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Ortho-para Protonation of Toluene and Ethylbenzene in Super Acids.

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grow single crystals suitable for X-ray structure analysis have
hitherto been thwarted by the poor thermal stability (loss
of oxygen and sublimation of AS406 above 300°C) and the
ease of hydrolysis of Asz05.
Single crystals of this arsenic oxide have now been obtained
by annealing of amorphous As205 under positive oxygen
pressure with careful exclusion of moisture. Space group:
P212121with a=8.64,, b=8.450, ~ = 4 . 6 2 ~ 1Z=4f21.
\;
For the
structure analysis, 1085 symmetry-independent reflections
were measured (Philips PW 1 1 00 automatic four-circle diffractometer, MoK,, graphite monochromator, w/20 scan). The
structure was solved with Patterson and Fourier synthesesf3];
refinement of the positional and anisotropic temperature factors converged at an R value of 5.3 %. The positional parameters and the A s 0 bond lengths are shown in Table 1.
Table I . Positional parameters of A s 2 0 5 and A s 0 bond lengths.
AsU)
As(2)
O(1)
ow
O(3)
o(4)
O(5)
X
v
z
0.1515
0.5336
0.2903
0.4953
0.5824
0.4990
0.2757
0.4025
0.7826
0.4334
0.1439
0.2655
0.4757
0.2561
0.1284
0.1 340
0.8451
0.6171
0.08 I7
0.4612
0.3000
As(l)-[0(1), 0 ( 1 ) , OC), 0(3), 0(4), 0 ( 5 ) ] =
(1.78, 1.80, 1.83, 1.82, 1.83, 1.82A)
- [0(2), 0(3), 0(4), 0 ( 5 ) ]=
(1.67, 1.66, 1.71, 1.69.k)
The structure of AszO5 resembles that of P-Ga203[41 in
that the two oxides both exhibit cations in adjacent tetrahedral
and octahedral oxygen environments (ratio 1 : 1). The composition M z 0 5 which for exclusively octahedral coordination
can only be attained by edge and corner linking of the octahedra is achieved in As205 by equal contributions of the
coordination and linkage principles of R e 0 3 and the S i 0 2
modifications stable under normal pressure, as expressed in
the formula ( A S O ~ / ~ ) ( A S O ~ , ~ ) .
Thus all the polyhedra are linked by corners: along [OOI]
there are strands of octahedra which are linked by tetrahedra
in such a way that tubular cavities are formed in the direction
of [OOl].
The strongly crosslinked crystal structure explains the crystal form and is in agreement with the findings of Thilo and
Wink[er that only half the As can be replaced at most by
P and Sb, respectively, on formation of mixed crystals of
As205 with P z 0 5 and Sb205[51.It could also account for
the reluctance of the compound to crystallize.
Experimental:
H3As04, prepared from sublimed AS^^^[^^, is completely
dehydrated first in a dessicator and then in high vacuum.
The finely powdered As205 is placed in a steel autoclave
under protective gas (corundum crucible, loosely closed with
a corundum stopper). The autoclave (volume 14m1, sample
volume including corundum crucible 5 ml) is charged with
5 ml of liquid oxygen and heated to 600°C for 4 weeks. After
prolonged cooling down (50"C/h) the sample was isolated,
once again with careful exclusion of moisture. In this way
As205 is obtained as homogeneous glass clear, largely agglomerated crystalline needles; maximum dimensions:
0.2 x 0.05 x 0.03 mm.
Received: February 16, 1977 [ Z 674 IE]
German version: Angew. Chem. 89, 326(1977)
Angew. Chem. I n t . Ed. Engl. 16 ( 1 9 7 7 ) Nu. 5
CAS Registry number:
Asz05, 1303-28-2
7: Bergmun- De Arsenico. Altenburg 1777 (nach Gmelins Handbuch
der anorganischen Chemie, 8th Edit. System-No. 17. Verlag Chemie,
Weinheim 1952).
Cf. J . W Visser, J. Appl. Crystallogr. 2, 89 (1969): Using the method
of I t o - d c Wd/f the values a = 8.645, b = 8.454, c =4.629 were determined
from powder data.
