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How High is the Barrier for the Valence Isomerization of Cyclobutadiene.

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Fig. 1. ORTEP perspective drawing of the molecular structure of [VZ(dfm)J.
toluene with ellipsoids at the 30% probability level. The carbon atoms of the
tolyl groups are drawn as arbitrarily sized spheres.
"formal shortness ratio"[" (FSR) supports the presence of a
Fdirly strong bond. The FSR of 0.808 is very similar to those
of the homologous series with metal-metal quadruple
bonds: 0.814 (Cr), 0.840 (Mo), and 0.838 (W). However, a
striking difference is found in the magnetic anisotropy, Ax of
the M-M bond. The change in the chemical shift of the
methine proton is very large. Relative to the nickel analogue
A6 for the title complex is 4.08. The calculated Ax value of
7300 x
m3 molecule- * is the highest known for any of
the dinuclear [M,(dfm),] compounds.[191
The electronic absorption spectrum (toluene solution)
shows two bands (24700, 21 300 cm-') and a shoulder
(1 8 200 cm- ') which following some preliminary SCF-XaSW calculations[201can probably be assigned to the N 6,
n + 6*, and n -+ F transitions, respectively. These are on the
edge of a rapidly rising absorption into the UV which probably arises from one or more L -+ M charge-transfer transitions.
Experimental Procedure
All syntheses were carried out under an inert atmosphere (argon) and with dry
and deoxygenated solvents. [Vz(dfm),] ' toluene was prepared by the reduction
of VCl,(thf), [21] (0.80 g, 2.14 mmol) with one equivalent of NaHBEt, in
20 mL of THF at -70 "C and further addition of a cold suspension of Li(dfm)
(4.28 mmol) in 20 mL of THF. After the temperature of the bath reached 20 "C,
the red solution was stirred for 30 min. The solvent was then removed under
vacuum. The dried solid was extracted with 35 mL of toluene and the solution
was kept at -70°C overnight. The long fiberlike, copper-colored crystals
formed were filtered and quickly washed with small amounts of hexane (0.55 g,
52% yield). Upon recrystallization from toluene and layering with hexane, red
block-shaped crystals of [V,(dfm),] . toluene formed. Both crystalline forms
gave the same NMR spectrum; that of the copper-colored crystals showed only
a negligible amount of toluene. The compound is diamagnetic (by NMR). 'H
NMR (200 MHz, C,D,, 25 "C): 6 = 1.99 (s, -CH,), 6.00 and 6.63 (d, -C6H4-),
10.24 (s, -NCHN-). UV/VIS(toluene): >.ma, [nm] = 550 (b. sh), 470. 405.
Received: January 14. 1992 [ZS12OIE]
German version: Anger,.. Chem. 1992, 104, 795
CAS Registry numbers:
[V2(dfm)J, 140633-84-7;[Vz(dfm,)]. toluene, 140633-85-8;[VCl,(thf),], 1955906-9; Li(dfm). 75344-35-3.
[l] F. A. Cotton, R. A. Walton, Multiple Bonds B e t w e n Metol Atoms. 1st ed..
Wiley, New York. 1982, 2nd ed., in press.
121 F. A. Cotton, R. Poli, Inorg. Chem. 1987, 26, 3652-3653.
[3] a) F. A. Cotton, M. Millar. J. Am. Chem. SOC. 1977, 99, 7886-7891;
b) F. A. Cotton, G . E. Lewis. G. N. Mott, Inorg. Chem. 1983,22,560-561.
141 F. A. Cotton, M. P. Diebold. I. Shim, Inorg. Chem. 1985, 24, 1510-1516.
[S] F. A. Cotton. G . Wilkinson, Advanced Inorgunic ChemisfrJ, 5th ed., Wiley,
New York, 1988. pp. 675-676.
Verla~sgesellsrhafrmhH. W-6940 Weinherm,1992
[6] For further discussion see for example F. A. Cotton. L. R. Falvello. R.
Llusar, E. Libby. C. A. Murillo, W Schwotzer, Inorg. Clrem. 1986, 25.
3423-3428, and references cited therein.
