High Pressure Food Science, Bioscience and Chemistry

495 Pages·1998·54.431 MB·English

by  N.S. Isaacs (Auth.)

High Pressure Food Science,
Bioscience and Chemistry
Edited by
Neil S. Isaacs
University of Reading, UK
SOCIETY OF
CHEMISTRY
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The proceedings of the 35th joint meeting of the European High Pressure Research Group and Food
Chemistry Group of The Royal Society of Chemistry on High Pressure Food Science, Bioscience and
Chemistry held at the University of Reading Reading on 7-1 1 September 1997.
Special Publication No. 222
ISBN 0-85404-728-X
A catalogue record for this book is available from the British Library
8 The Royal Society of Chemistry 1998
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Preface
It was a pleasure to welcome 125 delegates from some fifteen countries to Reading in
September, 1997, for the annual presentation of research in the application of high
pressures to serve scientific ends. As is now customary, the range of applications and the
number of practitioners in the field having grown so large, the meeting addressed
especially the topics of food science, bioscience and chemistry, in which fields major
applications are now apparent and the techniques pioneered by earlier workers have
assumed their recognized place. This publication records the original work presented at
the meeting, and if it stimulates the reader in exploring these techniques to the extent that
was evident in discussions at the conference, it will serve its purpose.
Neil S. Isaacs
Reading, May, 1998
Plenary Lecture
ORGANIC REACTIONS AT HIGH PRESSURE.
THE EFFECT OF PRESSURE ON CYCLIZATIONS AND
HOMOLYTIC BOND CLEAVAGE
F.-G. Klhe;', M. K. Diedrich, G. Dierkes, J.4. Gehrke
Institut fiir Organische Chemie, Universitat GH Essen, Universitatsstr. 5,
D-45 1 17 Essen, Germany
Telephone: [phone], Fax: [phone],
E-mail: klaerneraoc1 . orgchem.uni-essen.de
Pressure in the range of 1-20 kbar (units of pressure: 1 kbar = 100 Mpa =
0.1 Gpa = 987 atm) strongly influences the rate and equilibrium position of
many chemical reactions. Processes accompanied by a decrease of volume
(activation volume A F< 0) are accelerated by raising the pressure and the
equilibria are shifted toward the side of products (reaction volume AF < 0),
while those accompanied by an increase of volume (A v' > 0) are retarded and
the equilibria are shifted toward the side of reactants (AF> 0). Many Diels-
Alder [4+2] cycloadditions are accelerated under high pressure and this effect is
frequently exploited in synthetic work'). The finding, that the packing
coefficient q defined as the ratio of van-der-Waals volume to molar volume (q
= Vw/V) is larger for cyclic structures than for the corresponding acyclic
structures, explains the highly negative activation volumes found for the
pericyclic cycloadditions'*2), the preference of pericyclic cycloadditions over
4 High Pressure Food Science, Bioscience and Chemistry
stepwise reactions at high pressure and the negative activation volumes of many
pericyclic rearrangements 2,3,4). Recently, it has been shown that the activation
volumes of pericyclic rearrangements such as the Cope rearrangement of
various substituted 1,5hexadienes or the electrocyclization of Z- 1,3,5-
hexatriene to 1,3-cycIohexadiene involving monocyclic transition states are in
the range of A v' rz -9 to - 14 cm3mol-' whereas those of intramolecular Diels-
Alder reactions involving bicyclic transition states are found in the range
A f rz -20 to -38 cm3mol-' '). The absolute values of the latter reactions are
approximately twice as large as or even larger than those observed for the Cope
rearrangements or electrocyclization. From this finding it can be estimated that
each five- or six-membered ring newly formed in the rate-determining step
contributes to about -10 to -15 cm3mol-' to the activation volume of these
rearrangements. Here, we report on the relationship between the size of the
pressure effect and the size of the newly forming rings in cyclizations. The
utility of high pressure in the elucidation of reaction mechanisms will be
demonstrated by the example of the racemization and isomerization in 1,3,4,6-
tetraphenyl- 1,5-hexadiene indicating that a pericyclic Cope rearrangement
competes with a dissociative process involving free radicals. The investigation
of the pressure effect on homolytic C-C bond cleavage provides a first
indication that pressure may be a tool to distinguish between reactions of free
radicals within and out of the cage.
Chemistry: Presentations 5
Table 1. Volumes of Reaction (AF), van der Waals Volumes of Reaction (A&),
Enthalpies, Entropies, and Gibbs Enthalpies of Reaction Calculated for the Hypothetical
Cyclizations of 1- Alkenes to Cycloalkanes by Means of the Corresponding Thermodynamics
Parameters.
AVR" mb A.S~ A C ~
