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Differences in the Temperature Dependence of Reaction Parameters in Linear УFree EnergyФ Relationships.

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zene derivatives which also exhibit angles of similar size at
the twofold coordinated phosphorus atom'51. In contrast, the
complex (C0)5Cr[P(NCR3)N(SiR3)2] (R = CH3) which contains an aminoiminophosphane almost identical to ( 2 ) as
ligand is found to have an NPN angle of 119.7" and thus
a completely different hybridization at the
The other distances and angles in (2) correspond to expectation (Fig. 1). The average values of the Si-C and C--C
bond lengths are 1.850 and 1.518 A, respectively. As seen from
the size of the vibrational ellipsoids the tert-butyl group
attached to the imine nitrogen is not completely fixed sterically.
Hence localization of the H atoms was feasible only for the
other methyl groups.
The Taft-Hancock equation (b) has here proved its value
as a linear free energy relati~nship'~':
where 6 , (X=Br,CI) are the reaction parameters and E: the
ateric substituent constants for the different radicals R"6J.
Received: June 18, 1976 [Z 533 IE]
German version: Angew. Chem. 88, 723 (1976)
CAS Registry number:
( 2 1 , 53787-01-2
-~
[I] S. Pohl, E. Niecke, and 5. Krebs, Angew. Chem. 87, 284 (1975); Angew.
Chem. Int. Ed. Engl. 14, 261 (1975).
[2] 0. J . Scherer and N. Kuhn, Angew. Chem. 86,899 (1974); Angew. Chem.
Iut. Ed. Engl. 13, 811 (1974); J. Organornet. Chem. 82, C 3 (1974); and
references cited therein. I am indebted to Prof. Dr. 0. J . Schrrer for
supplying the substance.
131 Transition of the orthorhombic to a monoclinic phase occurs at ca.
-25°C. N o single crystals could he obtained below this temperature.
[4] Syntex P21 diffractorneter, MoKl radiation; Syntex XTL program system.
[5] D. E. C. Corbridgei The Structural Chemistry of Phosphorus. Elsevier,
Amsterdam 1974: and references cited therein.
[6] E. Niecke, G. Rinqel, and S. Pohl, to he published.
Differences in the Temperature Dependence of Reaction
Parameters in Linear "Free Energy" Relationships"']
By Bernd Giese"]
The linear free energy relationships (a) have proved their
value in the elucidation of reaction mechanisms and in the
planning and optimization of synthetic reaction steps''].
log kx
- - = pr'p.
ko
If equation (a) describes the effect of substituents (variation
of X) on the rate constant k ~ [ ' ]of a series of reactions, then
the substituent constants ps determined in standard reactions
and the experimentally determined reactivity differences
(logk, -logko) afford the reaction parameters p,, which are
thus typical selectivity
It has recently been shown that the selectivities of the radicals
R' in the competition system BrCCI3/CCI4 depend to such
an extent upon the temperature that they coincide in the
range 40-80°C and their order is reversed above 80"C'41.
It may therefore be expected that the reaction parameters
pr of various series of reactions become equal at certain temperatures and that the differences in the reaction parameters
(prl-pr2) consequently adopt different signs above and below
this temperature. This temperature dependence of the reaction
parameters can also be demonstrated with the aid of the
radical halogen abstractions in the competition system
BrCCI3/CCl4.
[*] Priv.-Doz. Dr. B. Giese
Chemisches Laboratorium der Universitat
Albertstrasse 21, D-7800 Freiburg (Germany)
[**I This work was supported by the Deutsche Forschungsgenieinschaft
688
Fig. I . Dependence of the differences in reactivity (logks,-logkci) for the
alkyl radicals ( I ) . ( 8 ) on the steric Ef constants between 0 and 130°C.
( l ) , C H ~ C ( C Z H S )( Z
2 1; , CsHiX(CH3)2; ( 3 ) , C ~ H ~ C ( C H ~ ) Z C
(41,
H ZC-;
C7H13: (S),c-CbHii; (6), 2-C8H17: ( 7 1 , I-ClHis; f a ) , CH3.
