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How High is the Activation Barrier to Helix Inversion of 1 4 5 8-Tetrasubstituted Phenanthrenes.

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172.1 ( s ; C I ) , 51.5 (q: OMe), 42.5 ( d ; C-3), 34.6 (t; C-2), 13.3 ( q ; 3-Me);
"C-NMR (CD2C12)for la+TiCI,: 6=215.0 ( d ; C-4), 180.9 (s; C-I),
58.4 (q. OMe), 43.3 (d; C-3). 35.9 ( t ; C-2). 14.1 (4; 3-Me). IR (CDCI,) for
la +TiCI,: C(CO)= 1658 e m - ' .
[9] For (presumably) chelate-controlled addition of cuprates to chiral aldehydes, see: W. Clark Still, J. A. Schneider, Tetrahedron Letr. 21 (1980)
1035; see also Y. Yamamoto, K. Maruyama, J Am. Chem. Sor. 107
(1985) 641 I .
[lo] a ) M. Cherest, H. Felkin, N. Prudent, Terrahedron Lett. 1968. 2199; N.
T . Anh, Top. Curr. Chem. 88 (1980) 145; b) a recent theoretical study
supporting the Felkin-Anh model: Y.-D. Wu, K. N. Houk, J. Am. Chem.
SOC.109 (1987) 908; c) recent experimental work: E. P. Lodge, C. H.
Heathcock, ;bid. 109 (1987) 3353.
[ I I] Since R ' = M e is expected to be sterically less demanding than the
C H 2 C 0 2 M egroup, the latter possibly exerts a dominating electronic effect as well.
[I21 For synthesis of 6a see Ref. [4] as well as C. Gunther, A. Mosandl, Liebigs Ann. Chem. 1986. 21 12, and references cited therein.
[I31 a) l a is accessible with 93% ee if an optically active auxiliary reagent is
used in equimolar amounts: A. Bernardi, S. Cardani, G. Poli, C. Scolastico, J. Org. Chem. 51 (1986) 5041; b) l a - c have been synthesized with
up to 50% ee by means of asymmetric catalysis: T. Kunz, H.-U. Reissig,
unpublished.
How High is the Activation Barrier
to Helix Inversion of
1,4,5,8-Tetrasubstituted Phenanthrenes?**
By Albrecht Mannschreck, * Erich Gmahl,
Thomas Burgemeister, Fritz Kastner, and Volker Sinn well
In 1947, the synthesis of (4,5,8-trimethyl-l-phenanthry1)acetic acid 1 and the enrichment of its (+) enantiomer were reported.[ZiThis kind of enantiomerism was
termed isomerism of the 4,Sphenanthrene type"] and must
be considered the starting point of helicene ~hemistry.'~'
This isomerism, described by equilibrium (a), contributed
substantially to our knowledge of what is presently called
molecular h e l i ~ i t y . 'The
~ ~ above enrichment of an enantiomer has been frequently cited, its significance being
shown by its appearance in several textbook^.'^]
Me5 Me'
I
Me5 MeL
The values obtained differ slightly[6-'01but are in agreement concerning the orders of magnitude (t(]s = 4 h,
AG' -99 kJ mol-I). These values are similar to the ones
obtained
for
4,5-dimethyI-9,10-dihydrophenanthrene
(2.8 h, 98.4 kJ mol-', 24.8"C, benzene).fh."l This result was
unexpected['] because under analogous conditions another
phenanthrene derivative had shown a considerably lower
activation barrier than the comparable 9,IO-dihydrophenanthrene.[l'] The AG' values of 4,5-bis(acetoxymethy1)phenanthrene (75.7 kJ mol - ', 24.8"C)"' and 4,5-dimethylphenanthrene (67 kJ mol - I , 25 oC)"O1are also considerably lower than the value of ca. 99 kJ mol-' obtained for
1 . It has been pointed out by several g r o ~ p s [ ' . ~that
.'~~
these differences remain to be explained. Toward the solution of these discrepancies, we report here the determination of the activation barrier to helix inversion ( M ) + ( P ) of
phenanthrenes 1-4.
