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Formation of Carbenium Ions in the Reaction of Aluminum Chloride with tert-Butyl Chloride in Liquid Hydrogen Chloride.

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found that cycloheptasulfur[31reacts smoothly with CF,CO,H
at - 20°C in dichloromethane to give S 7 0 in 452) yield :
S7
+ CF3C03H
+
S 7 0 + CF,CO,H
S 7 0 crystallizes from CH2CI2as orange, at -80°C yellow,
transparent monoclinic crystals having a density of 2.1 ,g.
(25 C).Elemental analysisand osmometricdetermination
(at 0°C in CHzC12)of the relative molecular mass (calc. 240;
obs. 244) confirm the formula.
S 7 0 decomposes completely within 24h at 25°C in diffuse
daylight, but only within several days in the dark. At - 50°C
thecompound can bestored for longer periods without decomposition. It melts at 55'C (SO, evolution), and on decomposition at normal pressure gives SO, and polymeric sulfur, which
according to its Raman spectrum contains only a small amount
of S,. The mass spectr~metrically[~~
investigated decomposition in a high vacuum proceeds, as in the case of S,O[",
according to the following equation :
5
s70+s20+
-s.
The ions SO', S,O+, and S: ( I T = 1 to 8; S: most abundant,
followed by S;) were detected, but no molecular ion was
observed (sample temperature 60°C).
The Raman spectrum[41of S 7 0 recorded at -90°C could
be assigned by comparison with the spectra of S7l6l and SsO"'
and enabled some conclusions to bedrawn about the molecular
structure. The molecule S, belongs to the point group C,'8J,
to which S 7 0 must therefore also belong, i. e. all I8 normal
vibrations are Raman active. Of these, I7 could be identified
(Table I ) , ri:. vs0 (which in the crystal is split into three
components)[". 1 6,,,. 7
and the 8 deformation and torsion
Vibrations of the ring. Since the spectra of S 7 and S,O are
very similar in the region of the ring deformation vibrations,
both compounds must contain rings having the same conformation. On the other hand, there is no similarity in the SS
stretching vibrations, a fact which can be attributed to the
influence of the oxygen atom (cf. the pair S8/S80'7b.lol).
The
SS nuclear separations determined from the wave numbers
of the SS stretching vibrations[") are: 1.93, 1.97, 1.99, 2.1 1 ,
2.14. 2.17. and 2.19h [three short (s) and four long (I) bonds
compared with the single bond length of 2.06A[121].S,O
ized by X-ray structure analysis[' 3 J . The SO nuclear separation
obtained from vso using valence force constants" calculated
according to the two-body modellt4]is 1.48 A.
The synthesis of S,O raises the question whether the hypothetical S,S, isoelectronic with S,O and isomeric with cyclooctasulfur, might be considered as a component of liquid or
gaseous sulfur.
E.uperinlerlru/
( C F 3 C 0 ) , 0 (4.4ml) is added with vigorous stirring at 0°C
to a suspension of H,O, (0.91 g ; SOU,;) in CH,CI, (95 ml, purified by column chromatography on basic A1,0,). The reaction
mixture is warmed to 2 5 T and stirred until no droplets
of H 2 0 2are discernible. S, (272mg) is dissolved in ultrapure
CHZCIz (80ml) at 0°C and the solution protected against
exposure to light. The CF3C03H solution (5.6mI) is then
added dropwise with stirring within 15min at -20°C. The
golden-yellow mixture is stirred for one hour and then cooled
to - 70°C whereupon S 7 0 ( I 30mg, 45 %) crystallizes out.
The crystals are collected on a cooled glass frit at -30°C
in the absence of moisture, washed with a little cold CHZCIz,
and dried in an oil-pump vacuum. If necessary the product
is recrystallized from CH2CI2 (50mI) by dissolution at 0°C
and cooling to -80°C.
