Dichlorodiazirine: A nitrogenous precursor for dichlorocarbene

Article abstract of DOI:10.1021/ja055656c

The reaction of 3-p-nitrophenoxy-3-chlorodiazirine with a 1.1:1.0:1.6 blend of tetrabutylammonium chloride, cesium chloride, and the ionic liquid 1-butyl-3-methylimidazolium chloride at 40-50 °C under vacuum at 1 mm·Hg produced a melt from which dichlorod

Full text of DOI:10.1021/ja055656c

Published on Web 09/24/2005
Dichlorodiazirine: A Nitrogenous Precursor for Dichlorocarbene
Gaosheng Chu, Robert A. Moss,* and Ronald R. Sauers*
Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey,
New Brunswick, New Jersey 08903
Received August 25, 2005; E-mail: moss@rutchem.rutgers.edu; sauers@rutchem.rutgers.edu
Given its central role in the history1 and practice2 of carbene
attack of Cl- on an intermediate isodiazirine, to the formation of
chemistry, it is remarkable that we still lack a suitable diazo or
diazirine precursor for dichlorocarbene (CCl2). The carbene can
be photoextruded from the CCl2 adducts of styrene,3a 1,4-
dihydronaphthalene,3b indane,3c or phenanthrene,4 but these proce-
dures perforce also generate aromatic byproducts which can interfere
with spectroscopic studies of CCl2.4 A nitrogenous precursor is
therefore desirable.
Chlorination of diazomethane with tert-butyl hypochlorite at
-100 °C gives monochlorodiazomethane, which thermally decom-
poses above -40 °C.5 However, attempts to prepare dichlorodi-
azomethane by further chlorination lead to rapid nitrogen loss, even
at -100 °C.5,6 Difluorodiazirine7 and chlorofluorodiazirine8 are
available from “shatteringly explosive” fluorinated nitrogenous
intermediates,9 but extension of this chemistry to dichlorodiazirine
is neither trivial nor inviting.
Recently, we reported that the reaction of fluoride with 3-p-
nitrophenoxy-3-chlorodiazirine (1) followed three competitive
pathways: F/Cl exchange yielding 3-p-nitrophenoxy-3-fluorodi-
azirine (2), F/aryloxy exchange leading to 3-fluoro-3-chlorodiazirine
(3), and a remarkable ipso attack on the p-nitrophenyl moiety
affording diazirinone (4).10 If the reaction of 1 and chloride were
to follow an analogous course, dichlorodiazirine could be obtained.
We are pleased to report the success of this approach and to
document the preparation of dichlorodiazirine (DCD), the first
nitrogenous precursor for dichlorocarbene.
dichlorodiazirine (7, DCD).11
Competitively, ipso attack of Cl- on the p-nitrophenyl moiety
of 1 (path b) affords diazirinone (4) and product 5.
We did not verify the formation of diazirinone,12 but the
generation of DCD is supported by the following observations.
Repetition of the 1/TBACl reaction in the presence of cyclopentene
gave CCl2 adduct 8,13 as well as 5 and 6 (in a 6:4 ratio). The yields
of 8 and 6 were comparable.
Analogous experiments with added cyclohexene, tetramethyl-
ethylene, or trimethylethylene afforded the expected CCl2 adducts
9-11.13
To further substantiate the intermediacy of CCl2, we determined
the relative reactivities14 for the dichlorocyclopropanations of
tetramethylethylene versus cyclohexene and tetramethylethylene
versus trimethylethylene by running the 1/TBACl reaction in the
presence of known quantities of the paired alkenes. GC product
analysis led to krel values of 56.5 and 2.17, respectively, in excellent
agreement with krel ) 53.7 or 2.28 for CCl2 generated from
chloroform and potassium t-butoxide.15 The olefinic capture of CCl2
from the reaction of diazirine 1 and TBACl implies the prior
formation of DCD (Scheme 1, path a), which gradually decomposes
to CCl2 during the week-long reaction.16 Definitive evidence is
supplied by the isolation of DCD.
A key problem is the low nucleophilicity of chloride, which
necessitates the lengthy reaction time of diazirine 1 with TBACl.
We find that a more reactive chloride source is provided by a 1.1:
1.0:1.6 blend of TBACl, CsCl, and the ionic liquid 1-butyl-3-
methylimidazolium chloride (mp 55 °C).17 Combination of this
mixture with diazirine 1 in a little dry HMPA, followed by warming
to 40-50 °C under vacuum at 1 mm‚Hg, produces a melt from
which DCD distills into a cold trap containing pentane at -70 °C
over 6-7 h. (Chloride ion is present in >10-fold excess relative to
diazirine 1 in the melt; see Supporting Information for details.)
In pentane, DCD exhibits a broad, weak UV absorbance at 310-
390 (λmax 339) nm, a stronger absorbance at ∼260 nm (see
Supporting Information), and an IR band at 1560 cm-1. These
features are attributable to the diazirine moiety and disappear upon
photolysis at 350 nm. UV monitoring indicates that DCD is
moderately stable at 25 °C in the dark: about 10% of decomposition
occurs after 13 h. B3LYP/6-311+G(2d,p) calculations18-20 predict
As “proof of principle”, diazirine 110 was reacted with excess