SHEL-X program system (C. Sheldrick, unpublished) modified for C D C
3300. A . Hohenesrer and M . Junsen.
S. Geller, J. Chem. Phys. 33, 676 (1960).
4.M/inkler, E . Thilo, Z. Anorg. Allg. Chem. 339, 71 (1965); A . Winkler,
ibid. 350, 320 (1967).
C . Braurr. Handbuch der Prlparativen Anorganischen Chemie, Bd. 1.
F. Enke Verlag, Stuttgart 1975.
A
Ovtho-para Protonation of Toluene and Ethylbenzene
in Super Acids
By Dan Fiircagiu, M . 7: Melchior, and Lynne Craine[*]
Reports on the protonation of monoalkylbenzenes ( 1 ) in
super acids indicate an overwhelming preference for para protonation to (2)"'. Fast intramolecular exchange of the aromatic hydrogens implied a small equilibrium concentration of
meta and ortho protonated isomers (2)+ (3)+(4)""s21. On
cooling below -97°C only the 'H-NMR spectrum of the
para protonated isomer was "frozen out" in each case['a3b1;
no detectable amount of ortho protonated species ( 4 ) in equilibrium with 2 was evidenced. O n the other hand, a study
of the protonation of alkylbenzenes in the gas phase[31indicates
that a measurable amount of ( 4 ) should be present in the
mixture. We report here a reinvestigation of the protonation
of toluene ( l a ) and ethylbenzene ( l b ) in two super acid
media [acid A: HF-SbF5 (1:l) containing S 0 2 F C l ; acid
B HF-TaF5 (30: I)], by 3C-NMR spectro~copy[~].
Solutions ofethylbenzene (I b ) in both acids at low temperature exhibited the spectrum expected['","] for para protonation
to ( 2 b ) . However, the spectrum in acid A exhibited also
the signals of ortho protonated ethylbenzene ( 4 b ) at the locations to be expected on the basis of spectra of ( 2 b ) and
of protonated m-diethylbenzene (5)r'c1 (Table 1 and Fig. 1).
Characteristic changes of the spectrum were observed on
warming in either acid. The pattern of change, as well as
the fact that very similar chemical shifts were recorded at
the same temperature in both acids of significantly different
strengthF5],unambiguously established the interconversion of
para ( 2 b ) and ortho protonated ( 4 b ) isomers as the source
______
[*] Dr. D. Fsrcaqiu
['I
['I and Dipl.-Chem. L. Craine
Corporate Research Laboratories
Exxon Research and Engineering Company
P. 0. Box 45, Linden, N. J. 07036 (USA)
Dr. M. T. Melchior
Analytical and Information Division
Exxon Research and Engineering Co.
Author to whom all correspondence should be addressed.
315
Table 1. Chemical shifts in the I3C-NMR spectrum of alkylbenzenes in super acids [4]. The carbon atoms are numbered as in the formulas. br.=broad.
Precursor
Solvent[a]
T["C]
C-I
c-2
"C-NMR (6 values in ppm) [b]
c-3
c-4
c-5
Toluene
(1 a )
A
A
A
B
A
- 70
- 10
A
A
B
0
10
75
200.9 [I a]
br.
br
202.4
204.6 [c]
21 5.0 [d]
br.
207.8 br.
204.2 [c]
214 br. [d]
204.5 br.
206.7 br.
207.3
207.6
200.5
136.5
128.9
126.6
126.4
136.0
52.1
126.9
125.9
135.6
178.0
178.0
176.8
177.7
179.4
177.9 [el
178.3
178.4
179.1
46.4
66.6
66.0
67.5
47.3
133.2 [fl
br.
68.0 br.
47.0
135.5 br
126.6 br.
125.4
124.4
133.5 [fl
177.9
178.I
178.3
178.5
207.0
br.
66.6 br.
69.0
70.5
48.5
Ethylbenzene
(!hi
B
B
B
10
0
- 100
~
-
B
m-Diethylbenzene
(5)
A [1 c]
~
60
30
0
10
65
C-6
134.3 [fl
173.9 [el
133.9 [f]
172.8
c-fi
c-3
27.9
28.6
27.2
27.2
36.0
34.4
36.2
36.5
35.6
35.1
35.3
35 2
35.2
35.1
34.4
[a] Acid A: HF/SbF,/ArH, 7/7/1, with 1.5 vol. S 0 2 F C I per vol. of acid; acid B: HF/TaF,/ArH, 30/1/0.33. [b] From external (co-axial) TMS. [c] Values for ( Z b ) .
