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Intermolecular Ene Reactions in a High-PressureHigh-Temperature Flow Apparatus.

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The esters formed from the unsymmetrical ethers and the
reaction times and temperatures indicate the following reactivity sequence for the individual CH-bonds: benzyl > phenylalkyl > primary-alkyl > secondary-alkyl, methyl > phenyl.
In phenyl ethers the phenyl group deactivates the CH-group
of the alkyl residue.
Received: November 6, 1978 [Z I23 b [El
German version. Angew. Chem. 91. 78 (1Y79)
[l] L. M . Brrkowitz, P . N . Rylander, J. Am. Chem. SOC.80, 6682 (1958);
M. E. WoljJ, J . K . Kerwien, F. F. Ow,ings, B. B. Lewis, 8. Blank, 1.
Org. Chem. 28, 2729 (1963); D. G. Lee, M . c'an den Engh in W S.
Truhunousk): Oxidation in Organic Chemistry. Academic Press, New
York 1973, Part B, p. 222.
[2] I . 7: Harrison, S. Hurrison, J. Chem. Soc. Chem. Commun. 1966. 752;
W Huckel, H . Bretschneidrr, J . Prakt. Chem 11, 61 (1938); H. B. Henbesr,
B. Nicholls, J . Chem. SOC.1959, 227.
131 E . C . Juenge, M . D. Core), D. A. B e d , Tetrahedron 27, 2671 (1971);
E . C . Juetige. D . A. B e d , Tetrahedron Lett. 1968, 5819.
[4] N. C. Deno, N . H . Potter. J. Am. Chem. SOC.89, 3550 (1967).
[5] G. A . Oluh, J . Welch, 7: L. Ho, J . Am. Chem. Soc. 98, 6717 (1976).
[6] !-!F r a n z m , R. Edens, Justus Liebigs Ann. Chem. 735, 47 (1970).
[7] K. A. Pollart, R. E. Mdler., J. Org. Chem. 27, 2392 (1962).
[8] ?: Shono, Y Matsumura, J . Am. Chem SOC.91, 2803 (1969).
Intermolecular Ene Reactions in a High-Pressure/HighTemperature Flow A p p a r a t u s [ * * ]
By Jiirgen Metzger and Peter KOll[*]
In the thermal degradation of chitin in supercritical acetone['I we were able to demonstrate the presence of large
amounts ofdiacetamide (1 1,which can be formed by dimerization of the acetamide molecules in the sense of an ene reaction.
This finding encouraged us to investigate some ene reactions['], as example of the numerous types of thermal pericyclic
reactions[31, in the same high-pressure/high-temperature
apparatus ("HP-HT" apparatus)['], which enables relatively
unproblematical operation at pressures of up to cu. 500bar
and temperatures of up to ca. 700K, and is readily built
from commercially available HPLC and G C equipment. The
apparatus offers all the advantages of a flow reactor, such
as exceptionally rapid withdrawal of sensitive products from
the reaction zone and the possibility of quickly changing
the reaction parameters with slightest consumption of substances.
Whereas numerous intramolecular ene reactions, even with
nonactivated enophiles, have already been reported[41,intermolecular ene reactions generally require active enophiles[21. It
was predicted, however, that if the latter reactions were to
be carried out at high pressures, besides the usual conditions
employed for ene reactions, namely high temperatures and
long reaction times, then intermolecular ene reactions with
nonactivated enophiles should also be possible. Thus, the reactions carried out at high temperatures under pressure by
Nemtsov et al.''' in 1938 with the aim of polymerizing olefins
[*] Prof. Dr. P. KOII, Dr. J . Metzger
Fachbereich 4 (Naturwissenschaften) der Universitat
Ammerlinder Heerstr. 67--99, D-2900 Oldenburg (Germany)
[**I Part 2 of High-Pressure/High-Temperature Reactions in a Flow Reactor.-Part 1 : [t].
70
may be regarded as ene reactions. The reaction of acetylene
as moderately active enophile with olefins at 623 K and 170
bar to give 1,4-dienes-likewise in a flow reactor--also
demonstrates the influence of pressure in such reactions''!
In all the examples so far investigated by us (cf. Table 1)
the expected products were seen to be formed in the overall
main reaction. Even toluene was found to react as "deactivated" ene component with non-activated enophiles; reaction
with cyclohexene afforded o-cyclohexyltoluene (2).
