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The Coexistence of the ReactivityЦSelectivity Principle and of Linear Free Energy Relationships A Diffusion Clock for Determining Carbocation Reactivities.

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The Coexistence of the Reactivity - Selectivity
Principle and of Linear Free Energy
Relationships : A Diffusion Clock for Determining
Carbocation Reactivities**
Michael Roth and Herbert Mayr"
Dcdicared 10 ProfesJor Giinler Schayer
on t/w occciJion of'/iis 60th birthday
The rates of reactions of carbocations with uncharged nucleophiles are given by Equation (a), where electrophiles are represented by a single electrophilicity parameter E, while nucleophiles are characterized by the nucleophilicity parameter N and
the slope parameter s, which is usually close to I.['] For the
1g h = .Y ( N
+ E)
determination of the strengths of electrophiles, we usually measure the rates of the reactions (Ig k ) of these electrophiles with
nucleophiles of well-known .s and N , and we then use Equation
(a) to calculate the electrophilicity parameter E.['. 31 However,
this method is only applicable to well-stabilized carbocations,
that is cations that can be produced in organic solvents as persistent entities with lifetimes of more than one hour. We now
introduce a general method for the determination of absolute
rate constants and electrophilicity parameters of short-lived carbocations. It is based on the selectivities of carbocations that are
generated from suitable precursors by chemical ionization and
thus does not depend on the presence of certain chromophores
as, for example, in the case of the flash-photolytic generation of
We observed a decrease of the relative reactivities of the allylsilanes 3a and 3b toward the benzhydryl chlorides 1 a-e
(Scheme 1 ) from 208 to 2.6 with decreasing electron-releasing
4a - 4h
TiCI4. -70 "C
l a - Ih
2a - 2h
Sa - 5h
Scheme 1.
abilities of the aryl substituents (Table 1 ) . The competition experiments were performed as described p r e v i o ~ s l y , [ ~and
yielded competition constants that were independent of the ratios of the nucleophiles 3a:3b. Only in the case of 1 a (very large
value which is difficult to determine) did the standard deviation
exceed 10%. The agreement of the competition constants with
the directly measured rate constants is shown by the
entry for l c : While competition experiments yielded a value
of kJk, = 42.7 for the bis(pchloropheny1)carbenium ion Z c
(Table l ) , a k,,/k, value of 37 is calculated from the directly
measured rate constants (CH,CN, 20 0C).[71As these reactions
[*] Prof. Dr. H. Mayr, Dipl.-Ing. M. Roth
lnslitut fur Organische Chemie der Tcchnischen Hochschule
Petersenstrassc 22. D-64287 Darmstadt (Germany)
Telehx: Int. codc + (61 51) 165591
Wc thank the Bayer AG. Leverkusen, for financial support.
S 10.00+ .25;0
Aiipei+. Cheni. f n r . Ed. EnXl. 1995, 34, N o , 20
can expect the same diffusion rates in both solvents as a
consequence of the Stokes- Einstein equation. The similar selectivities. which were determined for the carbenium ion 2c by
competition experiments at 20°C ( k u / k h= 32.0) and -70°C
2ox 33
( k o / k b= 42.7), again demonstrate that an explicit treatment of
39.9 f 2.6
temperature dependence is unnecessary.
It, therefore, becomes possible to determine absolute rate
7.03 f 0.23
constants for reactions which cannot be followed directly. I n the
2.61 0.20
region where Equation (a) holds, that is, for "slow" reactions,
140 k13.5
2.41 0.25
k J k , is generally greater than 1000. When the alkyl cation 2f is
generated from the alkyl chloride 1 f and titaniuni(1v)chloride in
1.10 f 0 10
8.31 3.61
the presence of 3 a and 3b, the products 4f and 5f are formed.
3.85 f 0.12
8.0 - 8.5
Their ratio yields a competition constant of k,,X, = 2.41, indicating diffusion control of the faster reaction (2f + 3 a ) .
