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Hammett analysis of absolute carbene addition rate constants
0040-4039/83/070685-04$03.00/O 01983 Pergamon Press Ltd.
Tetrahedron Letters,Vol.24,No.7,pp 685-688,1983 Printed in Great Britain
HAMMETT
ANALYSIS
OF ABSOLUTE
CARBENE
RATE CONSTANTS
N.J. Turro*, I.R. Gould and N.P. Hacker Chemistry Department Columbia University New York, New York 10027
R.A. MOSS* and L.A. Perez Department of Chemistry Rutgers University New Brunswick, New Jersey 08903 Summary.
ADDITION
Absolute rate constants were determined for the additions of 5 L-X-substituted ArCCl (X=CF3, Cl, H, CH , CH30) to tetramethylethylene, trimethylethylene, trans-pentene, and 1-hexene; goo a Hammett correlations were obtained with p = +1.4 - 1.6.
In the absence
of absolute
kinetic
been assessed
by linear free energy 1 sets of alkenes. Hammett analyses
strates,
followed
by comparisons
now make it possible previously absolute
generally
of p values
to determine
absolute
studied only by competitive rate constants
pretations
of carbenic
Bensamidine
reactivity,
involved
of carbenes
reactivities
variation
carbenes.
rate constants
on arylalkene
Flash photolytic
for additions
The results
to alkenes
that some revisions
broadly
or 1-hexene
led to 20 cyclopropanes
sub-
techniques
of carbenes analysis
support previous
may ultimately
which were isolated
have
with standard
We report here the first Hammett
additions.
but suggest
selectivities
relative
substituent
for several
techniques.2
for carbene/alkene
the alkenic
of carbenic
of
inter-
be required.
and 4 e-substituted derivatives (CF3, Cl, CH3, CH30) were oxidized 3 Photolysis (A>300 nm) of each diazirine in tetramethylethylene,
chlorodiazirines. trans-pentene,
information,
comparisons
to 3-aryl-3isobutene,
and characterized
by nmr
spectroscopy parent
and elemental analysis. Using methods and equipment described in our studies of the 2c,d carbene, PhCCl, each diazirine was photolytically decomposed in 3-methylpentane matri-
ces at 77K affording
uv spectra
of the corresponding
arylchlorocarbenes.
The spectra
obtained
at
77X were in good agreement
with respect to shape and maximum with point-by-point transient spectra 2c,d 4 of the laser flash generated carbenes in aerated isooctane solution at 23OC. 2c,d Laser excitation of isooctane solutions of the diasirines gave transient absorptions of the corresponding bene "quencher"
carbenes
of the transient
absorption
order decay constant analysis. 5 mined
(ArCCl).
The bimolecular
rized in Table I. in Table I.
Using relative
and compared
2 these comparisons responsible
were obtained
methods
and l-hexene.
to Lrel derived
suffices
for product
to indicate
formation,
and 1-hexene)
appear
concentrations
in Figure
with ci substituent
better
of car-
the decay
Plots of the pseudo-first employed
of ArCCl by each quencher
versus quencher
of these results
of photolytically
trans-pentene,
order kinetics.
for quenching
>0.99, significantly
high concentrations
trans-pentene
were linear under the conditions
of decay constants
analyses
competition
reactivities
of quencher
rate constants
The best correlations coefficients standard
isobutene, Me2C=CMe
Hammett
of sufficiently
trimethylethylene,
followed pseudo-first
as a function
from the slopes of plots
correlation
In the presence
(tetramethylethylene,
for
were deter-
and are summa-
1, with
p values reported constants, 6a which gave
than those obtained
with
c
constants.6b
analysis, p-lb we determined
based on cyclopropane
product
generated
ArCCl toward tetramethylethylene,
Results
(X>300 nm) appear
in Table
II, where they are normalized
from the k --abs data of Table I. that the spectroscopically
Although
monitored
the two sets of data are not identical
685
the agreement
kinetic
within
to in
species are
the maximal
686
experimental
error
(+ 20%) in all cases.
