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Mechanism of the Thermal Conversion of 3 3-Bicyclo-propenyls into Benzene Derivatives.

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[956 independent reflections ("Syntex P21"); solution with
"Syntex XTL" ( R , =0.057)] of the complex confirmed the
second alternative, and especially the bridging C6H5C ligand
having sp2-hybridized carbon (cf. Fig. 1). Cell parameters:
space 'group P21/c; a=1330.9, b=1083.9, c=1817.1 pm;
p=134.6"; 1/=1866.1 x IO6pm3; Z = 4 ; d,,l,=2.725gcm-3.
gel and recrystallized from methylene chloride/diethyl ether:
black crystals, decomposing above 108"C ; yield 0.5 g (49%).
Received: December 22, 1975 [Z 376 IE]
German version: Angew. Chem. 88, 228 (1976)
CAS Registry numbers:
Re,(CO),C(OCH,)C,H,. 38x55-78-6: AI,Br,. 18898-34-5:
Re,(CO),CC,H,Br. 58384-14-8
[l] Transition metal carbene complexes, Part 87.-Part 86: H . Brunner,
J . Doppelberger, E . 0 . Fischer, and M. Lappus, J. Organometal. Chem.,
in press.
[2] E. 0. Fischer, E. Offlaus, J . Miiller, and D. Nothe, Chem. Ber. 105,
3027 (1972).
[3] F . Hug, W Mowat, A . C. Skapski, and G . Wilkinson, Chem. Commun.
1971, 1477.
[4] W A. Herrmann, Angew. Chem. 86, 895 (1974); Angew. Chem. Int.
Ed. Engl. 13, 812 (1974).
[5] N . 1. Gapotchenko, N . V Alekseeu, N . E . Koloboua, K . N . Anisimov,
I . A . Ronoua. and A . A . Johansson, J. Organometal. Chem. 35, 319
[6] F . A . Cotton and S. J. Lippard, Inorg. Chem. 4 , 59 (1965).
Fig. I . Molecular structure of Rez(CO)8(CC6H5)Br( 2 )
The four-membered ReBrReC ring in (2) also differs from
the MCMC rings found in the structures of the compounds
bis(p-trimethylsilylmethylidyne)tetrakis(trimethylsilylmethyl)d i n i ~ b i u m 'and
~ ] di-p-ethoxycarbonylmethylidynebis(tetracarbonylmangane~e)[~]
in that the methylidyne bridge, discovered
here to occur singly for the first time, between the two coordination centers contains a substituent devoid of a hetero atom.
The Re-C bond length is 214.4 (4.1) pm, and is thus 14pm
shorter than a calculated Re-C single bond (covalent radii:
Re=151 pm['], C=77pm). As expected, the Re-Br distance
is longer than in Re3Br9(254.3pm, rhenium as Re3 +@I); having
a value of 265.2 (0.5) pm it corresponds to the sum of the
single bond radii (Br= 114pm). Both bridge atoms are located
approximately in the plane of the equatorial carbonyl groups.
The phenyl ring is twisted by 72" relative to this plane.
The results of total elemental analysis and IR, 'H-NMR,
13C-NMR, and mass spectrometry are in accord with this
finding.-In the IR spectrum (in n-hexane) of compound (2)
four bands appear in the vco frequency range at 2081 m,
2023vs, 2013 s, and 1953 scm-'.-In
the 'H-NMR spectrum
the phenyl protons are observed as two multiplets in the
intensity ratio 3 : 2 at 7.50 and 6.84ppm (in CD2C12, -2O"C,
TMS as internal standard).-The proton noise decoupled I3CNMR spectrum (in CD2CI2, -2O"C,
relative to
CD2C12= 54.16 ppm) contains a total of seven signals: four
in the carbonyl and three in the aromatic range. The signal
at 201.20 ppm is assigned to the br,idging carbon atom, the
three signals at 186.85,186.10, and 185.88ppm to the carbonyl
carbon atoms, and the remaining signals at 129.78, 127.84,
110.69ppm to the phenyl carbon atoms.-In the mass spectrum the molecular peak appears at m/e= 768 (based on '"Re
and "Br, ion source T O 4, 50 eV).
