Tracking Alkylchlorocarbenes by σ f p Absorptions
A R T I C L E S
in pentane. From a correlation of kobs versus [TME] (Supporting
Information Figure S9), we obtained kadd ) 3.0 × 106 M-1 s-1
and, from the Y-intercept at [TME] ) 0, kins ) 3.3 × 105 s-1
for the 1,3-CH insertion of 2c to 1,1-dimethyl-2-chlorocyclo-
propane. The latter value is not corrected for carbene-diazirine
ylide formation but is in reasonable accord with kins ) 9.3 ×
105 s-1 obtained by analysis of pyridine ylide data.34 Note that
the addition of t-BuCCl to TME in pentane is ∼600 times slower
than the corresponding addition of MeCCl (see above). Presum-
ably, the difference in reactivity is due to steric hindrance to
addition originating at the t-butyl group of 2c.
LFP of 1-adamantylchlorodiazirine 4d35 in pentane gave
1-adamantylchlorocarbene 2d which displayed it σ f p absor-
bance at 545 nm, accompanied by carbene-diazirine ylide 5d
absorbing at 280 nm; cf., Supporting Information Figure S10.32
The carbene signal has been observed at 540 nm in an argon
matrix at 10 K35 but not previously in solution. Correlation of
the formation of carbene-solvent complexes. The kinetics of
the reactions of “free” carbenes and carbene complexes can be
directly followed; examples of the latter are provided for MeCCl
and PhCH2CCl. Particularly effective complexation is provided
by anisole and 1,3-DMB, which modulate the rates of inter-
molecular additions of MeCCl and PhCH2CCl to TME and
1-hexene. Computational studies aid in understanding the
carbene absorption spectra and the nature of the carbene-solvent
complexes.
Experimental Section
Diazirines. Diazirines 4a-4e were prepared by Graham oxidation6
of amidines, RC(dNH2)NH2·HCl. Acetamidine hydrochloride (R ) Me)
was purchased from Aldrich. Amidines with R ) benzyl, t-butyl,
1-adamantyl, and cyclopropyl were prepared from the commercially
available nitriles (RCN) by the Garigipati procedure.39 The experimental
procedure is described in detail for 1-adamantyl amidine hydrochloride
in ref 39b, and the other amidines are cited in the literature.40
Methylchlorodiazirine (4a). Acetamidinium chloride (1.5 g) and 3
g of LiCl were added to 20 mL of DMSO in a 500 mL round-bottom
flask, and the mixture was stirred magnetically for 20 min at 0 °C.
Then, 200 mL of commercial bleach (12% by weight of aqueous
NaOCl, saturated with NaCl, and cooled to 0 °C) was added with
stirring over 30 min from an addition funnel. The reaction vessel was
connected to a series of two traps; the first trap was cooled to -30 °C;
the second trap was cooled to 77 K by liquid nitrogen. The reaction
vessel was allowed to warm to ambient temperature as the system was
evacuated to 1 mm of Hg for 30 min. Most of the diazirine 4a
condensed in the second trap. The vacuum was relieved; the diazirine
was diluted with pentane or other solvent, and the diazirine solution
was warmed to 0 °C. Caution. Diazirines should be considered
explosiVe, and preparatiVe operations must be conducted behind shields.
Diazirine solutions seem to be safe.
k
obs for the decay of the 545 nm absorption as a function of the
concentration of TME in pentane gave kadd ) 1.4 × 107 M-1
s-1 for the addition of 2d to TME; cf., Supporting Information
Figure S11. This is ∼5 times faster than the corresponding
addition of t-BuCCl but ∼130 times slower than the addition
of MeCCl to TME.36
The Y-intercept at [TME] ) 0 of the correlation in Figure
S11 gives k ) 4.0 × 105 s-1 for decay of the carbene in the
absence of TME. Product studies reveal the decay product to
be azine 9, derived from reaction of 2d with diazirine 4d,
1-Ad-C(Cl)dNsNdC(Cl)-1-Ad
9
1H NMR (400 MHz, CDCl3, δ): 1.66 (s, 3H). 13C NMR (100 MHz,
CDCl3, δ): 24.58, 45.21. UV (pentane, nm): 325, 331, 348, 358 (max).
t-Butylchlorodiazirine (4c) was prepared in the same manner
probably via intermediate ylide 5d. Photolysis of carbene 2d
in an argon matrix gives chlorohomoadamantene by ring
expansion,35 but a thermal version of this reaction does not seem
to compete with diazirine capture of 2d in pentane solution at
25 °C.