termediate
(vide infra) --
competition
experiments
is involved
Indeed,
if reversible formation of a carbene/alkene in2d then the results of the steady state
in these reactions,
will not match those of the time resolved -I lifetime of the intermediate.
of a non-negligible
experiments
under the conditions
k for ArCCl/alkene additions varies sensibly with both aryl and alkenic substituents. abs From Table I, the CH30 to CF 3 carbenic substituent change enhances Labs by factors of 88-138 for the alkenes; CH-C4Hg
rate constant
comparison.
addition
ratio for the fastest
-SbS
(p-CH30C6H4CC1 -
electron-donating
spreads of 83-194 occur for the various
The k
+ CH2=CH-C4H9)
(ED) substituents
ents accelerate
the additions;
Qualitatively,
(p-CF3CSH4CC1 -
is 4 orders of magnitude.
in ArCCl retard,
p values
are ~1.5
whereas
carbenes
in the Me2C=CMe2/CH2=
+ Me2C=CMe2) The Hammett
vs. the slowest analyses
electron-withdrawing
with little dependence
show that
(EW) substitu-
on alkene.*
than EW substituted
p>o might be interpreted to mean that ED substituted ArCCl are more stabilized 10 ArCCl, and therefore react more slowly with a given alkene. Recent evidence
that carbene/alkene
additions
complex)
complicates
may proceed via intermediates (e-g ---L'- 1 in eq. (l), a charge transfer 11 but rationalization of the p values can still be made by focus-
the matter,
sing on the intermediate.
When X is EW, passage
of 1. to product
over "electrophilic"
state 2 is favored because
X stabilizes
negative
imposed on the carbenic
enhanced
charge
transition carbon.
Dissociation
of 1 to carbene and alkene is disfavored, however, because EW substituents destabilolize carbenes. When X is ED, these considerations reverse. Now 2 is destabilized and the dis-
sociation
of 1 to carbene
1, but ED groups
and alkene
favor reversion
is favored.
of 1. to ArCCl,
be positive.
Thus, EW groups
enhance product
formation
and p based upon the disappearance
1
from
of ArCCl
should
Ar
Ar\C/C1 i The absence is striking concerning
tivity
of a substituent 8 Although
the
[1+21 cycloaddition formed
temperature despite
resolved
-1 dependence
and unexpected.
of a reversibly unusual
FM
a large change
kinetic
tion is involved
results.
of the kinetics in reactivity,
For example,
in the rate determining
selective
if the factors which determine Furthermore,
state competition
product
ratio.
We believe
a carbene
thank Professors
We are grateful
to alkene: la,10
calculations) involved
the concept
a possible basis for explaining the 2d of these reactions, the absence of altered selec-
and the differences
selectivity
moiety
further
involved
alkene,
between
state and time
system complex
(high reactivity)
the passage
forma-
may still be
of the intermediate
to
very high, as it does for a steady
in a reversibly leading
new insight
formed carbene/alkene
to a concentration
into the seemingly
and experimental
Science Foundation
for helpful
the steady
carbene/alkene
becomes
thereby
theoretical
to the National
M. Plats and R.C.D. Breslow
control
of quencher
that our results provide
and serve to encourage
ACKNOWLEDGEMENTS.
orbital
to alkenes have not explicitly
step, a very fast reaction
might be "trapped" by a different
cycloaddition
of these ArCCl additions
(e.g., molecular
if for a particular
if the concentration
experiment,
ideas
such an idea provides
product.
termediate
on the selectivity
current
of carbenes
intermediate,
dependence
e
discussions.
in-
dependent
simple
[I+21
investigations.
for financial
support.
We
687
Table I.