Our preparation of 1 is directed toward cyclodehydrogenation as the last step (Scheme 1). This reaction was unknown at the time of the reported 20-step synthesis.[2' The
primary alcohol 2 and the secondary alcohols 3 and 4
were also prepared (Scheme 1) because their AG' values
should be similar to that of 1 . Contrary to (MI')-1 and
( M I ' ) - 2 , four stereoisomers of 3 and of 4 are possible
(( M P ) ( RS)-3 and ( M I ' ) ( R S ) - 4 ) .
The mass spectra of 1-4 were in agreement with the
constitutions. The 'H-NMR assignments of all aliphatic
(Table 1) and aromatic protons of 1 , 2, and 4 were obtained by nuclear Overhauser difference spectra. The
HPLC peaks for the enantiomers of 1 and 2 , obtained on
an optically active sorbent at -4O"C, coalesced to a single
averaged peak during HPLC at 15°C (Table I), owing to
enantiomerization.'". 14] The melting point given in the literature (142.8-143.6"C, corr.)"' is close to that of our acid
1 (137.5-138.5"C, corr.). Its UV absorption in CHCl3 resembles the spectrum depicted earlier1'] (solvent not specified). The AG' values for helix inversion (Table 2) were
obtained by dynamic 'H-NMR spectroscopy and/or thermal racemization. For the latter measurements the flow
during HPLC on an optically active sorbent was stopped
+
->
Table I . 'H-NMR and HPLC data for the phenanthrenes 1-4
1,R - CH2C02H ; 2. R=CH20H : 3 , R = CH(Me)OH : 4.R=CH(CMe3)OH
1
Although no information regarding the rate of racemization of ( + ) - l was given, a few optical rotations between
+ 0.1 1 & 0.02 and O.OO", measured in CHC13 with respect to
time, were mentioned.['] Since no other data on the enantiomerization of 1,4,5,8-tetrasubstituted phenanthrenes
have been available until now, the above optical rotations
have been used by several groupsL6-'"'to estimate the halflife t O s at room temperature and/or A G f for the inversion.
[*I Prof. Dr. A. Mannschreck, DipLChem. E. Gmahl,
Dr. T. Burgemeister, F. Kastner
lnstitut fur Organische Chemie der Universitat
Universitatsstrasse 31, D-8400 Regensburg (FRG)
Dr. V. Sinnwell
lnstitut fur Organische Chemie und Biochemie der Universitat
Martin-Luther-King-Platz 6, D-2000 Hamburg (FRG)
[**I Helical Phenanthrenes, Part 2. Support of this work by the Fonds der
Chemischen lndustrie is acknowledged. We thank Drs. W. hscher and
H Scherubl for helpful discussions and N . Pustet for experimental assistance.-Part I : [I].
270
0 VCH Verlagsgeseli.~chafrmbH. D-6940 Weinheim. 1988
2
2.52 ( 5 )
2.55 (4)
2.68 (8)
2.53 ( 5 )
2.56 (4)
2.70 (8)
3
2.52 (5)
2.55 (4)
2.70 (8)
4
2.53 ( 5 )
[81 (4)
2.69 (8)
4.08 ( s ) [d]
5.07 [el
5.16[e]
-
0.7 ( - )
I.I(+)
0.7
0.6 ( - 1
12(+)
0.5
-
-
5.65 [fl
5.67 [fl
1.71 [fl
-
-
5.32 [h]
5.41 [il
0.93 [i]
1.06 [hl
-
-
1.65
[fl
[a] 250 MHz; 2 4 T : CIZCDCDCl2.[b] Position of CH, group (4, 5, or 8) is
given in parentheses. [c] HPLC capacity factors k = ( a - u o ) / a l , at -40°C for
each enantiomer (retention volumes u ) and averaged capacity factor
R = ( U - u O ) / u c I at 15°C for both enantiomers (retention volume @ after coalescence [ 13, 141 of their peaks L'~, = retention volume of water that is not retained [ 151. Sorbent: (+)-poly(tritylmethacry1ate) 1151 coated o n silica.
