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Front Strain of and Radicals.

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This will be demonstrated for the acid-catalyzed esterification
of carboxylic acids.
c
\
[2]
[3]
,121
[4]
151
[6]
[7]
[XI
I
1
1
3
I
-
I
\
5
S, I I O ' + I ~ O I - ' I
Fig. 1 . Correlation of the Es values (=logk,.,) for acid-catalyzed esterification
of carboxylic acids (R-COOH) with 9,values; values corresponding to
the open symbols were taken from P . J . Sniegoski, J. Org. Chem. 41, 2058
(1976), and the other from [l]. Correlation coefficients: R=prim-alkyl (o),
r = -0.962 (9 values); R=sec-alkyl (A), r = -0.972 (9 values); R=tert-alkyl
(m), r = -0.994 (4 values). The differences in the intercepts are highly significant
according to the Student's T test (confidence limits 99.9%).
This reaction is subject to steric hindrance by an increase
in steric strain on transformation of the carbonyl C atom
of the carboxylic acid into a four-coordinate C atom in the
transition state['.61. In the ground state the alkyl groups R
can avoid the repulsion exerted by the flat carboxyl group
to various degrees, depending upon the number of substituents
present on the a-C atom ofR, by rotation about the R-COOH
bond. The a-substituent in primary R can always find a "conformational gap", the two wsubstituents in secondary R can
avoid interaction less easily, while the tertiary groups R experience the full effect of repulsion. These possible ways of evasion
are limited in the transition state and should therefore appear
as differences in reactivity. Correlation of the reaction rates
(logk,,,=Es) with the Yf parameters gives linear plots for
primary, secondary, and tertiary groups R with similar slopes
but significantly different intercepts (Fig. 1). This is to be
regarded as an indication that the Es parameters include
an additional effect depending upon the number of H atoms
attached to the a-C atom of R. Most likely this effect is
due mainly to the special conformational features of the carboxylic acids.
As may be seen from this and another example['Obl, Yf
values prove to be a useful tool for the analysis of F strain
effects on reactivity. They are easily calculated even for very
bulky groups, e.g. ( 2 4 ) and ( 2 5 ) , and numerous unsaturatedrlLb1
hydrocarbon groups.
Received: April 20, 1978 [Z 993a IE]
German version: Angew. Chem. 90,633 (1978)
CAS Registry numbers:
( 6 ) , 2229-07-4; ( 7 ) , 2679-29-0; ( 8 ) . 3170-58-9; ( 9 ) , 21246-84-4; (101,
3268-43-7; ( l l ) , 22904-47-8; ( l 2 ) , 58281-61-1; ( 1 3 ) , 19067-45-9; ( 1 4 ) ,
2669-89-8; ( l S ) , 2417-82-5; ( 1 6 ) , 2396-01-2; ( l ? ) , 30734-19-1; (18),
22904-45-6; XCCI, (X = Cl), 56-23-5; XCCI, (X = Br), 75-62-7
[l] Surveys: J . Shorter in N. B. Chapman, J . Shorter: Advances in Linear
Free Energy Relationships. Plenum Press, London 1972; S. H . Unger,
C . Hansch, Prog. Phys. Org. Chem. 12, 91 (1976); H . Forster. F . Vogtle,
Angew. Chem. 89, 443 (1977); Angew. Chem. Int. Ed. Engl. 16, 429
(1977).
594
[9]
[lo]
[ll]
[12]
1' was defined as the diameter of a substituent. However, the numerical
values of v for all alkyl groups (Me and tBu are exceptions) were
calculated from the rates of formation and hydrolysis of carboxylic
esters: M . Charton, J. Am. Chem. SOC.97, 1552, 3691 (1975).
C . G. Swain, E. C . Lupton, J. Am. Chem. SOC. 90, 4328 (1968). Since
conventional suhstituent parameters d o not distinguish energetic and
enthalpic effects, only isokinetic reactions have so far been dealt with
in linear free-energy relationships: cf. 0. Exner, Progr. Phys. Org. Chem.
10, 411 (1973).
a) N. L . Allinger, Adv. Phys. Org. Chem. 13, 1 (1976); b) E . M. Engler,
J . D . Andose, P . v. R . Schleyer, J. Am. Chem. SOC. 95, 8005 (1973);
c) N . L . Allinger, M. 7: Tribble, M. A . Miller, D. H . Werrz, ibid. 93,
1637 (1971); d) J . D . Andose, K. Mislow, ibid. 96, 2186 (1974).
R . C. Bingham, P . u. R . Schleyer, J. Am. Chem. SOC.93, 3189 (1971);
J . L . Fry, E. M . Engler, P . v. R . Schleyer, ibid. 94, 4628 (1972).