Analogous generation of the carbocation 2f in the presence of
3a and 3 c yields the ratio k J k , = 140, which c ; ~ nbe combined
do not have an enthalpy barrier,[*] the comparison of these
with the diffusion-controlled k, = 3 x 10' L mol s - I , to give
reactivity ratios is possible without correcting for temperature.
k L 2.1 x 10' L mol-' SKI.Since this value of X , is within the
The decrease of the competition constants k,/k, with increasing
scope of the linear free energy relationships, the clectrophilicity
reacthities of the benzhydryl cations (Table 1) is in accord with
parameter E (2f) = 7 S 0 can be derived by substitution into
the postulate of the reactivity-selectivity principle["- ''I and
Equation (a) or graphically from Figure 1 . Analogously. for the
opposes Equation (a) which predicts constant selectivities for
/eri-butyl cation 2 g the ratio k ~ k =, 8.31 is obtained, and for
competing nucleophiles with the same s parameter.
the prenyl cation 2 h the ratio k J k , = 1.85. Sincc none of these
This apparent contradiction is resolved in Figure I , where the
ratios refer to a reaction within the scope of the linear reactivity
results of direct kinetic measurements and of competition experrelationships. only approximate values of E for the correspondiments are presented side by side. A noticeable decrease of the
ing cations can be derived from Figure 1 .
The procedure presented herein basically
corresponds to Jencks's diffusion clock,"'- 1 9 ]
which employs the fact that many carbocations
undergo diffusion-controlled reactions with
the azide ion in aqueous solution ( k = Sx
lo9 L mol s- I )
Therefore. one can use
the alkanol/alkyl azide ratio, which is obtained
by solvolytic generation of carbocations in
aqueous azide solution. for calculating the rate
constant of the reaction of the carbocation
with water. As the azide clock uses a single pair
of competing nucleophiles. its acope for the determination of E parameters is limited, and it
cannot be used to establish E values greater
than 6.5 because of the large value of k-,,20
which then is outside the scope of the linear free
energy relationships. Our previous investigations on the reactivities of 7c n~iclcophiles,['~
the other hand. provide a large choice of reac-8
less reactive
more reactive
tion partners and allow the selection of suitable
pairs of nucleophiles for determining the elecFig. 1 ('hangcobel- from constant selectivity relationships to the reactivity~~selectivity
trophilicity parameter E for ;my electrophile.
An = p M c 0 C ' * , t i 4 . Fc = ferrocenyl. Fur = 2.3-dihydrobenzofuran-5-yL To1 = /I-MeC,H, Directl)
Table 1 , Rclatiw rc.ictivilic\ ol'carbocations t o n a r d allylsilanes (CH,CI,. -70 C)
.ind electrnphilicit) p'mineters.
mc.i\urcd i-;ite comtaiitr .ire marked by dots, results from competition expcriments b) shaded bars.
Received: M;I) 13. 19'15 (Z 79841E]
German version: Anprit. Chcwi. 1995. /07.2478 2430
selectivity docs not occur until the rate constant k of one of the
parallel reactions exceeds lo8 L mol-' s-', that is, when the
inore reactive competitor approaches the diffusion limit." 3 1 Figure 1 now unambiguously corroborates the conclusion that constant selectivity relationships and the reactivity -selectivity principle hold in different ranges of reactivity, as previously derived
from studies of' relative rea~tivities."~.
Kinetic investigations of laser-flash photolytically generated
carbocations i n acetonitrile at 20 "C showed that different types
of highly reactive uncharged nucleophiles react with almost
identical rate constants k = (2-4) x lo9 L mol-' s-',['I indicating that this value corresponds to the diffusion limit. As
acetonitrile and dichloromethane have almost identical viscosities at 20 C (qcH3cN= 0.39 mPa s, qCH2C12
= 0.43 mPa s), one
Keywords: carbocations kinetics linear frec energy relationships . reactivity-selectivity principle
[ I ] H . Mayr, M. Patr. A i i q w C ' h c i i i . 1994. /Oh. 9YO -1010. 41i,qiw. C'hwii. fin Ed.
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[12] E. Buncel, H. Wilson. J Cheni. E&. 1987, 64. 475-480.
