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Oxidation of 2-Propanol at TiCr2O3 Anodes.

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methanol results in a slight increase in the conversion of 1.
In this solvent, a maximum quantum yield of Q, = 0.02, depending on the concentration of 2, may be achieved for
the consumption of l b at 313 mm. Tetramethyl-3,4-dihydrodiazete dioxide['] inhibits the addition, which indicates
the involvement of the triplet excited state of l b . A plot of
Q , - ' against [2]-' results in a straight line and thus suggests a bimolecular interaction between electronically excited l b and 2. The rate constant for the quenching of the
triplet excited state of l b by compound 2 was estimated to
be k = 1.8 x 10' L mol-' s-'.['OI
The endo position of the morpholino group was confirmed for 3b by an X-ray structure determination'"l (Fig.
1). Noteworthy are the significantly elongated bond distances between C1 and C10, C4 and C9, and C9 and C10.
By analogy, the same configuration is also assumed to be
valid for 3a and 3c.
In chloroform at 50"C, 3b decomposes with a half-life
of 6 d to give the educts l b and 2.
Although 1-acylnaphthalenes can in principle undergo a
Paterno-Biichi reaction,"*"' the formation of oxetanes with
2 was not observed. This and the finding that l a - c each
form only a single photoadduct with 2, along with the preferred endo orientation of the morpholino group, as established for 3b, render the intermediacy of diradicals 4, 5
(captodative stabilized at one terminus1") unlikely, but do
not rigorously exclude this possibility.
Apparently, in the photocycloaddition described here,
the radical-stabilizing properties ascribed to the captodative terminus in 2"l are less important than the ability of 2
to undergo complex formation. Nevertheless, 2, with a
first ionization potential of 8.68 eV)'3a1is more like an enamine such as cyclopentenylmorpholine (7.60 eV['3b1)than
an a$-unsaturated nitrile (acrylonitrile: 10.91 eV[l3'I). A
(triplet) exciplex, in which the lone electron pair of the
morpholino group or the entire enamine portion of 2 lies
above the n-system of l a - c , is therefore conceivable.
2-Acetonaphthone does not give a photoadduct with 2,
nor are oxetanes formed.['2b1 Instead, a dimer of 2 is
formed in low yield. The captodative-substituted olefin atert-butylthioacrylonitrile also can undergo photoaddition
to l a , b and to 2-naphthaldehyde.""
Received: August 7, 1985;
revised: September 26, 1985 [ Z 1420 IE]
German version: Angew. Chem. 97 (1985) 1159
la, 66-77-3; Ib, 941-98-0; le, 642-29-5: 2, 5807-03-4: 3% 99033-58-6; 3b,
99033-59-7; k,99033-60-0; 2-naphthaldehyde, 66-99-9; a-;err-butylthioacrylonitrile, 72314-64-8.
[I] H. G. Viehe, Z. Janousek, R. Merenyi, L. Stella, Arc. Chem. Res. 18
(1985) 148.
(21 S. C. Temin, J . Org. Chem. 22 (1957) 1714.
[3] D. R. Arnold, L. B. Gillis, E. B. Whipple, Chem. Commun. 1969. 918,
describe the 1,4-photoaddition of methyl cinnamate to 2-acetonaphthone, which is very slow and affords poor yields of product.
[4] N. C. Yang, J. Libman, M. F. Savitzky, J. Am. Chem. SOC.94 (1972)
9226; N. C. Yang, J. Libman, bid. 94 (1972) 9228; R. M. Bowman, J. J.
McCullough, J. Chem. SOC.D 1970, 948; H.-D. Scharf, H. Leisman. W.
Erb, Pure Appl. Chem. 41 (1975) 581.
[5l See, e.g.: J. J. McCullough, W. K. Maclnnis, C. J. L. Lock, R. Faggiani,
J. Am. Chem. SOC.ID4 (1982) 4644 and numerous earlier publications;
C. Pac, K. Mizuno, H. Sakurai, Nippon Kagaku Kaishi 1984, 110 and
earlier publications.
161 W. H. F. Sasse, P. J. Collin, D. B. Roberts, G . Sugowdz, Ausf. 1. Chem.
24 (1971) 2339, 2151; T. Teitei, D. Wells, ibid. 28 (1975) 571; T. R.
Chamberlain, J. J. McCullough, Can. J. Chem. 51 (1973) 2578.
[7] I00 mL, 0.1 M in both 1 and 2, high-pressure Hg lamp TQ 150 Original
Hanau, Duran immersion well (A2280 nm, only 1 undergoes excitation), purged with Ar or N2.
