34
J . A m . Chem. SOC.1989, 11I , 34-40
Substituted Ethylene Dications. Structures of Phenylmethyl Cations Substituted with an Electron-Withdrawing Group on the Cation Center Tomohiko Ohwada and Koichi Shudo* Contribution from the Faculty of Pharmaceutical Sciences, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan. Received May 13, 1988
Abstract: Ab initio calculations on substituted ethylene dications, especially those bearing r-stabilizing groups such as an aryl, hydroxy, or methoxy group, were performed on the basis of STO-3G, 4-31G, and 6-31G* basis sets. Participations of 1,l -diaryl-2,2-dihydroxyethylenedications and 1 , l -diaryl-2-hydroxy-2-methoxyethylene dications (7)were previously proposed in the reactions of diphenylmethanol bearing a carbonyl group in the presence of a strong acid, trifluoromethanesulfonic acid (TFSA). These cations can be formed by the protonation of a-carbonyl- or a-cyanobenzyl cations. The following results were obtained: ( I ) A carbonyl or cyano substituent on the benzyl cation does not cause any significant electronic change of the aromatic moiety. (2) Protonation of the carbonyl or cyano group caused a substantial delocalization of the positive charge to the rings. (3) Energetical preference for perpendicular and planar geometries was found for simple substituted ethylene dications: this excludes a rigid planar structure for the ion, suggesting the existence of equilibrating rotated structures about the CI-C2 bond. This is consistent with the fact that the stable ion, 0-protonated methoxycarbonylbis(p-methoxypheny1)methyl dication 7 (R = OCH,), in TFSA has the two equivalent aromatic rings in the 'H and I3C NMR spectra. (4) The difference of substituent effects of the carbonyl and CN groups on the formation of 0- and N-protonated dications was also interpreted in terms of the calculated proton affinities.
Experimental and theoretical studies of carbocations with electron-withdrawing substituents, including carbonyl, cyano, and trifluoromethyl groups, have revealed the substituent effects on the formation of carbonium ions.',* These studies have shown a facile formation of the electron-deficient carbonium ions, suggesting that substitution by such groups for hydrogen on a cationic center is not destabilizing, but rather ~ t a b i l i z i n g . ~It was found that diphenylmethanol (e.g., 1 and2) bearing a carbonyl group
4/P 6 H3- C-C
,
OR
7
-5s 'C
CF3S03H
R=CH3.H
%. -
in the case of 2) in high yield in the presence of trifluoromethanesulfonic acid (TFSA) a t -50 0C,4 while the diphenylmethyl cations 6 substituted with the groups were shown to be stable species formed by silver ion-assisted ionization of the corresponding diphenylmethyl chlorides. The reaction of these diphenylmethyl cations bearing an ester or a ketone group to form the fluorenes and phenylphenanthrol requires the presence of a strong acid. Thus, we proposed the participation of the ethylene dications 7 (R = H) with adjacent carbenium centers, the 0R
/P
OR
K
\ /
3
R
6
R. OCH, OH P ?
2
4
5
such as a methoxycarbonyl, carboxyl, or benzoyl group gave the 9-substituted fluorene (3 or 4) and 9-phenylphenanthr-10-01 (5 ( 1 ) (a) Takeuchi, K.; Kitagawa, T.; Okamoto, K. J . Chem. SOC.,Chem. Commun. 1983, 7. Creary, X. J . Org. Chem. 1979, 44, 3983. Creary, X. J . Am. Chem. SOC.1981, 103, 2463. Creary, X.; Seiger, C. C. J . Am. Chem. SOC.1982, 104, 4151. Daudry, D.; Charpentier-Morize, M. Tetrahedron LRtl. 1973, 32, 3013. Begue, J. P.; Charpentier-Morize, M. Acc. Chem. Res. 1980, 13, 207. Pardo, C.; Charpentier-Morize, M. J . Chem. SOC.,Chem. Commun. 1982, 1037. (b) Gasman, P. G.; Talley, J. J. J . A m . Chem. SOC.1980, 102, 1214. Gassman, P. G.; Talley, J. J. Ibid. 1980, 102, 4138. Gassman, P. G.; Talley, J. J. J . Am. Chem. SOC.1980, 102, 1214. Gassman, P. G.; Saito, K.; Talley, J. J. J . Am. Chem. SOC.1980, 102, 7613. Gassman, P. G.; Saito, K. Tetrahedron Lett. 1981, 22, 1311. Gassman, P. G.; Talley, J. Ibid. 1981, 22, 5253. Gassman, P. G.; Guggenheim, T. L. J . Org. Chem. 1982, 47, 3023. Olah, G. A.; Arvanaghi, M. J . Am. Chem. SOC.1980, 102, 6640. Olah, G. A,; Arvanaghi, M.; Prakash, G. K. S . Ibid. 1982, 104, 1628.
(2) (a) Dixon, D. A.; Eades, R. A,; Frey, R.; Gassman, P. G.; Hendewerk, M. L.; Paddon-Row, M. N.; Houk, K. J . Am. Chem. SOC.1984, 106, 3885. Paddon-Row, M. N.; Santiago, C.; Houk, K . J . Am. Chem. SOC.1980,102, 6561. Dixon, D. A.; Charlier, P. A,; Gassman, P. G.J . Am. Chem. SOC.1980, 102, 3957. Paddon-Row, M. N.; Houk, K.; Tidwell, T. T. Tetrahedron Lett. 1982, 23, 383. (b) Moffat, J. B. Chem. Phys. Lett. 1980, 76, 304. Reynolds, W. F.; Dais, P.; Taft, R. W.; Topson, R. D. Tetrahedron Lett. 1981, 22, 1795. Reynolds, W. F.; Dais, P.; MacIntyre, D. W.; Topsom, R. D.; Mariott, S.; Nagy-Felsonbuki, E. V.; Taft, R. W. J . A m . Chem. SOC.1983, 105, 378. (3) Chau, H. S.-I., Berchtold, G. A. J . Am. Chem. SOC.1981, 103, 898.