[7] a) Ref. [6]; b) F. A. Cotton, C. A. Murillo. ing. Cienc. Quim. 1985, 9, 1-2.
c) F. A. Cotton. E. Libby, C. A. Murillo, G. Valle, Inorg. Synfh. 1990,27.
306-310. d) F. A. Cotton. R. Poli, Inorg. C / r i n i . Acru 1988, 141, 91-98.
e) F. A. Cotton. L. M. Daniels, M. L. Montero, C. A. Mnrillo, unpublished results.
[XI See for example a) P. Dapporto, F. Mani, C. Mealli, fnorg. Chem. 1978,17,
1323 -1329; b) J. J. H. Edema, A. Meetsma, S . Gambarotta, J. Am. Chem.
Soc. 1989, I f f , 6878--6880; c) J. J. H. Edema, W. Stauthamer, F. van Bolhuis, S. Gambarotta, W. J. 3. Smeets, A. L. Spek. Inorg. Chem. 1990, 29,
1302-1306; d) J. J. H. Edema, A. Meetsma, F. van Bolhuis, S. Gambarotta, ihrd. 1991, 3032056-2061 ; e) J. J. H. Edema, S. Gambarotta. A.
Meetsma, A. L. Spek, N . Veldman, ihid. 1991, 30, 2062-2066; f) J. J. H.
Edema. S. Gambarotta. S. Hao, C. Bensimon, ihid. 1991,30,2584-2586.
[9] F. A. Cotton, T. Ren, .l
Am. Chen?. SOC.,1992, I I 4 , 2237-2242.
[lo] F. A. Cotton, X. Feng. M. Matusz, Inorg. Chem. 1989, 28, 594-601.
[Ill F. A. Cotton. T. Ren. J. Am. Chem. Soc., 1992. l f 4 , 2495-2501.
[12] F. A. Cotton, T. Ren, Inorg. Chem. 1991, 30, 3675-3679.
1131 F. A. Cotton, T. Ren, I. L. Eglin, Inorg. Chem. 1991, 30, 2559-2563.
[14] J. L. Bear, C:L. Yao, R. S. Lifsey, J. D. Korp, K. M. Kadish, Inorg. C/iem.
1991,30, 336-340.
[15] F. A. Cotton. R. Poli. Pobhedron 1987. 6 , 1625-1628.
(161 F. A. Cotton. M. Matusz, R. Poli. X. Feng, J. Am. Chem. Soc. 1988, tf0,
1144- 1154.
[17] F. A. Cotton. X. Feng, M. Matusz, R. Poli, J. Am. Chrm. SOC.1988, IfO,
7077 -7083.
[18] Crystal data for V2N,C,,H,, . C,H,, T = 298 K: M, =1087.2, space
group P4/n, u = 13.214(6). c = 17.427(5) A, V = 3043(3) A3, Z = 2.
e,,,,, =1.19 gcm-'. ki(CuK,) = 29.4cm-', Rigaku AFCSR, 4" 5 20 I
120". 4971 reflections were collected; 2282 unique, 1179 having I > 3 4 4 .
The data were corrected for absorption ($-scans) and an overall decay of
14.4%. Initial positions were taken from the homologous structure of
[W2(dfm)]. toluene [9]. The coordinates and isotropic temperature factor
for the methine H atom were included in the refinement. Positions for
other hydrogen atoms in the complex were calculated. Disordered solvent
molecules reside on the fourfold axis between the metal complexes. The
toluene molecules were modeled as two rigid groups with the methyl
groups pointing in opposite directions along the fourfold axis. The final
cycles of refinement led to R = 0.049 and R,, = 0.068. Further details of
the crystal structure investigation may be obtained from the Cambridge
Crystallographie Data Centre, University Chemical Laboratory, Lensfield
Road, GB-Cambridge CB2 1EW (UK) on quoting the names of the authors and the journal citation.
[19] The magnetic anisotropy was calculated using the McConnell equation
A6 = Ax [(l - 3cos*O)/12xr3],where A6 = 4.08, 0 = 90", r = 3.62 A. See
ref. 191 for further discussion.
1201 The SCF-Xa-SW calculations were carried out for the model system
[V2(HNCHNH),] and will be reported elsewhere. Also see the work on
[MoZ(HPO,),I2- in M. D. Hopkins, V. M. Miskowski, H. B. Gray. J. Am.
