COMMUNICATIONS

Four-Center Two-Electron Bonding in a Tetrahedral Topology. Experimental Realization of Three-Dimensional Homoaromaticity in the 1,3-Dehydro-5,7-adamantanediyl Dication** By Matthias Bremer. Paul von Rague Schleyer.* Karl Schotz, Michael Kausch, and Michael Schindler Dedicated to Professor George A . Olah on the occasion of his 60th birthday

Four orbitals placed at the bridgeheads of adamantane extend towards the center of the cage. Of the four possible combinations of phases, only the nondegenerate one, I , is

I 1 1'

bonding. The occupation of this molecular orbital by two electrons (Fig. 1)-e.g., as in the dication l/l'-should re- sult in considerable stability, provided the overlap is suffi- cient.

Fig. I . Jorgensen plot of the highest occupied molecular orbital of I . (The STO-3G basis set employed underestimates the overlap of the bridgehead orbitals. The Jorgensen program cannot deal with higher basis sets.)

[*] Prof. Dr. P. von R. Schleyer, Dip1.-Chem. M. Ererner, Dr. K. Schotz, Dr. M. Kausch Institut fur Organische Chemie der Universitat Erlangen-Niimberg Henkestrasse 42, D-8520 Erlangen (FRG) Dr. M. Schindler Lehrstuhl fur Theoretische Chemie der Universitat Postfach 1021 48, D-4630 Eochum 1 (FRG)

[**I We thank the Deutsche Forschungsgemeinschaft, the Fonds der Chem- ischen Industrie, ARDEC, Dover, NJ, and the CONVEX Corporation (the most recent calculations were carried out on a CONVEX C1 com- puter) for support, Dr. Alan Reed (Erlangen) for discussions, and Dr. W. Bauer (Erlangen) for the NMR spectra.

Attractive ideas based on qualitative molecular orbital considerations often have not survived or have given dis- appointing results when subjected to experimental test. Homoaromaticity in neutral and carbanionic systems is a case in point.['] Hence, such ideas are best tested calcula- tionally at an adequate level of quantitative theory before investing the much greater effort needed for an experimen- tal investigation.

MIN- D0/3l4I was employed to evaluate the energies of the isodesmic reactions (a) and (b). The former predicts the

When this project was begun a decade

3 L 2

trishomoaromatic 1,3-dehydro-5-adamantyl cation 2, an intermediate which has been implicated in solvolysis reac- tions,['] to be stabilized by about 25 kcal/mol relative to the 1-adamantyl cation 3 and 1,3-dehydroadamantane 4. Equation (b) indicates that the extension of the two-elec- tron delocalization from three centers (as in 2) to all four bridgeheads nearly doubles the stabilization. While 1 is tetratrishornoaromatic, this large resonance energy, along with the delocalized electronic structure, qualifies it for the designation "aromatic," as well. While Olah et al. have not succeeded in preparing the 1,3-adamantanediyl dication 5,l6] nor, indeed, any other 1,3-carbodication in solution,131 the high degree of stabilization indicated for 1 by Equa- tion (b) encouraged us to seek experimental verification.

The problem of obtaining a suitable precursor for dicat- ion 1 was complicated by the known instability of 1,3-de- hydroadamantane derivatives, e.g., towards reaction with oxygen and polymerization.[s1 After a large number of preliminary experiments,['] a simple route was found. When reacted with mercury(ir) fluoride, 1,3,5,7-tetra- i ~ d o a d a m a n t a n e [ ~ ] gives a trisubstituted product, 1,3,5-tri- fluoro-7-iodoadamantane 6, almost exclusively. This ob- viates the need for tedious chromatographic separations of polyhaloadamantane mixtures. With butyllithium in ether/ pentane solution at -8O"C, 6 gave (presumably via I-Li exchange and LiF elimination) 1,3-dehydro-5,7-difluoro- adamantane 7 directly. As 7 only is stable a t room temper- ature in solution (the solid polymerizes with near explosive exothermicity above O T ) , workup had to be carried out below - 30°C. After crystallization from pentane at -7O"C, 7 shows I3C- and 'H-NMR spectra typical of de- hydroadamantanes.['. 17] Chemical characterization was achieved by reaction of 7 with iodine in hexane; 1,3- difluoro-5,7-diiodoadamantane 8 is formed in over 80% yield.['71