0002-7863/89/1511-0034$01.50/0
protonated carbonyldiphenylmethyl dications, in the reaction. The conjugate carbenium centers of the carbodications are significantly stabilized by enhanced delocalization of the positive charge over the aromatic rings as in the well-known carbodications, tetraarylethylene dications (e.g., S).5 The pentadienylium character CH-O
+
c-c;
+
OH
OCHS
Ctl,O
7 (R.OCH3)
6
facilitates the 47r electrocyclization to fluorenes or phenanthrol in the conrotatory manner.6 Direct N M R spectroscopic obser(4) Ohwada, T.; Shudo, K. J . Am. Chem. SOC.1988, 110, 1862. (5) Mandelung, W.; Oberwegner, M. Ber. Dtsch. Chem. Ges. 1927, 60, 2469. Wizinger, R. Ber. Dtsch. Chem. Ges. 1927, 60, 1377. Baenzziger, N. C.; Buckles, R. E.; Simpson, T. D. J . Am. Chem. SOC.1967, 89, 3405. Buckles, R. E.; Wonner, W. D. J . Am. Chem. SOC.1958, 80, 5055. Buckles, R. E.; Erickson, R. E.; Snyder, J. D.; Person, W. B. J. Am. Chem. SOC.1960, 82, 2444. Olah, G.A,; Grant, J. L.; Spear, R. J.; Bollinger, J. M.; Seranz, A.; Sipos, G. J . Am. Chem. SOC.1976, 98, 2501. Schrnidlin, J.; Escher, R. V. Ber. Dtsch. Chem. Ges. 1910, 43, 1153. Hart, H.; Sulzberg, T.; Rafos, R. R. J . A m . Chem. SOC.1963, 85, 1800. (6) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry; Academic: New York, 1970; Chapter 5. Deno, N. C.; Pittman, C. U., Jr.; Turner, J. 0.J . Am. Chem. SOC.1965,87,2153. Jorsensen, T. S. Can. J . Chem. 1964, 42, 2768. Ibid. J . A m . Chem. SOC.1967, 89, 3782, 3794.
Q 1989 American Chemical Society
J . A m . Chem. Soc., Vol. 111, No. 1 . 1989 35
Substituted Ethylene Dications vations of the stable 0-protonated methoxycarbonylbis(p-meth0xypheny)methyl dication 7 (R = OCH,) from a-methoxycarbonylbis@-methoxypheny1)methanol in TFSA demonstrate the formation of stable carbodications with adjacent carbenium centers. The equivalence of the two aromatic moieties of the dication 7 (R = OCH3) in the IH and I3C NMR spectra excluded a rigid planar structure of the dication, suggesting the participation of equilibrating rotated geometries involving rotation about the C,-C2 bond. In spite of the electron-withdrawing ability of the cyano group, no fluorene compound was obtained in T F S A even a t room t e m p e r a t ~ r e .The ~ reason is supposed to be the absence of protonation of the nitrogen atom of the cyano group (as in 10) in a strong acid: the cyanodiphenylmethyl cation 9 was found to be stable even in magic acid.Ib
4 6
CF 3 SOAH H~-c-cs~-
4 i
C - E N
d
9
H' --3tp
1.095
AH
H\ 121 8C
1086
H 083
H I092
7.092
13
14
4. d +
1''"-
1369
1099
ti
1444
\ / ?I??