[I31 The decrease of the competition constants cannot exclusively hc due to diffusion control of one of the pirrallel reactions. There are indications that in this
region the position of the transition state on the reitction coordinate is shifted
(Hammond effect), that is the classical rationalization of the reactivity sclectivity principles may hold in this range [lS].
[I41 R. Fa-Shma. Z. Rappoport. J ,4111 Cheni. Sol.. 1983. 105. 6082 6095.
[15] J. P Richard. T~rruheciroi~
1995. 5 1 . 1535-1573.
[I61 J. P. Richard. M. E. Rothenberg, W. P. Jencks. J A m . C h ~ mSo[..
. 1984, l06,
[I71 J. P. Richard, W. P. Jencks. J A m C ~ ~ ISol..
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[I81 J. P. Richard. W. P. Jencks. J Am Chern. Soc. 1982. 104. 4689 4691
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Chon. SIC.
1991. 113, 1009-1014.
la, l b
3a, 3b
Alteration of the Reactivity of a Tellurophene
Within a Core-Modified Porphyrin Environment :
Synthesis and Oxidation of 21-Telluraporphyrin**
Lechoslaw Latos-Grazynski,* Ewa Pacholska,
Piotr J. Chmielewski, Marilyn M. Olmstead, and
Alan L. Balch"
Recent work on core-modified porphyrins, in which one or
more heteroatoms 0, S, Se, o r CH replace some of the four
nitrogen atoms, has produced new macrocycles with altered
core sizes and metal ion binding properties.[' - 4 1 The 21 -thiaporphyrin has been utilized to produce y ~ 'coordination of the thiophene ring to a variety of transition metal ions.[" Other studies
have been concerned with the effect of heteroatom substitution
on the aromaticity of the m a c r ~ c y c l e . [Here
~ ~ we report on the
synthesis of the 21 -telluraporphyrins 1 and their conversion by
oxidation into the 21 -oxaporphyrins 2,[51which proceeds via the
novel tellurium hydroxy compounds 3 as isolable intermediates.
The reactions of the new macrocycle 1 demonstrate the remarkable ability of the porphyrin environment to alter the fundamental reactivity of the tellurophene portion.
The synthetic work is summarized in Scheme 1 . The 21 -telluraporphyrins 1 a and 1 b and the 21-oxaporphyrins 2a-c have
been synthesized by the condensation of pyrrole, a substituted
benzaldehyde (4a, 4b, or 4 c . respectively) and the appropriate
x,d-disubstituted tellurophene 5 or furan 6. These procedures
follow the methodology previously utilized for the preparation
of 21 -thiaporphyrin and 21 -selenaporphyrin.['. 31 After chro-
2a, 2b, 2c
Scheme 1 . Summary of the reactions of the core-modified porphyrins discussed in
the text.
matographic workup, the macrocycles 1 a and 2a were obtained
in 20 and 15 YOyield. respectively. Both 1 a and 2 c have been
characterized by X-ray crystallography (Figs. t and 2) .I6, 'I The
size of the tellurium atom (Te-C 2.083(9); Te-C4 2.065(9) A)
results in a distortion of the core of l a such that the non-
[*] Prof'. Dr. L. Latos-Graiybski. E. Pacholska. Dr. P. J. Chmielewski
Institute of Chemistry
University of Wroclaw
50353 Wroclaw (Poland)
Telethx' Int. code +(71) 222-348
e-mail: Il~(~i'
Prof. Dr. A. L. Balch, Dr. M. M. Olmstead
Dcpdrtment of Chemistry
University of California. Davis, CA 95636 (USA)
Telefax: lnt. code +(916) 752-8995
e-mail: alhalchlri
This work was supported by the State Committee for Scientific Research KBN
of Poland (Grant 2.0732 91 01) and thc U.S. National Science Foundation
(Grant INT-9114389).
Fig. 1. Two views of the crystal structure of the 21-telluraporphyrin 1 a (thermal
ellipsoids for all non-hydrogen atoms for 50% probability). The lower drawing. in
which the aryl groups are omitted. emphasizes the deviations from planarity.
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determinism, carbocations, free, reactivities, energy, clock, diffusion, relationships, reactivityцselectivity, principles, linear, coexistence
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