Angew Chem In1 Ed Engl 26 11987) No 8 0 YCH Verlagsgeselischafi mbH. 0-6940 Weinberm. 1987 0570-0833/87/0608-0761$ 02 SO/O 76 1

'WF 6 7 8

SbF5 1 T < -80°C S02CIF

Kept cold, solid 7 was added to a solution of SbF, in S02ClF (ca. 5 M) at temperatures below - 80°C in order to prepare the NMR sample. The resuIting dication 1 ap- pears to be rather stable in superacid media, as spectra could be taken successfully at temperatures u p to 0°C. The NMR parameters of 1 are remarkable, and constitute a proof of the nonclassical structure. The I3C-NMR chemi- cal shift of the bridgehead carbons, which share the posi- tive charge at least in a formal sense, comes upfield (6=6.6 at - 7 I "c, with internal [D,]acetone/TMS capillary) of the methylene carbon resonances (6= 35.6), despite the pres- ence of the two positive charges. Shielded values are char- acteristic of hypercoordinate carbocation centers. A good analogy is provided by the trishomocyclopropenyl cation 9, which exhibits similar NMR parameters: 6 ~ 4 . 9 (CH) and 17.6 (CH2).I8l

& 9

The triplet splitting of the CH2 group in 1, J(C.H)= 165 Hz, in line with the trend found for adamantane (125 Hz)19' and the 1-adamantyl cation 3 (148 Hz),[~' confirms not only the formation of dication 1 but also the strong defor- mation of the C-CH2-C angles predicted by the calcula- tions. Figure 2 gives the 3-21G ab initio geometry. The J(C.H) value, 168.4 Hz, for 9 is similar.'81 The single, sharp 'H chemical shift of 1, 6=3.8, also is unusual in compari- son with 6=4.2[,] for the a-CH, groups of the 1-adamantyl cation, and 6= 1 .74191 for the adamantane methylenes.

Fig. 2. The 3-21G-optimized structure 0 1 the dehydroadamantanediyl dicat- ion 1 !distances in A). The 6-31G*-optmized data are similar: C - C = 1.504, 2.052A,C-C-C=86.0, 118.2". H-C-H=114.2".

These chemical shifts can be analyzed in several ways. If 1 were a classical, rapidly equilibrating dication (an un- likely possibility!), the bridgehead carbon chemical shift should be an average of the values found for the carbocat- ion center in the I-adamantyl cation 3 (6=300)161 and a cyclopropane bridgehead in 1,3-dehydroadamantane 4 (6=37.3).'5' The chemical shift (6= 169), predicted for the bridgehead carbons by the classical model on this basis, deviates from the experimental value by over 160 ppm! Al- ternatively, the sums of all the I3C chemical shifts for l (240 ppm) and for 1,3-dehydroadarnantane 4 (454 ppm) may be used as a classical-nonclassical classification crite- rion."" For typical classical carbodications, the dication- neutral difference is on the order of + 800 ppm (i.e., about twice that found on the same basis for monocarbocations). For 1, the difference is negative, -214 ppm! Such negative values (but not as large) are found, e.g., for the related trishomocyclopropenyl cation 9 (-48 ppm),["] and for the 7-norbornadienyl cation ( - 96 ppm).l'O1

Clearly, the I3C-NMR data for 1 cannot be reconciled with a classical, rapidly equilibrating model. But how well d o the experimental shifts compare with those predicted by a nonclassical model? The IGLO (individual gauge for localized molecular orbitals) method has been demon- strated to give good to excellent agreement between calcu- lated and experimental chemical shifts for both classical and nonclassical carbocations.l"l For example, the IGLO (DZ basis set) and experimental (in parentheses) I3C-NMR chemical shifts for the I-adamantyl cation 3 are: 6=337.7 (300.0), 57.7 (66.6), 67.8 (87.6), and 37.2 (34.6). When ap- plied to 1 (without prior knowledge of the experimental results), bridgehead (6= 7) and methylene carbon (6= 33) chemical shifts were calculated (DZ basis on the 3-21G geometry). The remarkable agreement with the experimen- tal values (6=6.6 and 35.6) speaks for itself. The calcu- lated proton chemical shift, 6=3.0, is lower than experi- ment (6=3.8), but the latter, measured from an internal standard, depends on the magnetic anisotropy of the me- dium and is somewhat uncertain.

