Reactions of atomic carbon with acyl chlorides

Article abstract of DOI:10.1016/j.tetlet.2008.07.105

Reactions of arc-generated carbon atoms with acyl chlorides proceed by two distinct mechanistic pathways depending on the nature of the alkyl group in the substrate. Acetyl chloride affords vinyl chloride from the putative chloromethylcarbene produced by deoxygenation, whereas pivaloyl chloride gives t-butyl chloride as the predominant product via a chain reaction from the initially generated pivaloyl radical. When the alkyl group is isopropyl, both pathways are implicated.

Full text of DOI:10.1016/j.tetlet.2008.07.105

Tetrahedron Letters 49 (2008) 6036–6038
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reactions of atomic carbon with acyl chlorides
b,
Daniel B. Herrick a, Dasan M. Thamattoor a, , Philip B. Shevlin
*
*
a Department of Chemistry, Colby College, Waterville, ME 04901, United States
b Department of Chemistry, Auburn University, Auburn, AL 36849, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Reactions of arc-generated carbon atoms with acyl chlorides proceed by two distinct mechanistic
pathways depending on the nature of the alkyl group in the substrate. Acetyl chloride affords vinyl
chloride from the putative chloromethylcarbene produced by deoxygenation, whereas pivaloyl chloride
gives t-butyl chloride as the predominant product via a chain reaction from the initially generated
pivaloyl radical. When the alkyl group is isopropyl, both pathways are implicated.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 15 June 2008
Revised 17 July 2008
Accepted 19 July 2008
Available online 24 July 2008
The deoxygenation of carbonyl compounds by atomic carbon is
an effective way of generating carbenes whose chemistry is un-
tainted by precursor reactions.1 Although the C atom deoxygen-
ation of phosgene yields dichlorocarbene,2 there have been no
reports of the generation of alkylchlorocarbenes by the deoxygen-
ation of acyl chlorides. This investigation explores the feasibility of
generating these carbenes by C atom deoxygenation.
When acetyl chloride, 1, is co-condensed with arc-generated C
at 77 K at 10À4–10À5 torr,3 the formation of CO is indicated by a
pressure rise to 10À2–10À3 torr and vinyl chloride, 2, is produced.
We feel that 2 is the result of an intramolecular hydrogen shift in
the chloromethylcarbene intermediate, 3.4 Attempts to trap an
intermediate chloromethylketene by the addition of methanol
were unsuccessful.5,6
The reaction of pivaloyl chloride, 4, with atomic carbon follows
a quite different course giving tert-butyl chloride, 5, as the major
product along with a small amount of 2-chloro-1,1-dimethylcyclo-
propane, 6. We attribute the formation of 6 to the known intra-
molecular C–H insertion in tert-butylchlorocarbene, 7,7 which
presumably results from the deoxygenation of
4 by carbon
(Scheme 1). We were not able to detect any 2-chloro-3-methyl-
2-butene, 8, which often,7 but not always,8 accompanies the gener-
ation of 7 from its diazirine precursor. It has been suggested that
the 1,2-methyl shift in 7 to form 8 is a process that is not generally
favorable, although it could occur to a small extent at elevated
temperatures.7b,c An earlier report by Skell and Harris indicates
that the reaction of atomic carbon with tert-butyl chloride, 5,
produces 7 by carbon atom insertion into the C–Cl bond, which
then rearranges to give 6 and 8 in a 9:1 ratio.9
We propose that 6 results from the free radical chain reaction in
Scheme 2. Thus, an initial abstraction of chlorine from 4 by atomic
carbon produces the pivaloyl radical 9 and C–Cl. Loss of carbon
monoxide from 9 generates the stable tert-butyl radical 10.10
Abstraction of chlorine from 4 by 10 gives rise to the product 5
and regenerates the radical 9, which carries the chain.
H3C
Cl
H3C
Cl
H2C
Cl
C
O
-CO
1
3
2
Observations made during the reaction of 4 with atomic carbon
CH3
CH3
are consistent with this hypothesis. When the arc is struck at 10À4
–
(H3C)3C
Cl
C
+
O
10À5 torr, there is an increase in pressure to about 10À2–10À3 torr)
reflecting the formation of CO. However, when the arc was
stopped, the pressure took an unusually long time to return to
H3C
H2C
Cl
Cl
6 (5%)
H3C
5 (95%)
4
CH3
H3C
(H3C)3C
Cl
(H3C)3C
Cl
C
O
Cl
Cl
-CO
4
7
6
8 (not found)
Scheme 1.
* Corresponding authors. Tel.: +1 207 859 5765; fax: +1 207 872 3804.
E-mail addresses: dmthamat@collby.edu (D. M. Thamattoor), shevlpb@auburn. edu (P. B. Shevlin).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.105

