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Basis and Limitations of the Reactivity-Selectivity Principle.

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Volume 16 - Number 3
March 1977
Pages 125- 204
International Edition in English
Basis and Limitations of the Reactivity-Selectivity Principle
By Bernd Giese[*]
The reactivity-selectivity principle (RSP), which describes a linear relationship between the
decrease in selectivity and increase in reactivity of molecules X, in their reactions with Y1
and Y2, has been much criticized during the last few years. The present paper shows the
kind of reactions where the RSP does not hold and reveals how the individual types of
reaction can be distinguished by varying the solvent. The dependence of molecule selectivity
on temperature produces a further effect. An isoselective relationship (ISR) can be deduced
when this dependence is taken into account. This relationship is not concerned with the
connection between the temperature-dependent rate and competition constants, but with the
connection existing between activation enthalpies and activation entropies, which are temperature-independent over wide ranges. The application of the ISR to different types of reactions
makes it possible to characterize and distinguish between short-lived intermediates and underlines
the significance of the isoselective temperature ( Ts)in interpreting the reaction parameters
of linear free energy relationships and in planning syntheses.
1. Introduction
The aim of preparative chemistry is quick and uniform
synthesis of desired compounds. Formation of inconvenient
side-products is caused by the ability of initial substances
and/or intermediates X, to react in different ways, e.g. with
Y1 and Y2:
If the reactions proceed with the rate constants k l and k2,
the product ratio P1/PI depends on the concentration of the
reaction partners Y1 and Y2 and on the competition constant
xx. Thus, in planning a synthesis, competition constants of
[*] Priv.-Doz. Dr. B. Giese
Chemisches Laboratoriurn der Universitlt
Albertstrasse 21, D-7800 Freiburg (Germany)
Angew. Chewi. Inr. Ed. Engl. 16,125-136 ( 1 9 7 7 )
possible branched reaction pathways should be approximately
A different problem can arise in studies of reaction
mechanisms: more often than not, it is impossible to identify
postulated short-lived intermediates, e. g. X,, by their physical
properties. In many cases, however, such intermediates can
be detected chemically by competitive trapping with suitable
reaction partners, e. g. Y1 and Y2[ll. The reactivity-selectivity
principle (RSP) is applied in order to obtain information
on the rate constants of the intermediates k l and k z from
the competition constants xx:
I f molecules X , , X Z ,Xj ... X , react with
and yZ, selectivity
(log xX)[” decreases proportionally to increasing reactivity
(log k x ) .
This linear relationship is described by proportionality relationship (a) or by equation (b).
The symbol 6 indicates that only changes of reactivities and
selectivities on varying molecules X, of the reaction series
in question are compared. The constants a and b, i.e. the
intercept and the slope of linear equation (b), are larger than
zero (cf. Section 3). The RSP can be checked graphically
by a logx,/logkx plot (Fig. 1).
as in amine alkylation, the charge separation is more advanced
on the activated complex ( 3 ) of the slower reaction than
on that of the faster reaction [ ( 4 ) ] .
A change to a reaction partner which stabilizes or destabilizes charges has greater influence on transition state (3)
than on transition state ( 4 ) , because formation of the charges
is further advanced in (3) than in ( 4 ) . This is exemplified
by the methylation of cc-substituted pyridines (5)r41.
l g kxFig. 1. Graphical check of the RSP [Eq. (a)] in a selectivity-reactivity plot.
If the various molecules X, give a straight line with a
negative slope as in Figure 1, the reaction complies with
the RSP: in this case the more reactive molecule Xz is less
selective than molecule X1.
2. Basis of the Reactivity-Selectivity Principle (RSP)
2.1. Variation of Substituents
The connection between changes in reactivity and changes
in selectivity, which is supplied by the RSP, certainly appears
plausible, for the differences in the free activation enthalpies
AGf -Act are large in the case of large activation enthalpies
of the individual reactions: when a less reactive molecule
X I reacts with Y1 or Y z it has to overcome a larger free
activation enthalpy than the more reactive molecule Xz. Variation of the reaction partners (Y, and Yz) has greater influence
on the free activation enthalpy of the less reactive particle
X I , as can be shown by the Hammond postulate[31according
to which the slower reaction has a ‘‘later’’ transition state
than the faster reaction. Reorientation of electron distribution
and atom location is further advanced in a “late” transition
state than in an “early” transition state (Fig. 2).
Figure 3 demonstrates how the selectivity of the competing
reactions with methyl fluorosulfonate/methyl iodide decreases
with increasing reactivity of the pyridine derivative ( 5 ) (varying substituents R). The fluorosulfate leaving group effects
a better stabilization of the negative charge than iodide, and
this is more noticeable in the “later” transition state of the
slower reaction than in the “earlier” transition state of the
faster reaction, wherecharge separation is comparatively small.
Reaction coordinate
Fig. 2. Reaction profiles for reaction of Y, and Y2 with a) less reactive
molecule X I and b) more reactive molecule X1.