As indicated by population analysis at various theoreti- cal levels (MNDO, MIND0/3, STO-3G, 3-21G), the bonding and charge distribution in 1 are revealing. The positive charge resides not only on the bridgehead carbons but also on the 12 hydrogens; the methylene carbons are neutral or even negatively charged. The overlap among the bridgehead carbons at 3-21G//3-21G (0.38 vs. about 0.7 for a normal C-C single bond),"61 is significant, consistent with the dotted-line representation. Each bridgehead car- bon is involved with one electron, and each has a total bond order of 0.5. Charge delocalization in a nearly spher- ical topology, as well as partial bridging among the bridge- head positions, is responsible for the stability of 1.

The dehydroadamantanediyl dication 1 now takes its place along with pyramidal e.g., those based on (CH)? and (CH);@, as purely "organic" examples of three-dimensional aromaticity"51 which exhibit large reson- ance energies. Hiickelk 4n + 2 rule, involving interstitial in- stead of x electrons, can easily be extended to such sys- tems.['''

Received: February 27, 1987 [Z 2116 1E] Publication delayed at the authors' request

German version: Angew. Chem. 99 (1987) 795

[ I ] For recent references, see P. van R. Schleyer, E. Kaufmann, A. J. Kos, H. Mayr, J. Chandrasekhar, J . Chem. SOC. Chem. Commun. 1986. 1583; A. McEwen, P. van R. Schleyer, J . Org. Chem. 51 (1986) 4357. A recent claim 10 have demonstrated 4.5 kcal/mol of "homoaromaticity" in tri- quinacene (J. F. Liebman, L. A. Paquelte, J. R. Peterson, D. W. Rogers,

762 0 YCH Verlagsgesellscha~ mbH. 0.6940 Weinheim. 1987 0044-8249/87/0808-0762 $ 02.50/0 Angew. Chem. Int. Ed. Engl. 26 (1987) No. 8

J. Am. Chem. Soc. 108 (1986) 8267). is not “unequivocal” as no attempt was made to dissect electronic from strain effects. “Homoconjugation” may be a bet!er term to describe stabilizdtlons of such small magnitude, if present.

[2] We thank Michael Waeber. Herbert Gareis. and Roland Hacker, Erlang- en, for preliminary experimental investigations, and E. M. Engler for early STO-3G calculations. Further work is described in the Diplom- arbeit of M. B.. Universitat Erlangen-Nurnberg 1986.

[3] See a review on stable carbodications: G. K. Surya Prakash, T. N. Row- dah, G. A Olah, Angew. Chem. 95 (1983) 356; Angew. Chem. In!. Ed. C iy l . 22 (1983) 390. Also see: G. K. Surya Prakash, M. Formia, S. Keyonian, G. A. Olah, H. J . Kuhn, K. Schaffner, J . Am. Chem. SOC. 109

141 a ) R. C. Bingham, M. J . S . Dewar, D. H. Lo, J . Am. Chem. Soc. 97(1975) 1285; b) for reviews, see T. Clark: A Handbook ofComputationa1 Chem- i.7rr-i.. Wiley, New York 1986; c) D. F. V. Lewis, Chem. Revs. 86 (1986) I I I I

IS] a) R. E. Pincock, J. Schmidt, W. B. Scott, E. J. Torupka, Can. J . Chem. 50 (1972) 3958; b) C. S . Gibbons, J . Trotter, ibid. 51 (1973) 87; c) R. E. Pincock, F.-N. Fung, Tetrahedron Left. 21 (1980) 19; d) W. B. Scott, R. E. Pincock, J Am. Chem. Soc. 95 (1973) 2040; e) R. E. Pincock, E. J. Torupka, ihid. 91 (1969) 4593.

161 G. A. Olah. G. K. Surya Prakash, J. G. Shih, V. V. Krishnamurthy, G. D. Mateescu, G. Liang, G. Sipos, V. Buss, T. M. Gund, P. von R. Schleyer. J Am. Chem SOC. 107 (1985) 2764. The pagodane dication is of interest in this context: G. K. S . Prakash, V. V. Krishnamurthy, R. Herges, R. Bau, H. Yuan, G. A. Olah, W.-D. Fessner, H. Prinzbach, J . Am. Chem Soc. 108 (1986) 836.

171 G. P. Sollott, E. E. Gilbert, 1. Org. Chem. 45 (1980) 5405, and reference\ cited therein. A modified procedure using CSI and AII’, prepared in situ. was employed here.

