Absolute kinetics of mesitylmethylchlorocarbene reactions

Article abstract of DOI:10.1039/a607552e

Absolute rate constants were determined for reactions of mesitylmethylchlorocarbene, which affords intramolecular products derived from 1,2-H migration, 1,2-mesityl migration, and C-H insertion; at -35 or -70 °C, however, the intrusion of intermolecular channels affords azine and carbene dimer, complicating interpretations of the kinetics.

Full text of DOI:10.1039/a607552e

View Article Online / Journal Homepage / Table of Contents for this issue
Absolute kinetics of mesitylmethylchlorocarbene reactions
Robert A. Moss* and Dina C. Merrer
Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, USA
Absolute rate constants were determined for reactions of
mesitylmethylchlorocarbene, which affords intramolecular
products derived from 1,2-H migration, 1,2-mesityl migra-
tion, and C–H insertion; at 235 or 270 °C, however, the
intrusion of intermolecular channels affords azine and
carbene dimer, complicating interpretations of the kin-
etics.
[a,a-2H2] analogue point to a substantial primary kinetic
isotope effect (KIE) associated with the H(D) shift. This is
discussed below.
Photolyses of 3 in the presence of 2-methylbut-1-ene led to
4–7 and the carbene/alkene addition products, Z- and
E-1-chloro-1-mesitylmethyl-2-ethyl-2-methylcyclopropanes.
When photolyses of 3 or [2H2]-3 were performed in the presence
of increasing concentrations of the alkene in isooctane,
correlations of (addition/intramolecular products) vs. [alkene]
were curved for each of the intramolecular products. However,
the inverse correlations of (intramolecular/addition products)
vs. 1/[alkene] were each linear.
If we assume that the second product-forming intermediate
(in addition to carbene 2) demanded by these results is excited
diazirine 3*,3 rather than a carbene/alkene complex,2 then the
y-intercepts of the inverse correlations give the partitions of 3*
between N2-loss to 2 or direct formation of 4–7.1b,c,3 Product
formation from 3* is then (4 + 5) 17.5; 6, 1.4; and 7, 4.7%. From
[2H2]-3*, the corresponding values are 24.5, 3.4 and 10.6%,
respectively. The balance of each product comes from the
carbene, and the ‘raw’ product yields can be ‘corrected’ and
renormalized to afford carbene-only distributions:7 (from 2)
(4 + 5), 74.3; 6, 5.7; 7, 20.0%; (from [2H2]-2) (4 + 5), 45.4; 6,
12.7; 7, 41.9%. These data are used below to partition the global
rate constants for carbene decay.
Over the past dozen years, the chemistry of benzylchlorocar-
bene, 1, has provided a rich complement of contemporary
puzzles in carbene chemistry.1 Continuing kinetics studies have
raised questions about the relevance of carbene/alkene com-
plexes,2 excited carbene precursors3 and quantum mechanical
tunnelling4 during the 1,2-H migration of 1, as well as the
origin(s) of curved Arrhenius correlations4b,c,5 and abnormal
kinetic isotope effects4 in these same rearrangements.
Our own interest in these matters1b,d impelled us to open a
new investigation from the perspective of the related mesityl-
methylchlorocarbene, 2. Although our results partly accord with
findings in the benzyl system, they raise important questions
concerning the influence of competitive intermolecular carbene
reactions at moderately low temperatures.
Carbene 2 and [a,a-2H2]-2 were generated by photolysis or
thermolysis of the corresponding diazirine precursors,6 3 and
[a,a-2H2]-3: lmax (pentane) 342, 348 (346 for [2H2]-3), 358 nm;
e342 = 50 dm3 mol21 cm21 (CDCl3). For the generation of 2,
we initially employed diazirine solutions with A342 ca. 1.0–1.1;
[3] ca. 0.02 mol dm23. Photolysis of 3 (isooctane, 25 °C,
l > 300 nm) was continued until the diazirine was destroyed,
and afforded products 4–7, attributed to carbene 2 in > 96%
yield.
Global rate constants for the disappearance of carbenes 2 or
[2H2]-2 were obtained by laser flash photolysis (LFP) at 351 nm
of the diazirines in isooctane. Carbene decay was followed
directly at 310–313 nm and from 270 to 47 °C. Rate constants
are considered accurate to ±10%.
Fig. 1 illustrates the Arrhenius correlations of ln k vs. T—1 for
the carbene decays. Linearity is observed for 2 down to ca.
240 °C, and for [2H2]-2 to ca. 220 °C, after which the observed
rate constants begin to level off with convergence at k ca. 9 3
105 s21 (ln k ca. 13.7). This behaviour resembles that of 1 and
The product distributions from carbenes 2 and [2H2]-2 were
determined relative to an internal decane standard (±2%) and
appear below. We obtained 1,2-H-rearranged alkenes Z- and
E-(2-chloroethenyl)mesitylene (4 and 5), 1,2-mesityl shifted
(1-chloroethenyl)mesitylene (6) and methyl insertion product
2-chloro-1,3-dihydro-4,6-dimethylindane (7). Thermolysis
[pentane, sealed tube, 80 °C (3) or 100 °C ([2H2]-3), 36 h] led
to distributions that were quite similar to those from photolysis.
Comparison of the yields of (4 + 5) vs. (6 + 7) from 2 and its
18.0
16
17.5
17.0
k
14
16.5
MesCH2
Cl
..
..
N
N
MesCH2CCl
PhCH2CCl
16.0
15.5
15.0
12
3
4
/ 10
5
–1
–3 –1
k
1
2
3
T
K
Me
14.5
14.0
13.5
Mes
Mes
Mes
+
+
+
Me
Cl
H(D)
H(D)
(D)H
(D)H
Cl
3.5
4.5
4.0
/ 10
5.0
3.0
(D2)
–1
–3 –1
T
K
H(D)
Cl
H(D)
Cl
4
5
6
7
Fig. 1 Arrhenius correlations for the decays of carbenes 2 ( ≠ ) and [a,a-
2H2]-2 ( < ); A342 ca. 1.0. Inset: Arrhenius correlations for the decays of
carbenes 2 ( ≠ ) and [a,a-2H2]-2 ( < ); A342 ca. 0.5.
2:
58
19
14
5
18 (%)
39 (%)
[2H2]-2: 36
11
Chem. Commun., 1997
617