-
A
fi -1.7 -5.5 7.86 -7.0 9.95
-
-
H -2.5 -6.6 6.43 -10.3 9.50
- 0
-3.8 -14.7 -13.46 -13.1 -9.56
- u
n
-
w -4.4 -16.5 -19.47 -21.0 -13.21
- 0
-4.7 -21.2 -13.41 -19.6 -7.57
-- 0
7 -4.9 -25.6 -9.88 -18.8 -4.28
-0
-
-4.7 -30.9
-c3
- -4.6 -32.3
0
-
' --+ -4.7 -32.8
-0
-4.7 -32.3
-e-(+w--
-4.6 -27.6
2n
nzl
a cm3 mol-'. V (n-alkene) calculated by the use of Exner incrementsI6). V (cycloalkane)
determined from density measurements in n-hexane. b kcal mol-I. cal mol-' K-I.
6 High Pressure Food Science, Bioscience and Chemistry
A first indication for the ring-size dependence of volumes came from the
observation that the ring-enlargement of cis- and trans- 1,2-divinylcyclobutane
leading to 1,5-~yclooctadieneo r 4-vinylcyclohexene and 1,5-~yclooctadieneis
accompanied by substantial decrease in volume (volumes of reaction,
A F= -12.8 cm3mol-' or -9.6 and -17.4 cm3mol*', respectively)6). The volumes
of reaction calculated for the hypothetical cyclizations of n-alkenes to the
corresponding cycloalkanes by the use of experimentally observed partial molar
volumes confirm this trend (Table 1). They decrease continuously from the
formation of cyclopropane to that of cyclodecane and, then, seem to be constant
for the larger rings, whereas the van der Waals volumes of reaction (AVw) are
approximately equal, with the exceptions of the formation of the three-, four-,
and five-membered rings. Therefore the ring-size dependent decrease in volume
of the cyclizations results from the different packing of cyclic and acyclic states
rather fiov the changes in their intrinsic molecular volumes. This may be
explained by the restriction of rotational degrees of freedom during the
cyclization. Provided that the activation volumes depend similarly on the ring-
size, the formation of larger ring should be dramatically accelerated by pressure.
The intramolecular Diels-Alder reactions of (E)-1,3,8-nonatriene and (0-1 ,3,9-
decatriene, in which either new five- and six-membered rings or two new six-
membered rings are formed, seem to be the first examples of the validity of this
assumption. The activation volumes found for the reaction of the decatriene are,
indeed, more negative by -10 and -13 cm3moT' than those found for the
corresponding reactions of the nonatriene (Scheme 1). Furthermore, this ring-
size effect may explain why the activation volumes observed for the formation
of three-membered rings in cheleotropic reactions (A i"# m -1 5 ~m~mol-'a)r~e )
substantially less negative than those for the formation of five- and six-
Chemistry: Presentations 7
membered rings in 1,3-dipolar cycloadditions (A v" w -22 ~rn~mol-'o)r~ D) iels-
Alder reactions (Av" = -35 ~m~mol-')~).
Scheme 1.
-c 3.0 - (24.8 & 0.3) -32.3
0
..
153.2"C BCiS
1 - (24.8 f 0.3) -27.0
trans
-c 1.2 - (37.6 f 1.6) -45.4
..
CiS
172.5"C 8
1 - (35.0 f 1.3) -37.4
trans
~ ~~~
a) the reaction volumes are extrapolated by the use of temperature dependent density
measurements to each temperature of reaction.
Optically active tetraphenylhexadiene 1 obtained by the separation of the
enantiomers on a chiral KPLC c01umn'~)u ndergoes a facile racemization at
temperatures just above room temperature. At 90°C racemic 1 shows a mutual
interconversion to meso-1.") Whereas the racemization of optically active 1
may be the result of the pericyclic Cope rearrangement involving the chair-like
transition state 2' shown in Scheme 2, the mutual interconversion of ruc-1 into
meso-1 cannot be explained by one or a sequence of Cope rearrangements. The
8 High Pressure Food Science, Bioscience and Chemistry
effect of pressure leads to an unambiguous mechanistic conclusion. The
observation, that the racemization of optically active 1 is accelerated by
pressure and, therewith, exhibits a negative volume of activation (A? < 0), is
good evidence for a pericyclic Cope mechanism in this case. In the other case,
however, the finding that the mutual interconversion of rac-1 into meso-1 is
retarded by pressure (A? > 0), suggests a homolytic bond cleavage in the rate-
determining step (Scheme 2).
Scheme 2.
I
1 J
enf-1
A v' = -(7.4 k 0.4) cm3m ol-'
AH# = (21.3 ?E 0.1) kcal mol-'; A$ = - (13.2 ?E 0.3) cal mol-' K-i
rneso-l-+ ruc-1: Av'= + (13.5 k 0.1) cm3mol-'
AH# = (30.9 k 0.2) kcal mol-I; A$ = (2.4 J. 0.5) cal mol-' K-'
rac-l-+meso-1: Av'=+(11.5 f0.2)cm3mol-'
AH# = (30.7 f 0.2) kcal mol-I; A$ = (1.9 k 0.5) cal mol-' K-'
Chemistry: Presentations 9
Scheme 3.
NcP*h Ez [ Ph
L1300c 2Ph---(zy]-
('IN C
Ph Ph
meso-4 5 rac-4
meso-4+ rac-4: Al/f = + (10.7 f 4.8) cm3m ol-'
rac-4+ meso-4: Al/f = + (8.5. f 3.4) cm3m ol"
1
7
AJ'# = + (35.7 k 0.4) cm3m ol-'
AH# = (44.7 k 1.9) kcal mof'; Asf = (33.7 f 4.8) cal mol-' K-' 13)
A Y' = + 6 cm3m ol-' 14)
Finally, the effect of pressure on rearrangements involving free radical
intermediates (Scheme 2, entry 2 and Scheme 3, entry 1,3,4) shall be compared