The straight lines in Figure 1 show that in the temperature
range of 0 to 130°C the radical selectivities (logkB,-logkcl)
depend upon the group size of the radicals ( I ) to ( 8 ) in
a manner described by the Taft-Hancock relationship (b).
Polar effects do not play any role, although the electrophilic
CC13 radicall*Jis displaced by alkyl radicals of various nucleophilicities['] in these radical substitution reactions[7J.Release
of the same CClj radical on abstraction of bromine and
chlorine in the competition system BrCCI,/CCI, leads to
extensive compensation of the polar effects in the transition
states ( 9 ) and
so that essentially only the differing
steric interactions of bromine and chlorine transfer onto the
radicals ( 1 ) to (8) become effective.
191
(10)
The slopes of the straight lines in Figure 1 indicate the
differences in the reaction parameters ( 8 -&I).
~ ~ Their opposing signs at 0°C [ ( ~ B ~ - & I )-0.221
=
and 130°C [(&&I)= +0.19] show that the reaction parameters a ~ and
, 6c1
vary differently with temperature and are equal at about 60°C.
This accords with the experimentally determined isoselective
temperature ( Ts)at which the radical selectivities coincide
in the BrCC13/CC14 system" 'I. As for the selectivities[41 a
comparison of the reaction parameters leads to false conclusions if the isoselective temperature of the competing series
of reactions lies below the temperature of measurement. The
interpretation of reaction parameters therefore requires at
Angmw. Chem. l n t . Ed. Engl. f Vol. I S (1976) No. 11
least a qualitative knowledge of their variation with temperature.
Received: May 12, 1976;
revised: June 30, 1976 [Z 534 IE]
German version: Angew. Chem. 88,723 (1976)
CAS Registry numbers:
( I ) , 19067-45-9; ( 2 ) . 58281-61-1 ; ( 3 ) , 22904-47-8; ( 4 ) . 4566-80-7;
( S I , 3170-58-9; (6), 3268-43-7; (7), 3356-67-0; (81, 2229-07-4
N. B. Chopmun and J . Shorrer: Advances in Linear Free Energy Relationships. Plenum Press, London 1972.
Equilibrium constants can be treated in analogous manner.
The log k values should be defined as reactivities. Selectivities indicate
differences in reactivities.
B. Giese, Angew. Chem. 88, 159 (1976); Angew. Chem. Int. Ed. Engl.
15, 173 ( I 976).
R . W Tujt in M. S. Newman: Steric Effects in Organic Chemistry.
Wiley, New York 1956, Chap. 13; C.K. Huncock, E . A . Meyeus. and
B . J Yugur, J. Am. Chem. SOC.83. 4211 (1961).
T Fajita, C.Tukuyumu, and M . Nakajima, J. Org. Chem. 38%1623 (1973).
X. L'. Ingold and B. P. Roberts: Free-Radical Substitution Reactions.
Wiley, New York 1971.
W C. Dunen in E . S . H u y s e r : Methods in Free-Radical Chemistry,
Vol. 5. Dekker, New York 1974.
R. W Hendermn and R . D. Ward, J. Am. Chem. Soc. 96, 7556 (1974);
W. A . Pryor and W H . D a r k s , h i d . 96, 7557 (1974); A. A . Zacirsas
and C . M . Hannu, J. Org. Chem. 40, 3782 (1975).
K. Hrrwig, P. Lorenz, and C . Riichardt, Chem. Ber. 108, 1421 (1975).
€3. Giese, Angew. Chem. X8. 161 (1976); Angew Chem. Int. Ed. Engl.
15. 174 11976).
Fig. 1. Instantaneous photograph of an oscillating chemical system which
forms a pattern varying in space and time. The solution (see text for details)
is located in a Petri dish standing on light-sensitive paper (Agfa Brovira
BH II I ) . T o increase the contrast a cobalt glass was placed between the
light source (normal 100 W light bulb) and the Petri dish.
the experimental and theoretical study of chemical oscillations
in time and space. At present, however, too little is known
about the reaction course to explain the observed phenomena.