D. B. Herrick et al. / Tetrahedron Letters 49 (2008) 6036–6038
6037
The abstraction of chlorine by atomic carbon to generate C–Cl
has been observed in the reactions of C with CCl4 and chlorofluoro-
carbons.14 In these reactions, the C–Cl can be trapped with cyclo-
hexene. Since attempts to trap the C–Cl with cyclohexene in the
present reactions were not successful, we conclude that only a
small amount of chlorine abstraction occurs and the majority of
5 is the result of the chain reaction.
O
(H3C)3C
Cl
C
O
(H3C)3C
-CCl
4
9
CH3
H3C
H3C
Cl
-CO
5
4
Given the fact that decarbonylation to a methyl radical does not
manifest itself in these reactions while decarbonylation producing
a tertiary radical does, it was of interest to react C with isobutyryl
chloride, 14, in which chlorine abstraction followed by decarbonyl-
ation would generate a secondary radical. As illustrated in Scheme
4, the product mixture from the reaction of 14 + C showed the
presence of both isopropyl chloride 15, attributed to the chlorine
abstraction mechanism, and 1-chloro-2-methylpropene 16, from
the known 1,2-H shift in the chloroisopropylcarbene, 17,15 in a
1:2 ratio. Thus, products resulting from Cl abstraction and decarb-
CH3
H3C
CH3
10
Scheme 2.
O
H3C
Cl
C (- CCl)
?
O
onylation to an isopropyl radical (D
H = 9.9 kcal/mol)11 are less
H3C
1
important than those in the corresponding pathway involving a
tert-butyl radical. It is also possible that steric effects could be in-
volved with the bulky tert-butyl group directing the C atom away
from the oxygen toward the sterically accessible Cl atom. We feel
that these results demonstrate that C atoms will deoxygenate acid
chlorides to chlorocarbenes in a manner similar to the deoxygen-
ation of aldehydes and ketones. However, a competing Cl abstrac-
tion followed by decarbonylation renders carbene formation
inefficient when the decarbonylation leads to a stable radical. Since
alkylchlorocarbenes are ground state singlets16 and the analogous
carbonyl deoxygenations generate singlet carbenes,11 we feel that
the carbenes formed here are in the singlet state.
11
H3C Cl
13
-CO
?
(not found)
?
1
CH3
12
Scheme 3.
the previous lower levels. This lends credence to the notion that CO
was being released even when there was no arc. An analogous free
radical mechanism in the reaction of 1 with atomic carbon would
have produced chloromethane, 13, via the acetyl radical 11 and
methyl radical 12 (Scheme 3). We were, however, unable to find
any chloromethane in the reaction mixture of 4 + C. Further, when
1 is used as the substrate, in marked contrast to the behavior of 4,
the pressure rises when the arc is struck but falls quickly after it is
stopped. Control experiments show that 9 does not result from
pyrolysis of 4 in the arc.
Acknowledgments
We gratefully acknowledge support of this work by NSF
through Grant CHE-07719335 and the Colby College Division of
Natural Sciences. We also thank Professor Murray Campbell and
Mr. Charles Jones for their assistance with the reactor assembly.
D. B. Herrick is a recipient of the Frank and Maureen Wilkens Stu-
dent Research Fellowship.
References and notes
Thus, the reaction of C with 4 appears to follow two pathways,
initial chlorine abstraction and deoxygenation. The fact that the
former initiates a chain reaction makes it difficult to estimate the
