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Kinetics of intramolecular and intermolecular reactions of benzylchlorocarbene
J. Phorochem. PhotobioL A: Chem., 63 (1992) 115-118
115
PreIiminaxy Note
Kinetics of intramolecular benzylchlorocarbene Michael
T. H. Liu and Robert
Deparbnent of Chemistq
and intermolecular
reactions
of
G. Chapman
Universi~ of Prince Edward Isknd, Charlottetown, P.E.I., Cl.4 4P3
(Canada)
Roland
Bonneau
WA348 CNRS, Laboratoire Chimie Physhue, Universite’de Bordeaux 1, Talence 33405 (France) (Received May 28, 1991; accepted September 6, 1991)
Abstract
Photolysis of 3-chloro-3-benzyldiaiirine
in tetramethylethylene
gives E- and Z-fi-chloro-
styrenes and cyclopropans. Relative rate studies show that the styrenes to cyclopropane ratio increases with increasing diazirine concentration_ This permits the determination of the ratio k,/ki= 8 where kD is the rate constant for the reaction of carbene with diazirine and ki is the rate constant for the 1,Zhydrogen atom shift of carbene. Laser Aash uhotolvsis yields the first absolute rate constant for the reaction of a chlorocarbene with d&i&e and a value of 7.7 for k&,. 1, Introduction A considerable
body of information
has recently become
available
concerning
the
kinetics of intramolecular carbene reactions [l-7]. In these cases, the allcylchlorocarbenes are generated from the laser flash photolysis (LFP) of the diazirine precursors. If the carbenes are “invisible” due to the lack of a chromophore, then the methodology of Jackson ef al. [S], in which the carbene is intercepted by pyridine to form an ylide, can be applied. The time-dependent absorption of the ylide is used indirectly to monitor the kinetics of the carbene. In a recent experiment, Moss and Ho [1] demonstrated that in cases where a carbene’s intramolecular reaction is “slow” (k= 16-l@ s-l), reactions of azine and other intermolecular products can be important; in such cases, the pyridinium ylide technique will not provide an accurate measurement of the intramolecular kinetics of the carbene. Our present results on the continuous irradiation and LFP of 3-chloro-3-benzyldiazirine (1) permit the determination of a rate constant for the reaction of carbene with diazirine. This formation is important because diazirines are often used as carbene precursors in LFP. Product analysis suggests that 1,2hydrogen atom migration can also take place in the carbene-diazirine adduct. 2. Experimental
details
Gas chromatographic analyses were carried out on a Varian Vista 6000 gas chromatograph (fitted with a column (6 ft X0.125 in) packed with CSP-20M) whose
lOlO-4030/92/$5.00
0 1992 - Elsevier Sequoia. All rights reserved
116
flame ionization detector was interfaced to an HP 3390A recorder. Mass spectra were acquired with an HP 5890 gas chromatograph connected to an HP 5988 mass selective detector and an HP 9216 work station. The LFP set-up used a crossed-beam arrangement. The sample in a cell (10 mmX 10 mm) was excited at 355 nm by single light pulses (duration, 200 ps; energy, S-30 mJ) provided by a frequency-tripled, mode-locked Nd-YAG (Quantel) laser. The detection system included a pulsed xenon arc, a monochromator, a red-sensitive photomultiplier (Hamamatsu R446) and a fast transient recorder (Tektronix 7912) with a response time of around 5 ns. Relative rate studies were carried out using 350 nm UV lamps in a Rayonet photoreactor until all the diazirine was destroyed. The synthesis of 3-chloro-3-benzyldiazirine has been described previously [5, 91. Product analyses were carried out by nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass spectrometry (GCMS). The gas chromatograph traces were calibrated using authentic samples of the reaction products.