- 1.0
‘g IkFIrel
Bond lengths and angles have changed to a greater extent
in the activated complex (1) than in the activated complex
(2). Where partial charges occur at transition states, such
Fig. 3. Checking the reactivity-selectivity principle by reaction of various
substituted pyridines ( 5 ) with methyl fluorosulfonate and methyl iodide
in nitrobenzene at 30°C.
Anyuw. Chem. Int. Ed. Enyl. 16,125-136 ( 1 9 7 7 )
2.2. Variation of the Solvent
If the RSP holds, changing the solvent should have similar
effects on reactivity and selectivity as varying substituents:
An increase in reactivity caused b y changing the solvent is
accompanied b y a proportional decrease in selectivity of molecule
X , reacting with Y, and 5.
The oppositely directed influence of solvents on reactivity
and selectivity can also be exemplified by the alkylation of
tertiary amines.
Figure 4 demonstrates how the selectivity of 2,4-diazabic:yclo[2.2.2]octane (6) decreases and the reactivity increases in
the competition system phenethyl iodide/phenethyl chloride
as the solvent is changedC5].
The considerable scatter of the experimental values in Figure
4 is typical of interaction between solvents and dissolved sub-
Thus, the proportional connection between reactivity and
selectivity is valid if a linear relationship exists between the
reactivities of the individual reaction series. The selectivity
principle therefore turns out to be a specific form of linear
free energy relationship and therefore holds only for limited
variations of molecules X, and reaction partners Y 1 and
Y,[']. The positive sign of b, a result of the application of
the Hammond postulate (cf. Section 2.1), limits the validity
range still further: only those linear free energy relationships
in which variations of substituent and solvent exert a greater
influence on the slower reaction series comply with the RSP.
When testing the reactivity-selectivityprinciple numerically
or graphically, care must be taken that the competition constants are greater than unity, otherwise the sign of b must
change for the RSP to remain valid. On the other hand,
, kx.y2 can be inserted, for the rate constants
either k x , ~ or
are related according to equation (c).
3.1. Opposing Influence of Substituents on Bond Cleavage and
Bond Formation
In the last few years the RSP has been much criticized
because there are reactions in which selectivity does not change
0 Dioxone
Acetic ester
Nitromet hone.
2s log k,
Fig. 4. Checking the reactivity-selectivity principle by reaction of tertiary amine(6 j with 2-phenethyl
iodide and 2-phenethyi chloride in various solvents at 54.4"C.
stances, which is dependent on several parametersc6].Therefore, application of the selectivity principle is admissible only
if the reaction is examined in a sufficient number of solvents.
In spite of frequently poor correlations, a greater influence
of the solvent is observed on slower reactions than on faster
reactions[71 provided that similar reactions are considered.
in spite of increasing reactivity, or even increases proportionally to increasing reactivity[']. One example is the reaction
of arenesulfonyl chlorides (7) in the competition system aniline/3-chloroaniline.
3. Limitations of the Reactivity-Selectivity Principle
The connection between changes in reactivity and selectivity
expressed in equation (b) can be reformulated as equation
(c) on substituting competition constant xx by rate constants
kx.y, and k X , Y 2 .
Angew. Chem. Int. Ed. Engl. 16,125-136 (1977)
The positive slope in a selectivity-reactivity plot of experimentally determined values["' (Fig. 5) shows that the most
reactive sulfonyl chloride, nitrobenzenesulfonyl chloride, is
also the most selective in this particular competition system.
This finding is completely opposed to the prediction of the
selectivity principle.
when substituents are varied represent only a partial view
of the entire reaction. If reaction of A with BC consists in
bond formation A-B and bond cleavage B-C, the free
energy of activation AG* can be formally split up into two
partial values, uiz. for bond formation (AGzB) and bond cleavage (AG&) in the transition state [Eq. (d)].
Only if partial values AG& and AG& change proportionally
to AG* when substituents R are varied, does each of the
formal “partial reactions” describe stereoelectronic reorientation of the entire reaction [Eq. (e)]. Relationships (0 and
(g) are derived from (d) and (e). Equation (8) shows that
proportional change of AG*, AG&,, and AG& is possible
only if the ratio of bond formation and bond cleavage in
the transition state of the reaction is independent of substituents R.
+ lg k,
Fig. 5 . Failure of the RSP for reaction of various substituted benzenesulfonyl
chlorides ( 7 ) with aniline and 3-chloroaniline in methanol at 25°C.
The influence of the solvent on the reaction of benzenesulfonyl chloride with aniline/3-chloroaniline, however, gives an
entirely different picture from that of varying substituents.
The selectivity is seen to fall and individual reaction rates
to rise when the solvent is changed (Fig. 6)l”’.