LFP of cyclopropylchlorodiazirine 4e6 in pentane at 25 °C
provided cyclopropylchlorocarbene, featuring a σ f p absor-
bance at 488 nm, a carbene-diazirine ylide (5e) absorbing at
270 nm, and a σ f σC-Cl* carbene signal at 248 nm; cf.,
Supporting Information Figures S12 and S13.37 The two carbene
signals were previously observed in a nitrogen matrix at 14 K.38
Decay of the 488 nm absorbance gave kc ) 1.4 × 106 s-1 for
the 1,2-C ring expansion of 2e to 1-chlorocyclobutene, in
reasonable agreement with kc ) 9 × 105 s-1, derived from decay
of the 248 nm signal,38 and 6 × 105 or 1.5 × 106 s-1 obtained
by the pyridine ylide method.10,38
1
except that methanol was substituted for the DMSO. H NMR (400
MHz, CDCl3, δ): 0.99 (s, 12 H). 13C NMR (100 MHz, CDCl3, δ):
27.03, 37.45, 50.89. UV (pentane, nm): 326, 332, 341, 348, 359 (max).
Cyclopropylchlorodiazirine (4e) was prepared in the same manner
1
as 4a. H NMR (400 MHz, CDCl3, δ): 0.41-0.42 (m, 2H), 0.64-
0.66 (m, 2H), 1.58 (m, 1H). 13C NMR (100 MHz, CDCl3, δ): 5.13,
17.54, 25.89, 49.70. UV (pentane, nm): 340, 345, 355 (all equal, max),
377.
Benzylchlorodiazirine (4b). Benzylamidinium hydrochloride (2 g)
and 4 g of LiCl were added to 40 mL of DMSO, and the mixture was
stirred magnetically for 20 min at 0 °C. Then, 50 mL of pentane was
added. Next, 300 mL of commercial bleach (12% by weight of aqueous
NaOCl, saturated with NaCl, and cooled to 0 °C) was slowly added
with stirring over 30 min. Stirring was continued for an additional 30
min at room temperature. Then, the pentane layer was separated and
the aqueous phase was extracted twice with 50 mL portions of pentane.
The combined pentane fractions were washed with 200 mL of ice water,
dried over MgSO4, and concentrated on the rotary evaporator to ∼5
mL. The residue was purified by chromatography over a short silica
gel column, with pentane as the eluent, to give a pentane solution
of 4b.
Conclusions. Contrary to implications in the literature,3 the
σ f p absorptions of alkylchlorocarbenes are readily acquired
by LFP with UV-vis detection in solution at ambient temper-
ature. Specific examples include RCCl where R ) methyl,
benzyl, t-butyl, 1-adamantyl, and cyclopropyl. The σ f p
absorptions permit direct monitoring of carbene reactions and
1H NMR (400 MHz, CDCl3, δ): 3.33 (s, 2H); 7.27-7.29 (m, 2H),
7.37-7.41 (m, 3H). 13C NMR (100 MHz, CDCl3, δ): 43.74, 48.60,
127.91, 129.03, 129.76, 132.98. UV (pentane, nm): 341 (max), 349,
356.
(34) (a) Moss, R. A.; Ho, G.-J. J. Am. Chem. Soc. 1990, 112, 5642. (b) Moss,
R. A.; Liu, W. Chem. Commun. 1993, 1597.
(35) Yao, G.; Rempala, P.; Bashore, C.; Sheridan, R. S. Tetrahedron Lett. 1999,
40, 17.
(36) The steric substituent constants of t-butyl and 1-adamantyl are compa-
rable: Hellmann, G.; Beckhaus, H.-D.; Ru¨chardt, C. Chem. Ber. 1979, 112,
1808.
(37) These spectra were acquired 50 ns after the laser pulse.
(38) Ho, G.-J.; Krogh-Jespersen, K.; Moss, R. A.; Shen, S.; Sheridan, R. S.;
Subramanian, R. J. Am. Chem. Soc. 1989, 111, 6875.
(39) (a) Garigipati, R. S. Tetrahedron Lett. 1990, 31, 1969. (b) Moss, R. A.;
Ma, W.; Merrer, D. C.; Xue, S. Tetrahedron Lett. 1995, 36, 8761.
(40) (a) R ) PhCH2: cf., ref 25b. (b) R ) t-Bu: Moss, R. A.: Munjal, R. C.
Chem. Commun. 1978, 775. (c) R ) c-C3H5: Moss, R. A.; Shen, S.; Krogh-
Jespersen, K.; Potenza, J. A.; Schugar, H. J.; Munjal, R. C. J. Am. Chem.
Soc. 1986, 108, 134.
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J. AM. CHEM. SOC. VOL. 129, NO. 32, 2007 10027