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Flash Thermolysis of 1 2 3-Thiadiazoles A Simple Route to Thioketenes.

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D . Wendisch in Houben-Weyl-Muller: Methodender organischen Chemie.
Thieme, Stuttgart 1971, VOI. 4/3, P. 5 7 5 8 ; M . Charron in J . Zabicky:
The Chemistry of Alkenes. Interscience, New York 1970, Vol. 2, p.
512: R. J. O u k r e in W S. Trahanoskr: Oxidation in OreanicChemistrv.
Academic Press, New York 1973, Part 8 , p. 158.
H. Schaferand E. Steckhan, Angew. Chem. 81,532 (1969): Angew. Chem.
internat. Edit. 8, 518 (1969); H.Schafer, Chem.-1ng.-Tech. 42, 164 (1970);
B. BeUeau and K. Au-Young, Can J. Chem. 47, 2117 (1969); T. Shono
and A . Ikeda, J . Amer. Chem. SOC.94,7892 (1972); 7: Shono, Z Matsumura,
and L Nakagawa, J. Org. Chem. 96, 3532 (1974): L. Eberson and H.
Schifer, Fortschr. Chem. Forsch. 21, I (1971) especially pp. 85, 108;
N. L. Weinberg and H. R. Weinberg, Chem. Rev. 68, 449 (1968), especially
p. 469; D. Koch, H. Schafer, and E. Steckhan, Chem. Ber. 107, 3640
(1974); J. M. Frirsch, H. Weingarten, and J. D. Wilson, J. Amer. Chem.
SOC.92, 4038 (1970); E. Steckhan and H. Schhfer, Angew. Chem. 86,
480 (1974): Angew. Chem. internat. Edit. 13, 472 (1974).
'
I
Shono, Z Matsumurn, and Y. Nakagawa, J. Org. Chem. 36, 1771
( I 971).
leneC4l. However, he was able to isolate only tetraphenylthiophene and a compound which was later identified as a 1,3-dithiol derivative[51.
L
Flash Thermolysis of 1,2,3-Thiadiazoles:
A Simple Route to Thioketenes["]
2
12
Fig. I . Apparatus for flash thermolysis. 1 : ground joint; 2: t o pump: 3:
cold finger; 4 : coolable tap; 5: receiver; 6: heated quartz tube; 7: cooling
water;S:tomanometer;9: tap;lO:reservoir;ll:currentsupply;12:thermostat.
By Giinther Seybold and Christian Heibl[']
Thioketenes (2) were until now either difficultly accessible or
only detectable indirectly as intermediated2'. It is not surprising
therefore that this class of compounds has been little studied
and their synthetic potential rarely utilized.
We have now found that the flash thermolysis of 1,2,3-thiadiazoles ( 1 ) at 580°C/10-4 torr constitutes a simple, convenient, and generally applicable method for the synthesis
of thioketenes in good yields-simple, not least because the
starting compounds are readily accessible from aldehydes and
ketones[3!
The flash thermolysis was carried out with the apparatus
illustrated in Fig.
The thiadiazole ( I ) to be pyrolyzed
is placed in the reservoir 10 whose temperature is regulated
by a thermostat such that a slow but constant stream of
substance into the electrically heated quartz tube 6 takes
place. Immediately after leaving the hot zone-the residence
time is of the order of
s-the pyrolyzate is chilled
to - 196°C on the cold finger 3. During the pyrolysis a lowboiling solvent (e.g. CH2C12 or CFC13) is distilled into the
vessel oia tap 9. In this way dimerizations and oligomerizations
are largely avoided when the cold finger is thawed off and
dilute thioketene solutions are formed which drip through
the coolable tap 4 into the receiver 5. The thioketene solutions
obtained in this way are 0 . 1 4 . 4 molar and d o not contain
any by-products ;they can be used directly for further reactions
or spectroscopic investigations.
