New kinetics methodologies applied to carbene fragmentation reactions [16]

Full text of DOI:10.1021/ja002611x

11256
J. Am. Chem. Soc. 2000, 122, 11256-11257
trimethoxybenzene (TMB) capture12 of the daughter carbocations
(R+) formed from ROCCl. Together, these methods provide an
unusually complete natural history of carbene fragmentation and
the fate of the daughter carbocations.
We reexamined the fragmentation kinetics5 of 1, R ) PhCH2
and R ) 1-adamantylmethyl (AdCH2) in both MeCN (dielectric
constant ) 35.6) and in 1,2-dichloroethane (DCE, ꢀ ) 10.7). The
latter’s dielectric constant is similar to that of pyridine (ꢀ ) 12.3),
so that changes in solvent polarity due to pyridine dilution during
pyridine LFP determinations of kfrag are mitigated in DCE.9
New Kinetics Methodologies Applied to Carbene
Fragmentation Reactions
Robert A. Moss,*,† Lauren A. Johnson,† Shunqi Yan,†
John P. Toscano,*,‡ and Brett M. Showalter‡
Department of Chemistry
Rutgers, The State UniVersity of New Jersey
New Brunswick, New Jersey 08903
Department of Chemistry, Johns Hopkins UniVersity
Baltimore, Maryland 21218
Photolysis of 3-benzyloxy-3-chlorodiazirine (3) in MeCN led
to benzyloxychlorocarbene (1, R ) PhCH2), which fragmented
ReceiVed July 17, 2000
ReVised Manuscript ReceiVed September 22, 2000
to PhCH2 via ion pair 2,13 affording benzyl chloride and
+
+
N-benzylacetamide (from PhCH2 attack on MeCN); cf., eq 1.
Many alkoxychlorocarbenes (ROCCl, 1) readily fragment.1,2
When the “R” group affords a relatively stable carbocation,
fragmentation occurs via ion pair (2); cf., eq 1.1e-3
LFP studies using UV detection and pyridine ylide visualization6
kfrag
ROC¨ Cl
8 [R+ OC Cl-] f RCl + CO + other products
1
2
(1)
afford kfrag ) 3.6 ( 0.45 × 105 s-1 (Table 1). In DCE solvent,
where only PhCH2Cl is formed, the same LFP method gives kfrag
) 6.2 ( 0.2 × 104 s-1. A small solvent effect may operate here;
fragmentation is about 6 times faster in the more polar solvent,
reflecting the polar transition state for conversion of 1 f 2.14
However, when R is a simple primary alkyl group (e.g., n-butyl),
fragmentation becomes a bimolecular process driven by nucleo-
philic chloride attack on the carbene.4
We measured the rate constants (kfrag) of several fragmentation
reactions by laser flash photolysis (LFP).2,4,5 However, because
the ROCCl generally lacked sufficiently intense UV-active
chromophores, we relied on indirect kinetic monitoring of the
fragmentation, making use of the pyridine ylide carbene visualiza-
tion method.6 With this method, the fragmentation itself is not
directly observed, and the necessary dilution with pyridine,
sometimes to high concentrations (e.g., 7-8 M),7,8 can alter the
solvent polarity and distort the kinetics.9
We are pleased to report here the first direct observation of
carbene fragmentation, achieved by time-resolved infrared (TRIR)
monitoring10 of the appearance of CO; cf., eq 1.11 Additionally,
we disclose a second (indirect) kinetics method based on
LFP of 3 in MeCN or DCE was next monitored from 2000 to
2200 cm-1 by TRIR spectroscopy,10 using 0.5 mm IR cells and
A355(3) ) 0.5. A TRIR spectrum averaged from 7 to 9 µs
following the laser flash appears in the inset of Figure 1 (DCE
experiment), where the band at 2132 cm-1 represents the
formation of CO during fragmentation (verified with authentic
CO). The time dependence of CO formation at 2132 cm-1 is
depicted in Figure 1, where analysis of the principal growth curve
gives k ) 2.9 × 105 s-1 for CO formation, which we equate with
kfrag. In MeCN (νmaxCO ) 2140 cm-1) very similar results are
obtained, with kfrag ) 4.4 × 105 s-1. These directly observed
values (Table 1) are in reasonable agreement with the indirect
LFP-UV (pyridine ylide) results. To avoid overinterpretation of
our data, we do not attempt to analyze small differences between
kfrag values determined by our three different monitoring methods.