This condition, a constant ratio of bond formation to bond
cleavage, is met for reaction of a-substituted pyridines ( 5 )
with the competition system methyl fluorosulf&ate/methyl
iodide. If formation of a N-C bond is facilitated, as in the
case of unsubstituted pyridine compared with the 2-tert-butyl
derivative,the energymaximum (transition state of the reaction)
0 .o
2 + lg k,
Fig. 6. Checking the RSP by reaction of benzenesulfonyl chloride with aniline and 3-chloroaniline in aprotic and protic solvents at 45°C. TMH = tetramethylurea, DMF =dimethylformamide, HMPT = hexarnethylphosphoramide.
Why should the reactivity-selectivity principle fail for substituent variation and yet remain valid for solvent variation?
This apparent discrepancy is resolved when the various
types of interaction between substituents and individual atoms
of reacting molecules are taken into consideration. Reactivity
changes (that is, selectivity) estimated in a competition system
is already reached while the C-Z bond (Z=OSOzF, I) is
only slightly stretched. Bond formation and bond cleavage
are both slight in the transition state (8) but extensive in
the transition state ( 9 ) . The ratio of bond formation and
bond cleavage turns out to be almost independent of substituent variation in (5).
Angew. Chem. Int. Ed. Engl. 16,125-136 ( 1 9 7 7 )
state (12) in less polar isoamyl alcohol because of favorable
interaction between charges and ethylene glycol.
bond formation
bond cleavage
On the other hand, the ratio of bond formation and bond
cleavage is not constant when arenesulfonyl chlorides (7)
react with an aniline/3-chloroaniline system, because substituent variation on sulfonyl chlorides influences bond formation
and bond cleavage in opposite directions: a methoxy group
at position 4 of the arenesulfonyl chloride impedes formation
of a negative charge at the sulfur atom, but facilitates the
expulsion of the chloride ion. A nitro group on ( 7 ) , however,
promotes formation of the N-S bond and impedes cleavage
of the S-Cl bond. The ratio of bond formation and bond
cleavage is smaller in the transition state (10) than in the
transition state ( 1 1 ) . Therefore, the nitrogen atom of ( 1 1 )
will bear a higher charge than that of ( l o ) , so that the more
reactive nitrobenzenesulfonyl chloride is more selective than
the less reactive methoxybenzene sulfonyl chloride in the competition system aniline/3-chloroaniline (Fig. 5).
Ethylene glycol:
E l 66-
The RSP holds if varying the solvent influences bond formation and bond cleavage in the same direction: the selectivity
of benzenesulfonyl chloride decreases as reactivity increases
(Fig. 6).
3.2. Dominating Influence of Frontier-Orbital Interaction
With a number of reactions, however, it seems that other
reasons are responsible for simultaneous increase of reactivity
and selectivity: if reactions of molecules X, and Y, pass
through “early” transition states, and energy differences El
and EII(Fig. 7) between the highest occupied molecular orbitals
(HOMO) and the lowest unoccupied molecular orbitals
(LUMO) of the same symmetry are small, the course of the
reaction can be described in a satisfactory manner by means
of stabilization energies AEl and AE11[16].The smaller the
orbital differences El and EII,the greater are the subsequent
stabilizing interactions AEI and A&, and the faster are such
reactions. This point of view neglects, for instance, repulsive
energy resulting from the mutual approach of molecules X,
and Y,. Therefore, this model should only be applied to
reactions with “early” transition states, in which reacting molecules are still comparatively far apart“6. ’’].
Bond formation
Bond cleavage
= variable
The Hammond postulate indeed holds for these reactions
(the faster reaction has an “earlier” transition state), but the
RSP is not valid (the more reactive particle is also the more
selective one).
Therefore, great care should be taken when interpreting
competition measurements, if “central atom B” is varied. Only
an analysis of the influence of substituents[’*]on bond formation and bond cleavage[’31 can supply information on selectivity behavior.
Compared with variation of substituents on arenesulfonyl
chloride ( 7 ) , varying the solvent has quite a different effect
on the reaction, because solvents stabilize both the positive
charge on the aniline nitrogen and the negative charge on
the leaving chloride ion. Transition state (13) is reached
“earlier” in the more polar ethylene glycol than transition
Angew. Chem. Int. Ed. Engl. 16,125-136 ( 1 9 7 7 )
Fig. 7. Interaction between frontier orbitals (HOMO and L U M O ) of reacting
molecules X, and Y..
Assuming that substituent variation on X, and Y, produces
a greater effect on the position of frontier orbitals, and subsequently on stabilizing interaction, than on repulsive energy,
relationship (h) was derived for cycloadditions[’*].
