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Diastereoselective Radical Recombination and Variation in Thermal Stability of Diastereomeric Hydrocarbons.

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their dimers meso- and ~~-3,4-di(p-chlorophenyl)-2,2,5,5-tetraf5) cannot disproporsince the
tionate, owing to the fact that there are no p-H atoms['"]
and because blocking of the p-position leads to formation
of only the cl,cc-dimer ( 4 ) , but no ~,p-dimer[~l.
In addition,
the different thermal stability of the non-chlorinated dimers
corresponding to ( 4 ) and their different conformations were
known['] from X-ray structuralr7"] and NMR analysis[7b!
The configurational and conformational analyses of (4) could
therefore be carried out analogously as in Ref. [61.
[I] Unstable Intermediates in the Gas Phase, Part IO.--Part 9: H . Bock.
8. Solouki, J . Wirrmmn, Angew. Chem. 90, 985 (1978); Angew. Chem.
f4)3
Int. Ed. Engl. 17, 932 (1978).
[2] Lectures delivered by H . Bock, IUPAC Congress, Tokyo Aug. 8, 1977;
and by B. Solouki, GDCh General Meeting, Miinchen Sept. 14, 1977.
[3] Among further application the oxidizability of molecules, can be determined, cf., e. g. H . Bock, G. Briihler, G. Fritz, E. Marern, Angew. Chem.
88, 705 (1976); Angew. Chem. [nt. Ed. Engl. 1 5 , 699 (1976); H . Bock,
W K a i m , Tetrahedron Lett. 1977, 2343; J. Organomet. Chem. 135, C14
(1 977).
[4] C . J . J a m , J. Am. Chem. SOC.74,4529(1952).
tRu tRu
Diastereoselective Radical Recombination and Varia1 \ C-&
tion in Thermal Stability of Diastereomeric Hydrocar- c 1 0 H:
bans[**]
loci
\
kzy
H
meso - ( 4 )
By Karl-Heinz Eichin, Kevin J . McCullough, Hans-Dieter Beckhaus, and Christoph Riichardt[*]
Dedicated to Professor Heinrich Hellmann on the occasion of
his 65th birthday
The rate of dimerization of simple alkyl radicals in solution
is determined by their rate of diffusion[']; I-phenylethyl and
2-phenyl-3-methylbutyl each give two diastereomeric dimers
in the ratio 1 : l['a*21.
Bulky alkyl radicals such as di-tert-butylmethyl or triisopropylmethyl, on the other hand, are persistentu3]and d o not dimerize. The dimerization of medium-sized
radicals therefore ought to proceed via an activation threshold.
An indication of this was provided by kinetic investigation
of the thermolysis of highly-strained alkanes (I)r41, from which
it could be concluded that 40% of the strain enthalpy of
(1) is retained in the transition state (2) of homolytic decomposition.
R'R2R3C-CR1R2R3
(1)
-%
[R1R2R3C,-.CR'RZR3]* -+
(2)
k:x:;s
tBU
2 C&oc(BU
-
H
H
H
H
DL- ( 4 )
The thermolyses of meso- and of DL-(4) in l-methylnaphthalene with 10 vol- % thiophenol was investigated kinetically
and preparatively. At 300"C, 1-(p-chlorophenyl)-2,2-dimethylpropane is formed in 80% yield from meso-(4) and 75%
yield from DL-(4). In addition, the stable DL-(4) was also
formed from meso-(4) by cage rec~mbination[~]
(ca. 10% .
in 3 halflives). From DL-(4) we also obtained ca. 3 % of
a product which was most likely 1,2-di(p-chlorophenyl)-3,3dimethylbutane. Hence, the thermolysis of (4) proceeds almost
exclusively by cleavage of the central c-c bond; the cage
effect does not seriously interfere in the kinetic investigation.
The rate of decomposition was determined by measuring the
decrease in concentration of (4) by GC at 6 temperatures
over a range of 50-60°C. The results are listed in Table
2 R'R2R3C'
(31
Hence, the dimerization of (3) to (1) should also have
to overcome a corresponding activation barrier; the distribution of the diastereomeric dimers ( I ) is then no longer random.
1['01,
Table 1. Kinetics of the thermolysis of nieso-(4) and DL-(4) in I-methylnaphthalene with 10 vol-% thiophenol.
(4)
A H * k o [a]
[kcal/mol]
AG*(300"C)
[kcal/mol]
~
DL
meso
AS* + r i [a]
[cal/mol. K]
kr,i(300"C) [b]
["CI
= 1.0
301
279
18.5f0.9
18.0 f 1.9
[c]
-
~~
50.0 f0.6
47.8 1 .o
+
T ( r l / 2 = 1 h)
'I
rs-
~~
39.4
37.5
5.5
~-~~~
~
[a] Standard deviation, 1-10.999; [b] a i 2 . 5 %; [c] temperature for halflife
The ratio of the yields is determined by the different strain
enthalpy of the diastereomers of ( I ).As has been demonstrated
e~perimentally[~I,
themeso- and DL-formsof(1) with R' = tertbutyl, R2= cyclohexyl, and R3 = H, differ in strain enthalpy
and thermal stability. l-Cyclohexyl-2,2-dimethylpropyl,or
other similarly bulky radicals, should therefore recombine
diastereoselectively.