Absolute
Rate Constants
for Additions
of PX-C~H~CC~
to Alkenesa
X CH30
Alkenes Me2C=CMe2
CH
Cl
Hb
3
CF3
PC
1.4 x 10 7d
1.2 x 108
2.8 x lOa
3.3 x 108
1.5 x 109
1.5
Me2C=CHMe
7.7 x lo6
4.9 x lo7
1.3 x lo8
1.8 x lOa
6.8 x lo8
1.4
t-MeCH=CHEt
4.4 x lo5
1.8 x 106
5.5 x lo6
7.5 x lo6
4.9 x 10'
1.5
CH,=CH-fl-C4Hg
1.3 x 10 5d
6.2 x lo5
2.2 x lo6
2.3 x lo6
1.8 x lo7
1.6
aRate constants are in L/mol-set and were determined in alkene/isooctane solution at 23OC by laser flash photolytic methods; see text and refs. 2c and 2d. Reproducibilities ared<+ 10% unless otherwise noted. bThese data differ slightly from those previously reported due to modifications in equipment. All data reported here were determined under identical conditions. 'p values are rounded to 2 significant figures. All correlation coefficients were >0.99. For U+ constants, see ref. 6a. Reproducibility is f 15%. P Table II.
Relative
Reactivities
for Additions
of p-X-C6H4CC1
to Alkenes a,b
X Alkenes
CH30
CH
3
H
Cl
CF
3
Me2C=CMe2
1.00
1.00
1.00
1.00
1.00
Me2C=CH
0.37
0.26
0.25
0.22
0.24
2
-t-MeCH=CHEtC CH2=CH-;-C4Hg
C
0.038 (0.031)
0.028 (0.015)
0.028 (0.020)
0.029 (0.023)
0.033 (0.033)
0.017 (0.0093)
0.012 (0.0052)
0.012 (0.0079)
0.012 (0.0070)
0.012 (0.012)
a
Relative reactivities were determined at 25OC in neat alkene mixtures. Product separation and quantitation was achieved by reversed phase hplc on a Waters C-18 RCM column, using CH3CN or lo-15% H20/CH CN as eluents, a calibrated uv detector, and an electronic integrator. Results are genera 2 ly averages of 2 or more runs and were usually determined in competition against isobutene. Reproducibilities were <* 9%, and all but 3 cases were
AND NOTES
(a) R.A. Moss, Act. Chem. Res., 2, 58 (1980); (b) R.A. Moss in "Carbenes", Vol. I, M. Jones, Jr., and R.A. Moss, Ed., Wiley, New York, 1973, pp. 153f; (c) R.A. Moss and M. Jones, Jr., in "Reactive Intermediates", Vol. 1, M. Jones, Jr., and R.A. Moss, Ed., Wiley, New York, 1978, pp. 69f; (d) Vol. 2, 1981, pp. 59f. (2) (a) G.L. Gloss and B.E. Rabinow, 3. Am. Chem. Sot., 98, 8190 (1976); (b) J.J. Zupanic and G.B. Schuster, -ibid., -_ 102, 5958 (1980); (c) N.J. Turro, J.A. Butcher, Jr., R.A. Moss, W. Guo, R.C. Munjal, and M. Fedorynski, ibid., 102, 7576 (1980); (d) N.J. Turro, G.F. Lehr, J.A. Butcher, Jr., R.A. Moss,and W. GUO, ibid., 104, 1754 (1982). (3) W.H. Graham, J. Am. Chem. Sot., 87, 4396 (1965). For prior preparations and thermal decompositions of arylchlorodiazirines, see M.T.H. Liu and K. Toriyama, Can. J. Chem., so, 3009 (1972). (4) xmax values (77K, 23OC) for ArCCl were (run): p-CH,O, 346, 340; g-CH3,, 317, 317; H, 307, 310 (benzene); e-cl, 323, 320; g-CF3, 303, 298. For solution kinetic studies, carbene absorptions were monitored at 320 nm, except for p-CH3CC6H4CC1 (330 nm). (5) The use of kinetics to uncover unexpected aspects of the reactions of PhCCl with alcohols has D. Griller, M.T.H. Liu, and J.C.Scaiano, J. Am. Chem. Sot., 104, recently been reported: 5549 (1982).