Like substituents, solvents influence the energy of the frontier
an effect due to the appropriate “ d o n i ~ i t y ” [ ~ ~ ~
of the solvents. Thus, according to equation (h), selectivity
In equation (h), EI and Ell are the differences between HOMO
and LUMO (Fig. 7), x is the effect of the substituents on
the frontier orbitals, energy-decreasing (+ x) and energyincreasing ( - x), respectively, fi is the resonance integral and
K specifies the products of the atomic orbital coefticients
(p and K are assumed to be constant for this simplified consideration). The inversely proportional relationship between
reactivity and frontier orbital differences shows that the
influence of substituents is greater, the smaller the energy
differences El and El[ of the reacting molecules. The selectivity
increases with increasing reactivity of molecules X, and
According to equation (h), plotting reactivity against any
parameter varying proportionally to the energy of the frontier
orbitals, produces hyperbolas. If the frontier orbitals are
arranged in a symmetrical manner, as in Figure 7, a point
of intersection of two hyperbolas is obtained. This intersection
is attained for the Diels-Alder reaction of bis(dimethylamin0)cyclopentadienone ( 1 4 ) , X = N(CH3)2, with unsubstituted
styrenetz0] (Fig. 8). Any substitution of styrene causes an
increase in reactivity.
Fig. 8. Reactivities of cyclopentadienones ( 1 4 ) . X = Br, N(CHs)2, plotted
against cr-parameters of styrene substituents Y.
should increase with increasing reactivity for such cycloadditions if the solvent is changed. This is indeed the case, as
is shown by the solvent influence on reactivity values of some
Diels-Alder reactions[241and 1,3-dipolar cycl~additions[~~!
A simultaneous increase of selectivity and reactivity was also
observed in [2 21-cycloadditionsof diphenylketene and enol
ethers (Fig. 9)[261.
Fig. 9. Solvent influence on reactivity and selectivity of reaction of diphenylketene with butyl
vinyl ether and 2.3-dihydropyran.
Varying the substituents on the diene shifts the minimum,
according to equation (h). Dibromocyclopentadienone ( I 4 ) ,
X = Br, with lower electron density, reaches minimum reactivity only with styrene derivatives of low electron density. In
as competition system, selectivity of the fast reacting dibromo compound ( 1 4 )
is higher, because reactivity is determined at a steeper part
of the hyperbola[”] (Fig. 8).
On the other hand, reaction of tetracyanoethylene (TCNE)
with the same competition system shows decreasing selectivity
values on increasing reactivity of TCNE due to a change
of solvent (Fig.
In contrast to the one-step cycloaddition of diphenylketene[z61,tetracyanoethylene and enol ethers form zwitterionic
the formation rates of which are no longer
governed by the stabilizing interaction of frontier orbitals.
Angew. Chem. Int. Ed. Engl. 16,125-136 (1977)
- do
ILl k2
Fig. 10. Solvent influence on reactivity and selectivity of reaction of tetracyanoethylene with butyl vinyl ether and 2,3-dihydropropane.
Therefore, application of the reactivity-selectivity principle
to solvent variations can help to distinguish between one-step
reactions and formation of zwitterionic intermediates.
4. Dependence on Temperature
4.1. Temperature Dependence of Selectivity Values
1.' [K-']
Classification of chemical reactions according to the reactivity-selectivity principle as dealt with in Sections 2 and 3
completely ignores the temperature dependence of selectivity
However, the selectivity (that is, difference in reactivity['])
will depend on temperature according to equation (i) if the
competing reactions have different activation enthalpies.
log- k l = AHf-AHt
- ASf-AS?
- m3
Fig. t i . Reaction of alkyl radicals (rr-radlcals) in the competition system
BrCCIJCCL between 0 and 130°C.
selectivity[291of halogen abstraction in competition system
BrCCI3/CCl4 is plotted against reactivity[301 of chlorine
abstraction in CC14 for methyl and cyclohexyl radical at various temperatures. Reversal of selectivity is not accompanied
by reversal of reactivity.
Whereas the order of rate constants of reaction series does
not change when the temperature is variedr2*],the temperature
has such a drastic influence on competition constants that
selectivity values coincide within a narrow temperature range,
the isoselective temperature Ts,and reverse their order above
this temperature. Halide formation of alkyl radicals R. in
the competition system BrCC13/CC14 was the first example
of this selectivity behavior to be thoroughly examined['9!
Plotting radical selectivity measured between 0 and 130°C
against the reciprocal temperature produces straight lines
as shown in Figure 11. Their position corresponds to the
expected gradation of selectivity at 0°C : the methyl radical,
most reactive in CC14[30],is less selective than primary, secondary, and tertiary radicals; the most selective is the bulky
rert-undecyl radical. Between 40 and 80°C selectivities have
similar values and above 80°C the order is reversed. Thus
at 130°C the methyl radical is the most selective, while the
tert-undecyl radical is the least selective. A further surprising
result is the increase of selectivity differences with increasing
temperature above the isoselective temperature (Fig. 11).
The methyl radical remains the most reactive radical in
spite of reversal of selectivity, as shown in Figure 12, where
Angew. Chenr. l n t . Ed. Engl. 16,125-136 ( 1 9 7 7 )
Fig. 12. Selectivity-reactivity plot for reaction of methyl and cyclohexyl radicals
in the competition system BrCCls/CCI4 at three temperatures.
Like n - r a d i ~ a l s ' ~0-radicals
also reverse their orderr291
(Fig. 13). In this case T 3 is in the region of 60°C.