3. Results
and
discussion
Photolysis (350 nm) of 1 in tetramethylethylene (TME) gives E- and Z-&chlorostyrenes (2) and l-benzyl-l-chloro-2,2,3,3-tetramethylcyclopropane (3) (Scheme 1). A mechanism based on a carbene-alkene complex has been advanced [9] to explain the relative distribution of the products. Steady state approximation of this scheme leads to chlorostyrene
2/cyclopropane
3 = k; /b + (k/k)
1 /[TME]
(1)
where the overall trapping rate constant k, = k, k,l(k,+ k:+ k- & Alternatively, a mechanism involving an excited carbene [IO] or diazo [7] intermediate can also account for an inverse first-order dependence on TME. However, we favour [5] the “complex model” because it predicts an intercept dependence on various alkene substrates. When the above experiment is repeated with different initial concentrations of diazirine and TME concentrations in the range 0.2-2.5 M in isooctane at 25 “C, the values of kilk, change and the 2 to 3 ratio increases with increasing diazirine concentration. However, the intercept, killk,, is invariant as expected, because it should not depend on diazirine concentration. Least-squares analysis of 2/3 ‘us. l/[TME] gives the results shown in Table 1. The changes in slope may be rationalized by adding to the mechanism ;_ -
bw
_-
-
---
:
I
PhCH2 Cl
PhCH2 >(t
\I N
_hv_
\C-
Cl’
mm
____;___-_-,
,
’
ki __f .
; PhCH
=Cl-lCI 2 Z+E
1
Scheme 1.
a
117 TABLE
1 analysis of eqn. (1) using various initial
Slope and intercept data at 25 “C from least-squares concentrations of diazirine [Diazirine]
(m)
0.0302 0.0396 0.0730 0.1210
7.0
4.6
Slope
Intercept
0.0901 f 0.0005 0.0965 f 0.0043 0.1187*0_0011 0.1435 f 0.010
0.309 0.280 0.315 0.307
f * + +
0.0050 0.035 0.0111 0.0402
-
: 0
I
I
I
I
I
10
20
30
40
50
[Diazirine], mM Fig. 1. Plot of reciprocal at 25 “C.
lifetime of benzylchlorocarbene
as a function of diazirine concentration
a reaction of the carbene with diazirine [ll] with a rate constant kD. The steady state approximation of this scheme leads to eqn. (1) but with ki changed to ki+kD [I].The dope is now equal to ki/kt+ kD[lJ/k,_ A plot of the slope VS. [l]gives ki/k,=0.073*0.020 and k~/k,=0.59f0.028;hence, kD/ki=S.l+ 0.6. LFP (355 nm) of 1 in a degassed isooctane solution gives a transient absorption at 310 nm which can be attributed to the absorption of benzylchlorocarbene [S]. The lifetime of benzylchlorocarbene was measured at 25 “C as a function of diazirine concentration (Fig. 1; l/T= ki+kD[l]).Least-squares analysis of l/~ VS. [l] for six diazirine concentrations in the range 9-50 mM gives ki= (4.7f0.2)X lo7 s-l, kD = (3.6f0.4)x 10’ M-l s-l and kD/ki=7.7* 1.2,in excellent agreement with the results obtained from the relative rate studies and with the quenching rate constant (2.87X107 M-’ s-l) for adamantanylidene with diazirine in benzene [ll]. Photolysis of neat diazirine 1 gives Z- and E-2 (40%), azine (40%), l-phenyl-2,2dichloroethane (10%) and l-phenyl-1-chloroethylene (lo%, apparently from a 1,2phenyl shift). A calculation using ki,kD and [l]-6 M predicts that azine should represent more than 95% of the products, but only 40% was found. At lower diazirine concentration, [l]= 0.034M, calculation shows that azine should represent approximately
118
15%~25%
of the product
formed. However,
azine was not detected,
only chlorostyrenes,
in the products. Several photolysis experiments using various initial concentrations of diazirine (0.01-0.04 M) in isooctane were carried out with dibenzyl as the gas chromatograph internal standard. The mass balance shows that more than 95% of the diazirine gives the chlorostyrenes. The absence of azine formation at low diazirine concentrations
and the small amount of azine formed at high diazirine concentrations are unexpected. Our results indicate that the carbene is intercepted by the nitrogen atoms on the diazirine ring to form an adduct which can rearrange to give the chlorostyrene to a certain extent. In addition, in agreement with the relative rate data, the 2 to 3 ratio increases with increasing concentration of 1, indicating that the adduct does not undergo a cycloaddition reaction with TME, hilt a 1,2-hydrogen atom shift occurs to form the chlorostyrenes. Thus far, there is no general rule to be followed in the investigation of intramolecular carbene reactions. Moss and Ho [l] have pointed out the restriction of the applicability of the yiide methodology. The present work demonstrates that the speed at which the intramolecular rearrangement of the carbene occurs can control the type of product formed. Studies on the nature of the carbene-diazirine adduct are in progress.