Chem. SOC.1986, 108,959-963.
[21] L. Manzer, Inorx. Synth. 1982, 21, 135-140.
How High is the Barrier for the Valence
Isomerization of Cyclobutadiene?**
By Giinther Maier,* Reinhard Wolf;
and Hans-Otto Kalinowski
"Our current knowledge about cyclobutadiene['I allows
this chapter to be terminated. It is now secured textbook material," wrote one of us (G.M.) three years ago.[ldlThis statement did not remain unchallenged,r21although even at the
time two existing gaps were indicated. The first concerns the
UV spectrum of cyclobutadiene that still is not clearly established. The spectrum must show an absorption maximum
Prof. Dr. G. Maier, Dip1.-Chem. R. Wolf, Dr. H.-0 Kalinowski
Institut fur Organische Chemie der Universitat
Heinrich-Buff-Ring 58, D-W-6300 Giessen (FRG)
Small rings, Part 73. This research was supported by the Fonds der
Chemischen Industrie. -Part 72: G. Maier, D. Volz, J. Neudert, Synthesis,
in press.
Angew. Chem. Int. Ed. Engl. 31 (1992) No. 6
above 254 nm, because irradiation of cyclobutadiene with
longer wavelengths causes its rapid cleavage into two molecules of
The extinction of this band is so small
that experimental determination has failed so far. The second question still open concerns the activation energy of the
valence isomerization of cyclobutadiene. We report here on
the first reliable experimental determination of this barrier.
The "Masamune Route"[3a,b] for the synthesis of sterically
hindered cyclobutadienes can be applied to cyclopropenyldiazomethane derivatives 1 a-d. Irradiation of cyclobutadienes 2 a and 2 b, available from 1a and 1 b, respectively, led
to tetra-tert-butyl- (3 a)[4a]and tri-tert-butyl(trimethylsily1)tetrahedrane (3b).[4b1 The analogous deazotization of the
spectroscopic data given in Fable 1. Tetrahedrane 3c was
also characterized by X-ray structure analysis.[61
The ' 3CNMR spectra of the silyl-substituted cyclobutadienes 2b-d are temperature dependent. This is especially interesting since the constitution of these cyclobutadienes offers the possibility of observing the valence isomerization of
the two rectangular forms with dynamic NMR spectroscopy. If in cyclobutadiene 2d, for example, the rectangular
structures A or B are frozen, then the two diagonal atoms
C-1 and C-3 and their substituents are magnetically
nonequivalent; in a square structure they would be equivalent. However, a single signal would also be found if a Fast
equilibrium A+B existed, feigning a square structure. Until
now it has never been possible to observe separate NMR
signals for C-1 and C-3, not even for tri-tert-butylcyclobutadiene (2, R = H).['"I Application of Saunders' isotopic per-
c _ _
1a - d
M e2 f OC H M e 2)
2d .B
diazo compound l c (yellow crystals, m.p. 33 "C) and I d
(orange oil) in the presence of CuCl provided cyclobutadiene-CuCI complexes which were treated with 1,2-bis(dipheny1phosphino)ethane to furnish the free cyclobutadienes
2c (dark red crystals, m.p. 55"C, 40%) and 2d (dark red
crystals, m.p. 45 "C, 50%). Photoisomerization of 2c and 2d
yielded the corresponding tetrahedranes 3c (colorless plates,
m.p. 58 "C) and 3d (colorless, waxy crystals, m.p. 77 "C)
( 2 c + 3 c : /2>300nm, 1 0 % ; 2 d + 3 d : /2=254nm, 40%).
The structures of the two new stable tetrahedranes 3c and
3dr5Iand their precursors were assigned on the basis of the
Fable 1. NMR spectroscopic data ('H NMR: 400 MHz,
100.6 MHz, b values relative to TMS) for compounds lc,d, 2c.d and 3c,d. All
compounds provided correct elemental analyses.
