Absolute kinetics of alkoxychlorocarbene fragmentation

Full text of DOI:10.1021/ja962194h

9792
J. Am. Chem. Soc. 1996, 118, 9792-9793
and neat MeOH occurred with k ∼ 2 × 106 s-1 13
.
The
Absolute Kinetics of Alkoxychlorocarbene
Fragmentation
fragmentation of 5a may therefore also be slow.
The introduction of the pyridine ylide methodology for the
LFP visualization of carbenes that lack strong intrinsic UV
absorptions,14 together with the finding that MeOCCl reacts with
pyridine (albeit slowly) to yield an ylide (λmax 472 nm, ky )
9.0 × 105 M-1 s-1),15 now permit us to apply the pyridine ylide
LFP technique to the fragmentations of 5a, (1-adamantyl)-
methoxychlorocarbene (5b), and neopentoxychlorocarbene (5c).
The results constitute the first kinetic data for carbene frag-
mentation and demonstrate that this process occurs on the µsec
timescale.
Reactions and Products. Diazirines 4a-c were prepared
by Graham oxidations16 of O-alkylisouronium tosylates (6); the
latter were synthesized from the appropriate alcohol, cyanamide,
and anhydrous p-toluenesulfonic acid in CHCl3 or THF3a,4,6 and
fully characterized. In MeCN, diazirines 4a-c (λmax 356 nm)
had half-lives of 96 (4b)-155 (4a)4 min at 25 °C.
Robert A. Moss,* Chuan-Sheng Ge, and
Ljiljana Maksimovic
Department of Chemistry, Rutgers
The State UniVersity of New Jersey
New Brunswick, New Jersey 08903
ReceiVed June 28, 1996
The reaction of an alkoxide with a dihalocarbene produces
an alkoxyhalocarbene1 (1) that can fragment to an alkyl cation
with loss of halide and CO (“deoxideation”);2 cf., eq 1.
Alternatively, 1 can be directly generated from an alkoxy-
halodiazirine, 2, by thermolysis (or photolysis).3
Photochemical decomposition (λ > 330 nm) of 4a in MeCN
affords 63% of benzyl chloride via carbene 5a, followed by
fragmentation to ion pair 3a, and subsequent collapse.4 Ad-
ditionally, 37% of PhCH2NHCOMe forms in a Ritter reaction
initiated by benzyl cation capture by MeCN. Fragmentation is
therefore the entire fate of carbene 5a.
Product mixtures from carbenes 5b and 5c are more complex,
however. Photolysis of (1-adamantyl)methoxychlorodiazirine
(4b) (A356 ) 1.0 in MeCN at 25 °C) gave 7-12 with the
capillary GC distribution shown in eq 2; thermal decomposition
at 25 °C afforded the same products in similar yields. Excepting
11, products were characterized by 1H NMR, GC-MS, elemental
analysis, and GC-spiking with authentic samples. Dichloride
11 was identified by NMR and hydrolysis to formate 12.
Subsequent studies have shown that the cations of eq 1 arise
as ion pairs that afford substantial halide return even in neat
alcoholic solvents (e.g., R ) PhCH2, X ) C1).4 Moreover,
return occurs with stereochemical retention,5,6 whereas competi-
tive solvent capture takes place with inversion,6 as anticipated
for ion pair intermediates. 1,2-Carbon shift rearrangements of
the initial alkyl moiety intervene when R is (1-adamantyl)methyl
(l-AdCH2),7 cyclopropylmethyl,8 or neopentyl.2,9 Indeed, the
associated product ratios,8 label distributions,8 and stereochem-
istry of the 1,2-Me migration when R ) neopentyl,9 indicate
that ion pairs 3 must be very tight. In the limit, the conversion
of 1 to RX approaches the mechanistic paradigm of the SNi
reaction.10
Despite this considerable body of product-based research, the
kinetics of the actual carbene fragmentation are undetermined.