CAN1
164
2
I 1453
c-cEw
1089
10
In this paper we will discuss electronic states of the protonated carbonyl carbodications, a kind of ethylene dications (e.g., 7b), in comparison with monocations (e.g., 6) with a free electronwithdrawing group. T o simplify the calculations, a b initio calculations on the benzyl cations substituted with a C H O group or C N group and the protonated groups were performed. The parent benzyl cation 13 and the benzyl cations substituted with a C H O group (14 and 15) or C N group (16) were optimized on STO-3G basis sets. Subsequently, the two most probable structures of 0-protonated species (17 and 18) and N-protonated species (20) were also calculated. O n the basis of the minimal STO-3G basis sets, the calculations of the dications (17, 18, and 20) suggested substantial delocalization of positive charge over the aromatic rings, while the electron delocalizations of monocations (14-16) were similar to that of the benzyl cation 13.' In the case of the 0-protonated dication of CHO-substituted benzyl cation, both perpendicular (17) and planar (18) structures had a similar charge distribution in the benzene nucleus. The electronic structures of the dications are comparable with those of the heteroatom analogues of the benzyl cation, Le., phenylnitrenium ion 23* and phenoxenium ion 24.9 To allow accurate estimation of energetics, we carried out higher a b initio calculations (fulloptimization at 4-3 I G basis sets and single-point calculations a t 6-31G* basis sets) on the simple methyl cations bearing C H O or C N and their protonated groups. These calculations showed a preferred perpendicular structure of the 0-protonated formylmethyl dication, i.e., hydroxyethylene dication, as compared with the planar structure 27. T h e participation of equilibrating rotated geometries is compatible with the results of theoretical calculations on the parent ethylene dication: a b initio calculations on the potential energy surface showed that the C2Hd2+energy minimum is the perpendicular ethylene dication 11, which was lower in energy by 28.1 kcal/mol (MP3/6-31G**//6-31G* with inclusion of zero-point energy correction) than the planar structure (7) Cowie, G. R.; Fitches, H . J. M.; Kohnstan, G. J . Chem. SOC.1963, 1585. Casapieri, P.; Swart, E. R. J . Chem. SOC.1961, 443. Olah, G. A,; Porter, R. D.; Kelly, D. P. J . A m . Chem. SOC.1971, 93, 464. Freedman, H . H. Carbonium Ions; Olah, G. A,, Schleyer, P. v. R., Eds.; Wiley-Interscience: New York, 1973; Vol. IV, Chapter 28. Krimsw, W.; Kund, K.; Ritzer, E.; Dorigo, A. E.; Houk, K. N . J . Am. Chem. SOC.1986, 108, 6045. (8) Gassman, P. G.; Granrud, J. E. J . A m . Chem. SOC.1984, 106, 1498, 2448. See also: Creary, X.; Inocencio, P. A. J . Am. Chem. SOC.1986, 108, 5979. Shine, H. J. Aromatic Rearrangements; Elsevier: New York, 1967; Chapter 3, pp 182-190. Yukawa, Y. J . Chem. SOC.Jpn. 1950, 71, 547, 603. Heller, H . E.; Hughes, E. D.; Ingold, C. K. Nalure (London) 1951, 168, 909. (9) (a) Schwartz, M. A.; Rose, B. F.; Uishnuvajjala, B. J . Am. Chem. SOC. 1973, 95, 612. Schwartz, M. A,; Rose, B. F.; Holton, R. A,; Scott, S. W.; Vishnuvajjala, B., J . A m . Chem. SOC.1977, 99, 2571. Chalmer, D. J.; Thamson, R. H. J . Chem. SOC.C 1968, 848. Dimroth, K.; Umbach, W.; Thomas, H. Chem. Ber. 1967, 100, 132. (b) Abramovitch, R. A,; Inbasekaran, M. N.; Kato, S. J . A m . Chem. Soc. 1973, 95, 5428. (c) Endo, Y.; Shudo, K.; Okamoto, T. J . Am. Chem. SOC.1977, 99, 7721. Endo, Y.; Shudo, K.; Okamoto, T.J . A m . Chem. SOC.1982, 104, 6393. lijima, H.; Endo, Y.; Shudo, K.; Okamoto, T. Tetrahedron 1984, 40, 4981.
1083 H
1092
1.092
15
H 16
Figure 1. STO-3G-optimized geometries of parent benzyl cation and benzyl cations substituted with a formyl or cyano group.
12.1° These calculations also revealed the difference of substituent effects of C H O and C N groups on the formation of protonated dications (27 and 32) in terms of proton affinities. H
2+
\c-c
1
H
H
..,IH \H
\
c-c
H/
11
/
'+
H '
12
Results and Discussion Benzyl Cations Substituted with an Electron-Withdrawing Group. The geometry of the benzyl cation 13 was optimized on the basis of both the minimal STO-3G" and split-valence 4-31G basis setsI2 with the constraint of planar symmetry. The optimized geometry H
H
P
H
H
H
H
H
for 13 obtained in this study (Figure 1) essentially agreed with previous re~u1ts.l~The calculated charges and LUMO coefficients are shown in Figures 2 and 3, respectively. In comparison with neutral benzene, the small distortion in the benzene ring structure of the benzyl cation 13 suggested little perturbation effect arising from the carbenium center (CJ. T h e benzyl ion 13 has the greatest value of both the charge and the LUMO coefficient on the cy position among all the carbon atoms. From these factors, the electrophilicity of the cation 13 was assumed to be localized ( I O ) (a) Lammertsma, K.; Barzaghi, M.; Olah, G. A,; Pople, J . A,; Kos, A. J.; Schleyer, P. v. R. J . A m . Chem. SOC.1983, 105, 5252. (b) Koch, W.; Frenking, G.; Gauss, J.; Cremer, D. J . Am. Chem. SOC.1986, 108, 5808. (c) Koch, W.; Frenking, G.; Maquin, F.; Stahl, D. In!. J . Mass Spectrom. Ion Processes 1985, 63, 59. (d) Frenking, G.; Koch, W.; Schwarz, H . J . Comput. Chem. 1986, 7 , 406. (e) Koch, W.; Schwarz, H. Srructure/Reactiuiry and Thermochemistry of Ions; Ausloos, P., Lias, S. G., Eds.; Reidel: Dordrecht, The Netherlands, 1987; p 413-465. (11) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J . Chem. Phys. 1969, 51, 2657. (12) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J . Chem. Phys. 1971, 54, 724. (13) Ermler, W . C.; Mulliken, R. S.; Clementi, E. J . A m . Chem. SOC. 1976, 98, 388. Hehre, W. .I.Pople, ; J. A. J . Am. Chem. SOC.1972, 94, 6901.