688
Figure 1. Logarithms of absolute rate constants (L/mol-set) for additions of ArCCl to Me2C=CMe2, For ArCCl substituents and P values, see Me C=CHMe, t-MeCH=CHEt, and CH,=CH-;-C4H9 vs. "pT a2 le1. -
(6) (a) C.D. Ritchi: and W.F. Sager, Prog. Phys. Org. Chem., 2, 323 (1964); cf., pp. 334-337. The value for oE (CF3) is taken from C.G. Swain and E.C. Lupton, Jr., J. Am. Chem. Sot., 90, (b) J. March, "Advanced Organic Chemistry", 2nd Ed., McGraw-Hill, New York, 4328 (1968). 1977, p. 253. (7) C. Lewis and W.R. Ware, Mol. Photochem., 5, 261 (1973). the most (8) Although classic electrophilic selectivity-1atlb is apparent with all 5 ArCCl (&, highly alkylated olefins react most rapidly') the carbenes are all comparably selective over This lack of selectivity the olefin set; the four Hammett correlations are nearly parallel. variation is also apparent from the be1 data of Table II. (9) The anticipated electrophilic behavior is also apparent in additions of PhCCl to either simple alkeneslb or to ring-substituted styrenes, for which p varies from -0.56 to -1.06 decf., W. Briick and H. Diirr, Tetrahedron Lett., 23, 2175 (1982). pending on temperature: (10) Electron-donating substituents stabilize carbenes by r-type resonance interactions; "f:, N-G. Rondan, R.N. Houk, R.A. Moss, J. Am. Chem. Sot., 102, 1770 (1980) and references therein. and g-CH3D derivatives (11) For additions to Me2C=CMe2, we find (unpublished work) that both -CF of PhCCl exhibit kabs vs. l/T dependencies similar to that of PhCCl i.$self; i.e., "negative" activation energies which are kinetically consistent with the presence of an intermediate.
(Received
in USA 5 November
1982)
Tetrahedron Letters,Vol.24,No.7,pp 685-688,1983 Printed in Great Britain
HAMMETT
ANALYSIS
OF ABSOLUTE
CARBENE
RATE CONSTANTS
N.J. Turro*, I.R. Gould and N.P. Hacker Chemistry Department Columbia University New York, New York 10027
R.A. MOSS* and L.A. Perez Department of Chemistry Rutgers University New Brunswick, New Jersey 08903 Summary.
ADDITION
Absolute rate constants were determined for the additions of 5 L-X-substituted ArCCl (X=CF3, Cl, H, CH , CH30) to tetramethylethylene, trimethylethylene, trans-pentene, and 1-hexene; goo a Hammett correlations were obtained with p = +1.4 - 1.6.
In the absence
of absolute
kinetic
been assessed
by linear free energy 1 sets of alkenes. Hammett analyses
strates,
followed
by comparisons
now make it possible previously absolute
generally
of p values
to determine
absolute
studied only by competitive rate constants
pretations
of carbenic
Bensamidine
reactivity,
involved
of carbenes
reactivities
variation
carbenes.
rate constants
on arylalkene
Flash photolytic
for additions
The results
to alkenes
that some revisions
broadly
or 1-hexene
led to 20 cyclopropanes
sub-
techniques
of carbenes analysis
support previous
may ultimately
which were isolated
have
with standard
We report here the first Hammett
additions.
but suggest
selectivities
relative
substituent
for several
techniques.2
for carbene/alkene
the alkenic
of carbenic
of
inter-
be required.
and 4 e-substituted derivatives (CF3, Cl, CH3, CH30) were oxidized 3 Photolysis (A>300 nm) of each diazirine in tetramethylethylene,
chlorodiazirines. trans-pentene,
information,
comparisons
to 3-aryl-3isobutene,
and characterized
by nmr
spectroscopy parent
and elemental analysis. Using methods and equipment described in our studies of the 2c,d carbene, PhCCl, each diazirine was photolytically decomposed in 3-methylpentane matri-
ces at 77K affording
uv spectra
of the corresponding
arylchlorocarbenes.