[d] Values for ( 4 h ) . [el Assignment based on the observation that the ring carbon atoms ortho to the CH2 group resonate generally at lower field than the ring
atoms para to the C H I group. [fl Assignment might be reversed. [g] The spectra at higher temperature were measured from the signal for C-p, assumed of
constant chemical shift
Fig. 1. I3C-NMR spectra of protonated ethylbenzene. A: acid A: B: acid
B. External standard: * CDC12: 0 CH2Cl2; A CD,CN. The numbers on
the signals signify carbon atoms, o and p indicate orrho- ( 4 h ) and para-protonated isomer ( 2 b ) .
of the observed exchange process, which was reversible on
cooling. In the weaker acid B, the peaks are still somewhat
broadened at the lowest temperature which could be attained
( - 75"C)E6],but the signal for C-I in ( 4 b ) could be seen (Table
1). In this acid the peaks for C-2 and C-4 disappear around
-50°C and the coalesced peaks can be observed above -30°C.
Interestingly, the coalescence temperature is about 30°C
higher in acid A['], thus indicating that isomer interconversion
(2b)=$(4b) is faster in the weaker acid. The intramolecular
hydrogen shift ( 2 ) + ( 3 ) $ ( 4 )
should have the same rate
in both cases. O n the other hand, the published 'H-NMR
study of toluene (1 a ) in DF-SbF5-SO2FC1['"1 indicated
that the concurrent proton exchange with the solvent is slow
in that system. It appears then that the mechanism of isomerization ( 2 ) ( 4 ) involves an intermolecular proton transfer
from the protonated to the unprotonated alkylbenzene li.
(1 ) ( 2 ) + ( 4 ) + ( 1
Alternatively, the exchange might
occur by different mechanisms in the two media: intramolecular in acid A and intermolecular in acid B. It must be noted
that within the detection limits of the 13C-NMR method,
+
316
protonation is virtually complete in the HF-TaF5 acid system
(B) as well. This leads to the possibility that the exchange
process observed for a protonated aromatic substrate less
basic than (1 ) (e.g. benzene"", 2e1) might occur intermoleculady even in HF-SbF5.
The equilibrium ratio (2 b ) / ( 4 b ) could be determined by
the integration of the low temperature spectra (ca. 6 % ( 4 b )
in acid A at - 100°C, 14 % ( 4 b ) in acid B at -75"C), and
from the chemical shifts measured for C-2 and C-4 in the
region of fast exchange (0°C to 10°C for acid A['], -30°C
to 10°C for acid B) giving the values of AHo= - 1.3 kcal/mol,
ASo= -2.7 cal K - ' mol-'. The equilibrium constant at O T ,
calculated from the regression line is 3.0 [25 % ( 4 b ) ] . All
the values are approximate, in view of errors in the measurement of chemical shifts and especially in determining and
maintaining constant the temperature during the long intervals
required for spectra accumulation. (Discarding the experimental value obtained at - I O O T results in values AHo= -0.9
kcal/mol, ASo= - 1.0 cal K - mol.) In any event, the AHo
value is very close to that reported for toluene [ ( 2 a)+ f 4 a ) ]
in the gas phasef3]and somewhat lower than the result of
ab initio calculation^^^^.
For protonated toluene the same behavior was seen (Table
I), but the poorer solubility in acid B limited the scope of
investigation. Nevertheless, an estimate of the paru/ortho protonation equilibrium constant ( 2 a ) / ( 4 a ) could be made from
the chemical shifts for C-2 and C-4 at -10 to 10°C[6371,
using chemical shifts for the ortho protonated isomer ( 4 a )
calculated from those of ( 4 b ) and (2 a). The value [20-25 %
( 4 a ) in the mixture at 0°C) is very similar to that found
for ethylbenzene.
'
+
Received: December 6, 1976 [Z 650 IE]
In abbreviated form: January 14. 1977
German version: Angew. Chem. 89, 323 ( 1 977)
CAS Registry numbers:
( l a ) , 108-88-3: (1 b ) , 100-41-4; ( 2 b ) . 36510-22-2; ( 4 b ) , 61650-32-6: (5),
141-93-5
(2.