The yield of thioketene reached its maximum between 500
and 580°C (temperature of the quartz tube 6). Above 600°C
decomposition into nitrogen, sulfur, and acetylene derivatives
occurred as side reaction. The products found by Staudinger
could not be detected. This is understandable since the flash
thermolysis was carried out at
torr, so subsequent second
The idea to prepare thioketenes from thiadiazoles is not new.
As long ago as 1916 Staudinger made attempts to obtain
diphenylthioketenes by thermolysis of ( 1 ) in boiling naphtha-
Table I . Yields and physical properties of some thioketenes (2).
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cpd.
Yield
[a1
["<,I
IR [cm-'I
[bl
'H-NMR
6
A,,
627
2.31
in CHzCll
624
2.51
in CHZCI2
[nm] Ig
E
Halflife [c]
~_~___~__________________________~___
(20)
70
1725
-
(26)
73
I725
-
(2dl
65-75
I758
I . I7 (s. 9 H )
4.35 (s, I H )
in CFC13
575
1.00
in CFCln
in CH2C12at
40°C: 6. I0 h
in CHzCll at
30°C: 6.13 h
40°C: 3.80 h
in CFCIJ at
22°C: 0.98h
[a] Determined by trapping with dimethylamine. The structures of the thioamides were confirmed by elemental
analysis, 'H-NMR and IR spectra, and by comparison with authentic samples.
[b] Recorded a t -50°C in a low-temperature cuvette in CH,CI,.
[c] Calculated for 0.01 M solutions [9].
[*] Dr. G. Seybold and Dipl.-Chem. Ch. Heibl
lnstitut fur Organische Chemie der Universitat
8 Munchen 2, Karlstrasse 23 (Germaqy)
[**I .Flash Thermolysis of Organic Compounds, Part 3.-Part
248
2: [I].
order reactions of the particles initially formed can be ruled
out.
As a rule, we worked at the gram level; however, if the
cold finger is made rotatable by means of a ground joint
Anyeir.. Chcm. iiircrnot. E d r l .
i
Vol. 14 ( 1 9 7 5 )
I No. 4
1,amountsofupto IOgcan be reacted becauseof the increased
cooling surface.
The yields and physical properties of a few thioketenes
( 2 ) which we have prepared by this method are listed in
Table I .
The structure of the reaction products ( 1 ) follows unequivocally from the IR bands between 1 700 and 1 800 cm- I [ I . 2.
which are characteristic for thioketenes, from NMR, UV and
mass spectra, and from chemical behavior. Reaction of (2a) to
( 2 r ) with dimethylamine at--80"C leads to immediate and
quantitative formation of the thioamide (3). Since neither
starting compounds nor secondary products of thioketenes
react with dimethylamine in this way, this reaction is suitable
for the quantitative determination of thioketene in solution.
Thioketenes (Za) and ( 2 b ) react with alcohols at room temperature to give the 0-alkylthiocarboxylic esters ( 4 ) .
Reductive Fixation of cis- and rrclrts-l,4-Diazabutadienes[ '
By Heindirk tom Dieck and Klaus-Dieter Franz"]
Like butadiene itself in the free state, simple 1,4-dialkyl-1,4diazabutadienes such as ( I ) exist in the ( E ) conformation
( l a ) . No interaction between lone pairs (as expected for
( I b ) ) is observed in the photoelectron spectra, and the IR
and Raman spectra indicate the presence of a center of inversion[? ( I a ) is readily reduced to the paramagnetic anion
( I a - ) by potassium in dimethoxyethane (DME)I3].The anion
( l a - ) displays a highly resolved ESR spectrum in which,
at line widths below 0.1 G, not only the splitting by the
two H and N atoms of the glyoxal bis(diimine) system (15
lines) but also the hyperfine structure of the 18 equivalent
rert-butyl protons is visible (aN 5.62, aH 4.37, U H (tBu) 0.15
G)[4.51.