Received: October 14. 1976 [Z 5x0 I[:]
German version: Angew. Chem. .YS. X54 119761
CAS Registry numbers:
S - 0 . 60828-66-2: lCF3CO)20.407-25-0: S-. 21459-04-1
[I]
[2]
[3]
\vSs.
[4]
[S]
[6]
[7]
[XI
(19751.
[Y]
[lo]
.+
r
[I I]
[I?]
[I31
[I41
[I51
thus resembles the isoelectronic cation S71+, which was
recently isolated as the hexafluoroantimonate and characterTable I . Wave numbers a n d iiiteiisities of the normal vibrations in the
Raman spectrum or crystalline S - 0 lcm I: sample temperature -90°C.
spectral split width I S c m - ' . intensities: \'s \ e r y strong. s strong. in moderate.
u weak. \nu very weak: !' stretching \ibration. 6 deformation vibration.
T torsion ~ i b r a t i o n .
~
. .
I113vu
I102 M,
109x vw
575 in
534 u
517m
402 x
I
1
VVJ
\i5
390 u
372 w-m
345 m
325 \'b
292 s
281 m
232 s
]
6550
v55
209 u
193 m
165 b
1% m
129 w-m
L,
and
L"p
Part 45 of Sulfur Compounds. Part 44. R. Srcwld. I.. RIJ.W.and
J . I'kl~mdr.Z. Anorg. AIlg. Chem.. 111 press.
R. SIciidd a n d J . Llirre. Angeu. Chem. XO. 648 119741: Angew. Chem.
Int. Ed. Engl. 13. 603 11974): Chem. Bcr.. in press.
.M. SdIfJlidl.B. Bled. If. D. B / O d . H . K ( i y / . and E. W i / / w / m ,Angeu.
Chem. NO. 660 11968). Angew. Chem. Int. Ed. Engl. 7 . 632 1196X).
Perkin-Elmer I R spectrometer 3 2 5 : Varian Cary 82 Ram;in spectrometer
with krypton liiser (647.1 n m ) : Varian 71 I maas bpectromcter (70eV
electron energy. 220°C ion soiircc tcmpcrature).
R. S r r d d a n d .W. R c h . d i . Z, Anorg. AIIg. Chem. 413. 222 11975).
M . C m / f r e r a n d 4 . Ro</\rd. J . Chem. Soc. Dalton Trans. l Y 7 3 . 599:
R. Srcldcl and F. Sdrmrrr. unpublished resiilts.
;I) R. Srvldd a n d M . Rch\di. J . Mol. Spectroac. 51. 334 119741; bl
R. Sretrdd a n d D . F . E q < / m . Spectrochim. Acta 3l 4. X71 11975).
I . K w i ~ ~ iand
i h E . Hc,//frrr.Angew. Chem. X2. 390 (1970): Angeu. Chem.
Int. Ed. Engl. Y. 379 (1970): R. Srrlrdrl. Spectrochim. Acta 21 4. 1065
S 7 0 is readily aoluble i n CS2:this solution shows only o n e S O stretching
tibration in the IR spectrum at 1129cm-'.
P . Lii</er. H . B r d i c A . R. Sfcii~lcl.and M . Rch\cli. Chem. Bcr. I O Y .
I X0 ( 1976).
R. S r w d d . Z. Naturforsch. 20h. 2x1 119751.
R. S / c w / d .Angew. Cliem. 87. 6x3 119751: Angeu. Chem. Int. Ed. Engl.
14. 655 119751.
J . P ~ i w r i ~P.
~ rEiihw.
~ ~ . T W h i d h i . a n d P. Whir(,. .I. Chem. Soc. Chem.
C o m m u n . 1976. 689 and pribate communication.
As regards the justification of this approximation. see R. Sfcwdd. Z
Naturrorsch. 25h. 156 (1970): 26h. 750 (1971).
J . Ci//l,.\pic, and E. .4. Rohirt.wi. Can. J. Chem. 41. 2074 11963).