H. Schlosherg, R. D. Porter, Y K . M o , D. P. K e l l y ,
G. D. MareescLr. J. Am. Chem. SOC.94, 2034 (1972): b) I . B. R r p i m h y a ,
A . I . R e z w k h i i i . V A . K o p t w y , Zh. Org. Khim. S. 1965 (1972); c) G.
A . Olah, R. J . Spear, G. Mrssina, P. W Westerman, J. Am. Chem. Soc.
97, 4051 (1975).
[l] a) G. A . Olah, R.
Anyew. Chem. l n f . Ed. Eflyl 16 (1977) No. 5
[2] a ) C. MurLean, E . L. Mackor, Discuss. Faraday Soc. 34, 165 (1962);
b) E. L. Mackor, C . MacLean, Pure Appl. Chem. 8, 393 (1964); c) D.
M . Brouwer, C . MacLean, E . L . Mackor, Discuss. Faraday Soc. 39,
121 (1965): d) 7: Birchall, R. J . Gillespie, Can. J. Chem. 42, 502 (1964);
e) G. A. Olah, R . H . Schlosberg, D. P. Kelly, G . D. Matrescu, J. Am.
Chem. SOC.92, 2546 (1970).
[3] J. L. Derlin, I l l , J . F . WOK R . W Taf, W J . Hehre, J. Am. Chem.
Soc. 98, 1990 (1976). Additivity of alkyl substituent effects was assumed
in that work.
[4] The NMR spectra were obtained in the Fourier transform mode, at
25.2 MHz. using a Varian XL-100 instrument with a variable temperature
probe.
[ 5 ] Acid A is well over lo' times stronger than acid B.
161 The solution in acid B partially froze at lower temperature.
[7] The peaks are still broad in acid A at + 10°C.
Facile Synthesis of Arabinoadenosine
3'-Phosphate[**I
By Rudolf Mengel and Harald Wiedner"]
Only few examples of oxirane ring opening by phosphate
ions are known in nucleoside chemistry"]. We demonstrated
that adenosine can be transformed into its lyxo-epoxide ( I )r21
and that this product can be opened with mineral acid or
alkali halide in the presence of boron trifluoride-etherate to
give predominantly 3'-substituted halon~cleosides'~~.
We have now examined the cleavage of the oxirane ring
of ( 1 ) with phosphoric acid in hexamethylphosphoric triamide. Fourteen hours' heating to 100°C affords arabinoadenosine 3'-phosphate ( 2 ) which can be precipitated as the barium
salt. On electrophoresis[4a1and chromatography[4b1the salt
behaves in a manner resembling adenosine 5'-monophosphate
and is cleaved by alkaline phosphatase''"] to arabinoadenosine
together with traces of xyloadenosine[4'l. After reprecipitation
of the salt and treatment with phosphatase, xyloadenosine
can no longer be detected. Treatment with 5'-nucleotida~e[~~]
(18 h) has no effect on ( 2 ) , while 3'-nu~leotidase[''~cleaves the
compound to the extent of 90% within 18 h.
The 90 MHz 'H-NMR spectrum of (2) was measured
against sodium 2,2-dimethyl-2-silapentanesulfonate
as internal standard in D,O to which had been added one drop of trifluoroacetic acid (to shift the HOD signal away from
the signals due to the sugar protons). It showed signals at
O=+O
Arabinoadenosine 3'-phosphate (2) could serve as an "arabino" building block in oligonucleotide synthesis and as starting material for the synthesis of arabino-cAMP[61.Compound
(2) also warrants interest in connection with the fact that
arabinoadenosine has an inhibitory action on DNA polymerase, ribonucleotide reductase, adenyl cyclase, various viruses,
and various animal tumors.