1 c :'HN M R : S =7.6-7.3(m,SH:Ph),I.l (s,18H;21Bu),1.0(s,9H;rBu),0.5
(s. 6 H ; SiMe,); "C NMR: 6 =138.7, 134.1, 129.1, 127.7 (Ph), 128.1 (C = C),
42.4 (CN,), 40.1 (quart. C), 36.8 (CMe,), 31.1 (2CMe,), 30.9 (2CMe,), 30.4
(CMe,), - 0.9 (SiMe,)[a]
1d:'H NMR: S = 4.0 (sept, 1 H, J = 6.0Hz: CHMe,), 1.2 (s, 18H; 2rBu), 1.1
(d. 6H. J = 6.0Hz; CHMe,). 0.9 (s, 9 H ; tBu), 0.3 (s. 6H; SiMe,); I3C NMR:
6 = 128.0 (C=C). 65.5 (CHMe,), 42.3 (CN,), 39.9 (quart. C), 36.2 (CMe,), 31.6
(2CMeJ 31.4 (2CMe,), 30.4 (CMe,), 25.7 (CHMe,), 0.0 (SiMe,)[a]
Zc: 'H NMR:S =7.8-7.2(m,5H;Ph),1.3(s,9H;rBu),1.2(s,18H;2~Bu),0.5
(s, 6 H ; SiMe,); " C NMR: S = 168.4(C-l/C-3), 149.1 (C-2), 142.3 (Ph), 140.4
(C-4). 134.4. 128.9, 128.1 (Ph), 34.0 (2CMe,), 32.5 (CMe,), 31.0 (CMe,), 30.1
(2CMeJ 2.6 (SiMe,)[b]
2d: 'H NMR: 6 = 3.9 (sept. 1 H, J = 6.OHz; CHMe,), 1.4 (s, 18H; 2rBu), 1.3
(s, 9 H : fBu). 1.2 (d, 6H , J = 6.0 Hz; CHMe,), 0.4 (s, 6H; SiMe,); I3C NMR:
6 = 168.1(C-l/C-3), 148.4(C-2), 141.9 ('2-4). 64.9(CHMe2), 35.5 (2CMe,), 33.3
(CMe,). 32.6 (CMr,), 31.1 (2CMe,), 27.0 (CHMe,), 3.0 (SiMe,)[b]
3c:'H N M R : 6 =7.8-7.2(m, 5H;Ph). 1.2(s. 27H; 3tBu),0.6(s,6H;SiMeZ)
" C NMR: 6 =140.6, 134.1, 129.0, 127.8 (Ph). 31.3 (3CMe,), 27.1 (3CMe,),
13.9 (?C-fBu), 0.3 (SiMe,). -22.1 (C-SiMe,)[b]
3:'H NMR: 6 = 4.1 (sept, 1 H, J = 6.0 Hz; CHMe,), 1.2 (s, 27H; 3tBu), 1.15
(d, 6 H , J = 6.0 Hz; CHMe,), 0.4 (s, 6 H ; SiMe,); ',C NMR: 6 = 64.7
(CHMe,), 31.2 (3CMe,), 27.1 (3CMe,), 26.2 (CHMe,), 14.0 (~C-IBU),
(SiMe,). -22.4 (C-SiMe,)[b]
[a] In CDCI,. [b] In C,D,.
Angew. Chem. Int. Ed. Engl. 31 (1992) N o . 6
turbation method to the same cyclobutadiene derivative with
a perdeutero tert-butyl group in the 1 position proved that
this system has a double minimum.[7b1
If a solution of 2d in propane/[D,,,]diethyl ether (ca.
80:20) is cooled to -6O"C, a single I3C NMR signal at
6 = 167.7 is found for C-I/C-3 and at 6 = 35.3 for C-5jC-7.
As the temperature is lowered these signals become broader
and each gradually resolves into two separate signals, which
at - 158 "C again have the usual half width. The downfield
signal (6 =267.7) is split into two signals at 6 =178.4 and
157.3; the second signal (6 = 35.3) now appears as two signals
at 6 = 36.0 and 34.6. The coalescence temperature T, for C-I/
C-3 is - 123 "C (1 50 5 K), for C-5/C-7, the T, is - 145 "C
(128 13 K ; this determination for C-5jC-7 is somewhat more
precise than that for C-1/C-3). Thus there are two independent probes for the determination of the free energy of activation for the valence isomerization of 2d. The values provided
are 5.810.2 (C-5/C-7, Av =134 Hz) and 6.0k0.2 (C-l/C-3,
Av = 2120 Hz) kcalmol-'. The line-shape analysis in the region of the intermediate exchange of the C atoms 5 and 7
furnished the following activation parameters : A H * =
5.3k0.3 kcalmol-', A S * = - 4.8k0.1 calK-'mol-', AC*
= 5.9k0.3 kcalmol-', E, = 6.6k0.3 kcalmol-'.