All operations are to be performed under nitrogen with
dried (Na, P,O,,) and N,-saturated solvents.-Compound
( I ) (I.oog, 1.34mmol) and &Br6 (0.72g, 1.34mmol) are
stirred together in toluene (10ml)at - 30°C for 30min. Unconsumed A12Br6 is decomposed at -70°C with methanol and
the solution decanted. Complex (2) in the residue is purified
by chromatography in methylene chloride/pentane over silica
Mechanism of the Thermal Conversion of 3,3-Bicyclopropenyls into Benzene D e r i v a t i v e s [ * * ]
By James H . Davis, Kenneth J. Shea, and Robert G. Bergman[*]
Possible mechanisms for the highly exothermic ( A H , =
thermal conversion of 3,3'-bicyclopropenyls
[e.g. ( I ) ] into benzene derivatives fall into two general
groups[']. The first (which we shall refer to here as group
A) postulates some sort of initial bonding interaction between
the two cyclopropenyl rings (examples are concerted double
ring expansion, the classical Breslow''"] prismane mechanism,
and the recently suggested tricyclic diradical mechanism
offered by Weiss and Kobl['g]). The other group (B) invoi*.es
cyclopropene ring bond cleavage as the initial critical step[2].
Both Weiss and Kob/['gl and de Wolfet a/.["] have recently
reported that, in addition to aromatization, substituted bicyclopropenyls also undergo Cope rearrangement[31. 3.3'Dimethyl-3,3'-bicyclopropenyl ( I ), for example, rearranges
to diastereomers (2) and ( 3 ) in competition with its conversion into xylenes["! During a recent chemiluminescence
of the aromatization of ( I ) , we, too, observed its
conversion into (2) and (3). It occurred to us that the stereochemical relationship of these two molecules was such that
an examination of their aromatization might allow us to decide
between the mechanisms A and B discussed above, since pathways involving initial ring cleavage (B) destroy the stereochemical distinction between ( 2 ) and ( 3 ) .
We have therefore (i) found methods to preparatively separate (2) and ( 3 ) from ( I ) and xylene products, (ii) carried
out chemical correlations to determine which of these diastereomers is the DL and which the meso form, and (iii) determined
- 120kcal/mol)
[*] Prof. R. G. Bergman ['I and J. H. Davis
Contribution 5072
Laboratories of Chemistry
California Institute of Technology
Pasadena, California 91 125 (USA)
Prof. Dr. K. J. Shea
Department of Chemistry
University of California, lrvine
Irvine, California 92665 (USA)
['I To whom correspondence should be addressed.
[**I We thank the National Science Foundation (Grant No. MPS74-1471 l ) ,
the Camille and Henry Dreyfus Foundation, and the Chevron Research
Corporation for financial support of this work.
Angew Chem. I n t . Ed. Engl. J Vol. 15 ( 1 9 7 6 ) No. 4
Table 2. Rate constants [a] (in s - ' , x lo5) for interconversion of bicyclopropenyls ( I ) , (2), and ( 3 ) and for their conversion into xylenes.
0.14 0.02
25 + 2
3.6 i 1 .O
1.7 i0.5
[a] Determined by computer simulation of concentration-us.-time curves
(at both low and extensive conversion for ( I ) , ( 2 ) , (3) and total xylenes),
using the MSIM4 program written by Ms. Frances Houle in collaboration
with Professor D. L. Bunker at the University of California, Irvine. We
are grateful t o Ms. Houle and Professor Bunker for providing us with a
copy of the program.
[b] Summation rate constants for appearance of all xylenes. For individual
(relative) rates of formation. see Table 1
The data are clearly inconsistent with group A mechanisms[' a . g l , since these should give dramatically different xylene distributions from diastereomers (2) and ( 3 ) . Initial (reversible) ring opening to diradicals ( 4 a ) and ( 4 b ) (or their
vinylcarbene relatives[']), however, nicely accounts for both
the similarity of product distributions from (2) and (3) and
for the competitive interconversion of these compounds, if
one assumes that the relative rates of formation of ( 4 a ) and
( 4 b ) from (2) and ( 3 ) are very slightly different. This
mechanism also accounts for the fact that the percentage
of m-xylene formed directly from (I) at "zero time" is zero
within experimental error. These results, taken together with
the recent observation that Dewar benzenes are required intermediates in the ar~matization[~],
lead us to the overall
mechanism outlined. We suggest that ring opening to diradicals (or carbenes) ( 4 ) followed by cyclopropenylmethyl ring
expansion['o* 'I, closure to Dewar benzenes, and subsequent
concerted" 1' ring opening of these intermediates, is the aromatization mechanism most consistent with presently available
experimental evidence.