Eluent: CH,OH. Dual detection by photometry (350 nm) and polarimetry
(365 nm). At 15°C no polarimetric detections were obtained. [dl In C5D5N
(400 MHz; 22°C)- 6('H)=4.38 and 4.41, 'J(AB)= 15.4 Hz. [el 'J(AB)= 12.3
Hz. [fJ 'J=6.5 Hz Diastereomeric ratio for 3 = I : I . [g] 6=2.54 and 2.56 for
the diastereomers of 4 (ratio 2.2 : I). [h] Minor diastereomer of 4. [il Main
diastereomer of 4 .
0570-0833/88/0202-0270 $ 02 50/0
Angew. Chem. lnt. Ed. Engl. 27 (1988) No. 2
Me
Me
Me
Me
Ue
Me
Me
58%
Me
Me
1
NaOH
b.4r
(MPI-2
Scheme I . Synthesis o f the 1,4,5,8-tetrasubstituted phenanthrenes 1-4. I n most cases, the substituted stilbenes were ( E ) / ( Z ) mixtures
when the cell of the polarimetric detector contained one
enriched enantiomer. Subsequently, the cell was thermostated to a temperature suitable for on-line racemizati*n.lla
I7.IXl
Since the known entropies of activation are not far from
zero,"."'1 it is permissible in most cases to compare AG'
values determined at different temperatures. The AG'
value of our acid 1 (78 kJ mol-', Table 2) corresponds to a
half-life of enantiomerization of only 3 s at 243°C and is
incompatible with the above estimated values of ca. 99 kJ
mol - ' and ca. 4 h. At the same time, our result clarifies all
reported discrepancies.'6-8. I' We h ave not yet been able to
attempt a crystallization of the diastereomeric saltsLZ1
of
our acid 1, since the amount available was low owing to
the low yields of the cyclodehydrogenation (Scheme 1).
Our AG' value for 1 is confirmed by the results for its
analogues 2-4. A comparison of 1 and 4,5-dimethylphenanthrene 5 or 2 and 5 shows a decrease of AG' by about
10 kJ mol-' (Table 2) when the 1- and 8-substituents are
removed. A similar decrease of AG' is found when going
from 1,3,4,5,6,8-hexamethylphenanthrene 6 to 3,4,5,6tetramethylphenanthrene 7. These observations indicate
that the interaction of the 1- and 8-substituents with the 10and 9-hydrogens contributes significantly to the helicity of
1,4,5&tetrasubstituted phenanthrenes.
Received: August 10, 1987 [Z2394 IE]
German version: Angew. Chem. 100 (1987)299
Table 2. A G i for the helix inversion described by (a)
Act
[kJ m o l '1~
1
2
3
4
5
6
7
78.3 t 0.7
77.9 t 0.4
78.0+ 0.6
79.4i-0.4
80+ 1
842 I (I)
XI +I(I1)
67+6[10j
105.I t0.4 [ 161
95.8t0.2 [ I ]
T
["CI
Solvent
-11
CH,OH
CSDSN
CH,OH
CI2CDCDCl2
78
- 10
96
90
Method
Cl2CDCDCI2
I08
Cl2CDCDCIZ
25
49
30
n-CeHi4
CHCll
CHCI,
[a] On-line racemization, monitored by polarimetry, after enrichment of one
enantiomer in solution by HPLC on silica coated with (+)-poly(trity1rnethacrylate) [Is]: cf. text and [l4, 17, 181. [b] Coalescence (400 MHz) of the 'HNMR C'H: absorption. *J(AB)= 15.4Hz, 6A-68=0.016 ppm extrapolated to
78°C: cf. Table I . [c] Coalescence (250 MHz) of the 'H-NMR C H 2 absorption upon decoupling from the O H proton at 6=1.83 (broadened).