What does the experimentally determined barrier AG* =
5.8 kcalmol- (24.4 kJmol- ') imply? This value is less than
that calculated for the parent compound ( A E * = 8.1 13.4 kcalmol-l),[lds81 but agrees remarkably well with the
latest results of CI calculations (CI = Configuration Znteraction). Multiconfigurational reference wave functions form
the basis of this calculational method, and the zero-point
vibration is also taken into account in the determination of
A H * . The CAS(4,4)-CEPA calculational method provides,
according to Janoschek,18b%c1
an activation enthalpy of
A H * = 6.2 kcalmol-'. If one considers that the bond alternance in the cyclobutadiene ring system depends very little
on the substitution
91 then the good agreement
between the value determined for. 2d and the calculated activation barrier for the parent compound is not surprising.
Low-temperature 3C NMR measurements with a highest
field NMR spectrometerf"] confirmed the barrier of
5.8 kcalmol-' for 2d. In addition, an analogous coalescence
phenomenon was observed for C-I and C-3 of 2 c under these
measurement conditions. At temperatures above - 150 "C
Veriagsgeseilschaft mhH. W-6940 Wernherm. 1992
0870-0833/92j0606-0739 $3.80+ .28/0
only a single broad signal is observed; at - 168 “C the resolution into two separate signals is recognizable. The coalescence temperature T , = - 158 “C (1 15 5 K) and the shift
difference A6 =17.1 furnish a free energy of activation of
4.5 _+ 0.2 kcalmol-’ for 2c. In the case of 2b, coalescence
cannot be observed even with the highest field NMR spectrometer. It is estimated that the coalescence temperature of
2 b is probably another 35K lower, that is, at about 80K. If
the shift difference is similar to that of 2c, this would correspond to a barrier of AG* z 3.5 kcalmol-I. The activation
barriers determined for 2 d and 2c have an important consequence. Carpenter’s kinetic study”’] of the automerization
of 1,2-dideuteriocyclobutadiene ( A H * between 1.6 and
10 kcalmol-’, A S * between -17 and -32 calK-’mol-’)
kindled a discussion[”] on the extent of the effect of heavyatom tunneling in this process. According to most calculations[’21the tunneling rate is so high that the “thermal”
barrier is practically inconsequential.
This prognosis certainly does not hold for the valence
isomerization of cyclobutadienes 2, whose ring C atoms must
be moved along with their large substituents. In this context,
one should recall that heavy-atom tunneling has not been
proved experimentally by means of dynamic NMR spectroscopy, not even for the migration of unsubstituted C atoms.[131
The upshot is that the valence isomerization of cyclobutadiene 2 d proceeds with an energy barrier of AG* =
5.8 kcalmol-’. For 2c the free energy of activation is even
lower (4.5 kcalmol-’), and for 2 b a further decrease of
about 1 kcalmol-’ is estimated. These reactions are faster
than earlier calculations had predicted. Nevertheless, we rule
out the participation of heavy-atom t ~ n n e l i n g . ~ ’ ~ ]
Received: January 20. 1992 [Z5132IE]
German version: Angeiv. Chem. 1992, 104, 764
CAS Registry numbers:
I c , 140633-74-5; I d , 140633-75-6; 2c, 140633-76-7; Zd, 140633-77-8; 3c,
140633-78-9; 3d. 140633-79-0; 3, R = SiHMe,, 140633-80-3; 3, R = CHMe,,
[l] Summaries of the cyclobutadiene problem: a) M. R. Cava, M. J. Mitchell,
CydobufadieneundRelated Compounds, Academic Press, New York, 1967;
b ) G . Maier. Angeiv. Chem. 1974. 86. 491-505; Angew. Chem. h/.
Engl. 1974, 13. 425-438; c) T. Bally, S . Masamune, Tetrahedron 1980, 36,
343-370; d) G. Maier, Angew. Chem. 1988, 100,317-341; Angew. Chew.