View Article Online
[2H2]-1, where leveling occurs at 240 and 210 °C, re-
Note, in
correlations for both carbenes that are linear to 270 °C (inset,
Fig. 1).
spectively, with convergence at k ca. 1.2 3 107 s21 4b
.
both cases, that Arrhenius curvature begins at a higher
These new correlations afford Ea = 4.8 kcal mol21 (1
cal = 4.184 J), log A = 10.7 s21 and DS*(298) = 211.4 e.u.
for 2, and analogous values of 4.6 kcal mol21, 10.2 s21 and
213.7 e.u. for [2H2]-2. Of course, these data contain contribu-
tions from all carbene decay channels, but they are similar to the
activation parameters determined for carbene 1.4b Importantly,
with the disappearance of Arrhenius curvature, the temperature
dependence of the KIE for 2/[2H2]-2 no longer exhibits a
marked decrease with decreasing temperature. It is also
interesting to note from the activation parameters that kH/kD for
the disappearance of 2/[2H2]-2 (at least from the global rate
constants) is controlled by entropic, not enthalpic factors.
In the cases of 1 and [2H2]-1, Arrhenius curvature was
attributed to increasing intervention of carbenic reaction with
the isooctane solvent and/or 1,2-H shift tunnelling.4b However,
no products of carbene/isooctane reactions were observed.4b
Moreover, in preliminary experiments, we have observed the
formation of azine (up to 30%) in photolyses of benzyl-
chlorodiazirine (A344 = 0.7) at 270 °C in isooctane, under
conditions analogous to those used in the photolysis of 3. In the
light of our findings with carbene 2, it seems likely that azine
formation is a major cause of the Arrhenius curvature observed
in the H-shift reactions of 1.
temperature for the [a,a-2H2] carbene.
At 25 °C, the global rate constant for decay of 2 is 2.4 3 107
s21, which can be partitioned as 1.8 3 107 s21 for H-shift to 4/5,
1.4 3 106 s21 for 1,2-mesityl shift to 6, and 4.8 3 106 s21 for
insertion to 7. For [2H2]-2, the corresponding total rate constant
is 8.2 3 106 s21, and the partitioned rate constants are 3.7 3 106
s21 (1,2-D shift), 1.0 3 106 s21 (1,2-mesityl shift), and 3.4 3
106 s21 (insertion). The global rate constants are somewhat
smaller than those reported for 1 (6.2 3 107 s21) and [2H2]-1
(2.1 3 107 s21) at 25 °C.4b
Our partitioned rate constants afford a large KIE for 1,2-H or
1,2-D shifts of 2 at 25 °C; kH/kD = 4.8. Product distributions
were not corrected for incursion of 3* at other temperatures, so
that those global rate constants cannot be accurately partitioned
to reflect carbene-only product formation. From the global rate
constants for decay of 2 or [2H2]-2 (Fig. 1), kH/kD appears to
decrease with decreasing temperature, as was reported for 1.4b
As shown below, however, this picture is deceptive.
Thus far, the chemistry of mesitylmethylchlorocarbene
closely parallels that of benzylchlorocarbene.1a,2,4a,b An im-
portant divergence occurs, however, for product formation at
low temperature. Photolyses of 3 and [2H2]-3 were carried out
at 30, 0, 235 and 270 °C. At the two higher temperatures, 4–7
accounted for > 96% of the products from either 2 or [2H2]-2.
However, at 235 or 270 °C, product mixtures became
increasingly complex, with up to six new products formed. Of
these, we have identified 8–11.
We do not exclude tunnelling as a contributory factor to the
Arrhenius curvature. Tunnelling is surely involved in the
rearrangements of 1 in argon matrices at 20–30 K,4a and may be
important in the rearrangements of CH3CCl and CD3CCl at
temperatures that are much closer to ambient.4c,d
In conclusion, the intramolecular chemistry of mesityl-
methylchlorocarbene parallels that of benzylchlorocarbene, but
the increasing product complexity and azine formation at low
temperatures suggest important revisions to previous inter-