Laser flash photolysis (LFP) of benzyloxychlorodiazirine (4a)
failed to afford a transient absorption for benzyloxychlorocar-
bene, 5a. Thermolysis of 4a in methanol gave PhCH2Cl and
PhCH2OMe, from return and solvolysis of ion pair 3 (R )
PhCH2, X ) Cl), but no products derived from methanolic
capture of carbene 5a.4 If 5a reacted with methanol at rates
approaching diffusion control, like PhCOMe11 or PhCCl,12 then
the fragmentation of 5a must have occurred with kfrag ∼ 1010
Products 7-10 arise by fragmentation (81%) of carbene 5b
via ion pair 3b. Most of the 1-AdCH2 moiety ring expands to
the homoadamantyl cation, from which 7 (Cl- return), 9 (Ritter
attack on MeCN), and 10 (adventitious water) derive. 1-AdCH2-
Cl (8), which retains the original alkyl moiety, also descends
from 3 but is not a displacement product since its yield does
not increase when 4b is decomposed in the presence of 0.1 M
benzyltriethylammonium chloride. Products 11 and 12 represent
interception (19%) of carbene 5b by HCl (released during
fragmentation) and water, respectively. When 4b is decomposed
in 4 M pyridine-MeCN, HCl is scavenged, and 11 disappears
in favor of 12, formed by hydrolysis of a carbene-pyridine ylide
(see below). Fragmentation or pyridine capture are the two fates
of carbene 5b in pyridine-MeCN.17
s-1 4
. However, we now know that ambiphilic carbenes, such
as 5, react “slowly” with methanol: the reaction of MeOCCl
(1) Hine, J.; Pollitzer, E. L.; Wagner, H. J. Am. Chem. Soc. 1953, 75,
5607.
(2) Skell, P. S.; Starer, I. J. Am. Chem. Soc. 1959, 81, 4117.
(3) (a) Smith, N. P.; Stevens, I. D. R. J. Chem. Soc., Perkin Trans. 2,
1979, 1298. (b) Smith, N. P.; Stevens, I. D. R. J. Chem. Soc., Perkin Trans.
2, 1979, 213.
Decomposition of diazirine 4c affords the products (and
(4) Moss, R. A.; Wilk, B. K.; Hadel, L. M. Tetrahedron Lett. 1987, 28,
1969.
capillary GC distribution) illustrated in eq 3;18 product identities
(5) Moss, R. A.; Kim, H-R. Tetrahedron Lett. 1990, 31, 4715.
(6) Moss, R. A.; Balcerzak, P. J. Am. Chem. Soc. 1992, 114, 9386.
(7) Tabushi, I.; Yoshida, Z-i; Takahashi, N. J. Am. Chem. Soc. 1971,
93, 1820.
(8) Moss, R. A.; Ho, G. J.; Wilk, B. K. Tetrahedron Lett. 1989, 30, 2473.
(9) Sanderson, W. A.; Mosher, H. S. J. Am. Chem. Soc. 1966, 88, 4185.
Sanderson, W. A.; Mosher, H. S. J. Am. Chem. Soc. 1961, 83, 5033.
(10) Likhotvorik, I. R.; Jones, M., Jr.; Yurchenko, A. G.; Krasutsky, P.
A. Tetrahedron Lett. 1989, 30, 5089.
(11) Moss, R. A.; Shen, S.; Hadel, L. M.; Kmiecik-Lawrynowicz, G.;
Wlostowska, J.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1987, 109, 4341.
(12) Griller, D.; Liu, M. T. H.; Scaiano, J. C. J. Am. Chem. Soc. 1982,
104, 5549.
(13) Du, X-M.; Fan, H.; Goodman, J. L.; Kesselmayer, M. A.; Krogh-
Jespersen, K.; LaVilla, J. A.; Moss, R. A.; Shen, S.; Sheridan, R. S. J. Am.
Chem. Soc. 1990, 112, 1921.
(14) (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. Reaction
Kinet. 1994, 19, 93.
(15) Ge, C-S.; Jang, E. G.; Jefferson, E. A.; Liu, W.; Moss, R. A.;
Wlostowska, J.; Xue, S. Chem. Commun. 1994, 1479.
(16) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
(17) Carbene dimer is not observed in MeCN, but in hexane, where
fragmentation is slower,4 dimer is the major product (55-60%).