Int. Ed. EngI. 1988, 27, 309-332.
[2] a) B. R. Arnold, J. Michl, Spectroxopy qfCydohuludiene in Kinerics and
Spectroscopy of Carbenes and Biradicals (Ed. : M. S. Platz), Plenum, New
York, 1990, pp. 1-35; b)H. Hopf, Angen.. Chem. 1991, 103, 1137-1139;
Angew. Chem. Int. Ed. Engl. 1991,30, 1117.
[3] a) S. Masamune, N. Nakamura, M. Suda, H. Ona, 1 Am. C k m . Soc. 1973,
95,8481-8483; b) P. Eisenbarth, M. Regitz. Chem. Ber. 1982,115, 37963810.
[4] a) G. Maier, F. Fleischer, Tetrahedron Lett. 1991,32. 57-60; b) G. Maier.
D. Born, Angen.. Chem. 1989, 101, 1085-1087; Angen. Chem. Int. Ed.
Engl. 1989, 28, 1050-1052.
[5] Besides the four stable tetrahedrane derivatives 3a-3d (isotopomers of 3 a
have been prepared in which either the quaternary center of one of the
terr-butyl groups is ”C-labeled or the hydrogens are replaced by deuterium atoms [Id], we have synthesized two other less stable tetrahedranes: In
the reaction of 3d with lithium aluminum hydride the isopropoxy substituent IS replaced with a H atom. The tri-tert-butyl(dimethylsilyl)tetrahedranethusformed[3,R = SiHMe,: ‘HNMR(C,D,):6 = 4.8(sept. l H ,
J = 3.6 Hz: SiHMe,). 1.1 (s, 27H; 3 tBu). 0.3 (d, 6H. J = 3.6 Hz; SiHM e , ) ; I3C NMR(C,D,): 6 = 31.2 (3 C M e , ) . 27.0 (3 CMe,), 14.3 (3 CtBu), -0.8 (SiMe,), -24.7 (C-SiMe,)] is indefinitely stable only below
-25 C, in accord with the “corset principle”[l d]. The same holds for
tri-tert-butyl(isopropy1)tetrahedrane 13, R = CHMe,: ‘H NMR(C,D,,):
6 = 2.7 (sept, lH, J = 6.8 Hz; CHMe,), 1.2 (s, 27H; 3 lBu), 1.15 (d, 6H,
J = 6.8 Hz; CHMe,): ‘,C NMR(C,D,,): 6 = 31.9 (3 CMe,), 27.6 (3
CMe,), 24.8 (CHMe,). 21.9 (CHMe,), 10.1 ( 3 C-tBu), 4.8 (C-CHMe,)].
which is accessible by the reaction sequence 1 + 2 + 3 but decomposes
within a few minutes at room temperature in the presence of air (F. Fleischer, planned dissertation, Universitit Giessen).
Verlugsgesellschalr mhH, W-6940 Weinheim, 1992
[6] R. Boese, unpublished results. We thank Priv. Doz. Dr. R. Boese, Universit~t-GesamthochschuleEssen, for the structure determination
[7] a) G. Maier, U. Schifer, W. Sauer. H. Hartan, R. Matusch, J. F. M. 0th.
Tefruhedron Left. 1978, 21. 1837-1840; b) G. Maier, H.-0. Kalmowski,
K. Euler, Angekv. Chem. 1982. 94, 706-707; Angen. Chem. I n / . Ed. Engl.
1982.21, 693-694.
[XI a ) H. Agren, N. Correra, A. Flores-Riveros, H. J. A. Jensen. I n / . J. Quanfum Chem. Quan/um Chem. Symp. 1986,19,237-246; b) R. Janoschek, J.
Kalcher, ibid. 1990. 38. 653-664; c) R. Janoschek, Cliem. tinserer Zeit
1991 2s. 59-66.
[9] The determination of the bond alternance in compounds like 2 a and 2 b by
X-ray diffraction is difficult because of disorder. The X-ray data obtained
for 2 a are consistent with the calculated differences in the bond lengths of
the parent compound (0.22 A) if the data are “averaged” by superimposing one rectangular ring on another one which is rotated by 90.: .I.D.