Formation of Carbenium Ions in the Reaction of Aluminum Chloride with rerr-Butyl Chloride in Liquid Hydrogen Chloride
By Frunz Ku/ch.schmitl and Erwin Mayer[*l
The catalytic action of aluminum chloride in Friedel-Crafts
reactions is usually explained in terms of intermediary occurrence of carbenium ions. However, a spectroscopic identifica[*] Dr. F. Kalchschmid and Dr. E. Mayer
['I
['I
Institiit fiir A n o r g a n i s h e und Analytische Chemie der Uni! ervtht
lnnraiii 52. A-6020 Innsbruck (Austria)
Author to u l i o m correspondence should be addressed.
773
tion of these intermediates has, to our knowledge, so far
proved unsuccessful. At low temperatures weak donor-acceptor complexes between alkyl chloride and aluminum chloride
have been detected['! In ,the reaction of terr-butyl chloride
with AlC1, at -78°C a crystalline 1 : 1 complex could be
isolated[21, which according to the 'H-NMR signal of the
tert-butyl group, however, does not contain a carbocation.
Only reaction of alkyl chlorides with SbFj, FS03-SbFj, and
HF-SbF5 gave stable carbenium ions['].
We have established that trrt-butyl chloride reacts with
excess AICI, in anhydrous liquid hydrogen chloride quantitatively with formation of the carbenium ion ( I ) .
hence of the [C(CH,),]'
1.5 mol/l.
Received: October I . 1976 [Z 571 IE]
German version: Angew. Chem. N K . 849 (1976)
CAS Registry numbers:
( I ) , 60804-16-2: rrrf-butyl chloride, 507-20-0: AlCl 1. 7446-70-0
~
[2]
[3]
[4]
[6]
'
Experimental:
The reactions were carried out in sealed glass ampoules.
Thick-walled ampoules had to be used since liquid hydrochloride develops a vapor pressure of 60 atm at 36 "C. 5-mm NMR
tubes having a wall-thickness of 0.7mm have proved to be
sufficiently stable up to 30°C. For safety, however, the NMR
spectra were recorded at or below - 30°C. Typical concentrations for an NMR experiment: 1 mmol AI2Cl6, 0.5 mmol tertbutyl chloride, and 7mmol HC1. The substances were sealed
in tubes under vacuum at - 196°C in the absence of moisture.
The concentration of tert-butyl chloride in this mixture, and
774
.
~
[I]
[5]
The reaction was carried out in sealed glass ampoules and
was complete within a few minutes at 25°C. ( 1 ) was identified
'H-NMR spectroscopically at 25°C (singlet at -4.38 ppm,
TMS as external standard; cf. -4.45ppm
for
[C(CH&][Sb,F;,]
in pure SbFjL3])and by a low-temperature IR spectrum of the product remaining after removal of the
hydrogen chloride at - I 12"C[41. The IR absorptions
(2958cm-' w, 2780 m, 2720 sh, 1452 m, 1285 vs, 1265 vs,
1195 m, 1110 sh, 1068 s, 957 s, 900 s) were very similar
to the values published by U/ah[3351except for differences
in intensities and band splitting. From the absence of the
intense IR bands of tert-butyl chloride it could be concluded
that the reaction was quantitative. Investigation of the Raman
spectrum was rendered dificult by strong fluorescence of the
HCI solution above 400 cm- ; nevertheless, characterization
of the anion as A12CI; (bands at 317 cm-' vs, 160 m, 97
m)16]was possible.
The rate of formation of [C(CH&]+ depends on the purity
and particle size of the AICI3 employed. For Raman investigations specially purified AIC13 reacts within hours at -6O"C,
less carefully purified samples react only slowly even at - 30°C.
The course of the reaction can be followed 'H-NMR spectroscopically by the shift of the terr-butyl singlet at -1.94 ppm
(tert-butyl chloride in HCI at -60°C) to the value for
[C(CH3)3]+ (-4.38 ppm). As in investigations carried out
by Olah, exchange between carbenium ion and unreacted (errbutyl chloride leads to only one singlet, whose chemical shift
lies between these two values, depending on the concentration.
