On the substituent effects of the thermal ethenylcyclopropane-to-cyclopentene rearrangement: Gas-phase kinetics of ethoxy-, methylthio-and trimethylsilyl-substituted ethenylcyclopropanes

FULL PAPER

On the Substituent Effects of the Thermal Ethenylcyclopropane-to-

Cyclopentene Rearrangement: Gas-Phase Kinetics of Ethoxy-, Methylthio-

and Trimethylsilyl-Substituted Ethenylcyclopropanes

Gregory McGaffin,^[a]Bernd Grimm,^[a]Ute Heinecke,^[a]Holger Michaelsen,^[a]

Armin de Meijere,*^[a]and Robin Walsh*^[b]

Dedicated to Professor Siegfried Hünig on the occasion of his 80th birthday

Keywords: Isomerization / Cyclopropanes / Kinetics / StructureϪreactivity relationships / Gas-phase reactions

A series of 1- and 2-substituted ethenylcyclopropanes were

prepared in high yields and subjected to gas-phase pyrolytic est rate accelerations, consistent with biradical mechanisms.

kinetic investigations. All ethenylcyclopropanes rearranged A previous finding of rate retardation by 1-trimethylsilyl sub-

cleanly to the correspondingly substituted cyclopentenes, al- stitution is attributed to steric hindrance. 2-Ethoxy and 2-

Trimethylsilyl and 1-methylthio substituents produced mod-

though in the case of the 2-substituted compounds, cis/trans

isomerization was additionally observed (both stereoisomers

were investigated in these cases). All rearrangements

obeyed first-order kinetics independent of pressure and sur-

face-to-volume ratio. Reasonable Arrhenius parameters were

methylthio substituents [in both (E)- and (Z)-configurations]

produced greater rate accelerations, inconsistent with birad-

ical mechanisms. The steric effects on the kinetics of the (Z)-

isomer rearrangements appear relatively unimportant. The

methylthio substituent effects are documented and analyzed

obtained for these homogeneous, unimolecular reactions. 1- for the first time.

Introduction

vived discussion and triggered off further intense research

work^[17bϪ17p]on the mechanism to the present day;^[17eϪ17q]

Ever since the discovery of the thermal ethenylcyclopro- (v) the discoveries of the low-temperature oxy anion driven

pane-to-cyclopentene (ECP-CP) rearrangement independ- rearrangement by Danheiser et al.^[18aϪ18e]in the early 1980s

ently by Neureiter and Vogel in 1959,^[1]this C₃Ǟ C₅ring and the low-temperature cation-radical-accelerated re-

expansion process has enjoyed the considerable attention of arrangement by Dinnocenzo et al.^[18f]in the late 1980s.

both organic and physical chemists. During the course of Both discoveries greatly simplified the reaction conditions

the following four decades the synthetic potential of this for more feasible organic preparations.

reaction as a powerful cyclopentene annelation scheme has

been successfully exploited. Today’s knowledge can, in

terms of historical development, be roughly divided into the

following stages of evolution: (i) fundamental gas-kinetic

studies of the thermal rearrangement during the 1960s, es-

tablishing the mechanism of smaller non-heteroatom-sub-

stituted ECPs in static reactors;^[2Ϫ12](ii) similar studies of

smaller heteroatom-substituted ECPs in static and flow re-

actors in the 1970s and early 1980s, documenting the rate-

accelerating effects of alkoxy,^[13a]dimethylamino,^[13b]cy-

ano^[14]and silyloxy^[15]groups; (iii) at almost the same time

a parallel branch of qualitative thermal studies on larger

heteroatom-substituted ECPs was performed;^[16](iv) the

discovery of evidence for concerted effects in the thermal

rearrangement, beginning in 1976 with Baldwin’s^[17a]le-

gendary experiment with an enantiopure ECP which re-

Besides these milestones a myriad of new and interesting

modifications have been devised by other researchers (e.g.

the transition metal-, acid- and base-induced ECP-CP re-

arrangement), many of which have turned out to provide

indispensable steps in natural product synthesis. The com-

plete arsenal of today’s known synthetic methodologies has

been excellently reviewed by Hudlicky et al.^[19]