The yields quoted in Table 1 are not optimized and might
easily be increased to preparatively satisfactory values by
increasing the residence time of the reactants (incorporation
Table 1 Some intermolecular ene reactions with non-activdted enophiles
Reaction conditions 673 K, 450 bar, 10 min residence time, ene enophile
=10 1, conversion based on enophile Characterization of the products
by GC-MS
Ene
Enophile
Reaction product(s)
ConverSlOll[
UO]
~
I-Hexene
Cyclohexene
Acetone
Toluene
Toluene
1 -Hexene
Cyclohexene
Cyclohexene
Cyclohexene
Tolan
Mixture of isomeric dodecenes
3-Cyclohexylcyclohexene
Cyclohexyldcetone
o-Cyclohexyltoluene
1,2-Diphenyl-l-tolylethylene
5
45
05
12
15
of a longer reaction capillary or reduction in the rate of flow
of reactants). A typical example here is the synthesis of 3-cyclohexylcyclohexene from cyclohexene (see Experimental). Such
a dimerization apparently proceeds regiospecifically, corresponding to the expected pericyclic mechanism. (With longer
residence times, however, about 5 % of the primary product
isomerizes to 1-cyclohexylcyclohexene.) Thus, the thermal olefin-dimerization and the alkylation of toluene described here
are superior to the catalytic methods, at least as far as regioselectivity is concerned.
Experimental
Synthesis of 3-cyclohexylcyclohexene: Cyclohexene (20 g)
is pumped through the "HP-PT" apparatus['] at 673 K and
460bar at a flow-rate of 0,12ml/min and with a residence
time of ca. 50min. A 15-m long stainless steel capillary (0.D.
1.6mm, I.D. 0.7mm) can be used as reactor. Unreacted cyclohexene (16g) is removed by distillation. The residue (3.4g,
17 %) mainly consists of pure 3-cyclohexylcyclohexene,which
is further purified by distillation: b.p. 98"C/10 torr,
n ~ o = l . 4 9 1 2(b.p. 224"C/760 torr, nio= 1.4941['1). The IR,
NMR, and mass spectra are consistent with the given structure.
Received: September 29, 1978 [ Z 13O;t IE]
German version: Angew. Chem. 91.74 (1979)
[l] P. K611, J . M e t z g e r , Angew. Chem. YO, 802 (1978); Angew. Chem Int.
Ed. Engl. 17, 754 ( I 978).
[Z] H. M. R. Hofjmann, Angew. Chem. 81, 597 (1969); Angew. Chem. Int.
Ed. Engl. 8, 566 (1969).
[3] J. B. Hendrickton, Angew. Chem. 86, 71 (1974); Angew. Chem. Int.
Ed. Engl. 13, 47 ( I 974).
Aiigew Chem. I n t . Ed. Engl. 18 11979) N O . 1
cal groundsr6].The formation of the major products can best
be described by the general scheme:
[4] W Oppolzer, V Snwckus, Angew. Chem. 90, 506 (1978);Angew. Chem.
1111. Ed. Engl. 17, 476 (1978).
[ 5 ] M . S. Nemisoa. T V Nizookina, E. A . Soskiria, J. Gen. Chem. (USSR)
8 , 1303, 1324 (1938): Chem. Abstr. 33, 4206 (1939).
161 N . F. C w i n s k i , J . Org. Chem. 30, 361 (1965).
[7] W H i i c k d . R . Bross, 0. Fechrig ei al., Justus Liebigs Ann. Chem. 624,
142 (1959).
Thermal Pericyclic Reaction between Alkynes and
Alkanes"'
As side reaction an alkyl group obviously can migrate (with
cleavage and reformation of a CC o-bond) instead of a
hydrogen atom. This would explain the formation of octene,
nonene, and decene in the reaction of n-hexane with 1 -hexyne,
as well as the fact that undecene is not observed:
By Jiirgen Metzger and Peter KOllC']
From results of intermolecular ene reactions[21 in a highpressure/high-temperature flow reactor ("HP-HT" apparatus)
it was predicted that hitherto presupposed but unobserved
thermal pericyclic reactions ought to be realizable at high
pressures and temperatures with rapid removal of reaction
products. We first examined the reaction of I-hexyne in n-hexane (1 : 10) under such conditions, expecting to observe not
only the trimerization of the alkyne to benzene derivatives
but also a dimerization in the sense of an ene reaction, which
should lead to allene derivatives. A hexyne dimer was
indeed detectable in small amounts by combined GC-MS.
Surprisingly, however, we obtained alkenes as major products
(0.9 y) octene, 0.9 % nonene, 1 % decene, 6 % dodecene, 2.6 %
tributylbenzene; in each case isomeric mixtures; yields based
on starting 1-hexyne). These alkenes could only have been
formed by reaction of 1-hexyne with n-hexane.