110°C (Fig. 15). As in radical halogen abstraction (Fig. 1 1
and 13) the more reactive anthracenes with higher electron
density are more selective above the isoselective temperature
than the less reactive anthracenes (with lower electron density).
T-' IK-'l
Fig. 13. Reaction of cr-radicals in competition system BrCCI3/CCl4 between
-20 and 130°C.
Temperature also has a pronounced influence on the selectivities of primary, secondary, and tertiary alkyl radicals in
their addition reaction with the 3-chlorostyrene/4-methylstyrene competition system[32].Distinct differences in selectivity
exist at -2O"C, but at f40"C the radicals have almost identical competition constants (Fig. 14).
T" [K-'l
. 10-3
Fig. 15. Temperature dependence of selectivity of anthracene derivates reacting
with the competition system N-(3-nitrophenyl)maleimide/N-(p-tolyl)maleimide between 25 and 130°C.
These examples demonstrate how the temperature dependence of selectivity values may falsify discussions of competition data if T',lies within or below the range of measurement.
4.2. Temperature Dependence of Reaction Parameters in Linear
Free Energy Relationship
Equation (j)is the general form of a linear free energy
+ 40 OC
-20 OC
T-' CK"1
LJO . 1 0 - 3
Fig. 14. Reaction of primary, secondary, and tertiary radicals with competition
between - 55 and +4O"C.
system 3-chlorostyrene/4-methylstyrene
Diels-Alderreaction of substituted anthracenes with N-arylmaleimide[171also shows a reversal of selectivity at about
Linear free energy relationships describe the effect of substituents (variation of X) on the rate constant kx of a reaction
series. The substituent constants ps determined in standard
reactions and the experimentally determined reactivity differences ( l o g k x - log k o ) afford the characteristic reaction parameters, which are dependent on temperature, being typical
selectivity values[34! If variation of reactants X, or their
partners Y, can be described by substituent parameters of
linear free energy relationships, the reaction parameters of
the individual series of reactions are essentially equal at the
isoselective temperature
This temperature dependence
of reaction parameters can be exemplified by the reactions
discussed in Section 4.1.
Angew. Chem. I n t .
Ed. Engl. 16,125-136 (1977)
The Taft-Hancock equation (k) has proved its value for
halogen abstraction by rc-radicals (15)-(22)[351:
where Fx (X=Br, C1) are the reaction
Eg the steric substituent constantsr3'!
their signs at about 60°C. This value accords with the isoselective temperature (Fig. 11). Plotting the differences (68, - F a )
directly against l/Taffords a straight line. (hr&a)becomes
zero at the isoselective temperature (Fig. 17).
Addition of alkyl radicals to alkenes (Fig. 14)'32,381
Diels-Alderreaction ofanthracene derivatives with N-arylmaleimides" 1' (Fig. 15) can be described by the Hammett relationship (1):
In equation (1) the p-values are the temperature dependent
reaction parameters, the a-values are the substituent cons t a n t ~ ' ~Plotting
dienophile selectivity against or-parameters
affords almost straight lines, their slopes being of opposite
sign at 25 and at 130°C (Fig. 18).
* 0.02
31) -
pl - p ,
= - 1.5
25 25
- 3.0
- 1.0
Fig. 16. Dependence of differences in reactivity (log ks, - logkcl) for alkyl
on steric EC constants between 0 and 130°C. (15).
radicals (15)-(22)
C H ~ C ( C ~ H ~(16),
) I ; C ~ H I ~ C ( C H ~(1) Z
7),; C?H~C(CHJ)ZCHZ;
(la), cC ~ H I( 1~9;) . C-CLHII;(20), 2-C~H17;(21), l-C?Hls; (22), CH3.
The various slopes of the straight lines in Figure 16 indicate
that the differences of reaction parameters (68, -&I)
Ti s
+ 0.19
- 0.1 -
A Er
Fig. 18. Dependence of differences in reactivity (logkl -1ogk2) on a,-parameters for reaction of substituted N-arylmaleimides with 9-methylanthracene
and 9-chloroanthracene at 25 and 130°C.
5. The Isoselective Relationship
5.1. Deduction of the Isoselective Relationship
This temperature dependence of reaction parameters makes
it evident that comparative interpretation of linear free energy
relationships is only possible with at least qualitative knowledge of their temperature dependence.
2.0 -
Fig. 11. Temperature dependence of reaction parameter differences ( 8 ~ . - 6 ~ ~ )
for halogen abstraction by radicals (I5)-(22)
in the competition system
BrCCl K C I I .
Angew. Chem. Inc. Ed. Engl. 16,125-136 (1977)
The reactivity-selectivity principle (RSP) is applicable only
above or below the isoselective temperature because of the
temperature dependence of selectivity values. Which equation,
then, is suitable for correct description of the behavior of
molecules X, reacting with Y, over the entire temperature
The temperature dependence of selectivity for every individual particle X, is described according to the Eyring relationship, equation (i). If boundary condition (m), accounting for
coincidence of selectivity values within the immediate vicinity
of Ts,is introduced, equation (n) evolves, which can be rewritten as the isoselective relationship (o)[~'].