Our motivation for the investigation of various het-

eroatom-substituted ECPs was to probe previously un-

known substituent effects on the homogeneous thermal re-

arrangement, and in particular to dissect these effects in

terms of activation parameters. Both 1- and 2-substituted

ECPs were considered to see what differences might arise

between substitution at the anchor point and at the migrat-

ing carbon atom in the ECP-CP process. The rearrange-

ment of 1-substituted ECPs (1-X) is shown in Scheme 1. We

extend our previous study of 1-OEt^[20]to 1-SiMe₃and 1-

SMe. 1-Trimethylsilyl ring substitution is of interest since it

has been reported to exert a retarding effect^[16c,16d]on the

ECP-CP process, in contrast to its rate-enhancing effect on

the parent cyclopropane ring opening reaction.^[21a]1-

Methylthio ring substitution will quantitatively reveal for

[a]

Institut für Organische Chemie der Georg-August-Universität,

Tammannstraße 2, 37077 Göttingen, Germany

Fax: (internat.) ϩ49-551/39-9475

E-mail: Armin.deMeijere@chemie.uni-goettingen.de

Department of Chemistry, University of Reading,

[b]

Whiteknights, P. O. Box 224, Reading RG6 6AD, UK

E-mail: R.Walsh@reading.ac.uk

Eur. J. Org. Chem. 2001, 3559Ϫ3573

WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001

1434Ϫ193X/01/0918Ϫ3559 $ 17.50ϩ.50/0

3559

G. McGaffin, B. Grimm, U. Heinecke, H. Michaelsen, A. de Meijere, R. Walsh

FULL PAPER

the first time the effect of a sulfur substituent on the re- cyclopropane (5)^[22bϪ22f]in only two steps, following stand-

arrangement.

ard literature procedures.^[22a]

Scheme 3

Scheme 1

The versatility of the ethenylcyclopropane 5^[22f]also

made the 1-methylthio-substituted ethynylcyclopropane 6-

SMe, a precursor to 1-SMe, easily accessible in three

steps.^[22g]Reaction of 5 with two equivalents of n-butylli-

thium and subsequent quenching with chlorotrimethylsil-

ane afforded 1-chloro-1-(trimethylsilylethynyl)cyclopropane

which, upon lithiation and reaction with S-methyl methane-

thiosulfonate gave 1-methylthio-1-(trimethylsilylethynyl)cy-

clopropane in high yield (90%). Subsequent protiodesilyl-

ation of the terminal acetylene with K₂CO₃in methanol

solution yielded 6-SMe (79%). In contrast to 6-SiMe₃, the

ethynylcyclopropane 6-SMe was hydrogenated extremely

slowly to 1-SMe. Obviously, the Lindlar catalyst was prone

to poisoning by the sulfur-containing reactant itself or by

other minor sulfide contaminants (in the absence of quinol-

ine no reaction occurred). Nevertheless 6-SMe was slowly

(within 144 h), but steadily converted into 1-ethenyl-1-

methylthiocyclopropane (1-SMe) without forming any by-

products under a hydrogen pressure of 3.5 bar (97%

yield).^[23a]

The rearrangement of 2-substituted ECPs (3-X) is shown

in Scheme 2. We investigate here compounds 3-OEt and 3-

SMe with 2-ethoxy and 2-methylthio substituents. Previous

reports^[13a]on the kinetics of 1-ethenyl-2-methoxycyclopro-

pane suggested a negligible participation of the (Z)-isomer

in the ring expansion. By careful analytical studies of the

system time evolution starting with each pure stereoisomer,

combined with computer modeling, we anticipated ob-

taining reliable values for all constants of this four-rate-con-

stant scheme (for each substituent). This study will there-

fore permit us to document the effect of these substituents

in either starting stereochemistry on the ECP-CP process,

and additionally on the stereoisomerization process itself.

For the preparation of the diastereopure ethenylcyclopro-

panes (E)- and (Z)-3-OEt two different and well-known

procedures were chosen, each predominantly giving either

the (E)- or the (Z)-ethenylcyclopropane. Thus, (Z)-2-

ethoxy-1-ethenylcyclopropane [(Z)-3-OEt] was accessible

by Rh₂(OAc)₄-catalyzed decomposition of ethenyldiazome-

thane (7)^[24]in the presence of an excess amount of ethenyl

ethyl ether (8) (Scheme 4). (E/Z)-3-OEt was obtained in

62% yield with an isomeric ratio E/Z ϭ 1:3.5. The major

(Z)-isomer was separated from the E/Z mixture by prepar-

ative GC (de ϭ 98.0%).