J . Am. Chem. SOC.,Vol. 11 I , No. I , 1989
36
Ohwada and Shudo -0 084
+0154
+O 149
\
H 10 104
10 097 -0 02C
H\
Table I. STO-3G Calculated Ring Charges of Benzyl Cation, &-Substituted Benzyl Cations, Phenylnitrenium Ion, and Phenoxenium Ion
0
+0154
x2:
/
+0119
10071 I I 10141 H
Il+0.063
+0121
10136 H
13
14 .O
124
10.149 10 101
125
10
H
+O
H 10123
.O OZ5
129 H& , H
+0133
037 -0062
+0.126 H
10122 10 122 +O 139 H
+O 144 H
15
16
Figure 2. STO-3G-calculatedcharge of benzyl cations and benzyl cations substituted with a formyl or cyano group. +O
'OH H
-0.395
H .0.430
.0.430
-0323
+O 636
647
ion
substituent benzyl cation + 1.oo CHO (planar) + 1.oo CHO +1.00 CN +1.00 CHOH' +2.00 CHOH' (planar) +2.00 CHOH' (planar) +2.00 CNH+ +2.00 CH2+ +2.00 CH2+(planar) +2.00 phenylnitrenium ion + 1.oo Dhenoxenium ion + 1.oo "Values based on 4-31G//4-31G.
13 14 15 16 17 18 19 20 21 22 23 24
ring charge STO-3G/ 4-31G// STO-3G STO-3G +0.639 +0.594" +0.601 +0.635 +0.6 13 +0.647 +0.68 1 +0.685 +0.986 +0.943 + 1.030 + 1.063 +1.104 +1.065 + 1.048 + 1.065 +1.014 +0.989 +1.241 +1.247 +0.859 +1.035a +1.373' + 1.002
+Q 037
tO
-0065
total charge
.o 393
H
H
-0 18853 au
-0 13875 au
13
14
that of planar 14 (+0.601), this also suggesting the small resonance participation of the C H O group. This conclusion was also supported by geometrical considerations: no substantial geometric distortion in bond angles and bond lengths was apparent in the ions 14-16 relative to the parent 13; e.g., the C2-C, bond length was almost constant a t 1.364 8, in these cations. Phenyl-Substituted Ethylene Dications. Further calculations, however, showed that 0-protonation of the planar C H O group of 14 or the perpendicular C H O group of 15 and N-protonation of the cyano group of 16 caused the positive charge to delocalize substantially over the aromatic rings. In the case of the C H O substituent, two possible geometries of 0-protonated species, perpendicular 17 and planar 18, were optimized by STO-3G,
+OB31
18a -0.395
18b
18c
-0.396
H
H
Hq+H +0.462
-0 15640 au
H 0 ; C . E L
2oa
-
19 H O c ' C + = N H -
+'
H O c ' C + = t 4 H
.._I
20 b
20c
16
Figure 3. STO-3G-calculatedLUMO coefficients and energies of benzyl cation and substituted planar monocations.
on the a-carbon atom, which is a n experimental reality.' Substitution effects of electron-withdrawing groups such as C H O and C N groups a t the carbonium center on the distribution of charge and L U M O of the aromatic ring were to be evaluated. Therefore, the geometries of the benzyl ion bearing a C H O group (planar 14 and perpendicular 15) or a C N group (16) at the cationic center (C,) were optimized on the basis of minimal STO-3G (Figure 1). The charge distributions and the L U M O coefficients are also shown in Figures 2 and 3, respectively. A total ring charge (RC) was obtained by summing Mulliken net charges associated with the carbon atoms and the hydrogen atoms belonging to the benzene ring. The R C values of 14 and 16 (Table I) were comparable with that of the parent benzyl cation 13 (Table I). It was reasonably concluded that small perturbing effects on the charge densities and the L U M O coefficients arose from the substitution of C H O and C N for H . In the case of the C H O substituent, it was apparent that the R C value of perpendicular 15 (+0.613; STO-3G//STO-3G) was essentially the same in magnitude as
whereas single-point calculations on the optimized geometries were performed with the 4-31G basis set. The geometry of the Nprotonated dication 20 was also optimized on the STO-3G level with the constraint of planar (C,) symmetry. In the process of protonation of the C H O group, the transoid protonation (as in 18) and the cisoid protonation (as in 19) were calculated by optimization on the basis of the minimal basis sets, which showed that the transoid 18 was lower in energy by 3.34 kcal/mol (STO-3G//STO-3G) or 4.45 kcal/mol (4-3 l G / / S T O - 3 G ) than the cisoid 19 (Table IV; Experimental Section). This energetic preference of transoid protonation was also found in the case of 0-protonation of the simple a-formylmethyl cation 25 at higher calculation levels (vide infra). Thus, we will discuss the species formed by transoid protonation in detail. The geometries and the charge densities of dications 17 and 18 are summarized in Figures 4 and 5, respectively. T o clarify the effect of the phenyl group on delocalization of charge in the ethylene dication, we also performed calculations on the phenylethylene dication in both perpendicular 21 and the planar 22 geometries (Figure 6). The geometric distortions of the benzene ring of these ions to quinoid structures suggest sub-
J . A m . Chem. SOC.,Vol. I l l . No. I , 1989 37
Substituted Ethylene Dications H I
1.003
H I
1.003 H
115.7 ' 0
H
1093
21
22
&-o"k H
1110
I 1 352
11.352
1 356
1.087
H 1102
1101
19
18 1005
I049
u
I
H
H 1.100
17
20
Figure 4. STO-3G-optimized geometries of 0-protonated formyl cation
H -.+..,,
\c
1207
stantial delocalization of positive charge over the ring; e.g., the C2-C, (C,-C,) bonds were shortened by 0.018 A and the CI-C2 (C&) bonds were lengthened by approximately 0.046 A (Figure 4). The values of the total ring charge (RC; Tables I) and the LUMO coefficients on the ring, especially on the p-carbon atom (in the case of planar dications, Figure 7) were increased by protonation. Similarly, positive charge in the perpendicular dication 17 was also delocalized over the aromatic ring; Le., the R C value was f 0 . 9 4 3 (STO-3G//STO-3G), which was comparable with that of planar 18 (the R C value was +1.030), indicating that delocalization of positive charge was attributed to the presence of the additional carbenium centers. These calculations provided a theoretical basis for significant contribution of the resonance structures (17c, 18c, and 20c). The L U M O orbitals of the planar dications 17, 18, and 20 are lowered in energy to become comparable to that of the phenoxenium ion, indicating enhanced electrophilic or dienylium characters, which lead to facile 4 a electrocyclizations of the diphenylmethyl cations to the fluorene. Phenylnitrenium Ion and Phenoxenium Ion. T h e substantial delocalization of positive charge over the aromatic ring could also be caused by replacement of the central carbon atom by heteroatoms such as nitrogen and oxygen atoms in the benzyl cation 13. Thus, we carried out a b initio calculations on the phenylnitrenium ion (23) and phenoxenium ion (24) both on STO-3G
'
1496
0\ 0
0._..4
-
0
and N-protonated cyanobenzyl cation.