c H,
the zero-time (kinetic) ratio of xylenes formed from each diastereomer. Our results provide strong evidence for the ringcleavage mechanism (group B).
Separation of (2) and (3) was achieved by preparative
VPC at 25°C on a 2 0 x 1 / 4 ' glass column packed with 30 %
SE-30 on chromosorb WAW (60/80). The stereochemistry of
each diastereomer was determined by chemical correlation
with DL- and rneso-3,4-dimethyl-2,5-hexanediones,
structures were assigned using chiral shift reagents[5a1 and
confirmed by asymmetric hydroboration of the DL-diket~nel~~l.
Pyrolysis of ( I ) , ( 2 ) , and ( 3 ) , in a 200-ml vessel made of
lead-alkaline glass['] (temperature range 16&200"C), along
with VPC analysis of all three xylenes at very low percentage
conversion['] provided information about both the Cope rearrangement and aromatization kinetic product distributions
(Table 1). The chemical correlations confirmed earlier judgTable 1. Extrapolated zero-time xylene distributions formed in the gas phase
pyrolysis [a] of dimethylbicyclopropenyls ( 2 ) , ( 3 ) , and ( I ) at 169.5"C.
~ _ _ ~ -____
( 2 ) [bl
20* 1
Xylene isomers [ %]
ments['e.'l that the Cope process favors the chair transition
state. The ( 2 ) / ( 3 ) ratio extrapolated to zero time (130+10)
showed no variation with
and is very similar
to that observed in acyclic systems[*] (one therefore needs
no "special geometric and bonding
to account for
Pyrolysis of isolated (2) and (3) (92 % and 95 % pure,
respectively) demonstrated (i) that the diastereomers interconvert under the reaction conditions (by a pathway different
from one involving reconversion into (I)) at rates (cf. Table
2) competitive with xylene formation, and (ii) the zero-time
(kinetic) distributions of 0-, m- and p-xylene formed from
each diastereomer are very similar.
/ Vol. 15 (1976) No. 4
CAS Registry numbers:
(I), 31707-64-9; (2), 58374-90-6; ( 3 ) , 55930-24-0; p-xylene, 106-44-5;
m-xylene, 108-39-4; o-xylene, 95-48-7
[a] Pyrolyses were carried out in a well-conditioned static system (see text)
at a total pressure of ca. 150 torr (15 torr of substrate and 135 torr of
pentane as bath gas). Samples were vaporized into the reaction vessel and
removed using a conventional vacuum line system connected t o the vessel.
[b] Extrapolated from VPC data (see ref 171) taken at 4 points ranging
from reaction times of 300 s (6.4% loss of starting material: 0.86%, conversion
to xylenes) to l0oOs (23.5 % conversion of starting material, 1S O % conversion
to xylenes). Data for ( 3 ) and ( 1 ) were taken over similar reaction times.
Angrw. Chrm. Ini. Ed. Engl.
Received: December 2, 1975;
revised: January 12, 1976 [Z 384 IE]
German version: Angew. Chem. 88, 254 (1976)
[ l ] a) R. Breslow, P . Gal, H.-W Chang, and L. J . Altman, J. Am. Cbem.
SOC. 87, 5139 (1965); b j R. Weiss and H. P . Kempcke, Tetrahedron
Lett. 1974, 155; c) R. Weiss and S. Andrae, Angew. Chem. HS, 145,
147 (1973); Angew. Chem. Int. Ed. Engl. 12, 150, 152 (1973); d) W
H . de WOK W Stol, I. J. Landheer, and F. Bickelhaupt, Rec. Trav.