2.303.R 'Es
6 ( A H f - AH?) = X,.&(ASf- AS?)
The isoselective relationship (ISR) signifies the proportional
alteration of the differences of activation parameters AH$ AH? and AS$ -AS? when particles X, are varied. 6 indicates
variation of molecules X,, the isoselective temperature ( TS)
is the proportionality factor of the ISR.
Equation (0)is formally very similar to the isokinetic relationship (p),derived from the temperature dependence of reactivity (logk) of a series of reactions (varying X, and constant
pis - 10.4
In contrast to selectivity, coincidence of reactivity values at
a certain temperature (in this case the isokinetic temperature
) reversal of order of reactivity above this temperature
has not yet been observed unequivocally for simple series
Statistically correct evaluation of results of
measurement affords p-values as computational quantities,
the physical significance of which is a matter of disputef28,43!
O C H 3 C0+H3
Fig. 19. Linear free energy relationships between activation enthalpy differences and substituent parameters for a) radical halogen abstraction, b) DielsAlder reaction.
/ YC
5.2. Application of the Ismelective Relationship to Elucidation
of Reaction Mechanisms
The RSP relates temperature dependent reactivity and selectivity values, the ISR on the other hand compares activation
enthalpies and activation entropies, which are temperatureindependent over wide ranges. If, for instance as in the
examples of Figures 16 to 18,the large temperature dependence
of selectivity values makes interpretation of reaction parameters of linear free energy relationships dubious, the problem
can be evaded by plotting differences of activation enthalpies
against substituent parameters (Fig. 19).
In Figure 19 the reaction parameters of Taft-Hancock and
Hammett relationships for radical halogen abstraction and
Diels-Alder reaction are constant quantities with no temperature dependence over wide ranges.
A further application of the ISR makes it possible to distinguish between intermediates with different bonding properties. It was shown that x- and o-radicals afford two straight
lines in AAH +/AAS* diagrams[401.As activation enthalpy
and activation entropy are evaluated from the same experiments according to the Eyring relationship and are thus
mutually dependent128*431,
the selectivity of x- and o-radicals
was plotted against 1/T in Figure 20. This representation
Fig. 20. Temperature dependence of selectivities of K - and a-radicals for
halogen abstraction in competition system BrCCI3/CCI4.
( 2 3 ) . l-C&,,; ( 2 4 1 , CH2=CH; (251, c-C3H5: (261, 7-norbornyI; (271,
C ~ H S(28),
Angew. Chem. Int. Ed. Engl. 16,125-136 ( 1 9 7 7 )
is statistically sound, for selectivity and reaction temperature
are independent variables[2s! The two fans of straight lines
in Figure 20 confirm the existence of two separate isoselective
relationships corresponding to n-radicals and o-radicals, respectively. Thus estimation of competition constants in a
BrCCl3/CCI4 system allows distinction of these types of radica1‘44~.
Similarly, it was possible to distinguish[471between different types of intermediate[461occurring when halogens and
benzenesulfonyl chlorides are added to alkenes. Figure 21
shows the different changes in selectivity for norbornene derivatives bridged by X=C1, Br, I or by a phenylthiyl group
for intramolecular endo trapping reactions with the adjacent
ester groups, the temperature ranging from -60 to 60”C[471.
on copolymerization parameters r l and r2. The values of
rl and r 2 describe the ability of growing nuclei ( 2 9 ) and
(30) to react competitively with monomers M 1 and M2[481.
Thus copolymerization parameters are competition constants. The course of copolymerization is specified by the
absolute values of r l and r 2 and their product (rl.r2). It
has been shown for various radicals[32.381 how selectivities
in olefin mixtures change to different extents when temperature
is varied and become equal at relatively low temperatures
(Fig. 14). In the case of radical copolymerization, therefore,
copolymerization parameters are temperature-dependent to
different extentd4’I. Reaction behavior in the vicinity of the
isoselective temperature where “ideal copolymerization”[481
is possible, should prove interesting.
6. Conclusion
( D Y )
1.’ [K”]
Fig. 21. Different dependence ofselectivity of intermediates bridged by halogen
(X =CI, Br, I ) or a phenylthiyl group for intramolecular trapping reaction
with adjacent ester groups.
The ISR could develop into a diagnostic criterion for distinguishing between different structures of reactive intermediates.