'J(AB)= 12.3 Hz, 6n-6n=0.065 ppm extrapolated to 96°C;cf. Table I. [d]
Coalescence (250 MHz) of two ' H - N M R singlets, originating from the methyl groups bonded to sp' C atoms of the diastereomers I and 11; cf. Ref.
[ 191. The spectrum of 4 exhibits two singlets for these methyl groups; the
spectrum of 3 consists of two singlets when decoupled from the methine
absorption. 16,-6,,1=0.045
(3) and 0.121 ppm (4), extrapolated to the coalescence temperatures. The equilibrium constants [l]:[II]= l : l (3)and 2.2:l
(4)did not depend appreciably upon temperatures; cf. Table I . [el Off-line
racemimtion, monitored by circular dichroism, after HPLC enrichment of
one enantiomer in solution; cf. Ref. [lo]. [fJOff-line racemization, monitored
by polximetry, after preparative enrichment of one enantiomer; cf. Ref.
"1.
A n g e r Chem I n r . Ed Engl. 2711988)
No. 2
[I] H. Scherubl, U. Fritzsche, A. Mannschreck, Chem. Ber. 117 (1984)336.
121 M. S. Newman, A. S Hussey, J . Am. Chem. SOC.69 (1947)978, 3023.
[3] M.S. Newman, D. Lednicer, J. Am. Chem. Soc. 78 (1956)4765: W. H.
Laarhoven, W. J. C. Prinsen, Top. Curr. Chem 125 (1984)63.
141 Cf. G. Krow. Top. Srereochem. 5 (1970) 31.
[S] See, e.g., E. L. Eliel: Srereochemrstry of Carbon Compounds, McGrawHill, New York 1962, p. 176; H. A. Staab, Einfihrung in die theorerische
organrsche Chemie, 4th ed., Verlag Chemie, Weinheim 1966, p. 5 5 5 ; W.
J. le Noble, Highlighrs ofOrganic Chemisrr.y, Dekker, New York 1974,p.
154; H. Kagan, La sreriochimre organrque. Presses Universitaires, Paris
1975, p. 142; S. Hauptmann, Organrsche Chemie, H. Deutsch, Frankfurt/M. 1985, p. 100.
[6] K. Mislow, H. B. Hopps, J. Am. Chem. Soc. 84 (1962)3018.
171 R. Munday, I. 0. Sutherland, J. Chem. SOC.81968. 80.
[S] R. E. Carter, P. Berntsson, Acta Chem. Scand. 22 (1968) 1047.
[9] W. H. Laarhoven, W. H. M. Peters, A. H. A. Tinnernans, Tetrahedron 34
(1978)769.
[lo] R. N. Armstrong, H. L. Ammon, J. N. Darnow, J. Am. Chem. Soc. 109
(1987)2077.
[I I] Cf. 0. Yamamoto, H. Nakanishi, Tetrahedron 29 (1973)781.
[I21 D. M. Hail, E. E. Turner, J . Chem. SOC.1955. 1242.
[I31 A. Eiglsperger, F. Kastner, A. Mannschreck, J. Mol. Slruct. 126 (1985)
421.
[I41 A. Mannschreck, D. Andert, A. Eiglsperger. E. Gmahl, H. Buchner,
Chromarographia 23 (1988),in press.
1151 Y. Okamoto, K. Hatada, J. Liq. Chromarogr. 9 (1986)369.
[I61 A. Mannschreck, E. Hartmann, H. Buchner, D. Andert, Tefrahedron
Lett. 28 (1987)3479.
[I71 M. Minras, Z. OrhanoviS, K. JakopEiS, H. Koller, G. Stuhler, A.
Mannschreck, Tetrahedron 41 (1985)229.
[IS] Cf. C. Roussel, A. Djafri, Nouueau J. Chrm. 10 (1986) 399.
(191 A. Jaeschke, H. Mtinsch, H. G. Schmid, H. Frieboiin, A. Mannschreck,
J. Mol. Speclrosc. 31 (1969) 14.
0 VCH Verlagsgesellschaji mbH. 0-6940 Weinherm. 1988
0570-0833/88/0202-0271S 02.50/0
27 1
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