Acknowledgments We thank Professor M. S. Platz for useful discussions. NSERC (Canada) is gratefully acknowledged.
Support
of this work by
References 1 R. R. 3 R. 4 M. 5 M. 6 R.
A. Moss and G.-J. Ho, J. Am. Chem. Sot., 112 (1990) 5642. A. Moss, G.-J. Ho, S. Shen and K. Krogh-Jespersen, J. Am Chem. Sot., 112 (1990) 1638. A. Moss and G.-J. Ho, Tetrahedron Lett., 31 (1990) 1225. T. H. Liu and R. Bonneau, _7.Am. Chem. Sot., 111 (1989) 6873. T. H. Liu and R. Bonneau, J. Am. Chem. Sot., 112 (1990) 3915. Bonneau, M. T. H. Liu and M. T. Rayez, J. Am. Chem. Sot., 111 (1989) 1973. 7 J. E. Jackson, N. Soundararajan, W. White, M. T. H. Liu, R. Bonneau and M. S. Platz, J.
2
Am. 8
Chem.
Sot.,
Ill
(1989)
6874.
J. E. Jackson, N. Soundararajan, M. S. Platz and M. T. H. Liu, J. Am. (1988)
Chem.
5595.
M. T. H. Liu, J. Chem. Sot., Chem. Commun. (1985) 982. P. M. Warner, Tetrahedron Lett., 25 (1984) 4211. 11 S. Morgan, J. E. Jackson and M. S. Platz, J. Am. Chem. Sot.,
9 10
113 (1991)
2782.
Sot.,
110
115
PreIiminaxy Note
Kinetics of intramolecular benzylchlorocarbene Michael
T. H. Liu and Robert
Deparbnent of Chemistq
and intermolecular
reactions
of
G. Chapman
Universi~ of Prince Edward Isknd, Charlottetown, P.E.I., Cl.4 4P3
(Canada)
Roland
Bonneau
WA348 CNRS, Laboratoire Chimie Physhue, Universite’de Bordeaux 1, Talence 33405 (France) (Received May 28, 1991; accepted September 6, 1991)
Abstract
Photolysis of 3-chloro-3-benzyldiaiirine
in tetramethylethylene
gives E- and Z-fi-chloro-
styrenes and cyclopropans. Relative rate studies show that the styrenes to cyclopropane ratio increases with increasing diazirine concentration_ This permits the determination of the ratio k,/ki= 8 where kD is the rate constant for the reaction of carbene with diazirine and ki is the rate constant for the 1,Zhydrogen atom shift of carbene. Laser Aash uhotolvsis yields the first absolute rate constant for the reaction of a chlorocarbene with d&i&e and a value of 7.7 for k&,. 1, Introduction A considerable
body of information
has recently become
available
concerning
the
kinetics of intramolecular carbene reactions [l-7]. In these cases, the allcylchlorocarbenes are generated from the laser flash photolysis (LFP) of the diazirine precursors. If the carbenes are “invisible” due to the lack of a chromophore, then the methodology of Jackson ef al. [S], in which the carbene is intercepted by pyridine to form an ylide, can be applied. The time-dependent absorption of the ylide is used indirectly to monitor the kinetics of the carbene. In a recent experiment, Moss and Ho [1] demonstrated that in cases where a carbene’s intramolecular reaction is “slow” (k= 16-l@ s-l), reactions of azine and other intermolecular products can be important; in such cases, the pyridinium ylide technique will not provide an accurate measurement of the intramolecular kinetics of the carbene. Our present results on the continuous irradiation and LFP of 3-chloro-3-benzyldiazirine (1) permit the determination of a rate constant for the reaction of carbene with diazirine. This formation is important because diazirines are often used as carbene precursors in LFP. Product analysis suggests that 1,2hydrogen atom migration can also take place in the carbene-diazirine adduct. 2. Experimental