\
H 23
24
basis set and 4-31G basis sets with the constraint of planar (C,) symmetry. The results are shown in Figure 8. Geometrical distortions in the heteroatom analogues 23 and 24, relative to 13, indicated substantial contributions of the quinoid resonance structures: the ring-heteroatom bonds, the C-N bond a t 1.311 8, (in 23; STO-3G), and the C - 0 bond a t 1.241 8, (in 24; STO3G) are close to those calculated for methylene imine (1.273 A; STO-3G)14 and for acetone (1.219 A; STO-3G),14 indicating nearly full double-bond character in these positions. The previous semiempirical M N D O calculations on the phenylnitrenium ion are consistent with this c o n c l ~ s i o n . ~The ~ calculated charge distributions (Figure 9) and LUMO coefficients (Figure 10) for these ions unambiguously indicate different electronic states in the rings from that of the benzyl cation 13: the ring charge (RC) increased in magnitude on the S T O - 3 G level from 13 ( f 0 . 6 3 9 ) to 23 (+0.859) and to 24 (+1.002) (Table I). While the largest L U M O coefficients were calculated on the carbon atom in the benzylium ion 13, the values of the L U M O coefficients on the aromatic rings, in particular that of the p-carbon atom, increased in going from the phenylnitrenium ion (23) to the phenoxenium ion (24). These calculations supported the postulation that the electrophilic reactivity on the aromatic ring depended on increasing the value of charge and L U M O coefficient in the ring: while the benzylium ion (13) exhibited electrophilicity predominantly a t the C, carbon atom,' the heteroatom analogues, the phenylnitrenium and phenoxenium ions (23 and 24), have electrophilic reactivities on their aromatic carbon^.^,^ W e can reasonably consider that these diverse reactivities of 23 and 24 can be attributed to significant perturbing effects of the greater electronegativities of the nitrogen and oxygen atoms. Formyl- or Cyano-Substituted Methyl Cation and the Protonated Dications. In order to get a more accurate estimation of the substituent effects on the geometries and energetics of ethylene dications, we carried out corresponding calculations on simple (14) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. A b initio Molecular Orbital Theory; Wiley: New York, 1986; Chapter 6. ( 1 5 ) Ford, G . P.; Scribner, J. D. J . A m . Chem. Soc. 1981, 103, 4281.
n/H
.0120
.o
+0399
to390
o/
122
C. H
10158
H +0118
10 164
H
-0.052
.O
045
-0047
H 10156
H
+3'58
10 129
060
tO
+O 163
1 0 161
H
+3 134
10191 H
+ O 184
18
17
10 376
H \ ~ -0113
441
I
+O
162 H
10 C83
10 437
H
10136
c0 165 H
+O 072
H
+ O 057
-0 133
10 c35
-0045 +O 165
H
H
10 155
+C 161
+C 164
H
+0 162 1 5 :92
13195
19
H
20
Figure 5. STO-3G-calculated charges of phenyl-substituted ethylene dications.
methyl cations substituted with a CHO or C N group or the protonated group based on higher a b initio levels such as 4-31G full-optimization and 6-31G* single-point calculations.16 The planar formyl-substituted methyl monocation 25 and its 90' rotated conformation 26 were optimized both on STO-3G and 4-31G, where the cyano-substituted methyl monocation 31 was (16) Hariharan, P. C.; Pople, J . A. Theor. Chirn. Acta 1973, 28, 213. (17) Binkley, J. S.; Whiteside, R. A.; Krishnam, R.; Seeger, R.; De Frees, D. J . ; Schlegel, H. B.; Topiol, S.; Kahn, L. R.; Pople, J. A. QCPE 1981, 13, 406.