Chim. Pays-Bas 90, 405 (1971); e) W H. de WOK I . J . Landheer, and
F . Bickelhaupt, Tetrahedron Lett. 1975, 179; f) R. Weiss and H. Kiihl,
J. Am. Chem. SOC.97, 3222 (1975); gj 97, 3224 (1975).
[2] E. J . York, W Dittmar, J . R. Stevenson, and R. G . Bergmarl. J. Am.
Chem. SOC.94, 2882 (1972); 95, 5681 (1973).
[3] See ref. [ l a ] and a) L . 7: Scott and M. Jones, Jr., Chem. Rev. 72.
181 (1972); b j A . T Balahan, Rev. Roum. Chim. 17, 883 (1972).
[4] N . J . Turro. G. B. Schusrer. R. G. Bergman. K . J . Sheo. and J . H
Davis, J. Am. Chem. Soc. 97, 4758 (1975).
[5] a) M. Kainosho, K . Ajisaka, W H. Pirkle, and S. D. Bears, J. Am.
Chem. SOC. 94, 5924 (1972); b) H. C . Brown, N . R. Ayyangar, and
G. Zweifel, ibid. 86,397 (1964). The early-eluting bicyclopropenyl diastereomer proved t o be DL-(2).
[6] G. Beasley, Ph. D. Dissertation, Yale University 1970.
[7] VPC resolution of the three xylenes required the use (with temperature
programming) of an "internal tandem" 2 0 x 114" glass column packed
with 15' of 3 0 % SE-30 on chromosorb WAW (60180) followed by
5' of 5 % DC-555 and 5 % Bentone-34 on chromosorb WAW (60/80).
[8] W uon E . Doering and W R. Rorh, Tetrahedron 18, 67 (1962); b)
M. J . Goldstein and M. S. Benzon, J. Am. Chem. Soc. 94, 7147 (1972);
c) M . J . Goldstein and M. R. Decamp, ihid. 96, 7356 (1974).
[9] Because of the unusual symmetry of ( I ) , the chair and boat transition
states may be interconverted by a simple rotation rather than a ring
flip, and the rigidity of the system prevents rearrangement by a number
ofotherconformationsavailabletothemoreflexibleacyclicsystem [Sb.c]
It therefore seems likely that the ( 1 ) + ( 2 ) and ( 1 ) + ( 3 ) AAF* of
4.3 kcal/mol (1 66°C) reflects the true chair-boat transition state free
energy difference for this system.
[lo] This pathway is analogous to the ionic mechanism suggested by Weiss
er a / . [I b, 1 c] for the silver-catalyzed conversion of bicyclopropenyls
to Dewar benzenes, except that due to the extreme difficulty of generating
charge-separated species in the gas phase, neutral diradicals rather
than ionic intermediates are probably involved here; see, for example,
a) L . Salem and C . Rowland, Angew. Chem. 84, 86 (1972); Angew.
Chem. Int. Ed. Engl. 1 1 , 92 (1972); b) E . F. Hayes and A. K . Q .
Siu, J. Am. Chem. Soc. 93, 2090 (1971); c) L. Salem and W-D. Stohrer,
to be published; d) W A . Coddard and J. H.Davis, unpublished results.
[ l l ] Interestingly, we have been unable to find a study in the literature
ofcyclopropenylcarbinyl radicals to use as a model for the ring expansion
of ( 4 a ) and ( 4 h ) . It seems likely, however, that the strain relief and
incipient allylic delocalization generated in the transition state for such
an expansion would render it an extremely facile process.
[I21 R. Breslow, .I.
Napirrski, and A. H . Schmidr, J. Am. Chem. Soc. 94,
5906 ( 1 972).
as a Bridging p3 Ligand in a Transition Metal Complex[**]
By Gottfried Huttner, Hans-Georg Schmid, Albin Frank, and
O K Orarna[*l
In an attempt to synthesize the arsinidene complex ( I ) [ ’ ] ,
according to:
A s - C r : 2 5 3 pm
As-Cr’: 2 6 4 pm
Fig. 1. Molecular structure of the arsenobenzene complex (2).
In the only example of a R2As2 complex yet known,
the ligand is merely stabilized by x addition to a carbonylmetal fragment.