The spectra
obtained
at
77X were in good agreement
with respect to shape and maximum with point-by-point transient spectra 2c,d 4 of the laser flash generated carbenes in aerated isooctane solution at 23OC. 2c,d Laser excitation of isooctane solutions of the diasirines gave transient absorptions of the corresponding bene "quencher"
carbenes
of the transient
absorption
order decay constant analysis. 5 mined
(ArCCl).
The bimolecular
rized in Table I. in Table I.
Using relative
and compared
2 these comparisons responsible
were obtained
methods
and l-hexene.
to Lrel derived
suffices
for product
to indicate
formation,
and 1-hexene)
appear
concentrations
in Figure
with ci substituent
better
of car-
the decay
Plots of the pseudo-first employed
of ArCCl by each quencher
versus quencher
of these results
of photolytically
trans-pentene,
order kinetics.
for quenching
>0.99, significantly
high concentrations
trans-pentene
were linear under the conditions
of decay constants
analyses
competition
reactivities
of quencher
rate constants
The best correlations coefficients standard
isobutene, Me2C=CMe
Hammett
of sufficiently
trimethylethylene,
followed pseudo-first
as a function
from the slopes of plots
correlation
In the presence
(tetramethylethylene,
for
were deter-
and are summa-
1, with
p values reported constants, 6a which gave
than those obtained
with
c
constants.6b
analysis, p-lb we determined
based on cyclopropane
product
generated
ArCCl toward tetramethylethylene,
Results
(X>300 nm) appear
in Table
II, where they are normalized
from the k --abs data of Table I. that the spectroscopically
Although
monitored
the two sets of data are not identical
685
the agreement
kinetic
within
to in
species are
the maximal
686
experimental
error
(+ 20%) in all cases.
termediate
(vide infra) --
competition
experiments
is involved
Indeed,
if reversible formation of a carbene/alkene in2d then the results of the steady state
in these reactions,
will not match those of the time resolved -I lifetime of the intermediate.
of a non-negligible
experiments
under the conditions
k for ArCCl/alkene additions varies sensibly with both aryl and alkenic substituents. abs From Table I, the CH30 to CF 3 carbenic substituent change enhances Labs by factors of 88-138 for the alkenes; CH-C4Hg
rate constant
comparison.
addition
ratio for the fastest
-SbS
(p-CH30C6H4CC1 -
electron-donating
spreads of 83-194 occur for the various
The k
+ CH2=CH-C4H9)
(ED) substituents
ents accelerate
the additions;
Qualitatively,
(p-CF3CSH4CC1 -
is 4 orders of magnitude.
in ArCCl retard,
p values
are ~1.5
whereas
carbenes
in the Me2C=CMe2/CH2=
+ Me2C=CMe2) The Hammett
vs. the slowest analyses
electron-withdrawing
with little dependence
show that
(EW) substitu-
on alkene.*
than EW substituted
p>o might be interpreted to mean that ED substituted ArCCl are more stabilized 10 ArCCl, and therefore react more slowly with a given alkene. Recent evidence
that carbene/alkene
additions
complex)
complicates
may proceed via intermediates (e-g ---L'- 1 in eq. (l), a charge transfer 11 but rationalization of the p values can still be made by focus-
the matter,
sing on the intermediate.
When X is EW, passage
of 1. to product
over "electrophilic"
state 2 is favored because
X stabilizes
negative
imposed on the carbenic
enhanced
charge
transition carbon.
Dissociation
of 1 to carbene and alkene is disfavored, however, because EW substituents destabilolize carbenes. When X is ED, these considerations reverse. Now 2 is destabilized and the dis-
sociation
of 1 to carbene
1, but ED groups
and alkene
favor reversion
is favored.
of 1. to ArCCl,
be positive.
