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Atropisomerism of Transition Metal Carbene Complexes.

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10 ml, and chromatographed on a silica gel column (Merck
0.05 to 0.2 mm; column dimensions 40 x 2 cm) at 0°C with
hexane/benzene (4 :1). The yellow zone is collected, the
solvent removed in a high vacuum, and the residue
recrystallized twice from n-pentane : pale yellow needles,
m. p. 45 "C ; yield 250 mg (4%).
Received: July 26,1971 [Z 504 IE]
German version: Angew. Chem. 83, 1021 (1971)
Atropisomerism of Transition Metal Carbene
By Henri Brunner, Ernst Otto Fischer, and
Manfred Lappust*l
o-Substituted derivatives of biphenyl and similar compounds exist in atropisomeric formsrz1.Appropriately substituted carbene complexes of transitions metals were also
expected to exhibit atropisomerism.
Reaction of (methoxy-I-naphthy1carbene)pentacarbonyl~hromiurn(O)[~~
with ( + )-a-phenylethylamine in diethyl
ether yields [(a-phenylethy1amino)-I-naphthylcarbenelpentacarbonylchromium(0). Of the two possible cis-trans
isomers (with regard to the C-N bond) ( I ) and (2), the
cis isomer (1) is formed preferentially. The barrier to free
rotation between the carbene carbon and the 1-naphthyl
group gives rise to a further kind of isomerism: For the
Figure. 'H-NMR spectrum of the isomeric mixture (15% solution in CD,OD)
cis and trans products ( I ) and (2) there are two atropisomers each, viz. ( l a ) , ( J b ) and (2a), (Zb), which differ in
the spatial arrangements of the I-naphthyl group.
Nuclear magnetic resonance is particularly suitable for
identification of the isomers. The 'H-NMR spectrum of a
1 :1mixture of the cis-trans isomers ( I ) and (2) in CD,OD
(Fig.) exhibits four doublets for the diastereoisomers ( I a ) ,
( I b ) , ( 2 a ) , and (2b) in the methyl region; the signals are
grouped as two pairs each with similar chemical shifts for
the cis and trans compounds. The differences between the
chemical shifts of the methine protons in the cis-trans isomers are greater than those for the methyl protons. The
Prof. Dr. H. Brunner and Dip1.-Chem. M. Lappus
Fachbereich Chemie der Universitat
84 Regensburg, Universitatsstrasse 31 (Germany)
Prof. Dr. E. 0. Fischer
Anorganisch-chemisches Laboratorium
der Technischen Universitat
8 Munchen 2, Arcisstrasse 21 (Germany)
low intensity and poor resolution of the methine signals,
however, precludes their use in quantitative studies or for
distinguishing between the atropisomers. Separation of
the cis-trans isomers or the atropisomers on the basis of
differing solubilities, however, can readily be followed
with the aid of the methyl signals.
The cis and trans forms can be recognized from the change
in chemical shift of the N-CH-CH,
group on transition
from acetone to chloroform solvent[41:The signals of the
group display a larger upfield shift than
the trans-N-CH-CH,
group when the solvent is changed.
It may thus be seen from the Table that the signal systems
of the methyl and methine protons at higher field are to be
assigned to the cis compounds ( I a ) and ( I b), and those
at lower field to the trans isomers ( 2 a ) and (2b).
The cis-trans isomers are formed in the ratio of about 6 :1
on preparation of [(a-phenylethy1amino)-1-naphthylcarbene]pentacarbonylchromium(0) at room temperature.
Angew. Chem. internat. Edit. 1 Vol. I0 (1971) No. 12
Recrystallization of the cis-trans mixture from diethyl
ether/pentane after chromatography on SiO, affords a
pure sample of the more sparingly soluble cis compound
( I ) ; in contrast, the more readily soluble trans isomer ( 2 )
has not yet been separated completely from the cis isomer.
We therefore limited our attempts to resolve the atropisoTable. T values from the 'H-NMR spectrum of the isomeric mixtures
in various solvents (15% solutions).
to give five-membered ring systems, the mesomeric sixmembered ring betaines ( I ) ought to be suitable for 1,4dipolar cycloadditions leading to six-membered heterocycles.
This is in fact shown to be the case in the reaction of ( I )
with dimethyl acetylenedicarboxylate ( 2 ) and maleic anhydride (3)IS1.Thus, ( l a ) and ( I b ) react with ( 2 ) with
loss of phenyl isocyanate to give, respectively, the 2pyridones ( 5 a ) and ( 5 b ) in good yields. As in the case of
the analogous reactions with acetylene derivativesF4, the
tendency to rearomatization leads to elimination of a
neutral molecule (in this case C,H,N=C=O)
from the
primary addition product (4).
mers to the cis compounds ( I ) . After repeated fractional
crystallization (18 x ) from diethyl ether/pentane the more
sparingly soluble fraction containing the levorotatory diastereoisomer exhibited a specificrotation of [a]& = - 168".