38 J . Am. Chem. SOC.,Vol. 1 1 1, No. 1 , I989
Ohwada and Shudo Geometries 1110
1126
H
H
25
26
31
also calculated. The optimized geometries obtained in this study agreed reasonably well with the previous results a t these levels.2a The two possible 0-protonated dications of 26 in perpendicular structures, i.e. the transoid 27 and the cisoid 28, and the planar H
I
1106 H
1101 H
22
21 27
28
29
30
rO 233
+O 265
Charge
H
Y
H \
+
C-CEN-H
/ H
32
counterparts 29 and 30, as well as the N-protonated 32, were also optimized on these levels. For accurate estimation of energies, single-point calculations on 4-3 1G-optimized geometries were performed with the d-polarized 6-31G* basis set. The 4-31Goptimized geometries of ions and calculated charge distribution are shown in Figures 11 and 12, respectively. Total energies are summarized in Table V (see the Experimental Section). In the protonation of the C H O group of the monocation 25, the transoid protonation (as in 27 and 29) was favored over the cisoid protonation (as in 30) by 9.94 kcal/mol (6-31G*//4-31G). This result provided support for the previous conclusion based on the STO-3G calculations that transoid protonation of the phenylsubstituted ethylene dication 18 was favored over the cisoidprotonated 19. Thus, we will concentrate on the species formed by the transoid protonation of a perpendicular structure 27 and a planar structure 29. In the case of the CHO-substituted monocations the planar conformation 25 was more stable in energy by 0.88 kcal/mol (6-31G*//4-31G) as compared with the perpendicular one 26 (Table 11). On the contrary, in the case of the 0-protonated dication, the perpendicular geometry 27 is lower in energy than the planar 29 at all employed levels (Table 11): by 6.57 kcal/mol (4-31G//4-31G) or by 5.67 kcal/mol (6-31G*//4-31G). A previous study on the C 2 H 4 0 2 +potential energy surface showed the results of the calculations on the dications 27 and 28, which are reasonably consistent with our results.'b*e Consequently, the hydroxyethylene dication exhibited a similar geometry preference to the parent ethylene dication 9,'O although the energy difference between planar and perpendicular geometries was significantly decreased by introduction of the stabilizing substituent. A further energy decrease was also seen in the 1,I-dihydroxyethylene dication: perpendicular 33 was more stable than planar 34 only u
H
L
i
i+
/
\
+ c-c H
33
/
H +0161
H tO176
r0178H
rO 141
I r0.109 H
+0211H
22
21
Figure 6. STO-3G-optimized geometries and calculated charges. H
1
H
H
Oib' 0 I?
02111 .O 1'5
H
H
+2
12
0 34"5 a"
12
0 29534 au
18
0 396C5 a i
20
21
Figure 7. STO-3G-calculated LUMO coefficients of planar phenylsubstituted ethylene dications. 1 057
NH
0g2H@r1 1311
1241
; JyH
?
120 4
1197 121 0
1366
1197 1083H
H
1421
1418
1427
i 083 I
H 1.097
H 1.095
H
24
23
Figure 8. STO-3G-optimized geometries of phenylnitrenium ion and
O
H
-0 050
tO162 H
34
by 3.73 kcal/mol (4-31G//4-31G) or 3.65 kcal/mol (6-31G*/ /4-31G), indicative of the possible intervention of the planar conformation (Figure 13). T h e bond rotation not only affects the energy but also the geometry of the hydroxyethylene dication: the C,-C, bond length increases from 1.490 8, in 27 to 1.540 8, in 29. This lengthening can be attributed to the absence of hyperconjugation in 29 in the greater electrostatic repulsion between in-plane vicinal hydrogens. Io Similar geometric changes were also calculated in the case of the parent ethylene dication: the C-C bond length is increased from 1.432 to 1.587 8, by rotation of the perpendicular structure 11 to the planar conformation 12.'O On the basis of the results of these calculations on these simple substituted ethylene dications a t higher levels, substitution of a-stabilizing groups such as a n aryl, O H , or methoxy group on the ethylene dication tends to stabilize the conjugating planar geometry relative to the perpendicular geometry, which was in-
phenoxenium ion. -0.002
/H
-0.09,
+0239
0
N 10.156r 0H064
+O ;036 H
a
+0132
+1@0 H053
.O 369 +O
132 H
10 099
I
10154 H
23
H t0.132 t O 138
+0.124
H
I +O 163 H
24
Figure 9. STO-3G-calculated charges of heteroatom analogues of benzyl cation.