181 3b: IR (KBr): v=2230 (CN), 1705 (C=O). - 80 MHz-'H-NMR (CDCI3,
TMS): AB (&A=1.92, 6.=2.21, I2J1= 12.6 Hz, 10-CHl), ABX [6A=6.94
(2-H), 6 ~ = 6 . 7 6(3-H), 6x=4.46 (4-H), 'JAs=7.8 Hz, 4JAx=1.3 Hz,
'JEx=6.0 Hz], 2.54 (s, COCH,), multiplets at 2.39-2.92 (4H), 3.4-3.7
(4H) and 6.9-7.4 (4H).-UV (CH3CN): A(&)=284 (29). 260 (227), 254
[9] P. Singh, D. G. B. Boocock, E. F. Ullmann, Terrahedron Len. 1971.
3935. ET=35 kcal/mol, see also ref. [9] in: C. M. Lok, M. E. den Boer, J.
Cornelisse, E. Havinga, Tefrahedron 29 (1973) 867.
[lo] We thank Dr. H. Corner, Max-Planck-Institut fur Strahlenchemie, Mulheim a. d. Ruhr, for these measurements.
[ I l l 3b: a=8.5.817(6), b = 19.287(1), c=9.8179(5) A, p = 105.766(3)",
V=I563.87A3, pcalcd=I.31g cm-', 2 = 4 , .u=6.47 cm-': space group
P2,/c, total number of reflections 3056, observed reflections 2161, refined parameters 288, R =0.040,R,=0.044. Further details of the crystal
structure investigation are available on request from the Fachinfomationszentrum Energie, Physik, Mathematik GmbH, D-7514 EggensteinLeopoldshafen 2, by quoting the depository number CSD 51533, the
names of the authors, and the journal citation.
1121 a) R. R. Sauers, A. D. Rousseau, B. Byrne, J . Am. Chem. SOC.97 (1975)
4947; b) N. C. Yang, W. Chiang, ibid. 99 (1977) 3163 and earlier publications.
[13] a) D. Dopp, J. Walter, Heterocycles 20 (1983) 1055; b) L. N. Domelsmith, K. N. Houk, Tefrahedron Lett. 1977, 1981; c) K. N. Houk, J. Sims,
R. E. Duke, Jr., R. W. Strozier, J. K. George, J . Am. Chem. Soc. 95
(1973) 7287; R. Sustmann, R. Trill, Angew. Chem. 84 (1972) 887: Angew.
Chem. Inf. Ed. Engf. 11 (1972) 838.
[I41 All new compounds gave correct elemental analyses. The spectral results
support the assigned structures.
Oxidation of 2-Propanol at Ti/Cr,OJ Anodes**
Fig. 1. Molecular structure of re/-(I S.4S,9S)-I-acetyl-9-morpholino-1,4-dihydro-1,4-ethanonaphthalene-9-carbonitrile3b in the crystal. Important bond
lengths [A]and angles ["I: CI-CIO 1.572(3), C4-C9 1.575(3), C9-CIO 1.567(3),
C1-C2 1.520(3), C2-C3 1.320(3), C3-C4 1.515(3), C4-C4a 1.520(3), C4a-CSa
1.396(3), Cl-C8a 1.531(3); C9-C4-C4a 107.0(1), C9-C4-C3 105.4(2), C3-C4C4a 108.0(2), NI-C9-C4 l13.3(1), NI-C9-ClO 109.5(1), NI-C9-CN 110.9(1),
C4-C9-C10 107.4(1), C4-C9-CN 106.5(1), CIO-C9-CN 109.1(2), Cl-ClO-C9
110.6(1), CIO-CI-C8a 105.1(2), CIO-Cl-C2 106.2(2), ClO-CI-CO 112.5(2),
C8a-CI-C2 106.7(2), C8a-CI-CO 112.6(2), C2-CI-CO lI3.1(2), CI-C8a-C4a
112.8(2), C4C4a-CSa 1 I3.3(2), C4-C3-C2 I14.7(2), CI-C2-C3 114.9(2).