As model compounds for testing this hypothesis we chose
the 1-(p-chlorophenyl)-2,2-dimethylpropylradicals (5) and
[*] Prof, Dr, Ch. Richardt, Dip[,-Chem. K,-H. Eichin, K,
Dr. H.-D. Beckhaus
Chemisches Laboratorium der Universitiit
['I,
J.
McCullough
Albertstrasse 21, D-7800 Freiburg (Germany)
[ '1 Visiting Scholar from the University of Strathclyde, Glasgow (Scotland).
[**I Thermolabile Hydrocarbons, Part 9. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.-part
8 : G . Hellmaim, H . - D . Beckhaus, c. Riichardt, Chem. Ber., in
press.
934
I
(61
(5)
tBu
c 1 o + - + o c 1
tBu
c cl+-N=,,
-
I , , ~ =I
h.
For determination of the diastereoselectivity of the recombination of ( 5 ) , 1-(p-chlorophenyl)-2,2-dimethyl-l
-azopropane (6) was thermolyzed at 135.2"C in n-decane; (4) was
obtained in 98 % yield and, according to G C analysis, consisted
of DL-(4) and meso-(4) in the ratio 1.66k0.05 :1. This constitutes the first demonstration of the diastereoselectivity of a
radical recombination["]. For the recombination of ( 5 ) it
is calculated that AAG* (135.2"C)=0.4 kcal/mol. Attempts
to determine the temperature dependence of the yield of diastereoisomers by photolysis of ( 6 ) have so far failed because
Of the photo1abi1ity"21 Of f 4 ) towards 253.7-nm light. From
the activation parameters of the thermolysis of meso- and
of DL-(4) it follows that AAG* (135.2"C)=2.0 kcal/mol for
homolytic decomposition. Accordingly DL-(4),
with
BAG * = 2.4 kcal/mol, is thermodynamically more stable than
meso-(4). Since AS* of the thermolysis of meso- and DL-(4)
is Constant within the error of measurement, a difference in
strain enthalpy of AAH=2.4 kcal/mol is obtained for the
Angew. Chem.
Inr. Ed. Engl. I 7 (1978) N o . 12
two diastereomers of (4)-in good agreement with the result
of force field calculations[' 31 and the known structures['. 'I.
Received: August 1 1 , 1978 [Z 97 IE]
German version: Angew. Chem. YO, 987 (1978)
Publication delayed at authors' request
CAS Registry numbers:
m e s o - ( / ) , 62678-51-7; DL-(I ), 68525-39-3; ( 3 ) - 68525-40-6; m e s o - ( 4 ) , 6852541-7; D L 4 4 ) . 68525-42-8; ( 5 ) , 68525-43-9; (6). 68525-44-0
a) M. J . Gibiun, R . C. C o r k y , Chem. Rev. 73, 441 (1973); b) H . Schuh,
H . Fischer, Int. J. Chem. Kinet. 8, 341 (1976); c) P . R y s , Acc. Chem.
Res. 9, 345 (1976).
N. Leri, D. S. Molumenr, J. Chem. SOC.Perkin Trans. I I , 1249 (1976);
F. D. C r e m e , M. A . Berwick, J . C. Stomell, J . Am. Chem. SOC. 92,
867 (1970); M . J . Gibiun, R . C. C o r k y , ibid. 94, 4178 (1972).
D . Griller, K. I / . Ingold, Acc. Chem. Res. 9, 13 (1976).
C . Riichardt, H.-D. Beckhaus, G. Hellmann, S. Weiner, R . Winiker, Angew.
Chem. 89, 913 (1977); Angew. Chem. Int. Ed. Engl. 16, 875 (1977).
Cf. r.y. K . S . Skinner, H . S. Hochsrer, J . M. McBride, J. Am. Chem.
SOC.96, 4301 ( I 974).
Dissertation by G. Hellmunn, Universitat Freiburg 1977. and Part 8
of this series [**I. The DL diastereoisomer decomposes at 300°C with
2.5 kcal/mol higher free activation enthalpy (AG') than the meso form.
a ) H . J . Lindner, B. Kirschke, unpublished results. b) H . Fritz, unpublished results; the meso compound is present in the onri-conformer,
the DL-compound in the gauche-I-structure [S].
H.-D. Beckhuus, G. Hellmann, C. Riichurdt, B. Kitschke, H . J . Lindner.
H . Fritz, Chem. Ber., in press.
T Kornig, H . Fischrr in J . K . Kochi: Free Radicals, Vol. I , p. 157.
Wiley-lnterscience, New York 1973.
The p-chloro substitution leads in m e s o - ( 4 ) to 1.1-fold acceleration,
but in D L 4 4 ) to 1.8-fold acceleration (300°C) relative to the thermolysis
of meso- and ~~-3,4-diphenyl-2.2,5,5-tetramethylhexane[6]. An
increase of the dipole moment by substitution is expected only in
the case of D L - ( 4 ) because of its gauche conformation 16, 71.