tetrabutylammonium chloride (TBACl) in CD3CN at 25 °C for 1
week in the absence of light. H NMR monitoring revealed the
slow disappearance of 1 and a concomitant build up of p-
chloronitrobenzene (5) and p-nitrophenol (6) in a ratio of 7:3. The
formation of products 5 and 6 suggests that the reaction of 1 and
Cl- proceeds analogously to that of 1 and F-10 (cf. Scheme 1).
1
Scheme 1
λ
max values of 240 and 361 nm for DCD, and Ea ) 28 kcal/mol for
cleavage to CCl2 and N2, in fair agreement with experiment.21
DCD is expected to be an efficient progenitor of CCl2. Indeed,
photolysis of DCD in the presence of cyclohexene leads to
dichloronorcarane 9.13 Even more convincing is laser flash pho-
Here, SN2′ attack of Cl- at diazirine N (path a) leads to the ejection
of p-nitrophenoxide (later protonated to 6) and, via a second SN2′
9
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10.1021/ja055656c CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
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(6) Some dichlorodiazomethane might have been transiently formed.5
(7) (a) Mitsch, R. A. J. Org. Chem. 1968, 33, 1847. (b) Mitsch, R. A. J.
Heterocycl. Chem. 1966, 3, 245. (c) Robertus, R. L.; McBrady, J. J.;
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(10) Moss, R. A.; Chu, G.; Sauers, R. R. J. Am. Chem. Soc. 2005, 127, 2408.
(11) Ejection of Cl- from 1 is degenerate and leads back to the initial diazirine.
(12) The very slow rate of the 1/TBACl reaction, coupled with the instability
of 4,10 makes the detection of diazirinone impossible under these
conditions.
Figure 1. The transient absorption of ylide 12 formed after the laser flash
photolysis of DCD in 0.4 M pyridine in pentane. The spectrum was recorded
200 ns after the laser pulse at 351 nm.
(13) This product was identified by 1H NMR, GC, and GC-MS comparisons
to an authentic sample.
(14) Moss, R. A. In Carbenes; Jones, M., Jr., Moss, R. A., Eds.; Wiley: New
York, 1973; Vol. 1, pp 153f.
tolysis of DCD in 0.4 M pyridine/pentane, whereupon the known
UV spectrum of the pyridinium dichloromethyl ylide 12 is observed
with λmax ) 387 nm (lit.,4 390 nm); see Figure 1.
With the obtention of DCD, a practical nitrogenous precursor
of dichlorocarbene, various spectroscopic and absolute kinetics
investigations become possible. Indeed, preliminary laser flash
photolysis experiments with DCD in pentane at room temperature
reveal a transient absorbing at 480-520 nm, consistent with the
absorbance at 440-560 nm attributed to (matrix isolated) dichloro-
carbene.22 Further reports will appear in due course.
(15) Doering, W. v. E.; Henderson, W. A., Jr. J. Am. Chem. Soc. 1958, 80,
5274.
(16) A control experiment was performed to rule out the formation of CCl2 by
reaction of p-nitrophenoxychlorocarbene (ArOCCl, from thermolysis of
1) with chloride ion. Photolysis of 1 with excess TBACl and cyclopentene
in 1:2 MeCN/THF led to adducts of ArOCCl and cyclopentene, as well
as ArOCHCl2, but not to CCl2 adduct 8. Moreover, the adducts of ArOCCl
and cyclopentene were not converted to 8 when stirred with excess TBACl
for 5 days at 25 °C.
(17) For anion nucleophilicity in ionic liquids, see: Landini, D.; Maia, A.
Tetrahedron Lett. 2005, 46, 3961 and references therein.
(18) All structures were fully optimized by analytical gradient methods using
the Gaussian 03 suite19 and density functional (DFT) calculations at the
6-311+G(2d,p) level, the exchange potentials of Becke20a and the
correlation functional of Lee, Yang, and Parr.20b Activation energies were
corrected for zero-point energy differences (ZPVE, unscaled) and thermal
effects at 298.150 K. Vibrational analyses established the nature of all
stationary points as either energy minima (no imaginary frequencies) or
first-order saddle points (one imaginary frequency).
Acknowledgment. This paper is dedicated to the memory of
Professor Gerhard L. Closs. We thank the National Science
Foundation for financial support, and the National Center for
Computer Applications for time on the IBM P Series 690 (to
R.R.S.).
(19) Frisch, M. J. et al. Gaussian 03, revision B.03; Gaussian, Inc.: Pittsburgh,
PA, 2003. See Supporting Information for the full reference.
Supporting Information Available: Experimental procedure for
DCD, complete ref 19, UV spectrum of DCD, and computational data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(20) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.;
Paar, R. G. Phys. ReV. B 1988, 37, 785.
(21) Ea is calculated to be only 11.2 kcal/mol for the decomposition of
dichlorodiazomethane, in accord with the lower thermal stability of this
molecule.5
(22) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1967, 47, 703.
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