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Orbital symmetry conservation in the cycloaddition reactions of tetrafluorodisilylene
Tetrahedron Letters No. 17, pp 1581 - 1584, 1973.
Psrgwaon Ress.
Printed in Great Britain.
ORDITAL SYMFXl'RYCONSERVATION IN THE CYCLOADDITION REACTIONS OF TEZRAFLUORODISILYLENE c
s. LlU
Department of Chemistry, National Tslw
Hua University, Rsinchu, Tawan.
J. C. Thompson De;artment of Chemistry, Ulllversltyof Toronto, Toa,nto, Ontano, (Received in Japm
Canada.
17 Fsbrusry 1973; reoeived in UK for publioation 20 lkroh 1973)
The recent Interest In the addition reactions between silicon difluoride and unsaturated l-5 has led to a successful synthesis of a sexnes of new orgsnosilicon comorganic compounds pounds.
The most reactive species m
these reactions has been the dimeric tetrafluorodisily-
lene dlradlcal .SIF~SIF~*. Almost In all these reactions there is a cyclic product which has been postulated as the result of a cycloadditlon of the *SiF2SlF2* diradical to the organic molecules.
The general molecular structures of these compounds can be represented as I for
the alkyne reactions, II for the alkene reactions end III for the product from the reaction of hutadlene:
H
F Si-SlF 2
2
2
I
II
The reaction with cyclopropane was of interest because the result could be compared with that of the reactlon with ethylene. However, no detectable volatile products were observed.7 The present work deals with the analysis of these reactrons by Woodward and Hoffmann's symmetry conservation rule8 and reports the rationalization of some experimental data mth the results The analysis of orbital symmetry 1s based on the assumption that there 1s no, or only mrlvnlflcant amount of, d-orbital involvement In the bandings of the *SlF2SlF2* species.
1581
The
1562
No. 17
microwave spectroscopic orbltal
character
Kth either
sp
and the X-ray dlffractlon
data
10
showed no evidence
for d-
in the Sl-F bonds
d-orbltal 2
data’
playing
no part UI the bondlngs,
or sp3 hybrldisatlon
we can assume that the slllcon
to form the *J~F~.S~E’* species 2
It 1s probably
atoms use
more convenient
n
to use epL hybridrzatlon
In this
the unpaired electron. The analysla c-c
and Sl-51
for
situation
the reactlon
allowed
the sucrafaclal
and the suprafacial
between silicon
difluonde
path,
axes for
the antarafaclal
the four-membered mng products
to accommodate In this
and a C2 symmetry dxls
of ethylene:
addition
addition
analysis.)
path which 1s thermally molecule
of alkyl-substituted
path.
forbidden.
of
Thus the addition
Thxs means, as far allowed
perpendicularly alkynes,
the
at the centers
only the antarafaclal
1s thewally
the acetylene
and a series
p-orbital
the same result
addition
the only reactlon
SIF~SIF~* urut approaches
aves
third
1s based on a .emJetry plane bisecting
to that of the dlmerlzatlon
as the symmetry is concerned, which the
atom uses Its
with acetylene
to the C-C and Si-S1
1s slmllar
1s thermally
silicon
(The use of sp3 hybridization
axes for
perpendicular
case,
1s the one ln
In the reactlons
the relative
yields
of
were found to be the sequence:
HGCH < CH3C=CH< CH3CH2ECH (< (CH3)3CECH It 1s clear
that as the substituent
This is in agreement with the analysis larger,
the parallel
approach of
alkyl
group becomes bulkier
described
the two reactants
and the perpendicular
approach becomes relatively
the thermally
approach becomes higher.
allowed
The same symmetry argument 1s also for
this
analysis
the thexmally
applicable
above.
antarafaclal
should be IV end V respectively
reactlon
path,
As the substltuent
18, to a greater more favored
extent,
Therefore,
to the reactlomwlth
would be the study of the reactions
allowed
the relative alkyl
Increases.
.ggup becomes
sterrcally
hindered,
the “frequency”
alkenes
with trans and -cls-butene. -
one would expect
yield
of
A good test According
that the reactlon
croducts
to
MO. 17
1583
trans-butene+
*SiF2SiF2.
-
r5
cis-butane+ -
*SiFSiF' 2 2
-
SiF2 V / SiF2
Both reactionswith -transand cis-butenedo show the fou~memberedring productswith the same molecularfomula C4HSSi~4. The lgF nmr spectrashow that the compoundsfrom the two 6 reactionsare different. The facts that thereare only two possibleisomers
(IV and V) ia
this case and that the lgF nmr spectrashow completelydifferentpatternslead to the belief that the productsfrom-_ transand cis-butenereactionseach containone compoundinsteadof a mixtureof isomers. The spectraare very complicated end so far it has not been possibleto the reactionsappearto be completely tell which one is IV and which one is V. Nevertheless, stereospecific. the symmetryelementtakeninto consideration is the For the reactionof cyclopropane, plane bisectingthe triangular plane end the Si-Si ~~18.
