 +11.9 64.0 $11.6
4 -15.0 +6.7 63.6 -15.7 +5.9 64.0 -9.9 +16.0 63.7 +15.4
8 -9.9 -19.8 +2.4 63.8 -21.4 +OS
64.3 -14.7 f10.9 63.9 +11.7
9 -2.3 -14.7 +5.8 63.9 -15.5 +4.9 64.2 -9.7 +15.0 63.9 $9.9
Basis: D95. Basis: 6-31G. Basis: 6-31G*.
TABLE 6: Calculated Geometries for the Transition States 2 and 4 (Numbering as in 3) 4
2
method AM1 PM3 D95 6-31G 6-31G* 153.0
O(3a-7a-1-8) 154.6 162.6 150.3
O(7a-3a-3-2) -5.8 -7.6 -10.3
41-21 1.458 1.721 1.714
e(2-1-7-9) 176.9 149.1 178.7 178.9 178.8
e(i-2-8-10) 107.1 - 149.0 91.4 91.4 90.3
1
e149.0
:
'1.147.0 0
Figure 9. Structure of 10 (AM1 minimum).
0
3
._
145.0
C v
transition state 2 should be reoptimized by methods which are beyond those described here. The calculations have been performed with the program systems MOPAC6 ( P C - V e r ~ i o n )and ~ ~ GAUSSIAN34 at the Rechenzentrum der Universitit Kiel using a CRAY Y-MP M92 and a CRAY Y-MPL.
143.0 0
141.0
i
IS9.O 137.01.20
1.30
r
(o('i;O-o(z))
1 -50
1.60
Figure 8. Variation of AI& with 40(1)-0(2)) for 10 (AM1 results). In conclusion one may remark that ab-initio methods with inclusions of electron correlation seem to be capable of giving a qualitatively correct picture of the benzofuroxan-o-dinitrosobenzene equilibrium. The energy (and structure?) of the
References and Notes
(1) The earlier work in this field has been reviewed adeq~ately?~ (2) Kaufman, J. V. R.;Picard, J. P. Chem. Rev. 1959,59, 429. (3) Boyer, J. H. In Heterocyclic Compounds; Elderfield, R. C., Ed.; Wiley: New York, 1961; Vol. 7, p 462. (4) Behr, L. C. In Chemistry OfHeterocyclic Compounds;Weissberger, A., Ed.; Wiley: New York, 1962; Vol. 17, p 283. ( 5 ) Mallory, F. B.; Wood, C. S. Proc. Nat. Acad. Sci. 1961,47, 697. (6) Excellent reviews in this area are available.'~8
Benzofuroxan-o-Dinitrosobenzene (7) (a) Boulton, A. 3.; Ghosh, P. B. Adv. Heterocycl. Chem. 1969, 10, 1. (b) Gasco, A. J.; Boulton, A. J. Adv. Heteroycl. Chem. 1981, 29, 251. (8) Katritzky, A. R.; Gordeev, M. F. Heterocycles 1993, 35, 483. (9) Recent compilations: (a) Sliwa, W.; Thomas,A. Heterocycles 1985, 23, 399. (b) Sliwa, W.; Mianowska, B. Chem. Pup. 1989,42, 697. (c) Friedrichsen, W. In Methoden der Organischen Chemie (Houben-Weyl); Thieme Verlag: Stuttgart, 1994; Bd. E 8c. (10) Green, A. G.; Rowe, F. M. J . Chem. Soc. 1913, 103, 897. (11) Earlier trapping experiments were unsuccessful.l* (12) Kresze, G.; Bathelt, H. Tetrahedron 1973, 29, 1043. (13) Dunkin, I. R.; Lynch, M. A.; Boulton, A. J.; Henderson, N. J . Chem. Soc., Chem. Commun. 1991, 1178. (14) Hacker, N. P. J. Org. Chem. 1991, 56, 5216. (15) Murata, S.;Tomioka, H. Chem. Lett. 1992, 57. (16) See also: Calzafem, G.; Gleiter, R.; Knauer, K.-H.; Martin, H.D.; Schmidt, E. Angew. Chem. 1974,86,52;Angew. Chem., Int. Ed. Engl. 1974, 13, 86. (17) (a) Bulacinski, A. B.; Scriven,E. F. V.; Suschitzky, H. Tetrahedron Lett. 1975,3577. (b) Hasiotis, C.; Gallos, J. K.; Kokkinidis, G. Electrochim. Acta 1993,38,989. (c) Gallos, J. K.; Malamidou-Xenikaki,E. Heterocycles 1994, 37, 193. (18) Ab-initio calculations (STO-3G, 3-21G) with partial geometry optimization revealed the existence of two conformers of o-dinimsobenzene (cisltruns, trans/trans).'g
(19) Maniott, S.;Topsom, R. D.; Gowenlock, B. G. J. Phys. Chem. 1990, 94, 5220. (20) (a) Stewart, J. J. P. In Reviews in Computational Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH Publishers, Inc.: New York, 1990; p 45. (b) Zemer, M. C. In Reviews in Computational Chemistry; Liokowitz. K. B.. Bovd. . .D. B.. Eds.; VCH Publishers. Inc.: New York. 19'91; Vol. 2, p 313. (21) See, for example: Hehre, W. J.; Radom, L.; Schlever, P. v. R.; Pople, J. A. Ab initio Molecular Orbital Theory; Wiley: New York, 1986.
J. Phys. Chem., Vol. 98, No. 49, 1994 12937 (22) See, for example: Feller, D.; Davidson, E. R. In Reviews in Computational Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH
Publishers, Inc.: New York, 1990; p 1. (23) Britton, D.; Olson, J. M. Acta Crystallogr. 1979, 835, 3076. (24) Seminario, J. M.; Concha, M. C.; Politzer, P. J. Comput. Chem. 1992, 13, 177. (25) See also ref 9c, pp 717, 770. (26) Even on the MF'2/6-31G* level there is a gross error in the calculated bond lengths. (27) All stationary points ("a transition states) have been characterized by calculation and diagonalizationof the Hessian matrix.2s (28) (a) Komomicki, A,; McIver, J. W. J. Am. Chem. Sac. 1972, 94, 2625. (b) Foresman, J. B.; Frisch, a.Exploring Chemistry with Electronic Structure Methods. A Guide to Using Gaussian; Gaussian, Inc.: Pittsburgh, PA, 1993. (29) It is not entirely clear whether there are hyo conformers for 3 on the PM3 level (see footnotes of Table 4). (30) The same is true in the furoxan-1,2-dinitrosoetheneseries? (31) For an early work on the geometry of t h i s transition state see: Hoffmann, R.; Gleiter, R,; Mallory, F. B. J . Am. Chem. 1970, 92, 1461. The resemblance to the HNO dimeri~ation~~ is obvious. (32) Luttke, W.; Skancke, P. N.; Traetteberg, M. Theor. Chim. Acta 1994, 87, 321. (33) QCMP 113: QCPE Bulletin 1992, 12, 72. (34) Frisch, A. M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P.M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A,; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Rhagavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. (GAUSSIAN 92, Revision A; Gaussian, Inc.: Pittsburgh, PA, 1992.