pretations of the rearrangement kinetics of the latter carbene.
We are continuing our studies of carbenes 1, 2 and their fluoro
analogues.
We are grateful to the National Science Foundation for
financial support, to Professors J. L. Goodman, M. T. H. Liu and
M. S. Platz for helpful discussions, and to Dr L. Maksimovic for
technical assistance.
MesCH2CHCl2
MesC CH
MesCH2CCl CClCH2Mes
8
9
10
MesCH2CCl
N
N
CClCH2Mes
11
Acetylene 8, also present in 2% yield in the 25 °C photolysis
of 3, appears to be an elimination product of 4, 5 or 6. Dichloride
9 results from reaction of 2 with HCl. The chromatographically
inseparable mixture of 15:85 carbene dimer 10 and azine 11
(from 2 + 3) was identified by NMR, GC–MS, microanalysis
and exact mass measurements. Yields of 8 and 9 were low
(@2%) under all conditions; whereas (10 + 11) constituted 2.5
and 11.2% of the products at 235 and 270 °C, respectively. For
[2H2]-2, dimer/azine formation was already apparent at 0 °C
(2.1%), and increased to 8.5 and 16% at 235 and 270 °C.
Analogous azine formation has also been observed from
benzylfluorodiazirine and benzylfluorocarbene.8
Azine formation was discounted as a complicating factor in
the chemistry of carbene 1,4b but it cannot be ignored here.
Moreover, azine formation will be more marked, and com-
mence at higher temperatures with [2H2]-2, which has a three-
fold lower rate constant for decay at 25 °C than does 2;
intramolecular reactions compete less efficiently with inter-
molecular reactions for the deuteriated carbene.
Azine (and dimer) formation are initially observed for 2 at
235 °C and for [2H2]-2 at 0 °C. Arrhenius curvature (Fig. 1)
becomes apparent at similar temperatures (240 and 220 °C).
These correspondences are not coincidental; with decreasing
temperature the intramolecular reactions of the carbenes are
slowed sufficiently so that the effectively temperature inde-
pendent intermolecular azine formation gains in importance,
causing significant curvature and levelling in the Arrhenius
correlations. Consistent with this explanation, we find that two-
fold dilution of diazirines 3 and [2H2]-3 (A342 = 0.5 in
isooctane, [diazirine] ca. 0.01 mol dm23) leads to Arrhenius
References
1 Reviews: (a) M. T. H. Liu, Acc. Chem. Res., 1994, 27, 287; (b)
R. A. Moss, in Advances in Carbene Chemistry, vol. 1, ed. U. H. Brinker,
JAI Press, Greenwich, 1994, p. 59; (c) J. E. Jackson and M. S. Platz, p. 89;
(d) R. A. Moss, Pure Appl. Chem., 1995, 67, 741.
2 (a) H. Tomioka, N. Hayashi, Y. Izawa and M. T. H. Liu, J. Am. Chem.
Soc., 1984, 106, 454; (b) M. T. H. Liu, J. Chem. Soc., Chem. Commun.,
1985, 982; (c) R. Bonneau, M. T. H. Liu, K. C. Kim and J. L. Goodman,
J. Am. Chem. Soc., 1996, 118, 3829 and references cited therein.
3 J. A. LaVilla and J. L. Goodman, Tetrahedron Lett., 1990, 31, 5109;
W. R. White III and M. S. Platz, J. Org. Chem., 1992, 57, 2841;
D. A. Modarelli, S. Morgan and M. S. Platz, J. Am. Chem. Soc., 1992,
114, 7034; R. A. Moss and W. Liu, J. Chem. Soc., Chem. Commun., 1993,
1597.
4 (a) S. Wierlacher, W. Sander and M. T. H. Liu, J. Am. Chem. Soc., 1993,
115, 8943; (b) M. T. H. Liu, R. Bonneau, S. Wierlacher and W. Sander,
J. Photochem. Photobiol. A: Chem., 1994, 84, 133; (c) E. J. Dix,
M. J. Herman and J. L. Goodman, J. Am. Chem. Soc., 1993, 115, 10 424;
(d) J. W. Storer and K. N. Houk, J. Am. Chem. Soc., 1993, 115,
10 426.
5 M. T. H. Liu and R. Bonneau, J. Am. Chem. Soc., 1990, 112, 3915.
6 R. A. Moss, W. Ma, D. C. Merrer and S. Xue, Tetrahedron Lett., 1995,
36, 8761.
7 R. A. Moss and G.-J. Ho, J. Phys. Org. Chem., 1993, 6, 126.
8 L. Maksimovic, unpublished work in this laboratory.
Received, 4th November 1996; Com. 6/07552E
618
Chem. Commun., 1997