Thus, EW groups
enhance product
formation
and p based upon the disappearance
1
from
of ArCCl
should
Ar
Ar\C/C1 i The absence is striking concerning
tivity
of a substituent 8 Although
the
[1+21 cycloaddition formed
temperature despite
resolved
-1 dependence
and unexpected.
of a reversibly unusual
FM
a large change
kinetic
tion is involved
results.
of the kinetics in reactivity,
For example,
in the rate determining
selective
if the factors which determine Furthermore,
state competition
product
ratio.
We believe
a carbene
thank Professors
We are grateful
to alkene: la,10
calculations) involved
the concept
a possible basis for explaining the 2d of these reactions, the absence of altered selec-
and the differences
selectivity
moiety
further
involved
alkene,
between
state and time
system complex
(high reactivity)
the passage
forma-
may still be
of the intermediate
to
very high, as it does for a steady
in a reversibly leading
new insight
formed carbene/alkene
to a concentration
into the seemingly
and experimental
Science Foundation
for helpful
the steady
carbene/alkene
becomes
thereby
theoretical
to the National
M. Plats and R.C.D. Breslow
control
of quencher
that our results provide
and serve to encourage
ACKNOWLEDGEMENTS.
orbital
to alkenes have not explicitly
step, a very fast reaction
might be "trapped" by a different
cycloaddition
of these ArCCl additions
(e.g., molecular
if for a particular
if the concentration
experiment,
ideas
such an idea provides
product.
termediate
on the selectivity
current
of carbenes
intermediate,
dependence
e
discussions.
in-
dependent
simple
[I+21
investigations.
for financial
support.
We
687
Table I.
Absolute
Rate Constants
for Additions
of PX-C~H~CC~
to Alkenesa
X CH30
Alkenes Me2C=CMe2
CH
Cl
Hb
3
CF3
PC
1.4 x 10 7d
1.2 x 108
2.8 x lOa
3.3 x 108
1.5 x 109
1.5
Me2C=CHMe
7.7 x lo6
4.9 x lo7
1.3 x lo8
1.8 x lOa
6.8 x lo8
1.4
t-MeCH=CHEt
4.4 x lo5
1.8 x 106
5.5 x lo6
7.5 x lo6
4.9 x 10'
1.5
CH,=CH-fl-C4Hg
1.3 x 10 5d
6.2 x lo5
2.2 x lo6
2.3 x lo6
1.8 x lo7
1.6
aRate constants are in L/mol-set and were determined in alkene/isooctane solution at 23OC by laser flash photolytic methods; see text and refs. 2c and 2d. Reproducibilities ared<+ 10% unless otherwise noted. bThese data differ slightly from those previously reported due to modifications in equipment. All data reported here were determined under identical conditions. 'p values are rounded to 2 significant figures. All correlation coefficients were >0.99. For U+ constants, see ref. 6a. Reproducibility is f 15%. P Table II.