details
Gas chromatographic analyses were carried out on a Varian Vista 6000 gas chromatograph (fitted with a column (6 ft X0.125 in) packed with CSP-20M) whose
lOlO-4030/92/$5.00
0 1992 - Elsevier Sequoia. All rights reserved
116
flame ionization detector was interfaced to an HP 3390A recorder. Mass spectra were acquired with an HP 5890 gas chromatograph connected to an HP 5988 mass selective detector and an HP 9216 work station. The LFP set-up used a crossed-beam arrangement. The sample in a cell (10 mmX 10 mm) was excited at 355 nm by single light pulses (duration, 200 ps; energy, S-30 mJ) provided by a frequency-tripled, mode-locked Nd-YAG (Quantel) laser. The detection system included a pulsed xenon arc, a monochromator, a red-sensitive photomultiplier (Hamamatsu R446) and a fast transient recorder (Tektronix 7912) with a response time of around 5 ns. Relative rate studies were carried out using 350 nm UV lamps in a Rayonet photoreactor until all the diazirine was destroyed. The synthesis of 3-chloro-3-benzyldiazirine has been described previously [5, 91. Product analyses were carried out by nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass spectrometry (GCMS). The gas chromatograph traces were calibrated using authentic samples of the reaction products.
3. Results
and
discussion
Photolysis (350 nm) of 1 in tetramethylethylene (TME) gives E- and Z-&chlorostyrenes (2) and l-benzyl-l-chloro-2,2,3,3-tetramethylcyclopropane (3) (Scheme 1). A mechanism based on a carbene-alkene complex has been advanced [9] to explain the relative distribution of the products. Steady state approximation of this scheme leads to chlorostyrene
2/cyclopropane
3 = k; /b + (k/k)
1 /[TME]
(1)
where the overall trapping rate constant k, = k, k,l(k,+ k:+ k- & Alternatively, a mechanism involving an excited carbene [IO] or diazo [7] intermediate can also account for an inverse first-order dependence on TME. However, we favour [5] the “complex model” because it predicts an intercept dependence on various alkene substrates. When the above experiment is repeated with different initial concentrations of diazirine and TME concentrations in the range 0.2-2.5 M in isooctane at 25 “C, the values of kilk, change and the 2 to 3 ratio increases with increasing diazirine concentration. However, the intercept, killk,, is invariant as expected, because it should not depend on diazirine concentration. Least-squares analysis of 2/3 ‘us. l/[TME] gives the results shown in Table 1. The changes in slope may be rationalized by adding to the mechanism ;_ -
bw
_-
-
---
:
I
PhCH2 Cl
PhCH2 >(t
\I N
_hv_
\C-
Cl’
mm
____;___-_-,
,
’
ki __f .
; PhCH
=Cl-lCI 2 Z+E
1
Scheme 1.
a
117 TABLE
1 analysis of eqn. (1) using various initial
Slope and intercept data at 25 “C from least-squares concentrations of diazirine [Diazirine]
(m)
0.0302 0.0396 0.0730 0.1210
7.0
4.6
Slope
Intercept
0.0901 f 0.0005 0.0965 f 0.0043 0.1187*0_0011 0.1435 f 0.010
0.309 0.280 0.315 0.307
f * + +
0.0050 0.035 0.0111 0.0402
-
: 0
I
I
I
I
I
10
20
30
40
50
[Diazirine], mM Fig. 1. Plot of reciprocal at 25 “C.