Agzun,
the orbitalcorrelation is
dependenton how the combiningreactantmoleculesapproachto each other. On approaclnng the 11 diradicalseeks the electron-rich part of the molecule cyclopropane molecule,the *SiF2SiF2' and most likelyit would approachthe triangularface of the cyclopropane ring where the electron cloudis concentrated, and then reactwith two carbonatomson one edge of the ring. From this point of view, it does not seem likelythat the .SiF2SiF2' specieswould approach which is the only path that the reaction one edge of the cyclopropane ring perpendicularly, becomesthermallyallowed. The situationfor the reactionof butadieneis very similarto the cucloaddition between 8 ethyleneand butatiene. A simpleanalysiswith respectto the symmetryplanebisectingthe middleC-C bond of butadieneand Si-Sibond of Si2F4 shows that it is a thewally allowed
1584
No. 17
the cycloedditlon oroductIII wae obtalned reaction. That is what wae foundexperimentally:4
Acknowledgement-The authorsthankProfessorK. Yates for helpful&scussions.
References (1) c. s. Llu, J. L. Margrave,J. C. Thompsonand P. L. Timme,Can. J. Chem.,50, 459 (1972). (2) c. s. Llu, J. L. Margreveand J. C. Thompson,Can. J. Chem.,z, 465 (1972). Chem.,z., 249 (1972). (3) c. S. Liu end J. C. Thompson,J. Organometall. (4) J. C. Thompsonand J. L. Mergreve,Inorg.Chem.,I& 913 (1972). (5) J. C. Thompeon,P. L. Timmeend J. L. Margrave,Chem. Commun.,566 (1966). results. (6) C. S. Liu and J. C. Thompson,unpublished results. (7) C. S. Liu and J. C. Thompson,unpublished AccountsChem.Res.,L, 17 (1968). (8) R. B. Woodwerdand R. HoffInann, (9) v. n.
Reo,
R. F. Curl,P. L. Timms and J. L. Margrave,J Chem.Phys , a, 2557 (1965).
(10) c. s. LlU, s. c. Nyburg,J. T. Szymenekiand J. C. Thompson,J Chem Sot., (Dalton) 1129 (1972). (11) C. S. Llu and J. C. Thompson,Inorg.Chem.,I& 1100 (1971).
Psrgwaon Ress.
Printed in Great Britain.
ORDITAL SYMFXl'RYCONSERVATION IN THE CYCLOADDITION REACTIONS OF TEZRAFLUORODISILYLENE c
s. LlU
Department of Chemistry, National Tslw
Hua University, Rsinchu, Tawan.
J. C. Thompson De;artment of Chemistry, Ulllversltyof Toronto, Toa,nto, Ontano, (Received in Japm
Canada.
17 Fsbrusry 1973; reoeived in UK for publioation 20 lkroh 1973)
The recent Interest In the addition reactions between silicon difluoride and unsaturated l-5 has led to a successful synthesis of a sexnes of new orgsnosilicon comorganic compounds pounds.
The most reactive species m
these reactions has been the dimeric tetrafluorodisily-
lene dlradlcal .SIF~SIF~*. Almost In all these reactions there is a cyclic product which has been postulated as the result of a cycloadditlon of the *SiF2SlF2* diradical to the organic molecules.
The general molecular structures of these compounds can be represented as I for
the alkyne reactions, II for the alkene reactions end III for the product from the reaction of hutadlene:
H
F Si-SlF 2
2
2
I
II
The reaction with cyclopropane was of interest because the result could be compared with that of the reactlon with ethylene. However, no detectable volatile products were observed.7 The present work deals with the analysis of these reactrons by Woodward and Hoffmann's symmetry conservation rule8 and reports the rationalization of some experimental data mth the results The analysis of orbital symmetry 1s based on the assumption that there 1s no, or only mrlvnlflcant amount of, d-orbital involvement In the bandings of the *SlF2SlF2* species.