In Figure 1, we note the rapid formation and decay of a second
transient immediately after the laser pulse, a feature we attribute
to (unstable) benzyloxychlorodiazomethane (4), formed by pho-
toisomerization of diazirine 3. Related diazirine to diazo isomer-
izations are known,15 and our assignment is supported by a
B3LYP/6-31G* calculation (gas phase, Gaussian 98, λ ) 1)16
which predicts the IR diazo band of 4 to fall at 2141.8 cm-1; that
is, making likely its overlap with CO.
† Rutgers University.
‡ Johns Hopkins University.
(1) (a) Hine, J.; Pollitzer, E. L.; Wagner, H. J. Am. Chem. Soc. 1953, 75,
5607. (b) Skell, P. S.; Starer, I. J. Am. Chem. Soc. 1959, 81, 4117. (c) Smith,
N. P.; Stevens, I. D. R. J. Chem. Soc., Perkin Trans. 2 1979, 213. (d) Smith,
N. P.; Stevens, I. D. R. J. Chem. Soc., Perkin Trans. 2 1979, 1298. (e) Moss,
R. A.; Wilk, B. K.; Hadel, L. M. Tetrahedron Lett. 1987, 28, 1969.
(2) Moss, R. A. Acc. Chem. Res. 1999, 32, 969.
(3) (a) Tabushi, I.; Yoshida, Z-i.; Takahashi, N. J. Am. Chem. Soc. 1971,
93, 1820. (b) Likhotvorik, I. R.; Jones, M., Jr.; Yurchenko, A. G.; Krasutsky,
P. Tetrahedron Lett. 1989, 30, 5089. (c) Moss, R. A.; Ho, G. J.; Wilk, B. K.
Tetrahedron Lett. 1989, 30, 2473.
(4) Moss, R. A.; Johnson, L. A.; Merrer, D. C.; Lee, G. E., Jr. J. Am. Chem.
Soc. 1999, 121, 5940.
(5) Moss, R. A.; Ge, C.-S.; Maksimovic, L. J. Am. Chem. Soc. 1996, 118,
9792. The present data are considered more accurate than the previous rate
constants.
(6) (a) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J.
Am. Chem. Soc. 1988, 110, 5595. (b) Platz, M. S.; Modarelli, D. A.; Morgan,
S.; White, W. R.; Mullins, M.; Celebi, S.; Toscano, J. P. Prog. React. Kinet.
1994, 19, 93.
Photolysis of 3-(1-adamantylmethoxy)-3-chlorodiazirine (5) in
MeCN gives mainly the fragmentation products homoadamantyl
chloride (6, 62%) and the corresponding N-homoadamantylacet-
amide (11%).5 Again, we formulate the reaction as the fragmenta-
tion of ROCCl (R ) 1-AdCH2), for which LFP-UV (pyridine)
(7) ROCCl are ambiphilic carbenes which react “slowly” with pyridine,8
thus requiring high [pyridine] to compete with fragmentation.
(8) Ge, C. S.; Jang, E. G.; Jefferson, E. A.; Liu, W.; Moss, R. A.;
Wlostowska, J.; Xue, S. Chem. Commun. 1994, 1479.
(9) Significant solvent effects on the fragmentation of several ROCCl have
been observed and will be reported in due course: Johnson, L. A.; Moss, R.
A., unpublished research.
(12) (a) Pezacki, J. P.; Shukla, D.; Lusztyk, J.; Warkentin, J. J. Am. Chem.
Soc. 1999, 121, 6589. (b) See also: Steenkin, S.; Ashokkumar, M.;
Maruthamuthu, P.; McClelland, R. A. J. Am. Chem. Soc. 1998, 120, 11925.
(13) Moss, R. A.; Kim, H.-R. Tetrahedron Lett. 1990, 31, 4715.
(14) For computational studies, see Yan, S.; Sauers, R. R.; Moss, R. A.
Organic Lett. 1999, 1, 1603.
(15) (a) Bonneau, R.; Liu, M. T. H. J. Am. Chem. Soc. 1996, 118, 7229.