5.3. Use of the Isoselective Relationshipfor Planning a Synthesis
When the occurrence of several intermediates leads to
branching of reaction pathways, the different temperature
dependence of selectivity values influences the yields and structures of reaction products. This is shown for example by
copolymerization, an extremely important synthetic reaction:
the structure of the polymer material is essentially dependent
mnr, +
The fact that the RSP does not hold for many series of
reactions does not necessarily render the Hammond postulate
useless, as frequently presumed”. 501. If substituents influence
bond cleavage as well as bond formation, a more reactive
particle can be more selective than a less reactive molecule
in spite of an “earlier” transition state (see Section 3.1). If
the Hammond postulate is valid, the RSP is also not applicable
at temperatures near or above the isoselective temperature
(’&) (see Section 4). This substituent and temperature dependence of selectivity values and reaction parameters must be
taken into account when interpreting kinetic data and planning
An unsolved problem is the question, for which series of
reactions the isoselective temperature might lie within the
range of measurement. According to equation (o),T sis low
if the activation entropies of reaction series Xm+Y1 and
Xm+Y2differ widely. It was concluded from studies on addition of carbenes to alkenes that variation of reactants in
reactions with low activation enthalpies (slight mutual
approach of molecules in the transition state) can have greater
influence on activation entropy than on activation enthalpyr5 l].
Adopting this concept, low isoselective temperatures are to
be expected in particular for reactions with “early” transition
states. T’,is actually low for radical addition to styrenes (Fig.
14), for radical halogen abstraction in the competition system
BrCCI3/CCl4 (Fig. 11 and 13), and for Diels-Alder reaction
of anthracenes and maleimides (Fig. 15), e.g. between 40 and
110°C; these are reactions with low activation enthalpies.
However, temperature dependence offurther series of reactions
must be determined in order to allow reliable predictions of the
types of reaction for which low isoselective temperatures are
to be expected.
I wish to express my gratitude to my co-workers Dip/.-Cheni.
Klaus Heuck, Dip/.-Chem.Jiirgen Meister, Dip1.-Chem. Joachim
Stellmach, and Mrs. Karla Keller. I owe special thanks to
my teacher Professor Rolf Huisgen and to Professor Christoph
Riichardt, whose advice and discussion contributions were of
great assistance for this work. Our research was supported
by the Deutsche Forschungsgemeinschaft.
Received: January 17, 1977 [A 157 IE]
German version: Angew. Chem. 89, 162 (1977)
Translated by Robert Winiker, Freiburg
Angew. Chem. I n t . Ed. € n g l . 16,125-136 ( I Y 7 7 )
[ l ] R. Huisgen, Angew. Chem. 82, 783 (1970); Angew. Chem. Int. Ed.
Engl. 9, 751 (1970).
[2] Reactivity is defined logk, selectivity indicates a difference in reactivity.
[3] G. S . Hammond, J. Am. Chem. SOC.77, 334 (1955).
[4] U . Berg, R . Gallo, J . Merzger, M . Chanon, J. Am. Chem. SOC.98,
1260 (1976).
[5] M . Auriel, E . de H o f f a n n , J. Am. Chem. SOC.97, 7433 (1975).
[6] I , A, ~
~ y A . palrn
in~ N , B,~ chapman,
J , ,shorrer: Advances
in Linear Free Energy Relationship. Plenum Press, London 1972, Chap.
[7] Reactions exhibiting such solvent influence are described in a) €3. Giese,
R . Huisgen, Tetrahedron Lett. 1967, 1889; b) Z Drougard, D. Decroocq,
Bull. SOC.Chim. Fr. 1969,2972; c) A . Euckley, N . B. Chapman, M . R. J .
Dack, J . Shorter, H . M . Wall, J. Chem. SOC.B1968, 631.
[8] For an extensive discussion of linear “free energy” relationships see
N . B. Chapman, J . Shorter: Advances in Linear Free Energy Relationships. Plenum Press, London 1972.
191 C. D. Johnson, Chem. Rev. 75, 755 (1975); and literature cited therein.
[lo] 0. Rogne, J.Chem. SOC.81971.1855.
[ l l ] B . Giese, K . Heuck, unpublished work.
1121 J . C . Harris, J . L. Kurz, J. Am. Chem. SOC. 92, 349 (1970); E. R .
Thornton, ibid. 89, 2915 (1967).
[13] The necessary desolvation of ions in reaction of charged particles can
also be regarded as bond cleavage. Pross [14] recently interpreted the
surprising selectivity behavior of cations [15] by this means.
1141 A . Pross, J. Am. Chem. SOC.98, 776(1976).
[15] C . D. Ritchie, D. J . W i g h t , D. Huang, A. A . Kamego, J. Am. Chem. SOC.
97, 1163 (1975); and literature cited therein.
[16] K . Fukui, Fortschr. Chem. Forsch. 15, 1 (1970).
1171 A . 1. Konoualou, B. N . Solomonov, A. N . Usfyugou, Dokl. Akad. Nauk
SSSR 213, 349 (1973).
[IS] R . Sustmann, H. Trill, Angew. Chem. 84, 887 (1972); Angew. Chem.
Int. Ed. Engl. 11, 838 (1972); R. Sustmann, Pure Appl. Chem. 40, 569
(1974); K . N . Houk, Acc. Chem. Res. 8, 361 (1975).