Evidence supporting formulation of the complex (2) with
an As=As double bond is provided by the arsenic-arsenic
interatomic distance (Fig. 1). which is 9pm shorter than
the As-As single bond in C5H5(C0)2Mn[PhHAsj 2Mn(CO),C,H5 ( 2 4 6 ~ r n ) [ ~inl , spite of the elongation due to
Received: December 30, 1975 [Z 38R IE]
German version: Angew. Chem 88.255 11976)
CAS Registry numbers:
15616-67-8: C,H,AsCI,. 696-28-6:
(OC),CrAs(C,H,)Li,. 55590-59-5; CI,(C,H,)AsCr(CO),.
[ I ] C . Hurmer and H.-G. Schmid, Angew. Chem. 87, 454 (1975); Angew.
Chem. Int. Ed. Engl. 14, 433 (1975).
[2] G. Hurtner and H . C . Schmid, to be published.
[3] 1157 diffractometer data (“Syntex P21”); solution with “Syntex XTL”
[4] P. S. Elmes, P. Levereft, and B. 0.Wesr, Chem. Commun. 1971, 747.
[5] G . Huttner, H . 4 . Schmid and H . Lorenz, unpublished.
Magnetic Nonequivalence of the C Atoms of Prochiral
Diphenylmethyl Groups in 3C-NMR Spectroscopy
we surprisingly isolated not ( I ) but instead the red complex
( 2 ) ,m.p. 142°C (dec.), among other hitherto unidentified products. The new compound (2) can also be prepared[’’ according to :
( C O ) , C r A s ( C s H 5 ) L i 2 + C12(C6H5)AsCr(CO)5
- LiCl
(2) +
The molecular ion of (2) does not appear in its mass
spectrum; however, intense peaks are observed for the fragment ions Ph2As2Cr(CO), (m/e=496) and Ph2As2 (rn/e= 304).
The IR spectrum exhibits absorptions which are characteristic
of phenyl groups; the band pattern in the vco region indicates
the presence of several differently coordinated Cr(CO), units.
The ‘H-NMR spectrum (solution in CD2C12) only shows
signals due to phenyl protons.
X-Ray structure analysis[31shows that arsenobenzene, PhAs=As-Ph, which is homologous to azobenzene, is stabilized
as a ligand in the complex (2). The two arsenic atoms both
form coordinative bonds to Cr(CO), groups. In addition.
the As=As double bond participates in x-interaction with
a third Cr(CO), group (cf. Fig. 1).
Doz. Dr. G . Huttner, H.-G. Schmidt, DipLChem. A. Frank, and 0.
Anorganisch-chemisches Institut der Technischen Universitat
Arcisstrasse 21. 800 Miinchen 2 (Germany)
This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
By Hans Otto Kalinowski, Bernd Renger, and Dieter Seehachl*l
The magnetic nonequivalence of diastereotopic groups
manifested in ‘H-NMR spectra has proved to be of value
in the elucidation of stereochemical problems[’]. Only very
few comparable studies have so far been performed in I3CNMR spectros~opy[~-~!
We here report the magnetic nonequivalence of the C atoms of diastereotopic phenyl groups
in tertiary alcohols of type ( I )[,I.
Table 1 shows thechemical shift differences of the 13C-NMR
signals of the phenyl carbon atoms of these compounds. The
relatively large difference of the signals for the ring C’ atoms
is not surprising since these atoms are located closest to
thecenterofchirality(except in thecaseof(1i)); themagnitude
of the difference remains practically constant within the classes
of compounds ( l a - c ) and (Id-h). The largest splittings are
observed for the carbonyl-containing compounds (I a-c), in
which effective hydrogen bonding could be responsible for
Dr. H. 0. Kalinowski, Dipl.-Chem. B. Renger, and Prof. Dr. D. Seebach
Institut fur Organische Chemie des Fachbereichs 14 der Universitiit
Heinrich-Buff-Ring 58, 6300 Giessen (Germany)
Anye%,. Chem. Iiir. Ed. Engl.
/ Vol. 15 ( 1 9 7 6 ) No. 4
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propenyl, thermal, bicycle, mechanism, benzenes, conversion, derivatives
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