Angew. Chem. Int. Ed. Engl. 24 (1985) No. 12
By Harry Schulz and Fritz Beck*
Chromic acid is a very important and frequently selective oxidizing agent in organic chemistry,"*21but separa[*] Prof. Dr. F. Beck, Dip1.-Chem. H. Schulz
FB6- Elektrochemie der UniversitBt-Gesamthochschule
Lotharstrasse 63, D-4100 Duisburg 1 (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft
0 VCH VerlagsgesellschaftmbH, 0-6940 Weinheim, 1985
0570-0833/85/1212-1049 $ 02.50/0
tion of the Cr"' salts formed in stoichiometric amounts
when it is used, and their electrolytic reoxidati~n,[~I
problems. The mechanism of the homogeneous redox reaction has been elucidated by Westheimer et al.[41(cf. ['I). Our
interest was aroused in the possibility of developing an
electrochemical method analogous to the chromic acidoxidation method. Some 20 years ago, Beer first described
the use of activated titanium electrodes in electrolysis[']
where they now play an important role in inorganic chemistry.
We have now developed a method for preparing Ti/
C r 2 0 3electrodes on the basis of a technique used for the
production of ceramics.['] The dark-green, excellently adhesive, microporous a-Cr203layer, ca. 1 pm thick, can be
stabilized by other oxides. Anodic polarization of the Ti/
C r 2 0 3 electrode in 1 M H2S04, leads to formation of a
layer of C r 0 3 on the surface, which after hydration (rate
constant k,) goes into solution (cf. Scheme 1). The rate
can be regarded as a special case of chemisorption. The
following experimental findings prove that the rate-determining step is that of homogeneous oxidation:
Fig. I. Quasistationary current voltage curves for a Ti/Cr20J electrode 16, 81.
A in 1 M &Sod, B in I M H 2 S 0 4 + 1 M 2-propanol, and C in 1 M H Z S 0 4 + 1 M
[D~]Z-propanol.Voltage scan rate: 5 mVs-'. j=current density. U,=potenlial referred to the Hg/Hg2S04 electrode in IM sulfuric acid.
l ) A comparison of curve B with curve A in Figure 1
shows the characteristic increase in current for the heterogeneous redox c a t a l y ~ i s . [ ~At
. ~higher
overvoltages, a limiting current density j , , , of 78 mA cm - 2 is etablished for the
oxidation of 2-propanol. If the rate constant khomfound
for the homogeneous case1"] is used in Equation (a) then a
j l , , =z%om[CrO~l,
Scheme I . Electrochemical oxidation of 2-prnpanol via heterogeneous redox
catalysis at Ti/Cr20, anodes (schematic). The complex between surfacefixed chromic acid and 2-propanol is shown in the box.
constant of the Cr"'-+CrV' electrochemical oxidation (k,)
increases exponentially with the anodic overvoltage. The
possibility of using the resulting C r 0 3 as an oxidizing
agent for the oxidation of 2-propanal to acetone was tested
(rate constant k ) ; Cr02 was formed, which either directly
reoxidized electrochemically or, as in solution, rapidly underwent further follow-up reaction^[^.^' to give Cr20,.
Turnover numbers of 100-10000 can be realized with catalytic amounts of C r 2 0 3 ("heterogeneous redox catalyS~S"['*~
~ ) . lifetime of the electrode is limited by the dissolution of C r 0 3 and decreases with increasing k,,,, but increases with k.[81
The Cr203,which adheres as a very thin passive layer to
the surface of chromium or chrome steels, has no catalytic
properties. In the anodic polarization of such electrodes in
sulfuric acid solutions of 2-propanol chromium dissolves
transpassively with 6 F/rnol."']
The initial reaction in the homogeneous oxidation of
sterically unhindered alcohols with chromic acid is a rapid
esterification; the rate-determining step is then a hydrogen
shift, in which significant H/D isotope effects O C C U ~ . [ ~' '~I , ~ .
Upon oxidation at Ti/Cr203 anodes, the esterification
takes place on the electrode surface (cf. Scheme 1); this
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value of only ca. 0.1 mA c m p 2 is obtained ( 2 ~ 2 CrV''Iv,
[CrO,], = surface concentration of CrO,['I). The discrepancy cannot be explained in terms of a roughness factor
alone, and, furthermore, the medium effect-k,,,
was determined in glacial acetic acid["]-only lowers the limiting
current to ca. 20 mA cm-2. The activation entropy plays
the decisive role:"31 Whereas AS in the homogeneous reaction is approximately -24 cal K - ' mol-',['4] AS at the
electrode must be larger. We obtain the observed limiting
current density of the reaction jllm
using a value of - 13.4
cal K - ' mol-' for AS.