[S] a) S. Masamune, M. Sahai, A. V. K. Jones, T. Nakashima, Can. J. Chem. 52 (1974) 8 5 5 ; b) G. A. Olah, G. K. Surya Prakash, D. Whittaker, J. C. Rees, J. Am. Chem. SOC. 101 (1979) 3935; c) G. K. Surya Prakash, M. Arvanaghi, G. A. Olah, ibid. 107 (1985) 6017; d) also compare the pen- tacyclononyl cation reported by R. M. Coates, E . R. Fretz, J . Am. Chem. Soc. 97 (1975) 2538.

(1987)911

191 R. C. Fort, P. von R. Schleyer, J . Org. Chem. 30 (1965) 789. [lo] P. von R. Schleyer, D. Lenoir, P. Mison, G. Liang, G. K. Surya Prakash,

G. A. Olah, J. Am. Chem. SOC. 102 (1980) 683. [ I I] M. Schindler, J. Am. Chem. SOC. 109 (1987) 1020. 1121 Reviews on pyramidal carbocations: H. Schwartz, Angew. Chem. 93

(1981) 1046; Angew. Chem. Int . Ed. Engl. 20(1981)991; P. Vogel: Car- bocatlon Chemistry. Elsevier, Amsterdam 1985.

1131 S . Masamune, M. Sahai, H. Ona, A. J. Jones, J . Am. Chem. Soc. 94 (1972) 8956; A. V. Kemp-Jones, N. Nakamura, S . Masamune, J . Chem. Soc. Chem. Commun. 1974. 109, and references cited therein.

[I41 H. Hogeveen, P. W. Kwant, Acc. Chem. Res. 8 (1975) 413. [ I51 E. D. Jemmis. P. von R. Schleyer, J . Am. Chem. Soc. I04 (1982) 4781,

and earlier papers in the same series. E. D. Jemmis, ibid. 104 (1982) 7017: J. Chandrasekhar, E. D. Jemmis, P. von R. Schleyer, in preparation.

1161 Natural population analysis (A. E. Reed, R. B. Weinstock, F. Weinhold, J . Chem. Phys. 83 (1985) 735) based on natural localized molecular orbitals (A. E. Reed, F. Weinhold, ibid. 83 (1985) 1736) and the ST0-3G//STOO-3G and the 3-21G//3-21C wave functions (W. J. Hehre, L. Radom, P. von R. Schleyer, J. A. Pople: Ab Initio Molecular Orbital 77wory. Wiley, New York 1986) were employed.

1171 Spectroscopic data of 6 , 7, and 8. (The multiplicities of the NMR sig- nals are taken from ‘H-decoupled ”C-NMR spectra; ‘Jle. , , l coupling constants from “gated” spectra): 6 , m.p. =72-73”C; ‘H-NMR (400 MHz, CDC13): 6=2.10, 2.19 (2mc, 6 H , CF-CHZ-CF), 2.50 ( s , 6H, CF-CHI-CI); “C-NMR (100 MHz, CDCI,): 6=25.5 (4, ‘ J l c F ) = 14 Hz. CI), 45.7 (t, ’J lc e,=20 Hz, CF-CHZ-CF, ’ J l c . ~ ) = 134 Hz), 53.6 (d, ‘J(< > , = 2 0 Hz, CF-CHI-CI, ‘ J , c H ) = 136 Hz), 90.2 (dt, ‘J,C,F,= 195 Hz, ‘Jl< ,,= 15 Hz. CF); IR (KBr): 3=2920, 2810, 1440, 1430, 1310, 1295, 1230, 1205, 1130, 1025. 1005,970,940, 885,860,780, 700cm-’.-7, ‘H- NMR (400 MHz, [D,ltetrahydrofuran): 6= 1.36 (d, 4 H , ’ J I H 10 Hz, CF-CHI-C), 1.64 (d, 2 H , ‘ J , e ~ ) = 8 Hz, C-CHZ-C), 2.01 (d, 4 H .

is)= 10 HZ, CF-CHI-C), 2.22 (t. 2 H , ‘ J 1 t ~ 1 = 4 Hz, CF-CHI-CF);

