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Origin of the Stabilization of Vinyldiazonium Ions by -Substitution; First Crystal Structure of an Aliphatic Diazonium Ion -Diethoxyethene-diazonium Hexachloroantimonate.

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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
ions have been observed in superacid mediaL3]and in the gas
phase.[4]Alkanedkazonium ions have been stabilized by transition metals, but in these complexes the diazonium ions are
bent and differ greatly from the free ions.''] Bott discovered
that various B,p-disubstituted ethenediazonium ionsr6]are
rather stable while the parent system is not.L21This was attributed to the resonance stabilization of the p substituents.
Bertrand et al. recently reported on the alkylation of a diazomethylenephosphorane which gives rise to stable C, Panalogues of ethenediazonium ions, best described as phosphonio-substituted diazoalkanes.[']
In this article, we report the first crystal structure analysis
of an aliphatic diazonium ion, B,p-diethoxyethylenediazonium ion ( I ) in I-SbCI,. With this crystal structure, weprovide
for the first time experimental information with which theoretical structural studies can be judged. The ab initio structures of 1 and its dimethoxy analogue 2, 1,l-dimethoxyethene (3). and 1,I -dimethoxyethenium ion (4) also have
been determined for the examination of the importance of
individual mesomeric resonance forms.
1, R = Et
1-SbCI, was prepared by direct alkylation of diazoacetic
ester with triethyloxonium hexachloroantimonate in 1,2dichloroethane as described by Bott.r61The white needles of
I-SbCI, obtained after recrystallization from 1,2-dichIoroethane were characterized by elemental analysis and by infrared and 'H and 13C NMR spectroscopy. IR spectra of
1-SbC1, were recorded in CH,CI, and in the solid state
(KBr). In the solution spectrum, the characteristic N-N
stretching vibration appears at 2193 cm-' while two
bands"] arise for this N-N stretching mode in the solid-state
spectrum at 2192 and 2161 cm-'. The 'H (italics) and I3C
NMR chemical shifts are summarized in Scheme 1. The 'H
4.72 CHz-0
75.4 CH,-0
Fig. 1. Crystal structures of the two symmetry-independent b,S-diethoxy
ethenediazonium ions A and B. For bond lengths and angles see text.
ing in a dry box under N,. Colorless single crystals formed
at the interface after three
The asymmetric unit of the orthorhombic unit cell of I-SbCI, contains two symmetry-independent cations A and B
(Fig. 1) as suggested by the solid-state IR spectrum. A and B
are surrounded by SbCl, ions, and all chlorine atoms are
more than 3.35 8, away from the cations. The jN atoms are
\ 6% 7
14.1 CH3
Scheme 1. "C and 'H (italics) NMR chemical shifts of 1
signals were assigned based on the observed NOE between
the vinylic H and the trans-CH, hydrogen atoms and the
two-dimensional 'H-'H and 'H-I3C COSY spectra. The
signal for the vinylic H at 6 = 6.14 is most diagnostic. Unexpectedly, the I3C NMR signal of the a C atom shows a lower
intensity than that of the quaternary C atom, but a DEPT
experimentr8]clarified the assignment of these signals.
Single crystals suitable for X-ray crystallography were obtained from a solution of 1-SbCl, in 1,2-dichloroethane
which was covered with a layer of dry hexane and left standAngrn.. Chmi. I n ! . E d EngI. 31 (1992) N o . 6
coordinated by four chlorine atoms, and some of these chlorine atoms occupy bridging positions between the p N and
c( C atoms. The p C atoms in A and B are relatively close to
two chlorine atoms (>3.4 8,). The ions A and B have the
same conformation, and deviate from local C, symmetry
only slightly. C-0 s-trans and s-cis conformations are preferred for the C,-O bonds that are cis and trans with regard
to the N, group, respectively, and all conformations are
s-trans about both the Et-0 and C-C bonds. Significant
structural differences are evident for A and B (Table 1). The
C-N bond in A is about 0.05 8, longer than in B while both
of the C,-0 bonds, and the cis C1-02 bond in particular, are
shorter. Angular differences are small except for the trans
ethyl group. A and B both exhibit Cl-C2 bond lengths of
1.37-1.38 A and N-N bond lengths of 1.10 A. The bond
angles involving the C1-C2 bond indicate attractive interactions between the N, group and the proximate ethoxy group.