2) As shown by the curve C in Figure 1, a strong isotope
effect is found with [D8]2-propanol.j,,,, and thus also k,18]
decreases by a factor of 2.9. Primary isotope effects of 6.57 were found for the homogeneous o ~ i d a t i o n . ] ~ '.~ '*]
. ~ I, so'
tope effects were also established in the reaction at the alkaline nickel oxide anode.[I5]
3) We have converted 2-propanol into acetone on a preparative scale with 100% current efficiency at j = 0 . 5 mA
cm-*. This high selectivity is also obtained upon oxidation
with chromic acid. In complete contrast, a current yield of
only 70% is achieved on platinum.["] The by-products of
the oxidation include, inter alia, acetic acid.
4) When oxidation at the anode is carried out in the
presence of 0.43 M MnS04, the limiting current density is
reduced to 25% of the normal value. In the homogeneous
reaction, a marked decrease in the conversion was established upon trapping the Cr'" products formed in the first
+ Mn"
+ Mn"'
oxidation step [Equation (b)], so that CrIV cannot be used
for the oxidation.["] The decrease injl,,,, could also be attributed, in part, to a partial coating of the electrode with
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Angew. Chem. Inr. Ed. Engl. 24 (1985) No. I2
5) Addition of acrylamide to the electrolyte (0.5 M) leads
to a decrease o f j l l mto 40% of the original value. The formation of polyacrylamide in the oxidation with chromic
acid has been regarded as proof for the occurrence of radical
In the oxidation at the Ti/Cr20,
anode, however, the possibility that layers of polymers on
the electrodes leads to a decrease in current cannot be
ruled out.
Summarizing, it has been established that 2-propanol is
oxidized to acetone at a Ti/Cr,03 anode via a mechanism
analogous to that postulated by Westheimer for homogeneous oxidation with chromic acid. The lifetimes of the electrodes are still too short for use on an industrial scale, but
we have already been able to achieve substantial improvements by using Sb20, as additional oxide component.[*]
Received: July 4, 1985:
revised: August 28, 1985 [ Z 1376 IE]
German version: Angew. Chem. 97 (1985) 1047
9 10
CH, -5-CH
Fig. 1. Sequence of the tetracyclic heterodetic 22-peptide antibiotic epidermin. Trypsin cleaves behind Lys" into the fragments PI and P2 (cf. Fig. 2).
All chiral amino acids without indicated configurations have the L-configuration, i.e. they belong to the (S)-series (exceptions are sulfur-containing amino acids such as cysteine: L-CYSis (R)-configurated).
[ I ] K. B. Wiberg: Oxidation in Organic Chemistry, Part A , Academic Press,
New York 1965.
[2] R. L. Augustine (Ed.): Oxidation. Vol. I, 2 , Marcel Dekker, New York
1969 and 1971.
[ 3 ] M. Kappel, Chem. Ing. Techn. 35 (1963) 386.
[4] a) F. H. Westheimer, A. Novick, J. Chem. Phys. I1 (1943) 506; b) F. H.
Westheimer, N. Nicolaides, J . Am. Chem. Soc. 71 (1949) 25; c) F. H.
Westheimer, Chem. Rev. 45 (1949) 419.
[5] P. Miiller, Chimia 31 (1977) 209.
[6] H. B. Beer, DDR-Pat. 55223 (Priority: 12. 5. 1965), Belg. Pat. 710551
[7] F. Beck, H. Schulz, Ber. Bunsenges. Phys. Chem. 88 (1984) 155.
[S] F. Beck, H. Schulz, Electrochim. Acta 29 (1984) 1569.
191 C. P. Andrieux, J. M. Dumas-Bouchiat, J. M. Saveant, J. Electroanal.
Chem. 123 (1981) 171.
[ 101 F. Beck, H. Schulz, B. Jansen, Electrochim. Acta. in press.
[ I l l K. B. Wiberg, Chem. Rev. 55 (1955) 713.
1121 K. 8. Wiherg, H. Schafer, 1.Am. Chem. SOC.91 (1969) 933.
[I31 We thank Prof. J. Koryta for a helpful remark made at the HeyrovskyDiscussion-Meeting in Liblice. CSSR, in May 1985.
[I41 P. Miiller, J.-C. Perlberger, Helu. Chim. Acta 57(1974) 1943.
[IS] M. Fleischmann, K. Korinek, D. Pletcher, J. Chem. SOC.Perkin Trans. 2
1972. 1396.
[I61 K. Elbs, 0. Brunner, 2. Elektrochem. 6 (1900) 609.