relative importance of the competing initial C atom reactions. It
is certainly possible that chlorine abstraction is a minor pathway
in which products are considerably amplified by the chain reaction.
While the decarbonylation of acyl radicals is endothermic, it is less
1. For a recent review, see: Shevlin, P. B. In Reactive Intermediate Chemistry; Moss,
R. A., Platz, M. S., Jones, Jr., M., Eds.; Wiley-Interscience: Hoboken, NJ, 2004;
Chapter 10, p 463.
2. Skell, P. S.; Plonka, J. H. J Am. Chem. Soc. 1970, 92, 2160.
3. The reactor is modeled after the one described in: Skell, P. S.; Wescott, LDJr;
Golstein, J. P.; Engel, R. R. J. Am. Chem. Soc. 1965, 87, 2829.
4. Moss, R. A.; Mamantov, A. J. Am. Chem. Soc. 1970, 92, 6951.
5. Ketenes have been proposed and trapped in the reaction of C with 2-butanone:
(a) Dewar, M. J. S.; Nelson, D. J.; Shevlin, P. B.; Biesiada, K. A. J. Am. Chem. Soc.
1981, 103, 2802; (b) Biesiada, K. A.; Shevlin, P. B. J. Org. Chem. 1984, 49,
1151.
so when the product is 10 (
D
H = 9.7 kcal/mol)11 rather than 12
(D
H = 11.9 kcal/mol).11 While these differences in enthalpies of
decarbonylation are not dramatic, they may be sufficient to slow
the decarbonylation of 11 enough that an observable chain reac-
tion is not initiated. Thus, it could be that 11, if produced, simply
does not decarbonylate fast enough to initiate a viable chain. A
search of the reaction mixture for biacetyl, in the unlikely event
that 11 dimerizes,12 and acetaldehyde, which could arise from
hydrogen abstraction by 11,13 was unsuccessful. It is also conceiv-
able that 11 and 1 can simply interconvert by a degenerate chlo-
rine exchange before the radical is eventually destroyed.
6. For an example of an alcohol adding to chloromethylketene, see: Tandon, V. K.
Tetrahedron Lett. 2001, 42, 5985.
7. (a) Ishitsuka, M. O.; Niino, Y.; Wakahara, T.; Akasaka, T.; Liu, M. T. H.;
Kobayashi, K.; Nagase, S. Tetrahedron Lett. 2004, 45, 6321; (b) Moss, R. A.; Liu,
W. J. Chem. Soc., Chem. Commun. 1993, 1597; (c) Moss, R. A.; Ho, G.-J. J. Am.
Chem. Soc. 1990, 112, 5642.
8. Zuev, P. S.; Sheridan, R. S. J. Am. Chem. Soc. 1994, 116, 4123.
9. Skell, P. S.; Harris, R. F. J. Am. Chem. Soc. 1965, 87, 5807.
10. (a) Jensen, C. M.; Lindsay, K. B.; Tanning, R. H.; Karaffa, J.; Hansen, A. M.;
Skrydstrup, T. J. Am. Chem. Soc. 2005, 127, 6544; (b) Fischer, H.; Paul, H. Acc.
Chem. Res. 1987, 20, 200.
(H3C)2HC
Cl
H3C
H3C
15 (34%)
H3C
H3C
16 (66%)
(H3C)2HC
Cl
C
C
-CO
(H3C)2HC
Cl
O
O
1. -CO
-CCl
Cl
2. Cl abstraction
18
14
17
Scheme 4.

6038
D. B. Herrick et al. / Tetrahedron Letters 49 (2008) 6036–6038
11. Viskolcz, B.; Bérces, T. Phys. Chem. Chem. Phys. 2000, 2, 5430–5436.
12. Gandini, A.; Hackett, P. A. J. Am. Chem. Soc. 1977, 99, 6195.
13. For a discussion of acyl radicals abstracting hydrogen, see: Chatgilialoglu, C.;
Ferreri, C.; Lucarini, M.; Pedrielli, P.; Pedulli, G. F. Organometallics 1995, 14,
2672.
14. (a) LaFrancois, C. J.; Shevlin, P. B. J. Am. Chem. Soc. 1994, 116, 9405; (b)
LaFrancois, C. J. Ph.D. Dissertation, Auburn University, Auburn, AL, 1996.
15. LaVilla, J. A.; Goodman, J. L. J. Am. Chem. Soc. 1989, 111, 6877.
16. Jones, Jr., M.; Moss, R. A. In Reactive Intermediate Chemistry; Moss, R. A., Platz,
M. S., Jones, Jr., M., Eds.; Wiley-Interscience: Hoboken, NJ, 2004; p 278.