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Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9793
follow from GC-MS and spiking experiments with authentic
samples.
Therefore, the extrapolated Y-intercept of correlation S-3 fairly
represents kfrag of (1-adamantyl)methoxychlorocarbene, 5b.23
The absorbance of ylide 18c (λmax 465 nm), formed from
LFP-generated 5c and pyridine was too weak to reliably deter-
mine kobs. However, the intensity of the absorbance did track
pyridine concentration; a double reciprocal correlation of 1/Irel
vs 1/[pyr] was linear (six points, r ) 0.996, supporting
information Figure S-5) and led to kfrag ) 1.3 × 106 s-1 for
neopentoxychlorocarbene, 5c, assuming that ky for the formation
of 18c is identical to ky for the formation of ylide 18b (see
above).
Dichloride 16 was characterized by NMR, GC-MS, and
hydrolysis to formate 17 in aqueous MeCN. Products 13-15
arise by fragmentation (44%) of carbene 5c, proceeding via ion
pair 3c (X ) Cl), where rearrangement to the tert-amyl cation
is total: loss of H+ then generates 13 and 14,19 while return of
chloride affords 15. Dichloride 16 and formate 17 reflect
interception (55%) of 5c by HCl or water, respectively; the large
alkene yield of fragmentation produces enough HCl to render
trapping product 16 dominant. In the presence of 4 M pyridine,
HCl is removed, 16 is entirely suppressed, and formate (74.5%)
from hydrolysis of the carbene-pyridine ylide becomes the major
product. Under these conditions, the fate of 5c is fragmentation
(23%)20 or pyridine capture.
Kinetics. Absolute rate constants for the fragmentations of
carbenes 5a-c were determined by LFP21 using pyridine ylide
methodology;14 carbene 5a is illustrative. LFP at 351 nm and
25 °C of diazirine 4a in MeCN (A356 ∼ 1.0) in the presence of
pyridine produced an absorption due to ylide 18a at 460 nm.
(Ylide 18, R ) Me, absorbs at 472 nm.15)
A referee has pointed out that this assumption, coupled with
the product ratio of 5c fragmentation to pyridine capture (∼1:3
in 4 M pyridine; see above) leads to an estimated value of kfrag
∼ 3 × 105 s-1, which is ∼4 times smaller than the value that
emerges from the double reciprocal analysis of the ylide
absorbance data. We believe that the double reciprocal method
is not as accurate as the direct correlation of kobs with [pyridine]
and agree with the referee that it may be suspect in the case of
carbene 5c. Alternatively, the assumption that carbenes 5b and
5c have equal rate constants for ylide formation (ky) may be
poor. In the absence of better data, an inclusive range for kfrag
of 5c is 0.3-1.3 × 106 s-1
.
For carbene 5c the product distribution, eq 3, is less favorable
than for 5b; only 44% of 5c products arise by fragmentation in
MeCN, where the major product (52%) is HCl-intercept
dichloride 16. However, in the presence of g4 M pyridine
(where most of the LFP data is collected), HCl and 16 are
suppressed, so that the kinetics should reflect a simple competi-
tion between ylide formation and fragmentation.24
The measured kfrag values are 0.69-1.3 × 106 s-1 (5a), 2.8-
5.2 × 106 s-1 (5b), and 0.3-1.3 × 106 s-1 (5c).25 Clearly,
ROCCl fragmentation at 25 °C is not extraordinarily fast; the
three carbenes have lifetimes of ∼0.2-3.3 µs. Most intrigu-
ingly, kfrag for the benzyloxycarbene, 5a, is rather similar to
A correlation of the apparent rate constants for ylide forma-
tion, kobs (8.35 × 105 - 3.27 × 106 s-1) vs pyridine concentration
(0.41-5.36 M) was linear (eight points, r ) 0.997) with a slope
of 4.85 × 105 M-1 s-1, equivalent to the rate constant for ylide
formation, ky,22 and a Y-intercept of 6.9 × 105 s-1 (see
supporting information, Figure S-1). The latter value can be
equated with kfrag for 5a f 3a (X ) Cl), because the product
studies indicate that only fragmentation products result from
the decay of benzyloxychlorocarbene.