D. F. DeTar, J. Am. Chem. SOC. 96, 1254 (1974); D . F. DeTar, C.
J . Tenpas, ibid. 98, 4567, 7903 (1976).
H . C . Brown, M. D . Taylor, H . Bartolomay, J. Am. Chem. SOC. 66,
435 (1944); J . Slutsky, R . C . Bingham, P . v. R . Schleyer, W C . Dickason,
H . C . Brown, ibid. 96, 1969 (1974).
a) H.-D. Beckhaus, C . Riichardt, Chem. Ber. 110, 878 (1977); b) C .
Riichardt, H.-D. Beckhaus, G . Hellmann, S . Weiner, R . Winiker, Angew.
Chem. 89, 913 (1977); Angew. Chem. Int. Ed. Engl. 16, 875 (1977);
c) H.-D. Beckhaus, G. Hellmann, C. Riichardt, Chem. Ber. 111, 72 (1978).
a) P . Miiller, J . C . Perlberger, J. Am. Chem. SOC. 97. 6862 (1975);
98, 8407 ( 1 9 7 6 ) ; b ) J . C . Perlberger, P . Miiller, ibid. 99, 6316 (1977).
a) B. Giese, Angew. Chem. 88. 723 (1976); Angew. Chem. Int. Ed.
Engl. 15, 688 (1976); b) B. Giese, H.-D. Beckhaus, ibid. 90, 635 (1978)
and 17, 594 (197X), respectively.
a) The energy unit lo4 J/mol was chosen in order to provide convenient
numerical values for the Yr parameters. b) The analogous differences
AHP(R-tBu) -AHP(R-Me)
calculated with a force field according
to Allinger (1971) [4c] could he correlated linearly with the 9,values
(slope a=1.06). Hence the calculation of 9,values for alkenyl groups
and groups containing phenyl rings [4d] is possible in principle.
a) A . Liden, C. Roussel, 7: Liljefors, M. Chanon, R . E. Carter, J . Metzger,
J . Sandstrom, J. Am. Chem. SOC. 98, 2853 (1976); h) H . K w a r t , S .
Alekman, ibid. 90, 4482 ( 1 968).
Front Strain of n and CT Radicals[**]
By Bernd Giese and Hans-Dieter Beckhaus[*]
Radicals are classified as planar 7c radicals (1) and nonplanar
G radicals (2).
It was deduced from the temperature dependence of the
13C and 'H coupling constants of ESR spectra"] that the
pyramidal conformation becomes energetically more favorable
with increasing alkylation of the radical carbon atom'']. While
the methyl radical is planar (K radical), the tert-butyl radical
should possess almost tetrahedral geometry (G radical)", 'I.
More recent studies have shown that the bending of the tertbutyl radical is caused by solvents at low temperatureL31.
In the absence of this matrix effect the tert-butyl radical should
have planar geometry, like the methyl radicalf3].
Evidence for a common structure of alkyl radicals above
273 K in solvents of low polarity such as CC14 is also provided
by our observation[41that the reactions of methyl and of
[*] Prof. Dr. B. Giese ['I, Dr. H.-D. Beckhaus
Chemisches Laboratorium der Universitat
Albertstrasse 21, D-7800 Freihurg (Germany)
['I T o whom correspondence should be addressed. New address:
Institut fur Organische Chemie und Biochemie der Technischen Hochschule
Petersenstrase 22, D-6100 Darmstadt (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. Engl. 17 ( 1 9 7 8 ) No. 8
prim-, sec-, and tert-alkyl radicals in the competition system
BrCCI3/CCl4
can be described by a common isoselective relationship[41
which differs significantly from the isoselective relationship
of G radicals.
In order to gain afurther insight into the preferred conformation of alkyl radials we have developed a method based on
the various degrees of steric shielding of the radical carbon
atoms. Thus, owing to their planar geometry n radicals (I)
should have a greater F strain than similarly substituted o
radicals (2) because the latters' substituents will be bent away
from the frontside. The F strain of the groups R can be
calculated from the differences between the heats of formation
of the hydrocarbons ( 4 ) and (5)Is1.A suitable radical reaction
is halogen capture according to eq. (a) which is controlled
by steric parameters of the radicals (3)[61.
Plotting the experimentally determined activation energies
AH&-AH& of the competing radical reaction (a) versus the
values of the F strain Yf calculated from ( 4 ) and (5) gives
two straight lines (Fig. 1).
The lower, less steep line represents the o radicals whose
carbon atoms do not have a planar conformation either in
the radical (3) or in the hydrocarbons ( 4 ) and (5). The
steeper slope of the upper line shows that the corresponding
radicals have a larger F strain than is calculated for the
nonplanar hydrocarbons ( 4 ) and (5). In the transition state
of halogen abstraction (a) the radicals (6)-(13)
therefore
exert a comparatively greater repulsion on the halogen transfer
agent XCCl3 (X=C1, Br) than the CT radicals (24)-(18).