Solutions of [C(CH3)3]+[A12C17]- in liquid HCI are stable
for weeks at 25°C and discolor only slowly. Solutions stored
at - 30°C remain colorless. Solid [C(CH3):][A12CIT],
obtained by removal of HCI at - 112"C, already decomposes
at ca. -50°C.
Reaction of aluminum bromide with tert-butyl bromide
in liquid HBr likewise furnishes the ion [(C(CH3)3]+ ('HNMR signal at -4.7ppm). Hence, it should be possible to
carry out the first complete investigation of the IR and Raman
spectra of the tert-butyl cation, since the [A12Br,]- bands
are strongly shifted compared to those of [A12C17]-.
present after the reaction, was about
C . 4 . Oldi. Angew. Chem. 85. 183 (1973); Angew. Chem. lilt. Ed. Engl.
12, 173 (1973): and literature cited therein.
S. Crsm. A . P r i i h t . and G . ferrctris. Makromol. Chem. 156. 325 (1972).
G . 4. 0/<1/t,
E. B. B ~ l k P t ' .J . C . E~(I,~.S.
W S. 7i>/<Jj'<,.>i.
J . S M(,/ii!j.rc,2
and I . J . Bmrirtt, J. Am. Chem. Soc. 86. I360 (1964).
KBr pellet at - 196°C. Details of this technique for the IR investigation
of substances that are unstable at higher temperatures will be published
later.
G . A. Oh11, J . R . DrMerirbo., 4 . C ~ J I I J I ~and
~ ) J.. ~L.~ Bribe\.
~ , ~ , J Am.
Chem. Soc. Y3.459 (1971).
H . A . O y , E. R!.ffer. P. Kkrhor. and S. J . C W ~ JActa
I . Chem. Scand.
25. 559 ( I97 I).
Cleavage of Esters and Ethers with Iodotrimethylsilane[**l
By fie-Lok H o and George A . Oh/z[*]
Reagents for nonsaponificative ester hydrolysis are generally
powerful nucleophiles which promote 0-alkyl cleavage. They
are exemplified by lithium iodide/alkylpyridines"I, potassium.
thiocyanate/dimethylformamide[21,
and alkali thiolates in
aprotic
We wish to report that iodotrimethylsilane
is a useful
reagent for mild ester cleavage. Selected examples are shown
in Table I . It is seen that difficulties are encountered only
with the highly hindered methyl pivalate; in this case a longer
reaction time is needed and the yield of free acid is somewhat
lower.
0
R-C,
OSi(C H 3l 3
H20
RCOzH
+ (CH3),SiOH
Table I . Yields of free acid on ester cleavage with iodotrimethylsilane.
___~
R'
R
~
Reaction time
Yield
[hl
[ 'XI
- -.
CH,
C2Hs
CIHS-CH~
CbHs
2-Br-ChH4
ChHSXH2
C9Hi9
cyclohexyl
(CH,),C
ChHs
ChHsXH2
ChHs
cyclohexyl
2
-7
2
2
2
18
4
4
2
2
-
80
81
78
75
80
55
72
70
86
90
The advantages of iodotrimethylsilane over lithium iodide
as demethylating agent are: neutral and milder reaction conditions (lower temperature, shorter reaction time) and the
absence of solubility problems in aprotic solvents.
[*] Dr. T.-L. Ho and Prof. Dr. G. Olah
Department of Chemistry
Case Western Reserve University
Cleveland. Ohio 44106 (USA)
[**I Synthetic Methods and Reactions, Part 29.-This work was supported
by the National Science Foundation.--Part 28: T - L . Ho, M . ff~~Jl~liJtyf'I'.
and G . 01r111 Synthesis. in prcss.
'
Ailyen. Chrrn. / i l l . Ed. E i l y l . J WJ/. 15 (1976) N o . 12
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hydrogen, carbenium, butyl, reaction, formation, tert, ions, chloride, liquid, aluminum
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