(b) Nigam, M.; Platz, M. S.; Showalter, B. M.; Toscano, J. P.; Johnson, R.;
Abbot, S. C.; Kirchhoff, M. M. J. Am. Chem. Soc. 1998, 120, 8055. (c) Moss,
R. A.; Chang, M. J. Tetrahedron Lett. 1981, 22, 3749. (d) Bayley, H.; Knowles,
J. R. Biochemistry, 1978, 17, 3420.
(10) We used the method of Hamaguchi, et al.: Iwata, K.; Hamaguchi, H.
Appl. Spectrosc. 1990, 44, 1431. Yuzawa, T.; Kato, C.; George, M. W.;
Hamaguchi, H. Appl. Spectrosc. 1994, 48, 684. For a description of our
instrumentation and methodology, see: Wang, Y.; Yuzawa, T.; Hamaguchi,
H.; Toscano, J. P. J. Am. Chem. Soc. 1999, 121, 2875.
(11) For low-temperature matrix photochemistry of MeOCCl and PhOCCl,
see: Kesselmayer, M. A.; Sheridan, R. S. J. Am. Chem. Soc. 1986, 108, 99-
107 and 844-845.
10.1021/ja002611x CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/26/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 45, 2000 11257
kfrag (R ) AdCH2), s-1
Table 1. Fragmentation Kinetics of Alkoxychlorocarbenes, ROCCla
kfrag (R ) PhCH2), s-1
method
MeCN
DCE
MeCN
DCE
LFP, UV (pyr)
LFP, TRIRc
LFP, UV (TMB)
3.6 ( 0.45 × 105
4.4 ( 0.03 × 105
9.3 ( 2.1 × 106
6.2 ( 0.2 × 104
2.9 ( 0.1 × 105
d
2.45 × 106 b
4.2 ( 0.4 × 106
8.26 ( 0.23 × 105
3.25 ( 0.07 × 105
1.1 ( 0.2 × 106
d
a At ambient temperature. b Compare, kfrag ) 5.2 × 106 s-1; ref 5. c TRIR rate constants are based on averages of two to four sets of 4000 or
16000 laser shots. d Not measured; see text.
from the UV (pyridine ylide) or TRIR methods (Table 1).
Analogous LFP experiments with diazirine 3 and TMB in MeCN
give signals for 7 (R ) PhCH2). Analysis affords kfrag for PhCH2-
OCCl as 9.3 ( 2.1 × 106 s-1 (three experiments), which appears
too high. However, the yield of 7 (R ) PhCH2) was quite low,17
and kfrag here is not very reliable. In DCE, accurate data could
not be obtained.
Further manipulation of the TMB data is possible.12a For the
LFP of 5 in MeCN, a Stern-Volmer plot of 1/absorbance of 7
vs 1/[TMB] is linear (r ) 0.995, 8 points). Assuming diffusion
control for the R+/TMB reaction, one can extract the lifetime of
R+ from the intercept and slope of the Stern-Volmer correla-
Figure 1. Intensity of CO vs time in the fragmentation of PhCH2OCCl
in DCE monitored at 2132 cm-1 by LFP-TRIR. The solid line is the
tion.6b,12a We find τ ) 2.6 ( 0.6 × 10-11 s for the homoadamantyl
calculated best fit to a single exponential for the growth of CO; kfrag
)
cation in MeCN/TMB. For comparison, Pezacki et al. report τ )
1.2 × 10-10 s for 2-Ad+ in aqueous MeCN.12a
2.9 × 105 s-1. The simultaneous diazoalkane formation and decay is
discussed in the text. Inset: TRIR spectrum (2000-2200 cm-1) averaged
over 7-9 µs following a 355 nm laser pulse (0.5 mJ, 90 ns) applied to
diazirine 3.
Finally, we determined Arrhenius parameters for the fragmen-
tation of 1-AdCH2OCCl using both the LFP (pyridine) and LFP
(TMB) monitoring methods for kfrag. With the former procedure,
and the reaction temperature varied from -39 to 21 °C, linear
correlation of ln kfrag versus 1/T gave Ea ) 3.6 ( 0.2 kcal/mol,
log A ) 9.2 ( 0.4 s-1 (MeCN) and Ea ) 4.6 ( 0.3 kcal/mol, log
A ) 9.3 ( 0.6 s-1 (DCE). With LFP (TMB) over -29 to 39 °C,
the Arrhenius correlation in MeCN gave Ea ) 5.4 ( 0.3 kcal/
affords kfrag ) 2.45 × 106 s-1, recorded in Table 1. In DCE, the
same methodology yields kfrag ) 8.26 ( 0.23 × 105 s-1
.