Relative
Reactivities
for Additions
of p-X-C6H4CC1
to Alkenes a,b
X Alkenes
CH30
CH
3
H
Cl
CF
3
Me2C=CMe2
1.00
1.00
1.00
1.00
1.00
Me2C=CH
0.37
0.26
0.25
0.22
0.24
2
-t-MeCH=CHEtC CH2=CH-;-C4Hg
C
0.038 (0.031)
0.028 (0.015)
0.028 (0.020)
0.029 (0.023)
0.033 (0.033)
0.017 (0.0093)
0.012 (0.0052)
0.012 (0.0079)
0.012 (0.0070)
0.012 (0.012)
a
Relative reactivities were determined at 25OC in neat alkene mixtures. Product separation and quantitation was achieved by reversed phase hplc on a Waters C-18 RCM column, using CH3CN or lo-15% H20/CH CN as eluents, a calibrated uv detector, and an electronic integrator. Results are genera 2 ly averages of 2 or more runs and were usually determined in competition against isobutene. Reproducibilities were <* 9%, and all but 3 cases were
AND NOTES
(a) R.A. Moss, Act. Chem. Res., 2, 58 (1980); (b) R.A. Moss in "Carbenes", Vol. I, M. Jones, Jr., and R.A. Moss, Ed., Wiley, New York, 1973, pp. 153f; (c) R.A. Moss and M. Jones, Jr., in "Reactive Intermediates", Vol. 1, M. Jones, Jr., and R.A. Moss, Ed., Wiley, New York, 1978, pp. 69f; (d) Vol. 2, 1981, pp. 59f. (2) (a) G.L. Gloss and B.E. Rabinow, 3. Am. Chem. Sot., 98, 8190 (1976); (b) J.J. Zupanic and G.B. Schuster, -ibid., -_ 102, 5958 (1980); (c) N.J. Turro, J.A. Butcher, Jr., R.A. Moss, W. Guo, R.C. Munjal, and M. Fedorynski, ibid., 102, 7576 (1980); (d) N.J. Turro, G.F. Lehr, J.A. Butcher, Jr., R.A. Moss,and W. GUO, ibid., 104, 1754 (1982). (3) W.H. Graham, J. Am. Chem. Sot., 87, 4396 (1965). For prior preparations and thermal decompositions of arylchlorodiazirines, see M.T.H. Liu and K. Toriyama, Can. J. Chem., so, 3009 (1972). (4) xmax values (77K, 23OC) for ArCCl were (run): p-CH,O, 346, 340; g-CH3,, 317, 317; H, 307, 310 (benzene); e-cl, 323, 320; g-CF3, 303, 298. For solution kinetic studies, carbene absorptions were monitored at 320 nm, except for p-CH3CC6H4CC1 (330 nm). (5) The use of kinetics to uncover unexpected aspects of the reactions of PhCCl with alcohols has D. Griller, M.T.H. Liu, and J.C.Scaiano, J. Am. Chem. Sot., 104, recently been reported: 5549 (1982).
688
Figure 1. Logarithms of absolute rate constants (L/mol-set) for additions of ArCCl to Me2C=CMe2, For ArCCl substituents and P values, see Me C=CHMe, t-MeCH=CHEt, and CH,=CH-;-C4H9 vs. "pT a2 le1. -
(6) (a) C.D. Ritchi: and W.F. Sager, Prog. Phys. Org. Chem., 2, 323 (1964); cf., pp. 334-337. The value for oE (CF3) is taken from C.G. Swain and E.C. Lupton, Jr., J. Am. Chem. Sot., 90, (b) J. March, "Advanced Organic Chemistry", 2nd Ed., McGraw-Hill, New York, 4328 (1968). 1977, p. 253. (7) C. Lewis and W.R. Ware, Mol. Photochem., 5, 261 (1973). the most (8) Although classic electrophilic selectivity-1atlb is apparent with all 5 ArCCl (&, highly alkylated olefins react most rapidly') the carbenes are all comparably selective over This lack of selectivity the olefin set; the four Hammett correlations are nearly parallel. variation is also apparent from the be1 data of Table II. (9) The anticipated electrophilic behavior is also apparent in additions of PhCCl to either simple alkeneslb or to ring-substituted styrenes, for which p varies from -0.56 to -1.06 decf., W. Briick and H. Diirr, Tetrahedron Lett., 23, 2175 (1982). pending on temperature: (10) Electron-donating substituents stabilize carbenes by r-type resonance interactions; "f:, N-G. Rondan, R.N. Houk, R.A. Moss, J. Am. Chem. Sot., 102, 1770 (1980) and references therein. and g-CH3D derivatives (11) For additions to Me2C=CMe2, we find (unpublished work) that both -CF of PhCCl exhibit kabs vs. l/T dependencies similar to that of PhCCl i.$self; i.e., "negative" activation energies which are kinetically consistent with the presence of an intermediate.
(Received
in USA 5 November
1982)