lifetime of benzylchlorocarbene
as a function of diazirine concentration
a reaction of the carbene with diazirine [ll] with a rate constant kD. The steady state approximation of this scheme leads to eqn. (1) but with ki changed to ki+kD [I].The dope is now equal to ki/kt+ kD[lJ/k,_ A plot of the slope VS. [l]gives ki/k,=0.073*0.020 and k~/k,=0.59f0.028;hence, kD/ki=S.l+ 0.6. LFP (355 nm) of 1 in a degassed isooctane solution gives a transient absorption at 310 nm which can be attributed to the absorption of benzylchlorocarbene [S]. The lifetime of benzylchlorocarbene was measured at 25 “C as a function of diazirine concentration (Fig. 1; l/T= ki+kD[l]).Least-squares analysis of l/~ VS. [l] for six diazirine concentrations in the range 9-50 mM gives ki= (4.7f0.2)X lo7 s-l, kD = (3.6f0.4)x 10’ M-l s-l and kD/ki=7.7* 1.2,in excellent agreement with the results obtained from the relative rate studies and with the quenching rate constant (2.87X107 M-’ s-l) for adamantanylidene with diazirine in benzene [ll]. Photolysis of neat diazirine 1 gives Z- and E-2 (40%), azine (40%), l-phenyl-2,2dichloroethane (10%) and l-phenyl-1-chloroethylene (lo%, apparently from a 1,2phenyl shift). A calculation using ki,kD and [l]-6 M predicts that azine should represent more than 95% of the products, but only 40% was found. At lower diazirine concentration, [l]= 0.034M, calculation shows that azine should represent approximately
118
15%~25%
of the product
formed. However,
azine was not detected,
only chlorostyrenes,
in the products. Several photolysis experiments using various initial concentrations of diazirine (0.01-0.04 M) in isooctane were carried out with dibenzyl as the gas chromatograph internal standard. The mass balance shows that more than 95% of the diazirine gives the chlorostyrenes. The absence of azine formation at low diazirine concentrations
and the small amount of azine formed at high diazirine concentrations are unexpected. Our results indicate that the carbene is intercepted by the nitrogen atoms on the diazirine ring to form an adduct which can rearrange to give the chlorostyrene to a certain extent. In addition, in agreement with the relative rate data, the 2 to 3 ratio increases with increasing concentration of 1, indicating that the adduct does not undergo a cycloaddition reaction with TME, hilt a 1,2-hydrogen atom shift occurs to form the chlorostyrenes. Thus far, there is no general rule to be followed in the investigation of intramolecular carbene reactions. Moss and Ho [l] have pointed out the restriction of the applicability of the yiide methodology. The present work demonstrates that the speed at which the intramolecular rearrangement of the carbene occurs can control the type of product formed. Studies on the nature of the carbene-diazirine adduct are in progress.
Acknowledgments We thank Professor M. S. Platz for useful discussions. NSERC (Canada) is gratefully acknowledged.
Support
of this work by
References 1 R. R. 3 R. 4 M. 5 M. 6 R.
A. Moss and G.-J. Ho, J. Am. Chem. Sot., 112 (1990) 5642. A. Moss, G.-J. Ho, S. Shen and K. Krogh-Jespersen, J. Am Chem. Sot., 112 (1990) 1638. A. Moss and G.-J. Ho, Tetrahedron Lett., 31 (1990) 1225. T. H. Liu and R. Bonneau, _7.Am. Chem. Sot., 111 (1989) 6873. T. H. Liu and R. Bonneau, J. Am. Chem. Sot., 112 (1990) 3915. Bonneau, M. T. H. Liu and M. T. Rayez, J. Am. Chem. Sot., 111 (1989) 1973. 7 J. E. Jackson, N. Soundararajan, W. White, M. T. H. Liu, R. Bonneau and M. S. Platz, J.
2
Am. 8
Chem.
Sot.,
Ill
(1989)
6874.
J. E. Jackson, N. Soundararajan, M. S. Platz and M. T. H. Liu, J. Am. (1988)
Chem.
5595.
M. T. H. Liu, J. Chem. Sot., Chem. Commun. (1985) 982. P. M. Warner, Tetrahedron Lett., 25 (1984) 4211. 11 S. Morgan, J. E. Jackson and M. S. Platz, J. Am. Chem. Sot.,
9 10
113 (1991)
2782.
Sot.,
110