Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 485–486
A search for dichlorocarbene ether solvent interactions
Stanislav I. Presolski,a Adelajda Zorba,a Dasan M. Thamattoor,a,* Eric M. Tippmannb
and Matthew S. Platzb,*
aDepartment of Chemistry, Colby College, 5750 Mayflower Hill Drive, Waterville, ME 04901, USA
bDepartment of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, OH 43210, USA
Received 11 August 2003; revised 4 November 2003; accepted 5 November 2003
Abstract—The absolute rate constant of reaction of dichlorocarbene with pyridine and with tetramethylethylene was determined in
Freon-113 (CF2ClCFCl2), dioxane, and tetrahydrofuran. No evidence of specific solvation was obtained.
ꢀ 2003 Elsevier Ltd. All rights reserved.
Singlet carbenes have closed shell electron configura-
tions with both a filled and empty orbital.1 This has led
chemists to wonder if singlet carbenes may form weak
complexes with coordinating solvents. Tomioka et al.,2
Ruck and Jones,3 and Moss et al.4 have probed for
evidence of complexation using product based studies.
Khan and Goodman5 have used photoacoustic spec-
troscopy to gather evidence of an interaction between
singlet methylene and benzene.
growth can be analyzed to yield an observed rate con-
stant, kobs, which can be equated to the elementary rate
constants of Scheme 1 in Eq. 1.
kobs ¼ k0 þ kpyr½pyridineꢀ
ð1Þ
A plot of kobs versus [pyridine] is linear (Fig. 2) with
slope kpyr
.
The presence of TME, at constant pyridine concentra-
tion increases kobs as shown in Eq. 2. We also carried out
independent product studies in all three solvents to
confirm that :CCl2 does indeed form the cycloadduct
with the trap, as expected, when TME is present in the
reaction mixture. In the absence of TME, the carbene
adds to the precursor itself and no products from
insertion of :CCl2 into the solvent were found.
We have reasoned that solvent complexation will reduce
the absolute bimolecular reaction rate constants of
carbenes. In previous work we found that the absolute
rate constant of reaction of arylhalocarbenes with
tetramethyethlylene (TME) is essentially the same in
coordinating and noncoordinating solvents.6;7 We
speculated that the stabilizing influence of the aryl group
weakened any carbene solvent interaction past the point
of kinetic significance. This led us to study a simpler
carbene––dichlorocarbene.
0.4
0.3
0.2
0.1
0.0
Dichlorocarbene was generated by laser flash photoly-
sis8 of diene 19 using 308 nm radiation. The carbene was
visualized by trapping with pyridine to form ylide 2,
which is easily detected at 400 nm.10
The intensity of absorption of ylide 2 increases expo-
nentially after the laser pulse (Fig. 1). The exponential
150
200
250
300
ns
350
400
450
Keywords: Singlet carbene; Laser flash photolysis; Solvent interaction.
* Corresponding authors. Tel.: +1-207-8723429; fax: +1-207-8723804;
Figure 1. Transient absorbance of ylide 2 at 400 nm in Freon-113.
Top: 0.005 M pyridine, bottom: 0.001 M pyridine.
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.11.009