itially favored in the case of the parent ethylene dication. Similar changes in energetical preference for planar and perpendicular geometries were found in the case of fluorine-substituted ethylene dications: the calculations predicted that C2F42+ and C2HF3*+
J . A m . Chem. SOC.,Vol. 1 I I , No. 1, 1989 39
Substituted Ethylene Dications +0.365 H
+0.500
/"
\
O .+C ,
tO.02)c-
H -0.436
H
4 349 H
H -025045 au
-0.219@4 au
23
+O455
H
1077
\
H
c-c ,*,
H
1231
\12031153/ 1194c-c
/
1490
/
0-H
10443
+0.459
g83H\ 1354
H
1094
J3.476
\ l)C-c\+O
O-H
+0430
/
1192
/
27
H
H
\c-c / H
+0378
I,,
" 1093
28
+OU1
+0439
H
H
+0464
\ / H
+0173-0081
C-C-N
+0153
+0378
1086
/ H
c-c 1540
1245
\H
1 083
1194
1088
H
c-c
29 1079
1021
I 158
/C-C-N
1202
I079
\H
\ /
+0593
H
1021
H
Geometry 1090
I087
/;l; 0-H
\1199
1081
/
I251
1513 lo':I I
c-C'
0 98.2
'y2Tn
H\
1539 1'68111 4
c-c
1199 127\
/(2281254\
;yo 0977
1 OgO
I125
09.e9
H
H
, ~ 4 6
1343 0
/
1086
1091
H 0976
C-C-N-H
34
33 Charge Density
1443
\
32
Figure 11. 4-3 1G-optimized geometries of a-substituted methyl cations
prefer the planar structures while, for 1 , 1 - C 2 H 2 F ~and + CzH3FZ+, the perpendicular structures are still more stable.'"*e These trends can be interpreted in terms of a more effective a-stabilization effect of conjugating groups a s compared with the hyperconjugative stabilizing effect of hydrogen atoms. However, an X-ray structure analysis of the tetra-p-anisylethylene dication (cf. 8) with a 41' twist around the central C-C bond supported a nonplanar structure of multisubstituted ethylene dications, presumably owing to steric r e p ~ l s i o n . ~Therefore, the preferred structure of t h e phenylsubstituted ethylene dication is concluded t o be nonplanar, a n intermediate between t h e perpendicular and planar geometries, with equilibration between possible conformations by rotation about the central C-C bond. This result agreed well with the
+a148
r0450
.a563 n-u
+a602
'\+a
,tS\o-H
.0.610
153
/-
\:,,,c.,
n
Table 11. Difference of Energies (kcal/mol) between Planar and Perpendicular Geometries of a-Formylmethyl and 0-Protonated
Formvlmethvl Cationso STO-3G//STO-3G 4-3 1G//4-3 1G 6-3 1G*//4-3 IG CHO 8.03 4.19 0.88 CHOH' -4.36 -6.51 -5.61 'Positive values indicate that planar geometries are more stable than the perpendicular ones.
-0566
/cyf>o
and the protonated dications.
+O 596
10 447
+O 450
31
+O 600
-0443
C-C-N-H
32
H
\
/ H
+0445
lZz3
H
1193
1359
H
H
+0316
+O 464
30
H
\
/"
1173
1553
\
Figure 12. 4-3 IG-calculated charges of a-substituted methyl cations and the protonated dications.
137 H\0g86 1
\12261231 121 1
-0481 461
tO
30
31 1088
/"
29
1165
1487
H
+0617
lZz8
c-c
H
I094
+O 457
\
+O 090
430
0
\12041279/
.,1252
'?
H
28
H
H 1077
10432
lZE4
" 1.089
H
1093
10.614
26 0 986
H
'*e
H
+O 1
25
1093
-O 505 +0.454
+OM7
+0618
1265
/
1082
/"
\c-c /
r0.435
'H
"\
H
1200
\12081150/ 1184
1072
H+0317
0
H
1082
1471
+0459
27
/l185116a /01212
/
/"-"
H
heteroatom analogues of benzyl cation.
1236
H
+060g
-0.513
\c-c /
24
-03%
+0265
26
4.101
Figure 10. STO-3G-calculated LUMO coefficients and energies of
c-c
10367
+0.452
H
1194
4317
H
H
/"
\c-c /
4049
25
+0.455
H
.0'270 2 15
H
-0.45
H
1078
t0.367 H
.06160
+OS02
33
t0.426 H
/ H 10597
34
Figure 13. 4-31G-optimized geometries and charge densities of di-
hydroxyethylene dications. Table 111. Calculated Proton Affinities (kcal/mol) to Ions 21, 29, and 32 ion STO-3G//STO-3G 4-3 1G//4-3 1G 6-3 lG*//4-3 1G 25 29 62.08 39.08 39.12 49.84 46.21 26 21 14.46 45.65 45.39 25 21 66.43 31 32 49.94 31.64 35.63
--
-
N M R observation of equivalent aromatic rings of the 0-protonated carbonylmethoxybis@-methoxypheny1)methyl dication 7 ( R = OCH,). T o evaluate the difference of the substituent effects of CHO and C N groups on protonation, the proton affinities of monocations 25, 26, and 31 were calculated (Table III), since we have postulated a higher energy barrier of protonation of the cyanodiphenylmethyl ~ a t i o n .T~h e proton affinity of the most probable process, planar CHO-substituted monocation 25 to perpendicular
J. Am. Chem. SOC.1989, I 1 1 , 40-46
40
carbenium center of the benzyl cation have no effect on the charge distributions or on the frontier orbital coefficients, relative to the parent benzyl cation, the protonation on the substituents acquired the ethylene dication character, which repels the positive charge and substantially delocalizes it over the aromatic ring. The charge distributions over the aromatic ring a r e comparable with those of phenylnitrenium ion and phenoxenium ion. T h e higher level calculations on the simple substituted ethylene dications revealed that a perpendicular structure of ethylene dication is favored over the planar one. The CHO group conjugating with the carbenium center is more basic than the CN group in a similar system. These results a r e consistent with the experimental observations on the reactions of diphenylmethyl cations in a strong acid.