[19] A . I . Konoualou, B. N . Solomonou, A . N . Ustyugou, Dokl. Akad. Nauk
SSSR 211, 102 (1973).
[20] A. I . Konoualou, B. N . Solomonou, 0.Z Chertou, Zh. Org. Khim. 11,,
106 (1975).
[21] Intersection of hyperbolas with opposite signs leads to a curve similar
to a parabola, the minimum of which is reached when frontier orbitals
are distributed symmetrically (Fig. 7). The smaller the energy differences
El and Ell, the steeper are such parabolas [Eq. (h)].
[22] The influence of solvent on the energies of frontier orbitals can be
shown, for instance, by UV spectroscopy; see H . A . Staab: Einfuhrung
in die theoretische organische Chemie, Verlag Chemie, Weinheim 1964,
p. 385. Electrochemical redox reactions supply further information, as
shown by unpublished work by H . Eainngirtel.
[23] T! Girtmann, Angew. Chem. 82, 858 (1970); Angew. Chem. Int. Ed.
Engl. 9, 843 (1970).
[24] M . E. Burrage, R . C. Cookson, S. S. Gypte, 1. D. R. Stevens, J. Chem.
SOC.Perkin Trans. 1975, 1325.
[25] R. Huisgen, L . Mobius, G . Miller, H. Stangl, G. Szeimies, J . M . Vernon,
Chem. Ber. 98, 3992 (1 965).
[261 R‘ Huissgen’ L’ A‘ Feiler’
[27] G. Sreiner, R . Huisgen, Tetrahedron Lett. 1973, 3769.
1281 0. Exner, Prog. Phys. Org. Chem. 10,411 (1973).
[291 B. Giese, Angew. Chem. 88, 159 (1976); A w w . Chem. Int. Ed. End. 15,
173 (1976).-Earlier work on the dependence of selectivity on solvents
also showed a reversal of the order of selectivity in a narrow temperature range: H . Pracejus, A. E l k , Chem. Ber. 96, 854 (1963).
[30] J . Currie, H . Sidebottom, J . Tedder, Int. J. Chem. Kinet. 6, 481 (1974).
[31] In x radicals a p orbital is occupied by a single electron, whereas in D
radicals the unpaired electron is located in a hybrid orbital. See also [44].
[32] B. Giese, J . Meister, unpublished work.
[33] Equilibrium constants can be treated in an analogous manner.
[34] 0.Exner, Collect. Czech. Chem. Commun. 39, 515 (1974).
[35] B. Giese, Angew. Chem. 88, 723 (1976); Angew. Chem. Int. Ed. Engl.
15, 688 (1976).
[36] R. W Taji in M . S. Newman: Steric Effects in Organic Chemistry.
Wiley, New York 1956, Chap. 13; C. K . Hancock, E. A. Meyers, B.
J . Yager, J. Am. Chem. SOC.83, 4211 (1961).
[37] 7: Fujita, C. Takayama, M . Nakajima, J. Org. Chem. 38, 1622 (1973).
[38] B. Giese, J . Meister, Angew. Chem. 89, 178 (1977); Angew. Chem.
Int. Ed. Engl. 16, 178 (1977).
1391 L. P. Hammett :Physikalische Organische Chemie. Verlag Chemie, Weinheim 1973.
[40] B. Giese, Angew. Chem. 88, 161 (1976); Angew. Chem. Int. Ed. Engl.
1 5 , 174 (1976).
[41] J . E . Lefler, E. Grunewald: Rates and Equilibria of Organic Reactions.
Wiley, New York 1963.
[42] For a discussion of experimental data indicating attainability of isokinetic temperature for certain series of reactions see [28] and 1431.
[43] 0.ExneF, Collect. Czech. Chem. Commun. 40,2762 (1975).
[44] Distinction of x and D radicals by different reaction behavior is very
interesting because pyramidal structureofthe r-butyl radical was derived
from spectroscopical measurements [45].
[45] 7: Xoenig, 7: B d k , W Snell, J. Am. Chem. SOC.97, 662 (1975); J .
B. Lisle,L . F. Williams, D . E. Wood, ibid. 98, 227 (1976); D. J . Krusic,
P. Meakin, ibid. 98, 228 (1976).
1461 8. Giese, Chem. Ber. 108, 2978 (1975).
[47] B. Giese, J . Stellmach, Angew. Chem. 88, 258 (1976); Angew. Chem.
Int. Ed. Engl. 15, 237 (1976).
[48] H.-G. Elias: Makromolekule. Hiithig and Wepf, Basel 1975.
[49] F. M . Lewis, C . Walling, W Cummings, E. R . Briggs, F . R . Mayo,
J. Am. Chem. SOC.70, 1519 (1948).
[SO] C . D. Johnson: The Hammett-Equation. Cambridge University Press,
Cambridge 1973, p. 152.
[5l] P. S. S k d , M . S. Cholod, J. Am. Chem. SOC.
91, 6035 (1969).
Angew. Chem. 1nt. Ed. Engl. 16,125-136 (1977)
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