The CI-C2-N1 and C2-C1-02 angles are less than 120",
whereas the corresponding angles C1-C2-H and C2-C1-01
are larger. Importantly, the Et-0-C, angles indicate approximately sp2-hybridized 0 atoms, and the C,-O distances are
comparatively short.
The C, structure of the diethoxyethenediazonium ion with
the conformation of 1 was optimized with the 3-21G and
6-31G* basis sets (Fig.2). Conformational analysis of 2 suggests than this conformation is the most stable one.[''] The
ab initio structures agree well with the average experimental
values. Bond lengths generally are over- and underestimated
at the lower and higher levels of calculation, respectively, but
these differences are of about the same magnitude as the
experimental uncertainties. The most significant deviations
between the calculated and the solid-state structure occur for
the Et-0-C, angles (larger by 3.6-5.8') and for the N-N
bond length (underestimated by 0.01 -0.02 A).1i21Note that
both the C,-O and the Et-0 bonds are consistently longer
for the cis ethoxy group. The C1-C2-H angle is predicted to
be significantly larger than the sp2 hybridization angle.
The structural parameters of the CNN units in diazo compounds and diazonium ions are similar," 31 and thus provide
little information regarding contributions of the mesomeric
resonance forms of I-IV (Scheme 2). The C-C and the 0-C
bond lengths probably are the parameters best suited for this
Verlugsgesellschuft mhH, W-6940 Weinheinl, 1992
0S70-0833/92/0606-0741S3.50+ ,2510
Table 1. Selected important structural parameters of the crystal and ab initio structures of ,4,b diethoxyethenediazonium ion [a].
Parameter [b]
ci c 2 - n
C 1-C2-N 1
c 2 - c 1-01
1.374( 14)
1.102( 15)
1.296( 13)
13 7.4(8)
1 .o
I .4
125 69
ah initio [c,d]
0.01 9
[a] Length in .& and angles in degrees. Standard deviations given in parentheses refer to the last digitis). [b] For atom numbering see Figure 1. X is a dummy atom
connected to N1, cis with C1; X-NI is perpendicular to C2-Nl. C1 = C,, C2 = C,: N, is attached to C,. [c] C, structure. [d] A values are the deviations of the ab iuitio
structures from the average values of the crystal structure.
analysis. The short C,-0 bonds and the large Et-0-C, angles
indicate that the Lewis structures of types 111 and IV contribute significantly. This importance of I11 and IV can be
- ’ - \FC\
Scheme 2. Resonance forms I-IV of 1
demonstrated convincingly by comparison[’41 of 2 with 3
and 4 (Fig. 2 ) . Note the structural similarities of 1 and 2. The
C,-O bond lengths in the carbenium ion 4 are only moderately shorter than in 2 (0.015-0.026 A) while the 0-CH,
bonds are slightly longer (0.008-0.012 A). In contrast, the
C,-0 bonds in 3 are significantly longer (>0.074 A), and the
H,C-0 bonds are shorter (>0.035 A) than in both 2 and 4.
Thus, the characteristic shortening of the C,-O bonds and
the concomitant lengthening of the H,C-0 bonds in the
carbenium ion 4 are also clearly evident in 2. This finding
stresses the importance of resonance forms 111 and IV. The
longer C-C bonds in 1 and 2 compared to 3 provides evidence for the importance of all of the resonance forms IIIV. Oxosubstituted carbenium (“oxenium”) ions[’ 51 as well
as dioxo derivatives“ 61 (“dioxenium” ions) are known, and
a few crystal structures of the former have been reported.
Childs et a1.[‘5a1determined the crystal structure of two hydroxycarbenium ions; their short C-OH bond lengths
(1.288-1.302 A) indicate significant contributions of the
oxenium ion resonance form.’”]