Table IV. Total Energies (au) for Ions 14-22 ion STO-3G//STO-3G 4-3 lG//STO-3G 14 -376.876 71 -38 1.031 73 15 -376.87 1 92 -381.02789 16 -356.18852 -360.067 46 17 -377.088 68 -381.18348 18 -377.097 12 -381.19361 19 -377.091 79 -381.18652 20 -356.389 33 -360.233 66 21 -303.161 38 -306.379 21 22 -303.181 94 -306.399 89 Table V. Total Energies (au) for Ions 25-34 ion STO-3GIlSTO-3G 4-3 1G//4-3 1G 25 -150.023 33 -151.711 61 26 -1 50.010 54 -151.70494 27 -1 50.129 20 -151.78437 28 -1 50.105 47 -1 50.769 1 5 29 -I 50. I22 25 -1 5 1.77390 -150.10547 -1 51.756 90 30 31 -129.32437 -1 30.748 45 -129.399 18 32 -1 30.808 44 33 -226.565 07 34 -226.559 13
6-3 1G*114-3IG -151.941 82 -151,94041 -152.014 15 -151.99982 -152.005 12 -1 5 1.98928 -130.94231 -130.999 10 -226.91087 -226.905 62
dication 27, was 45.4 kcal/mol (6-31G*//4-31G), and that of the CN-substituted ion 31 to 32 was 35.6 kcal/mol (6-31G*// 4-31G). T h e larger proton affinity of 25 (by 9.76 kcal/mol) provided a theoretical interpretation for the experimental substituent effects of carbonyl and cyano groups in the reactions of the diphenylmethyl cations in a strong acid.
:,-i \
M.
__t
L
d'
25
H
+
i\+C.+. /
0-H \I3
27
H
\
r
H' D
G
N
+
H\
-+GC=N--L1
/
H/
31
32
Conclusion The S T O - 3 G calculations showed that, although ?r-electronwithdrawing substitutions such a s CHO and CN groups on the
Experimental Section Calculation Methods. The calculations have been performed at the Computer Center of the University of Tokyo. The ab initio calculations were carried out by using a modified version of the GAUSSIAN 80 computer programs (GAUSSIAN E O H ) . ' ~ Structures of cations (benzyl cation 13, phenylnitrenium ion (23) and phenoxenium ion (24)) were completely optimized by using Martaugh-Sargent gradient optimization techniques and the split-valence RHF/4-31G basis set with the constraints of planar symmetry and C, symmetries (except 23). The a-substituted benzyl cations 14-22 were optimized on the minimal STO-3G level, and single-point calculations on the 4-31G basis set for the STO-3G-optimized geometries were performed because of the limited number of the allowed primitive shell functions on this Gaussian version. Total energies of these ions 14-22 are given in Table IV. The complete optimizations on simple substituted methyl cations and substituted ethylene dications 25-34 were carried out on both STO-3G and 4-31G basis sets. To estimate the energetics accurately, single-point calculations on 4-3 1G-optimized geometries were also performed at d-polarized 6-31G*. Total energies of these ions 25-34 are given in Table V. Acknowledgment. W e thank Dr. A. Itai, Y. Toriumi, and Dr. N. Tomioka, Faculty of Pharmaceutical Sciences, University of Tokyo, for their helpful advice and for the use of their calculation systems. Registry No. 13, 671 1-19-9; 14, 117687-20-4; 16, 65108-03-4; 17, 117687-21-5; 20, 117687-22-6; 21, 117687-23-7; 23, 117687-27-1; 24, 77417-34-6; 25, 41084-88-2; 27, 117687-24-8; 31, 34430-18-7; 32, 117687-25-9; 33, 117687-26-0.
Molecular Analogues of Surface Species. 2.? A Theoretical Study of Molybdenum Carbonyl Thiophene Complexes: Organometallic Models for the Chemisorption of Thiophene Fernando Ruette,* Norma Valencia, and Roberto Shchez-Delgado* Contribution from the Centro de Qujmica, Instituto Venezolano de Investigaciones Cientificas, IVIC, Apartado 21827, Caracas 1020-A, Venezuela. Received May 27, 1988 Abstract: Molecular orbital (CNDO) calculations on the interaction of Mo(CO), (n = 3, 5 ) fragments with thiophene suggest that formation of the hitherto unknown complexes Mo(qlS-thiophene)(CO), and Mo(qs-thiophene)(CO)3is possible. The nature of the thiophene-metal bond is mainly sp-sp with some d-d and sp-d contributions. Thiophene is more strongly bound in the q5 than in the q'S complex, and activation of the C-S bonds toward nucleophilic attack at the a-carbons is expected in the former case but not in the latter. This theoretical approach leads to conclusions which agree well with experimental organometallic and surface chemistry results.
T h e hydrodesulfurization ( H D S ) reaction is of prime importance in the petroleum industry. Despite this industrial impact and numerous studies devoted to this process, a fundamental understanding of the catalytic sites, reaction intermediates. and elementary steps of the H D S process is far from complete,l The most widely used catalysts are composed of molybdenum or tungsten sulfides supported on alumina and Promoted by cobalt '
Reference 12 is to be considered part 1 of this series.
0002-7863/89/1511-0040$01.50/0
or nickel. M a n y other metal sulfides in bulk or dispersed on a support have also proved to be active in H D S . * (1) (a) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (b) Satterfield, c. N.Hererogeneous Catalysis in Practice; McGraw-Hill: New York, 1980. (c) Grange, P. C a r d . Rev. Sci. Eng. 1977, 15, 249. (d) Schuman, S. C.; Shalit, H. Cafal. Reo. 1970.4. 245. (e) Mitchell, P. C. H. Cural. (London) 1977, I , 204 and 1981, 4, 175. (f) Kwart, H.; Schuit, G. C. A,; Gates, B. C J . Card. 1980, 61, 128.
0 1989 American Chemical Society