S
I1
[2a)-[2e) + I - I L V ( C H ~--+
) ~ R1R2CH-C-K(CH3),
(3al-(3e)
( 2 0 ) . (26)
+
s
HOC2H5 -+ R'R'CH-C-OCZH~
J.
(4ai f4b1
I
I
On warming (30"C), the diarylthioketenes ( 2 a ) and ( 2 b )
dimerize to bismethylenedithietanes ( 5 ) , which have already
been described by Sch6nberg' 'I.
In contrast, the thioketenes (2c)-(2e)
oligomerize at room
temperature to complex mixtures. (2 f) already begins to polymerize at - 120°C and could therefore only be detected indirectly.
Thedimerization or oligomerization ofthethioketenes ( 2 a ) ,
( 2 b ) and ( 2 d ) proceeds strictly according to a second order
rate law. The halflives (Table 1) and 13C-NMR spectra["]
of the thioketenes show that both electronic as well as steric
effects are responsible for their different kinetic stabilities.
Received: November 26.1974 [ Z 142 IE]
German rersion: Angew. Chem. 87. 171 (1972)
-~
[I]
[Z]
[3]
[4]
[S]
[6]
[7]
[XI
[9]
[ 101
[ I I]
G . Sri.ho1d. Tetrahedron Lett. 1974. 555.
For literature survey see: M. S. Rarrsdt. J. Org. Chem. 35. 3470 (1970):
37, I347 11972).
C. D. Hiird and R . 1. Mori, J. Amer. Chem. SOC. 77. 5359 (19.55):
L. Wo1ff: Liebigs Ann. Chem. 333. I (1904).
H . Simrdiiigrr and J. Sicywurr, Ber. Deut. Chem. Ges. 4 9 . 1918 11916).
K . P. Zi,//cv, H . M r i r r . and E . Miilln. Liebigs Ann. Chem. 766. 32
(1972).
G . S d d r 1 and C. J r r c ~ i h .unpublished.
A . Krrrizrr and J . Liriiroiti, J. Amer. Chem. SOC.Y6. 6768 (1974).
E. S~~/~IIII~~~II~I~I
and W Wi11rr.Chem. Ber. 107. 3562 (1974).
We thank Dr.. H. C . W~igiwrfor the spectroscopic investigations.
S. S(1riiithe.r.y. W Kittjfi1, E. F r e s r . and K . Prorfcke. Chem. Bcr. 103.
1705 ( 1969).
J . Fir/. Ch. H c M and G. Sri~bolrl,unpublished.
The neutral ( Z ) conformer ( 1 b ) is not present in measui..rle
concentration in solution; however, the conformational transition ( I a)=( I b ) occurs readily, as can be demonstrated by
rapid formation of numerous chelate complexes (2jr6].
Some of the complexes (2) are sensitive to reduction (u.g.
M(XY) = copper(1) halide"] or K-allylbromodicarbonylmolybdenurn[''). In the course of cleavage by potassium or solvated
electrons in DME the ( Z )conformer of the ligand is reduced;
the cis anion radical ( I b - ) corresponding to ( l a - ) , which
I S probably formed initially, rapidly complexes potassium ions
to form the conformationally stable neutral potassium complex
(3).
With the reduced compounds of type ( I ) the ( Z )or the
( E ) conformation can thus be fixed at will. No transition
( I a - ) = ( 3 ) is observed in the ESR spectrum at temperatures
up to +30"C; this is reasonable since reduction involves
occupation of a molecular orbital with bonding interactions
between the centers 2 and 3.
i;!
+
II
The potassium complex (3) displays an ESR spectrum which
can be "built up" by superposition of four spectra of the
[*] Prof. Dr. H. tom Dieck and Dip1.-Chem. K.-D. Frdnz
lnstitut fur Anorganische Chemie der Universitat
6 Frankfurt am Main 70. Theodor-Stern-Kai 7 (Germany)
249
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