In the dimerization of substituted benzyl radicals the meso dimer is
often isolated in higher yields as the DL-form. Conclusions on the
composition of the crude product, which was not analyzed in any
one of the cases, are unreliable. Cf. P . Gouoerneur, Ind. Chim. Belge
3Y, 467 (1974) and [6].
Contrathermodynamically, D L 4 4 ) is photochemically more labile than
meso-( 4 ).
Using the force field of Allinger (1971) and the phenyl parameters
of Mislow (SET-B),a difference in strain enthalpy of A A H = 2.9 kcal/mol
[6] was calculated for the non-chlorinated analogues of meso- and
DL-(4).
Synthesis and Characterization of Liquid-Crystalline
Polymers with Cholesteric Phase
By Heino Finkelmann, Johanna Koldehof, and Helmut Ringsdorf"']
We have already demonstrated by means of models that
it should be feasible to systematically produce polymers with
thermotropic liquid-crystalline phases by analogy with low
molecular-weight liquid crystals"'. The synthetic principle of
these polymers lies in a side-chain type linkage to a polymeric
backbone of known molecular units leading to a liquidcrystalline phase (mesogenic groups). It is essential that
the mesogenic groups be bonded via flexible spacer groups
(e.g. alkyl chains) to the polymer backbone, thus uncoupling
the motions of the main and side chains. Nematic and smectic
polymers have been synthesized in this way['].
The synthesis of cholesteric polymers['] requires that the
polymerizable molecules be chiral. Apart from a known copolymeric cholesterol derivativec4], all attempts to polymerize
cholesteric monomers led only to smectic polymers[31.
Studies on low molecular-weight liquid crystals have shown
that a cholesteric helical structure can be induced in a nematic
phase by addition of a chiral compound, even if the chiral
-.
[*] Dr. H. Finkelmann. J. Koldehoff, Prof. Dr. H. Ringsdorf
lnstitut fur Orgdnlsche Chemie der Universitat Mainz
J.-J.-Becher-Weg 18-20, D-6500 M a i m (Germany)
Angen.. Chem. lnt. Ed. Engl. 1 7 (1978) N o . 12
component itself is not liquid crystalline['! Thus copolymerization of a nematogenic monomer, exhibiting a nematic phase
as homopolymer, with a chiral monomer should yield induced
cholesteric polymers.
The known biphenylyl benzoate derivative ( 1 ) served as
nematogenic monomed'l. Its homopolymer exhibits a smectic
phase at low temperature in addition to a broad nematic
phase (see Table 1). The chiral component ( 2 ) , which is itself
not liquid crystalline as homopolymer, was chosen with the
following points in mind:
1) Owing to the amount of chiral (-)-1-phenylethylamine
present, small pitches pare expected for the cholesteric phase[']
at low concentration of (2) in the polymer.
2) In order to minimize perturbation of the nematic phase
of polymeric ( I ),compound (2) likewise contains a mesogenic
group (phenyl benzoate unit).
3) To ensure random incorporation of monomers ( I ) and
(2) in the polymer and that their contribution to the polymer
corresponds to the composition of the monomeric mixture,
the polymerizable groups should have approximately equal
reactivities. Thechiral monomer (2) is therefore also a methacrylate.
FH3
CHZ=C-C,
40
FH3
CHZ=C-C,
(1)
40
0-R2
0-R'
(2)
Both monomers ( 1 ) and (2) undergo radical polymerization. The phase transitions of the polymers are compiled
in Table 1. Studies under a polarizing microscope show that
above the glass transition temperature TG a layer of the
polymers located between glass slides spontaneously assumes
a cholesteric Grandjean texture. For polymers having
x12,>0.17 this texture is to be seen in the reflection of visible
circularly polarized light. It is particularly interesting that
the reflecting Grandjean texture remains fixed in the glassy
state of the polymer on cooling.
Table I . Composition, phase transitions, and reflection wavelengths ;.Rof
the polymers
1.000 (1.000)
0.906 (0.89)
0.836 (0.83)
0.798 (0.79)
0.753 (0.75)
-
0.094 (0.1I )
0.164 (0.17)
0.202 (0.21)
0.247 (0.25)
Tc 60 [d] s I33 n 271 i
TG 70 n* 247 i
T, 73 n* 229 i
T G 7 7 n * 216i
T,; 80 n* 203 i
~ ; i
1260
712
562
467
[a] Mole fractions based on monomer unit (mole fractions of monomer
mixtures given in parentheses).
[b] g= glass transition temperature (from DSC measurements; Perkin-Elmer
DSC I B); n, n*, s = transitions to the nematic, cholesteric, and smectic phase,
respectively; i =isotropic phase.
[c] At T=0.9; T,.,.".,.,,.,/r,,,,,,,,
point.
[d] Determined as softening point.
The cholesteric polymers were characterized like conventional cholesteric mixed phases by optical studies on the re935
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