[I71 W. Watanabe, F. H. Westheimer, J. Chem. Phys. 17 (1949) 61.
[IS] M. Rahman, Jan RoEek, J . Am. Chem. Soc. 93 (1971) 5462.
The acidic total hydrolyzate of epidermin contained
thirteen protein amino acids, two lanthionines, and one 3methyllanthionine. The meso- and (2S,3S,6R)-configurations of lanthionine and 3-methyllanthionine, respectively,
were determined by gas chromatography on chiral stationary phases."] In the nuclear resonance spectra of the antibiotic, two unsaturated amino acids could be detected;
these were completely destroyed upon acidic total hydrolysis (Table 1).
Elucidation of the Structure of Epidermin, a
Ribosomally Synthesized,
Tetracyclic Heterodetic Polypeptide Antibiotic
Starting point of the sequencing of the complex basic
polypeptide was the tryptic cleavage into an N-terminal
fragment P1 and a C-terminal fragment P2. These fragments could be separated from each other, since P2 precipitated upon enzymatic cleavage of epidermin. C-terminal lysine could be cleaved from the tryptic fragment P1 by
reaction with carboxypeptidase B (Fig. 2). A subsequent
desulfurization with Raney nickel['] afforded a bridge-free
dodecapeptide 1-12, which was sequenced by FAB-MS
and Edman degradation in a gas-phase sequencer. A further fragmentation of the dodecapeptide into the peptides
1-4 and 5-12 was achieved by further tryptic cleavage. The
positions of the sulfur bridges in P1 could be determined
by reactions with carboxypeptidase A and B, trypsin, and
endoproteinase Lys-C, in combination with Edman degradation and FAB-MS (Fig. 2).
The tryptic fragment P2, which is insoluble in water, was
N-terminally blocked with 2-oxobutyric acid and C-terminally blocked by the &-amino group of the amino acid S(2-aminovinyl)-~-cysteine.Tryptic cleavage furnished the
2-oxobutyryl residue, in which the dehydroaminobutyric
acid (Dhb) after LysI3 was converted.[31S-(2-Aminovinyl)D-CySteine (Cys(Avi)), which is likewise labile under the
conditions of acidic total hydrolysis, could be converted by
By Hermann Allgaier, Giinther Jung,* Rolf G. Werner,
Ursula Schneider, and Hans Zahner
From the culture filtrate of Staphylococcus epidermidis
Tii 3298, we have isolated a novel active compound which
is highly effective, especially against the pathogen Propzonibacterium acnes occurring in acne disease and against staphylococci and streptococci (Fig. 1). After adsorption o n
Amberlite XAD-8 and chromatography on Sephadex LH20 the antibiotic could be isolated in pure form via two
purification steps, by multiplicative countercurrent distribution.
[*I Prof. Dr. G. Jung, Dr. H. Allgaier
lnstitut fur Organische Chemie der Universitat
Auf der Morgenstelle 18, D-7400 Tiibingen (FRG)
Priv.-Doz. Dr. R. G. Werner
Abteilung Biotechnische Verfahren der Dr. Karl Thomae GmbH
D-7950 Biberach (FRG)
Prof. Dr. H. Zahner, Dr. U. Schneider
Institut fur Biologie, Mikrobiologie I der Universitat
Auf der Morgenstelle 28, D-7400 Tuhingen (FRG)
Angew. Chrm. I n t . Ed. Engl. 24 (1985) No. 12
Table I . Characterization of epidermin. Dhb =dehydroaminobutyric acid (2amino-2,3-didehydrobutyricacid).
Amino acid composition: L-Asn (I), L-Pro ( I ) , Gly (2), L-Ala ( 2 ) , ~ - I l e(2),
L-Tyr (I), L-Phe (2), L-LYS(2), Dhb ( I ) , meso-lanthionine (2), (2S,3S,6R)-3methyllanthionine (I), S-(2-aminovinyl)-~-cysteine
Thin-layer chromatography (silica gel 60 FZ54plates, Merck): chloroform/
methanol/ 17% ammonia (2 : 2 : I), RF= 0.73: I-butanol/glacial acetic acid/
water (4: 1 : I), RF=0.05
Mass spectra: Fragment PI [ M + H ] + : m/z 1302 (FAB-MS), fragment P2
[ M s N a ] ' : m/z904(FD-MS)
UV spectrum (c=O.15 mg/mL, water, pH =7): 1,,,=267 nm, E,,, = I I000
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oxidation, anodes, ticr2o3, propanol
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