k
frag for the adamantylmethoxy and neopentoxy species 5b and
5c. If fragmentation of 1 occurs in a single step, as in eq 1,
with concerted cleavage of R-O and C-X, then one might
expect benzyloxychlorocarbene to fragment (to delocalized
PhCH2+) most rapidly. By this analysis, the fragmentations of
5b and 5c seem unusually fast, suggesting acceleration due to
concerted fragmentation with alkyl participation.9,26
Alternatively, the similarity of all three fragmentation rates
might indicate that the rate limiting step is actually scission of
the C-X bond, forming [RO-C+ X-], which is followed by
rapid cleavage of the R-O bond, affording 3. However, the
complete fragmentation of carbene 5a in MeOH,4 as opposed
to the efficient trapping of isobutoxychlorocarbene in MeOH,3a
suggests that two-bond concerted fragmentation is the more
likely mechanism, at least for 5a. Now that fragmentation
kinetics can be readily visualized further mechanistic discrimi-
nation is possible and will be pursued.
We also determined kfrag by a double reciprocal analysis,14b
in which we measured the absorbance (Irel) of ylide 18a as a
function of pyridine concentration. A correlation of 1/Irel vs
1/[pyr] was linear (12 points, r ) 0.996); see supporting
information, Figure S-2. Division of the correlation’s intercept
by its slope gave 0.369, which represents the lifetime (τ) of 5a
multiplied by ky (4.85 × 105 s-1, see above). Thus, τ5a ) 7.6
× 10-7 s, and kfrag ) 1.3 × 106 s-1, in reasonable agreement
with the directly measured value.
Similarly, LFP afforded kfrag for carbene 5b as 5.2 × 106
s-1, with ky ) 2.0 × 105 M-1 s-1 for the formation of ylide
18b (λmax ) 468 nm); see supporting information, Figure S-3.
Here, the double reciprocal correlation gave kfrag ) 2.8 × 106
s-1 (see supporting information, Figure S-4). In contrast to
carbene 5a, where fragmentation in MeCN was quantitative,
fragmentation of 5b accounted for only ∼80% [eq 2], with
∼16% of the carbene diverted to 11 by HCl. However, in the
presence of pyridine, under conditions analogous to those of
LFP, HCl was removed, 11 was not formed, and fragmentation
or pyridine capture were the principal modes of carbene decay.
Acknowledgment. We are grateful to Professor M. S. Platz for
helpful discussions and to the National Science Foundation for financial
support.
Supporting Information Available: Figures S-1-S-5; see text (5
pages). See any current masthead page for ordering and Internet access
instructions.
JA962194H
(23) The enhanced yield of formate 12b, formed from carbene 5b in
place of 11 in the presence of pyridine, is mainly due to hydrolysis of ylide
18b by adventitious water. Formate yield increases with added water, and
direct LFP monitoring of the rate of ylide decay as a function of [H2O] at
[pyr] ) 6.2 M in MeCN gives k ) 2.1 × 106 M-1 s-1 for the reaction of
18b with water. The analogous hydrolysis of ylide 18a had k ) 6.6 × 105
(18) Photochemical and thermal decompositions were similar. Carbene
dimer (0.5%) and a trace of azine were detected. In hexane, dimer was the
major product (58-75%).
(19) The predominance of alkene 14 suggests an intraion pair proton
acceptor role for chloride.
M-1 s-1
.
(24) Ylide 18c eventually gives formate 17 upon hydrolysis; see above
and ref 23.
(20) Yields are 13 (7.5%), 14 (13.7%), and 15 (1.7%).
(21) See ref 11 for a description of our LFP system.
(22) Ambiphilic carbenes react “slowly” with pyridine; for MeOCCl,15
(25) The rate constants for 5a and 5b are cited as ranges of direct and
double reciprocal values. The range cited for 5c is discussed above.
(26) Cf. Liggero, S. H.; Sustmann, R.; Schleyer, P. V. R. J. Am. Chem.
Soc. 1969, 91, 4571.
ky ) 9.0 × 105 M-1 s-1
.