These results point out the crucial importance of the 0stabilized carbenium-type resonance forms 11-IV in understanding the stability of the 8-substituted alkenediazononium ion. Our conclusions are in agreement with topological
electron density analysis of other j-substituted ethenediazonium ionsLZd1
as well as of the unsubstituted
suggest electronic structures with overall neutral dinitrogen
Verluggesellschuft mhH. W-6940 Wcinheim, 1992
Fig. 2. RHF/6-31GC-optimizedC, structures of 1-4.
fragments. Moreover, the 13CNMR chemical shifts of 1 are
indicative of electron depletion at C, which is also in agreement with the additions of nucleophiles to the j carbon atom
in the chemistry of P,P-disubstituted ethenediazonium
Experimental Procedure
Caution: Oxonium salts are toxic and carcinogenic. A11 experimental work was
carried out in a glove box under dry N,. Solvents were dried over P,0,,; hexane
was dried over molecular sieves.
I : To a stirred solution of diazoacetic ester (2.08 g, 18 mmol) in 15 mL of
1,2-dichloroethane was added triethyloxonium hexachloroantimonate (2.6 g,
6 mmol). The resulting mixture was allowed to react 24 h a t room temperature.
Upon addition of 40 mL of CC1,l precipitated. The crude product was purified
by recrystallization. A saturated solution of 1 in dichloroethane at room tem-
0570-0833/92j0606-0742$3.50+ ,2510
Angew Chem. Int. Ed. Engl. 31 (1992) N o . 6
perature was treated with CCI, until the crystallization of 1 was observed.
Recrystallization at 5 "C furnished 2.08 g of the pure salt 1 as white needles;
m.p. 110-112°C: yield 24%.
'H NMR(500 MHz, CD2C12.25'C,TMS): 6 =1.53(t, J = 7 Hz. 3H), 1.62(t,
J = 7 Hz. 3H).4.65 (9. J = 7 Hz, 2H),4.72(q, J = 7 Hz, 2 H ) , 6 . 1 4 ( ~1H);
NMR (CD,CI,): 15 = 14.14(CH,), 14.28 (CH,), 55.75 (CH), 72.47 (CH,). 75.41
(CH,). 177.8 (Cl). - Correct elemental analyses (C,H,N,CI).
Received: December 4, 1991 [Z 5054 IE]
German version: AngeH-. Chem. 1992, 104, 749
CAS Registry numbers:
I , 3883-93-0; 2, 140834-35-1 ; 3. 116-11-0; 4, 41798-19-0, ethyl diazoacetate,
623-73-4; triethyloxonium hexachloroantimonate, 3264-67-3.
Reviews: a) K . Bott, Angew Chem. 1979, 91, 279; Angew. Chern Int. Ed.
ErrgI. 1979,18,259; b) K. Bott, Alkenediazonium Compounds in The Chemistry of tho Fun(rional Groups. Supp!. C. (Eds.: S . Patai, Z. Rappoport),
Wiley, New York, 1983. p. 671 ; c) K . Laali, G. A. Olah, Rev. Chem. Interm i d 1985. 6 , 237.
a) R. Glaser, J. Phys. Cheni. 1989.93.7993; b) R. Glaser J. Cornput. Chem.
1990. 11, 663: c) R. Glaser. G. S:C. Choy, M. K. Hall, J. Am. Chem. Soc.
1991, 113, 1109; d) R. Glaser, C. J. Horan, E. Nelson, M. K. Hall, J. OrK.
Chnii. 1992. 57, 215; e) R. Glaser, C. J. Horan, G. S.-C. Choy, ibid. 1992.
57, 995.
a) J. R Mohrig. K. Keegstra, J. Atn. Chem. SOC.1967, 89, 5492; b) J. R.
Mohrig. K . Keegstra, A. Maverick, R. Roberts, S. J. Wells, J. Chem. Soc.
Chrm. Conimun. 1974, 780; c) M. Avaro. J. Levisalles. J. M. Somnier,
Chmm. Commrm. 1968. 410; d) J. F. McGarrity, D. P. Cox, J. An?. Chem.