CH:-C-CH?), 37.2 (t, ‘Jlc 6 1-4 Hz, C-CHz-C, ‘Jlc..HI= 159 Hz), 45.3 (dd, ‘J(c 15 Hz, “ J , c , , = 3 Hz, CF-CHZ-C, ‘ J , c H l = 136 Hz), 46.0(t, ‘J,< g , = I8 Hz, CF-CHz-CF, ‘ J , C , ~ ~ = 132 Hz), 98.4 (dd, ‘J1c,,,=226

CDCli): 6=2.18 (t. ZH, ‘ J l b * , , = 5 Hz, CF-CHZ-CF), 2.52 (d, 4 H , ‘ J C t C t o= I I Hz, CF-Ctf-CI), 2.62 (d. 4 H , ‘J(H.H)= 11 Hz,

CDCI,): 6=28.8 (t. ‘J<c .11= 12 Hz, CI), 45.3 (t, ‘Jlc.+)=20 Hz, CF-CHI-CF, ’J,<. . ,~I= 136 Hz), 53.1 (dd, ‘ J l c . F I = 18 Hz, ‘JiC-F,=3 Hz, CF-CH?-CI, ‘ J ,< .HI= 135 Hz), 60.2 ( s , CI-CHI-CI, ‘ J ( c H 1 = 136 Hz), 89.7 (dd, ‘ J t c t i = 198 Hz, ‘Ji<.,,= 14 Hz, CF); IR (KBr): C=2910, 2820, 1430, 1310. 1290, 1250, 1215, 1205, 1095, 1015, 995, 960, 945, 930, 870, 815, 725, 675 cm ~ I .

“C-NMR (100 MHz, [D&etrahydrofuran): 6= 18.9 (t, ‘J,cFl=9 Hz.

Hz, ‘Jtc , , = 5 Hz, CF).-S, m.p.=132-134”C; ‘H-NMR (400 MHz,

CF-CHZ-CI), 2.91 (s, 2 H , CI-CH?-CI); “C-NMR (100 MHz,

Dimerization of 1,6-Dithiacyclodeca-3,8-diyne in the Presence of Cobalt Complexes. A Simple Synthesis of a [2.2](2,5)Thiophenophane Derivative** By Rolf Gleiter, * Michael Karcher. Bernhard Nuber. and Manfred L. Ziegler Dedicated to Professor Hermann Schildknecht on the occasion of his 65th birthday

Recently, we showed that cyclodeca- 1,6-diyne 1 under- goes dimerization in the presence of [CpCo(CO),] o r [CpCo(C8HI2)1 (Cp =q5-C5H5) to give a superphane of cy- clobutadiene, 3, doubly capped by CpCo units.“’

CP co

2 (x=s)

CO CP

3

4

To test the scope and limitations of this new reaction, we have investigated the reaction of 1,6-dithiacyclodeca-3,8- diyne 2IZ1 with [CpCo(CO),] and [CpCo(C8H ,?)I. Heating 2 (1.0 g, 5.95 mmol) with [ C ~ C O ( C O ) ~ ] (1.07 g, 5.95 mmol) in n-octane yielded a colorless product (120 mg) which con- tained no CpCo unit. The analytical data of the product (see Table 1) showed a molecular weight of 336, and the IR, UV,“’ and NMR data indicated the presence of thio- phene units and two nonequivalent CH2 bridges. When 2 was heated with less than the equimoiar amount (10%) of the Co complexes, we obtained the same product. Three structural possibilities, the anti and syn isomers 4 and 5 , respectively, and the cage compound 6, are compatible with the spectroscopic data.

5 6 7

From the observation that the most intense peak in the mass spectrum of the product is at m/z 168 ( M 0 / 2 ) , we favored 4 and 5 over 6.

[*] Prof. Dr. R. Gleiter, Dipl.-Chem. M. Karcher Institut fur Organische Chemie der Universitat Im Neuenheimer Feld 270, D-6900 Heidelberg (FRG) Prof. Dr. M. L. Ziegler, Dr. B. Nuber lnstitut fur Anorganische Chemie der Universitat Im Neuenheimer Feld 234, D-6900 Heidelberg (FRG)

Fonds der Chemischen Industrie, BASF AG, and Degussa AG. [**I This work was supported by the Deutsche Forschungsgemeinschaft, the

Angen, Chem. In! . Ed. Engl. 26 11987) No. 8 0 V C H Verlagsgesellschaji mbH. 0-6940 Wernheim, 1987 0044-8249/87/0808-0763 $ 02.50/0 763