The highest rotations recorded so far for the fractions of
the mother liquor enriched in the more readily soluble
dextrorotatory diastereoisomers are [a]&= +46". The
rotations were measured with approximately
solutions in benzene. The four doublets of the methyl
groups of ( l a ) , ( I b ) , ( 2 a ) , and ( 2 b ) are retained in the
'H-NMR spectrum at 70°C in CD30D solution. Like the
thermal interconversion of cis and trans isomeric amino
carbene complexes[51,the transition between the atropisomers ( l a ) - ( l b ) and ( 2 a ) - ( 2 b ) is so slow that it
cannot be determined from the temperature dependence
of the 'H-NMR spectrum.
Received: August 2,1971 [Z 508 IE]
German version: Angew. Chem. 83, 1022 (1971)
[l] Optically active transition metal complexes, Part 7. This work was
supported by the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industtie.- Part 6: H. Brunner and H.-D. Schindler,
J. Organometal. Chem. 24, C 7 (1970). Also Part 37 of the series: Transition metal carbene comp1escs.- Part 36: E . 0. Fischer, M . Leupold, C .
G. Kreiter, and J . Miiller, Chem. Ber., in press.
[2] G . Krow, Top. Stereochem. 5 , 31 (1970).
[3] E. 0. Fischer, C . G. Kreiter, H. J . Kollmeier, J . Miilfer, and R. D.
Fischer, J. Organometal. Chem. 28, 237 (1971).
[4] P . Lazlo: Progress in Nuclear Magnetic Resonance Spectroscopy.
Pergamon Press, Oxford 1967,Vol. 3, p. 231 ;E. Moser and E. 0.Fischer,
J. Organometal. Chem. 13, 387 (1968); J . A. Connor and E . 0. Fischer,
J. Chem. SOC.A 1969, 578.
[ 5 ] E. Moser and E. 0. Fischer, J. Organometal. Chem. IS, 147 (1968).
Cycloadditions with Mesomeric Pyrimidine
By Thomas Kappe and Wolfgang Lube"]
Recently, we reported on the synthesis of mesomeric
pyrimidine betaines ( A ) , whose structure can only be
described by a series of zwitterionic canonical
The distribution of charge in these compounds is best expressed in terms of formulation (B). Huisgen's "sextet
limiting formula" (C) I3]suggests that such compounds
are potential starting materials for 1,4-dipolar cycloadditions[3? Since five-membered mesoionic heterocycles
can, in many cases, be used for 1,3-dipolar cycloadditionsr4]
[*] Doz. Dr. Th. Kappe and W. Lube
Institut fur Organische Chemie der Universitat
A-8010 Graz, Heinrichstrasse 28 (Austria)
Angew. Chem. internat. Edit. 1 Vol. I0 (1971) No. I 2
In the reaction of ( I b ) with maleic anhydride, however,
we have been able to isolate the 1 : I adduct ( ~ 5 ) ~The
structure of (6) has been confirmed by mass spectrometry,
molecular weight determination (528), and NMR spectroscopy (in DMSO). The NMR spectrum contains one singlet
at 6 = 3.70 due to the methylene protons of the benzyl group,
two doublets .of the cis-protons in positions 7 and 8 at
6=4.32 and 5.53, 3=9.5 Hz, as well as signals for 20
aromatic protons at 6 = 6.8-8.8. The IR spectrum (in KBr)
with anhydride bands at 1860 and 1790 cm-' and C=O
bands at 1710 and 1670 cm-', is also in agreement with
this structure.
2-Pyridones ( 5 a ) and ( 5 b )
The solutions of ( I a ) and ( I b ) (5 mmol) in chlorobenzene
(25 ml) are heated under reflux with (2) (1.4 ml. 10 mmol)
for 16 h. The residue recovered after evaporation under
vacuum is digested with petroleum ether. ( 5 a ) : yield
83%, colorless needles recrystallized from methanol, m. p.
168-170°Cr71 ; free dicarboxylic acid (by hydrolysis with
1 N NaOH): m. p. 253-256°C[71. (5 b) : yield 67%, colorless needles recrystallizedfrom ethanol, m. p. 235-237 OC['].
Joctane-7,8-dicarboxylic anhydride ( 6 )
( l b ) (1.07 g, 2.5 mmol) and (3) (0.25 g, 2.5 mmol) are
vigorously ground together and then heated at 170°C for
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carbene, metali, atropisomerism, transitional, complexes
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