Sot.. 1983. 105, 3961.
a) M. S. Foster, J. L. Beauchamp, JT Am. Chenr. Sot.. 1972, 94, 2425:
b) M. S. Foster, A. D. Williamson, J. L. Beauchamp. Inr. J. Mass. Spect r u m fon P / I ~ , 1974,
. A . 15.429; c) T. B. McMahon, T. Heinis, G. Nicol, J. K.
Hovey. P. Kebarle, J. A m . Cheni. SOC.1988. 110. 7591.
a) M. F. Lappert. J. S. Poland. Chem. Commun. 1969, 1061; b) B. W. Day.
T. A. George, S. D. Iske. J. Am. Chern. SOC.1975, 97,4127; c) W. A. Herrmann. A n p w . Cheni. 1975,87, 358; Angew. Chem. Int. Ed. Engl. 1975, 14.
355, d ) G . L. Hillhouse, B. L. Haymore, Inorg. Chem. 1979, 18. 2423;.
e) W. A. Herrmann. M. L. Ziegler. K. Weidenhammer. Angew. Chem.
1976. 88. 379; A n g r w . Chem. I n t . Ed. Engl. 1976, 15, 368.
K. Bolt, Tetvuhedron 1966, 1251.
J. M. Sotiropoulos, A. Baceiredo, K. Horchler von Locquenghien, F. Dahan. ti. Bertrand. Angmi. Chem. 1991, 103, 1174; Angebc. Chem. lnr. Ed.
&ng/. 1991. 30. 1154.
a) D. M . Doddrell. D. T Pegg, M. R. Bendall, J. M a p . Reson. 1982, 48.
323. h) Delay time was 1 s. c) The lower intensity of the methine signal
mobt likely is the result of broadening due to quadrupolar coupling to I4N.
X-ray data of I : were collected at - 100 "C on an Enraf-Nonius CAD4
diffrlictometer with Mo,, radiation. The needle-shaped crystal was mounted o n a glass fiber approximately along the long axis of the needle. The
data waq corrected for Lorentz and polarization effects. An empirical
absorption correction was applied using $ scans, giving relative transmitance factors hetween 0.90 and 0.99. The structure was solved by Patterson
techniques and refined in the space group P m 2 , with Z = 8. Though the
antmionate ions are related by a pseudo center of symmetry, the organic
cations do not agree with this symmetry element. The acentric space group
reported is therefore correct. The polarity of the crystal could not be
determined, probably as a result of the pseudocentrosymmetrically related
iintimonates. Refinement included anisotropic thermal parameters for the
non-hydrogen atoms. The hydrogen atoms were included in the model at
calculated positions: the orientation o f the methyl hydrogen atoms were
established in a difference Fourier map. Full-matrix least-square refinements gave final R = 0.032. R, = 0.044 and S = 1.20 for 2203 reflections
. final Fourier map includes four peaks of about
with I P 2.5 ~ ( 0The
0.8 e - k 3 near the antimony atoms. but no other features above
0 . S e - k 3 . All calculations were performed with the NRCVAX suite of
programs.[lO] Further details of the crystal structure investigation are
available on request from the Director of the Cambridge Crystallographic
Dais Centre. University Chemical Laboratory, Lensfield Road, GB-Cambridge CB2 IEW (UK), on quoting the full journal citation.
[lo] E. J. Gabe, Y. Le Page. J.-P. Charland, F. L. Lee. J. App!. Chem. 1989.22,
[ I l l I t is certainly reasonable to assume that the methyl and the methylene
groups in the efhoxy fragments are staggered and in the s-rrms conformation with respect to the C3-C4 and C5-C6 bonds. The dimethyl compound therefore serves well in the investigation of the conformational
preferences about the four C - 0 bonds. R. Glaser. G. S. Chen, unpublished
[I21 The former is likely to be a crystal-packing effect requiring relatively little
energy (bending modes), and the latter reflects the importance of electron
correlation in predictions of the N-N bond length as discussed in ref. 12~1.