To check the possible generalization of this surprising observation we examined the reaction of cycloalkanes with alkynes
in the same HP-HT reactob3]. As shown in Table 1, in
each of the cases investigated the alkenylcycloalkanes are
formed as major products in a remarkably uniform reaction.
@/
/
- .+
4
H
The new pericyclic reaction might be of interest both for
the functionalization (vinylation) and for the fragmentation
(thermal non-radical cracking) of alkanes to alkenes.
General Procedure
The alkynes are dissolved in an excess of alkane: in the
case of gaseous, alkynes, the alkane is saturated in an autoclave
with the alkyne. The solution is pumped through the HP-PT
apparatus13? As reactor for the present series of experiments
we used a stainless steel capillary (length 12m, 0.D. 1.6mm,
I. D. 0.7mm). The following reaction conditions proved to
be suitable: T=623--673 K, p=350--500bar, flow rate 13 ml/min.
Table I . Reactions of alkanes and alkynes (reaction conditions: T=673K, in the case of Nos. 1 and 2, 6 3 3 K ; p=400bar). The products were identified by
GC-MS and determined gas chromatographically using an internal standard. The major products were isolated by distillation (Nos. 2, 4, 5 ) or by preparative
thin-layer chromatography (No. 3) and investigated by N M R and IR-spectroscopy.
-_
No.
Alkane
Alkyne
1
rz-Hexane
I-Hexyne
2
Cyclohexane
1-Hexyne
3
Cyclohexane [b]
Tolan
Alkane Alkyne
Residence
time [min]
Main product(s)
10: I
4
Dodecene isomers
6
20.1
2
rr0n.q-l -Cyclohexyl-1-hexene
cb-l -Cyclohexyl- 1-hexene
2-Cyclohexyl-l -hexene
7.5
2.6
0.8
100: 1
4.5
E- and Z-l-Cyclohexyl-I ,2-diphenylethylene
7
'
Yield
[XI [a]
20.0
4
Cyclohexane
Acetylene
0.5bar [c]
-
Vinylcyclohexane
0.2
5
Methylcyclopentane [d]
Acetylene
1.0bar [c]
4
Methyl(viny1)cyclopentane (isomeric mixture)
1.5
[a] Yields based on alkyne; in the case of Nos. 4 and 5 on alkane. [b] Same result on addition of 1 '% 2,6-d1-tert-butyl-4-methylphenol
(based on cyclohexane). [c] Saturated with acetylene a t the given excess pressure. [d] Same result o n addition of 1 % hydroquinone (based on methylcyclopentane).
Owing to the extremely drastic reaction conditions free
radicals were assumed to be involved in the reaction, particularly since the formation of vinylcyclohexane is known to
take place in the presence of radical-forming species'4, '1. Addition of radical scavengers, however, did not influence the
yield of main product. In our opinion, this finding rules out
a radical chain reaction.
The reaction is apparently another novel type of thermal
pericyclic reaction, which also ought to be possible on theoreti~__
[*] Prof. Dr.P. Ko11, Dr.J. Metzger
Fachbereich 4 (Naturwissenschaften) der Universitat
Ammerlander Heerstr. 67-99, D-2900 Oldenbnrg (Germany)
Angrw Chem. 1nt Ed. Engl. 18 ( 1 9 7 9 ) No. 1
Received: November 16, 1978 [ Z 130b IE]
German version: Angew. Chem. 91, 75 (1979)
[I] Part 4 of High-Pressnre/High-Temperature Reactions in a Flow Apparatus.-Part 3:P. Kd1, E.Steinweg, U . Lackmann, J . M e t z g e r , Tetrahedron
Lett., in press.
121 J . M e t z g e r , P. KO/!, Angew. Chem. 91, 74 (1979);Angew. Chem. Int.
Ed. Engl. 18, 70 (1979).
[3] P . Kdll, J . Meizyrr, Angew. Chem. 90, 802 (1978);Angew. Chem. Int.
Ed. Engl. 17, 754 (1978).
141 N. I. Shuikin, B. L. Lebedeu, V G . Nikol'ski, Neftekhimiya 6, 544 (1966).
[S] R. Srinivasan, K.H . Curlough, Can. J . Chem. 45, 3209 (1967).
161 J . B. Hendrickson, Angew. Chem. 86, 71 (1974); Angew. Chem. Ink.
Ed. Engl. 13, 47 (1974).
71
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