1581
The
1562
No. 17
microwave spectroscopic orbltal
character
Kth either
sp
and the X-ray dlffractlon
data
10
showed no evidence
for d-
in the Sl-F bonds
d-orbltal 2
data’
playing
no part UI the bondlngs,
or sp3 hybrldisatlon
we can assume that the slllcon
to form the *J~F~.S~E’* species 2
It 1s probably
atoms use
more convenient
n
to use epL hybridrzatlon
In this
the unpaired electron. The analysla c-c
and Sl-51
for
situation
the reactlon
allowed
the sucrafaclal
and the suprafacial
between silicon
difluonde
path,
axes for
the antarafaclal
the four-membered mng products
to accommodate In this
and a C2 symmetry dxls
of ethylene:
addition
addition
analysis.)
path which 1s thermally molecule
of alkyl-substituted
path.
forbidden.
of
Thus the addition
Thxs means, as far allowed
perpendicularly alkynes,
the
at the centers
only the antarafaclal
1s thewally
the acetylene
and a series
p-orbital
the same result
addition
the only reactlon
SIF~SIF~* urut approaches
aves
third
1s based on a .emJetry plane bisecting
to that of the dlmerlzatlon
as the symmetry is concerned, which the
atom uses Its
with acetylene
to the C-C and Si-S1
1s slmllar
1s thermally
silicon
(The use of sp3 hybridization
axes for
perpendicular
case,
1s the one ln
In the reactlons
the relative
yields
of
were found to be the sequence:
HGCH < CH3C=CH< CH3CH2ECH (< (CH3)3CECH It 1s clear
that as the substituent
This is in agreement with the analysis larger,
the parallel
approach of
alkyl
group becomes bulkier
described
the two reactants
and the perpendicular
approach becomes relatively
the thermally
approach becomes higher.
allowed
The same symmetry argument 1s also for
this
analysis
the thexmally
applicable
above.
antarafaclal
should be IV end V respectively
reactlon
path,
As the substltuent
18, to a greater more favored
extent,
Therefore,
to the reactlomwlth
would be the study of the reactions
allowed
the relative alkyl
Increases.
.ggup becomes
sterrcally
hindered,
the “frequency”
alkenes
with trans and -cls-butene. -
one would expect
yield
of
A good test According
that the reactlon
croducts
to
MO. 17
1583
trans-butene+
*SiF2SiF2.
-
r5
cis-butane+ -
*SiFSiF' 2 2
-
SiF2 V / SiF2
Both reactionswith -transand cis-butenedo show the fou~memberedring productswith the same molecularfomula C4HSSi~4. The lgF nmr spectrashow that the compoundsfrom the two 6 reactionsare different. The facts that thereare only two possibleisomers
(IV and V) ia
this case and that the lgF nmr spectrashow completelydifferentpatternslead to the belief that the productsfrom-_ transand cis-butenereactionseach containone compoundinsteadof a mixtureof isomers. The spectraare very complicated end so far it has not been possibleto the reactionsappearto be completely tell which one is IV and which one is V. Nevertheless, stereospecific. the symmetryelementtakeninto consideration is the For the reactionof cyclopropane, plane bisectingthe triangular plane end the Si-Si ~~18.
Agzun,
the orbitalcorrelation is
dependenton how the combiningreactantmoleculesapproachto each other. On approaclnng the 11 diradicalseeks the electron-rich part of the molecule cyclopropane molecule,the *SiF2SiF2' and most likelyit would approachthe triangularface of the cyclopropane ring where the electron cloudis concentrated, and then reactwith two carbonatomson one edge of the ring. From this point of view, it does not seem likelythat the .SiF2SiF2' specieswould approach which is the only path that the reaction one edge of the cyclopropane ring perpendicularly, becomesthermallyallowed. The situationfor the reactionof butadieneis very similarto the cucloaddition between 8 ethyleneand butatiene. A simpleanalysiswith respectto the symmetryplanebisectingthe middleC-C bond of butadieneand Si-Sibond of Si2F4 shows that it is a thewally allowed
1584
No. 17
the cycloedditlon oroductIII wae obtalned reaction. That is what wae foundexperimentally:4
Acknowledgement-The authorsthankProfessorK. Yates for helpful&scussions.
References (1) c. s. Llu, J. L. Margrave,J. C. Thompsonand P. L. Timme,Can. J. Chem.,50, 459 (1972). (2) c. s. Llu, J. L. Margreveand J. C. Thompson,Can. J. Chem.,z, 465 (1972). Chem.,z., 249 (1972). (3) c. S. Liu end J. C. Thompson,J. Organometall. (4) J. C. Thompsonand J. L. Mergreve,Inorg.Chem.,I& 913 (1972). (5) J. C. Thompeon,P. L. Timmeend J. L. Margrave,Chem. Commun.,566 (1966). results. (6) C. S. Liu and J. C. Thompson,unpublished results. (7) C. S. Liu and J. C. Thompson,unpublished AccountsChem.Res.,L, 17 (1968). (8) R. B. Woodwerdand R. HoffInann, (9) v. n.
Reo,
R. F. Curl,P. L. Timms and J. L. Margrave,J Chem.Phys , a, 2557 (1965).
(10) c. s. LlU, s. c. Nyburg,J. T. Szymenekiand J. C. Thompson,J Chem Sot., (Dalton) 1129 (1972). (11) C. S. Llu and J. C. Thompson,Inorg.Chem.,I& 1100 (1971).