Received: January 22, 1976:
in abridged form: August 10, 1976 [Z 5.36 IE]
German version: Angew. Chem. XN. 728 (1976)
CAS Registry numbers:
KBrO,, 7758-01-2; tris(2,4-pentanedionato)manganese(ni),14284-89-0
-~
-~-
Chemical Waves in the 2,4-Pentanedione/Potassium
Bromate System
By Wolfgung Jessen, Heinrich G . Busse, and Bent Hausteen[*]
A homogeneous solution in which a chemical reaction is
taking place usually remains monodisperse. We now report
an exception['!
Into the lid of a Petri dish (90mm diameter) at 10°C were
successively introduced: 2.0 M HzS04 (7.5ml), 0.3 M K B r 0 3
(3.75m1), H ,O (3.75 ml), and tris(2,4-pentanedionato)manganese(iii) (81.4 mg). After mixing, the bottom of the Petri dish
is placed in the solution. Glass spacers ensure that there
is a 2 mm-thick liquid layer between the two glass surfaces.
After about lOmin, a small yellow spot (pacemaker) suddenly appears in the initially homogeneous solution which
is kept at room temperature. The spot steadily expands while
the remaining solution stays colorless. Once it has reached
a diameter of about IOmm, the solution decolorizes starting
from the center of the circle, while the periphery continues
to grow. After a brief pause a yellow spot is reformed in
the center of the ring, and a second ring migrates through
the solution. Instead of a system of concentric rings, spirals
may appear or ring formation may simultaneously occur at
several places in the solution to give a pattern of the kind
shown in Figure 1. This Figure shows the state of a solution
20min after the admixture of 2,4-pentanedione (as its Mn"'
complex). About 40 min later, the pattern had disappeared
and the solution was again monodisperse. The solution
remained clear all the time; toward the end of the reaction
only a slight cloudiness appeared but no precipitate was
formed.
Compared with other known systems['], the one described
here has the advantage that it neither forms a precipitate
nor liberates a gas. Consequently, it can also be contained
in a sealed vessel. We believe it to be a suitable model for
[I]
C-Substitution of Nucleosides with the Aid of the
Eschenmoser Sulfide Contraction['
By Helmut Vorbriiggen and Konrad Krolikiewicz"]
Among the numerous chemical modifications of nucleosides
and nucleotides suitable for preparing analogs of biological
interest, very little attention has so far been paid to C-substitution in the a- or y-position to the heterocyclic nitrogen
atoms['].
We have therefore applied the Eschenmoser sulfide contractionL3Jto the readily accessible S-alkyl derivative^'^] of h-thiopurine and of 2- or 4-thiopyrimidine nucleosides.
The S-alkyl derivatives (I ) of 6-thiopurine nucleosides
reacted with strong bases and triphenylphosphane to give
the C-alkylated nucleosides (2), generally in good yield. The
products were hydrolyzed to give the free crystalline compounds (3)C51 (see Table 1 for details).
The 6-phenacylthiopurine afforded, analogously to ( I a ) ,
6-phenacylpurine (m. p. 248°C) in 88 % yield.
S-CH~-R'
CH~-R'
['I and K. Krolikiewicz
Research Laboratories of Schering AG, Berlin/Bergkamen
Postfach 6503 1 I . D-1000 Berlin 65 (Germany)
T o whom correspondence should be addressed
[*] Dr. H. Vorbriiggen
~
[*] W. Jessen, H. G . Busse, and Prof. B. H. Havsteen
Biochemisches lnstitut im Fachbereich Medizin der Universitar
Olshausenstrasse 40-60, D-2300 Kiel (Germany)
A n g e ~ Chem.
.
Int. Ed. Enyl.
CL A . 7: Winfree, Sci. Am., June 1974, p. 82.
/
Vol. 15 ( 1 9 7 6 ) No. 1 I
['I
689
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