LFP-TRIR kinetics were conducted on 5 as for 3. Multiscans
from 2000 to 2080 cm-1 in DCE tracked the time-dependent CO
formation attending the fragmentation of AdCH2OCCl. In DCE,
kfrag ) 3.25 ( 0.07 × 105 s-1 in reasonable agreement with the
LFP-UV (pyridine) value (Table 1). In MeCN, however, persis-
tence of the (putative) diazoalkane isomer of 5, formed after the
laser pulse, interfered with the analysis of CO production and
mol, log A ) 10.6 ( 0.4 s-1 18
. It is unclear how to interpret small
differences between low Ea’s measured with different monitoring
methods.
Using DFT methodology, in analogy to our previous study,14
we computed19 the Ea’s for the fragmentation of (cis)-1-AdCH2-
OCCl to the homoadamantyl cation, CO, and chloride as 4.38
(MeCN) or 5.58 (DCE) kcal/mol, in good agreement with the
observed values. We have suggested that the 1-AdCH2OCCl
fragmentation involves a concerted ring expansion of the 1-ada-
mantylmethyl group directly to the homoadamantyl cation.5 The
B3LYP/6-311+G** transition state, together with a B3LYP/
6-31G* intrinsic reaction coordinate calculation, support this idea,
although, in accord with the low activation energy, the transition
state is “early.” Details will appear in a full paper.
prevented accurate determination of kfrag
.
Pezacki et al. found that LFP of oxadiazolines generated
diazoalkanes which, in the presence of (e.g.) CF3COOH, very
rapidly protonated to give alkyldiazonium ions, which then lost
nitrogen, affording carbocations. These could be captured by
TMB, yielding chromogenic cyclohexadienyl cations (7), which
were readily observable in the UV.12a The additions of R+ to TMB
were diffusion-controlled.12a
LFP of 5 in MeCN or DCE containing 2.68 M TMB gave UV
signals for 7 (R ) homoad) at 385 nm (MeCN) or 390 nm (DCE).
Analysis of the growth of 7 with time gave apparent rate constants
(kapp) for the formation of 7. Accepting that the addition of R+ to
TMB is diffusion-controlled,12a the rate-determining step in the
In conclusion, we have applied the mutually supportive TRIR
and UV methodologies to the kinetics of ROCCl fragmentation.
In the case of 1-AdCH2OCCl, an unusually complete picture
emerges: carbene fragmentation occurs with kfrag ≈ (2-4) × 106
s-1 (MeCN) or ≈ 3-10 × 105 s-1 (DCE), and Ea ≈ 3-5 kcal/
mol, log A ≈ 9.1-10.4 s-1; while the homoadamantyl cations
produced by the fragmentation survive for ∼2 × 10-11 s in MeCN.
Applications of the multiple kinetic methodologies described here
to other alkoxychlorocarbenes are in progress.
ultimate formation of 7 must be the fragmentation of ROCCl to
17
generate R+. We therefore equate kapp with kfrag
.
We thus obtain kfrag ) 4.2 ( 0.4 × 106 s-1 in MeCN and 1.1
( 0.2 × 106 s-1 in DCE, in reasonable agreement with the values
(16) (a) Optimizations utilized Gaussian 98, revision A.7: Frisch, M. J.;
Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J.
R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J.
C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.;
Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.;
Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala,
P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox,
D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez,
C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M.
W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (b) DFT calculations
used Becke’s three parameter hybrid method using the LYP correlation
functional: Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
Acknowledgment. We are grateful to the National Science Foundation
for financial support, and to the Center for Computational Neuroscience
of Rutgers University (Newark) for computational support. J.P.T.
acknowledges both NSF Faculty Early Career Development and Camille
Dreyfus Teacher-Scholar Awards. We thank Dr. Dina Merrer and
Professor Ronald Sauers for helpful discussions.
JA002611X
(18) Arrhenius experiments with kfrag for PhCH2OCCl gave poor and
relatively flat correlations, indicative of a very low Ea. The computed Ea in
MeCN is 1.45 kcal/mol.14
(19) Single point SCI-PCM calculations were based on fully optimized
B3LYP/6-311+G** gas-phase geometries, and incorporated zero-point energy
and thermal corrections at the B3LYP/6-31G* level.
(17) Most likely, only those carbocations that escape from the ion pair,
possibly a small fraction, can react with TMB.