These differing steric effects indicate that there is only slight
pyramidalization, if any, in the radicals (6)-(13). Not only
the methyl radical ( 6 ) but also the prim-, sec-, and tert-alkyl
radicals (7)-(13) are therefore largely planar above 273 K.
A transition from planar to tetrahedral conformation with
increasing number of alkyl substituents[*]cannot be reconciled
with our experimental results.
Received: April 20, 1978 [Z 993b IE]
German version: Angew. Chem. 90, 635 (1978)
CAS Registry numbers:
(1),2229-07-4;(2),2025-56-1; (3),2143-61-5; (4),2492-36-6; (5),2672-01-7;
(6), 2679-29-0; (7), 3356-67-0; (a), 4606-96-6; (9), 20199-83-1 ; ( I O ) ,
67271-34-5; (ll), 4630-45-9; (12), 3744-21-6; (13), 22904-47-8; (14),
2025-55-0; (15), 2348-55-2; (16), 22903-92-0; (17), 3268-43-7; ( l a ) ,
23443-59-6; (19), 20693-38-3; (20), 67271-35-6; (21), 67271-36-7; (22),
41770-95-0; (23), 67271-37-8; (24), 1605-73-8; (25), 4348-35-0; (26),
24436-95-1; (27), 58281-61-1; (28), 19067-45-9; (29), 28013-53-8; (30),
24436-96-2; (31), 4548-06-5; (32), 67151-55-7; (33), 2417-82-5; (34).
3889-74-5; (35), 3170-58-9; (36), 4566-80-7; (37), 67271-38-9; (38),
2697-23-6; (39), 67271-39-0; (40). 30734-19-1; (41), 21246-84-4; (42),
2697-21-4; (43), 33968-73-9; (44). 2819-03-6; (45), 21517-94-2; (46),
16998-65-5 ; (47), 6727 1-40-3 ; (48), 67271-41-4
[l] J. B. Lisle, L. F . Williams, D. E. Wood, J. Am. Chem. SOC.98, 227
(1976); P. J. Krusic, P. Meakin, ibid. 98, 228 (1976); cf. also 7: Koenig,
7: Balk, W Snell, ibid. 97, 662 (1975).
121 P. J . Krusic, R. C. Bingham, J. Am. Chem. SOC.98, 230 (1976).
131 L . Bonazzola, N . Leray, J. Roncin, J. Am. Chem. SOC.99, 8348 (1977).
[4] B. Giese, Angew. Chem. 88, 159 (1976); Angew. Chem. Int. Ed. Engl.
I S , 173 (1976); ibid. 89, 162 (1977) bzw. 16, 125 (1977).
[5] H.-D. Beckhaus, Angew. Chem. 90, 633 (1978); Angew. Chem. Int. Ed.
Engl. 17, 593 (1978).
[6] B . Giese, Angew. Chem. 88, 723 (1976); Angew. Chem. Int. Ed. Engl.
15, 688 (1976).
T I - Radicals
Temperature Dependence of Carbene Selectivities:
A New Example of the Ismelective Relationshipl**]
By Bernd Giese and Jiirgen Meister"]
Hardly any absolute rate constants have been published
so far for carbene reactiond']. Selectivity values (relative reactivities) therefore usually have to be employed in interpreting
such reactions['! However, this can lead to serious errors
if temperature effects are disregarded. For example, several
series of reactions are known in which the selectivities of
molecules become equal in a narrow temperature range, the
-
I
I
I
I
I
0
2
4
6
8
9,[10'1 m o ~ " ~
Fig. 1. Variation of the differences in activation energies A H & - A H g r of
halogen abstraction (a) with the values of F strain Y f calculated from the
differences in heats of formation of (4) and ( 5 ) . (6): CH3, (7): 1-C6H;,,
(8): c-C,H;!,
(9): 2-bicyclo[2.2.2]octy1,
(10):
?-CBHI,, (11):
C ~ H K ( C H ~ ) I C H(12):
~,
C ~ H ~ ~ C ( C H(13):
~ ) Z , C H J C ( C ~ H ~ (14):
)~,
CHl=CH, (15): c-C,H;, (16): C6H;, (17): 7-norbornyl, (18): 2-tC4H&e,Hi.
Angew. Chem. Int. Ed. Engl. 17 (1978) N o . 8
[*] Prof. Dr. B. Giese ['I, Dr. J. Meister
Chemisches Laboratorium der Universitat
Albertstrasse 21, D-7800 Freiburg (Germany)
['I To whom correspondence should be addressed.
New address:
Institut f i r Organische Chemie und Biochemie der Technischen Hochschule
Petersenstrasse 22, D-6100 Darmstadt (Germany)
[**I This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
595
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