Dunitz, C. Kruger, H. Irngartinger. E. E Maverick, Y Wang, M. Nixdorf,
Angew. Chem. 1988,100,415-417: Angew. Chrm. In/.Ed. Engl. 1988.27.
[lo] 150.9 MHz I3C NMR spectrometer. We thank Bruker. Karlsruhe (FRG).
for conducting these measurements.
1111 a) D. W. Whitman. B. K. Carpenter, J. Am. Chem. Soc. 1980, 102. 42724274; b) ibid. 1982, 104. 6473-6474.
[I21 a) B. K. Carpenter, J. A m . Chem. Soc. 1983, 105, 1700-1701; b) M.-J.
Huang. M. Wolfsberg. ibid. 1984 106. 4039-4040; c) M. J. S. Dewar,
K. M. Merz. Jr., J. J. P. Stewart, ibid. 1984.106.4040-4041 ;d) P. &sky,
R. J. Bartlett, G. Fitzgerald, J. NOga, V. Spirko. J. Chem. Phys. 1988, 89.
3008-3015; e) R. Lefebvre. Moiseyev, J. Am. Chem. Soc. 1990,112.50525054.
[13] M. Saunders, C . S. Johnson, Jr., J. A m . Chem. Soc. 1987. 109,4401-4402.
According to these authors there is no need to discuss a heavy-atom tunneling effect (M. J. S. Dewar, K. M. Merz, Jr. ibid. 1986. 108, 5634-5635)
for the norbornyl cation ( C . S. Yannoni, V. Macho. P. C. Myhre, h i d .
1982, 104, 907-909, 7380-7381).
[14] For the unsubstituted cyclobutadiene a value of AG* = 5.8 kcalmol-’
would already require a rate constant that is consistent with the fast automerization at - 50 ’C as measured by Carpenter. but not with the results
of Michl (A. M. Orendt, B. R. Arnold, J. G. Radziszewski, J. C. Facelli,
K.-D. Malsch, H. Strub, D. M. Grant, J. Michl, J. An?. Chem. Soc. 1988.
110, 2648-2650) based on the estimated exchange rates
[ k ( 2 5 K ) t lo3 sec-‘1 from the solid-state ”C NMR spectrum of doubly
I3C-labeled cyclobutadiene. The influence of the substituents on the exchange rate is difficult to estimate. The reduction of the barrier in the series
2d to Zc to Zb supports the notion that the rate of valence isomerization
increases with increasing steric hindrance. This could be explained by the
fact that the substitution raises the energy of the rectangular ground-state
more than that of the transition state: a) W. T. Borden, E. R. Davidson, J.
Am. Chenz. SOL..1980,102,7958-7960; b) Acc. Chem. Res. 1981.14,69-76;
c) K . Mislow, W D. Hounshell, unpublished. See footnote [24] In ref.
Origin of the Stabilization of Vinyldiazonium
Ions by fi-Substitution ; First Crystal Structure of
an Aliphatic Diazonium Ion : fl,fl-Diethoxyethenediazonium Hexachloroantimonate **
By Rainer Glaser,* Grace Shiahuy Chen,
and Charles L. Barnes
Dedicated to Professor Andrew Streitwieser
on the occasion of his 65th birthday
In contrast to aromatic diazonium ions, most aliphatic
diazonium ions are highly reactive intermediates that sponmaking the characterization of these
taneously lose N,
important intermediates quite difficult. Aliphatic diazonium
Prof. Dr. R. Glaser, G. S. Chen, Dr. C. L. Barnes
Department of Chemistry, University of Missouri
Columbia, MO 6521 1 (USA)
This research was supported by the Petroleum Research Fund of the
American Chemical Society and the Research Council of the University of
Missouri (92-RC-023-BR). The X-ray diffractometer and the Bruker
500 MHr NMR spectrometer were partially funded by the National Science Foundation (CHE 90-1 1804 and CHE 89-08304. respectively). This
research IS part of the projected Ph.D. dissertation of G. S. Chen.
OS70-0833/92]0606-0740 $3.50+ ,2510
Angew. Chem. Int. Ed. Engl. 31 11992) N o . 6
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valence, high, isomerization, barriers, cyclobutadiene
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