Procedure
A solution of adenosine lyxoepoxide ( I ) (100mg, 0.4 mmol)
and phosphoric acid (crystalline, 98 %; 200mg, 1.7mmol) in
hexamethylphosphoric triamide (2 ml) is stirred for 14h at
100°C. After neutralization with conc. ammonia most of the
solvent is removed in uacuo at 100°C. After addition of 10 %
barium acetate solution (6 ml), triethylamine (0.4ml), and ethanol (12ml) a precipitate is formed. The mixture is allowed
to stand for 12h at 4°C and then centrifuged; the residue
is washed with ethanol ( 3 x IOml), and then with water
(7 x 10ml). The combined aqueous extracts are evaporated
to IOml, treated with ethanol (10ml), and left to stand for
10h at 4°C. The precipitate formed is filtered off and applied
to a Sephadex DEAE A 25 column (2.3cm x 35 cm). Elution
is performed with a gradient of triethylammonium hydrogen
carbonate (0.05 to 0.2 M ; 2 liter); 25-ml fractions are collected.
Fractions 33 to 47 contain 0.153 mmol of (2) (yield 38%).
They are concentrated and freeze dried.
Received: February 17, 1977 [Z 678 IE]
German version: Angew. Chem. 89, 328 (1977)
CAS Registry numbers:
( l ) , 40110-98-3: (Z), 54621-41-9
W E . Harue!, J . J . Michalsky, A . R . Todd, J. Chem. Soc. 1951, 2271:
K . Tadashi, A. Mituse, J . Motonobu, N . Tokrrro, Japan. Patent Kokai
74, 18879 (CI. 16E 461) 1974: Chem. Abstr. 80, 121290e (1974).
M. J . Robins, Y Fonron, R. Mengel, J. Org. Chem. 39, 1564 (1974).
R . Mengel, H . Wiedner, Chem. Ber. 109, 1395 (1976).
a) Triethylammonium hydrogen carbonate, pH =7.5, IOOOV: b) o n PEI
cellulose with I N NHaOAc/ethanol ( I : I ) ; c) xyloadenosine and arabinoadenosine can be separated by electrophoresis in borate buffer
(pH = 9) at 1000 V.
a) Boehringer (Mannheim); b) [E.C. 3.1.3.51, Sigma Comp.; c) [E.C.
3.1.3.61, Sigma Comp.
7: A. Khwaja, R . Harris, R . K . Robins, Tetrahedron Lett. 1972, 4681,
and references cited therein.-Cf. the analogous synthesis of 9-p-D-arabinofuranosylcytosine 3',5'-cyclophosphate: R. A . Long, G . L . Szekeres,
T: A . Kliwaja, R . W. Sidwell, L. N . Simon, R . K. Robins, J. Med. Chem.
1 5 , 1215 (1972).
Side Reactions in Peptide Synthesis: tert-Butylation of
Tryptophan
By Erich Wiinsch, Ernst Jaeger, Lajos Kisfaludy, and Miklos
LOW[*]
6=8.6 (s, 1, Hs), 8.46 (s, 1, HZ), 6.5 (d, Jt,-2,=5.2HZ, 1,
HI,),4.86 (m, 1, H3,),4.74 (q, Jz,-3.=4Hz, 1 , H2,),4.29 (quart,
1, H4.), 3.95ppm ("d, 2H, Hs,,, H5.b).The chemical shift
of the H atom attached to C-3' (shifted downfield by 0.5 ppm
relative to adenosine) and the coupling constant between 31P
and 3'-H, show the phosphate group to be located in position
3'.
Indoie and its derivatives are known to be very reactive.
The amino acid tryptophan contains an indole ring as a third
function, thus incorporating a relatively reactive moiety into
the peptide chain. Side reactions due to indole have been
described and postulated in peptide chemistry, e. g . also under
conditions of acidolytic removal of protective groups from
["I
p] Prof. Dr. E. Wunsch, Dr. E. Jaeger
Priv.-Doz. Dr. R. Mengel, H. Wiedner
Fachbereich Chemie der Universitat
Postfach 7733, D-7750 Konstanz (Germany)
[**] This work was supported by the Deutsche Forschungsgerneinschaft
and represents part of the Doctoral Dissertation by H . Wiedner (Universitat
Konstanz, 1976).
Anyew Chem. J a r . Ed. Engl. 16 11977) No. 5
Max-Planck-Institut fur Biochemie, Abteilung fur Peptidchemie
Schillerstrasse 42, 0-8000 Miinchen 2 (Germany)
Dr. L. Kisfaludy, Dr. M. Low
Forschungslaboratorium der Chemischen Werke Gedeon Richter AG
H-1475 Budapest X, Pf. 27 (Hungary)
317
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