Anpew. (%em. Int. Ed. Engl. 31 (1992) No. 6
[I31 R. Glaser, G. S. Chen, unpublished ah initio studies.
[14] a) 2-4 were optimized at the RHFi3-21G- and RHF/6-31G* levels. Analytic computation of the Hessian matrices at the lower level showed all of
these structures to be minima. b) Investigation of the protonation of vinyl
ethers: K. &pay, I. Delhalle, K. M. Nsunda, E. Rolli, R. Houriet. L.
Hevesi, J. Am. Chem. SOC.1989, i l l , 5028.
[15] a) R. F. Childs. R. Faggiani, C. J. L. Lock. M. Mahendran. S. D. Zweep,
J. Am. Chem. Soc. 1986, /OR, 1692; b) S. K. Chadda. R. F. Childs, R.
Faggiani. C. J. L. Look, rhid, 1986, 108, 1694.
[16] F. Perron-Sierra, M. A. Promo. V. A. Martin, K. F. Albizati, J. Orx. Chem.
1991,56, 6188.
(171 Conclusions based on a comparison of 1-4 with this ion are difficult to
impossible because of the difference in C suhstituents (phenyl and cyclopropyl as opposed to CHN,) as well as the number and type (HO or RO)
of 0 substituents.
(181 a) K. Bott, Chem. Eer. 1987, 120, 1867, b) Terruhalron Lert. 1985. 26.
Cyclohexadienyl-Type Cationic Intermediates
in the Friedel-Crafts Alkylation of Benzene
Derivatives with the 9-Fluorenyl CationObservation by Laser Flash Photolysis""
By Frunces Cozens, Jianhui Li, Robert A . McClelland,*
and Steen Steenken*
There have been a number of recent reports of the use of
laser flash photolysis to directly study carbocations under
conditions where they are actually found as intermediates in
organic reactions."] This approach allows the measurement
of absolute rate constants for the further reactions of the
cation. Previously, reactivities of short-lived cations had
been examined only in terms of relative rate constants, obtained, for example, from competition experiments in which
an intermediate not observed directly partitions between two
nucleophiles. One fundamental reaction that has seen considerable investigation in the latter regard is electrophilic
aromatic substitution. In this paper we demonstrate that
flash photolysis can be employed to directly study the key
steps in such a reaction. In particular, both cationic intermediates of a Friedel-Crafts alkylation can be observed, the
initial electrophile and the cyclohexadienyl cation, the ''0
complex" formed by addition to the arene.
The experiments employed the 9-fluorenyl cation (Fl +),
generated photochemically from 9-fluoreno1 (FI-OH) in
1,1,1,3,3,3-hexafluoroisopropylalcohol (HFIP)r2' (Scheme 1).
This system has several features that make it useful for
studying reactions with arenes: 1) The photoheterolysis of
9-fluoreno1 is very clean in this solvent with no competing
reactions such as photohomolysis. 2) The solvent itself is
relatively unreactive to cations, so that the added arenes can
compete for the cation. 3) 9-Fluorenol has sufficient absorbance to be excited at 300-310nm, a range in which
simple aromatic compounds have essentially no absorbance.
Indeed, the rates (k,,,,) for the exponential decrease of F1+
(due to reaction with solvent[']) increase in the presence of
alkenes and arenes, with the expected linear dependence on
the concentration of the additive. The second-order rate con[*] Prof. Dr. R. A. McClelland. F. Cozens, J. Li
Department of Chemistry, University of Toronto
Toronto, Ontario M5S 1Al (Canada)
Prof. Dr. S. Steenken
Max-Planck-Institut fur Strahlenchemie
Stiftstrasse 34-36, D-W-4330 Mulheim (FRG)
This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Petroleum Research Fund, administered by the American Chemical Society.
Vc.rlugsgeselfschaJtmbH, W-6940 Weinheim, 1992
0570-0833l92l0606-0743 $3.50+.25/0
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crystals, hexachloroantimonate, ion, substitution, diazonium, aliphatic, origin, ions, diethoxyethene, vinyldiazonium, structure, first, stabilization
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