Scheme 2

The recently published^[25]efficient 1,3-dehydrobromin-

ation of 5-bromo-4-ethoxy-1-trimethylsilyl-1-pentyne (9)

and subsequent base-induced Z Ǟ E isomerization of the

resulting E/Z mixture of 2-ethoxy-1-(trimethylsilylethynyl)-

cyclopropane gave the diastereopure (E)-isomer. Sub-

sequent protiodesilylation as mentioned above afforded (E)-

Results and Evaluation of Data

Synthesis of Ethenylcyclopropanes

Both 1-substituted ethenylcyclopropanes 1-X (X ϭ 2-ethoxy-1-ethynylcyclopropane [(E)-10] which, upon facile

SiMe₃, SMe) were prepared from their corresponding ethy- catalytic hydrogenation, was converted into diastereopure

nylcyclopropane precursors 6-X by high-yielding hydro- (E)-3-OEt on a multigram scale (85% isolated yield).

genation reactions in the presence of appropriately deactiv-

Initial attempts to prepare (E/Z)-1-ethenyl-2-methylthi-

ated (quinoline) Lindlar catalyst (Scheme 3). The ethynylcy- ocyclopropane [(E/Z)-3-SMe] in one step in analogy to pro-

[22a]

clopropane 6-SiMe₃

reacted extremely rapidly under cedures for similar compounds by Schöllkopf et al.^[26]and

(approx.) atmospheric hydrogen pressure, giving pure 1- Rynbrandt et al.^[27]were abandoned. The outcome of sev-

ethenyl-1-trimethylsilylcyclopropane (1-SiMe₃) in 85% isol- eral experiments was that a base-induced (KOtBu) genera-

ated yield. The ethynylcyclopropane 6-SiMe₃was prepared tion of methylthiomethylenoid from chloromethyl methyl

from commercially available 1-chloro-1-(trichloroethenyl)- sulfide and its subsequent trapping with excess 1,3-butadi-

3560

Eur. J. Org. Chem. 2001, 3559Ϫ3573

Substituent Effects of the Thermal Ethenylcyclopropane-to-Cyclopentene Rearrangement

FULL PAPER

1-SMe were investigated as nitrogen-diluted gaseous mix-

tures with cyclohexane (CH) as an internal standard. De-

pending on the reactant vapor pressures (see Table 11, Exp.

Sect.) these master mixtures contained a maximum of

1.03% of 1-SiMe₃(with 0.65% of CH) and 0.20% of 1-SMe

(0.50% of CH), respectively. Total reaction pressures were

in the range of 12Ϫ25 Torr (Scheme 1 and Table 11). The

thermolyses were performed over a temperature interval of

50 °C for both reactions, which allowed the determination

of rates at six approximately equidistant reaction temper-

atures. 1-SiMe₃was examined between 313.3 and 363.1 °C

and 1-SMe between 241.0 and 289.9 °C. After the recorded

reaction times, the gaseous reaction mixtures of partially

converted 1-X (for ranges of conversion see Table 11) and 2-

X were pressurized and further diluted with nitrogen before

analysis by GC. Integration of the individual reaction com-

ponents allowed a quantitative determination and time

tracking of the mixture composition.

Scheme 4

Total mass recoveries were achieved for 1-SiMe₃as both

the reactant and the ring-expanded cyclopentenylsilane 2-

SiMe₃proved volatile enough for quantitative gas handling.

However, in the pyrolysis of 1-SMe the thioenol methyl

ether product 2-SMe was incompletely recovered due to its

low volatility, with mass losses consistently increasing at

higher reactant conversions. Several checks and careful

product GC analyses verified, as was previously shown in

kinetic studies of similar ethenylcyclopropanes,^[20]that the

generation of additional side-products can be excluded as

possible sources of product mass loss. Because there was no

mass loss of reactant, 1-SMe, the kinetics were based on

reactant disappearance. Excellent linear first-order plots

(least mean-squares procedure) were found from the time

evolution studies for both 1-SMe and 1-SiMe₃. The temper-

ature dependence of rate constants derived from the slopes

of the first-order plots is shown in Table 1. Again, applying

least mean-squares procedures to the rate constants in

Table 1 gave linear Arrhenius plots (Figure 1) for both reac-

tions which obey Equation (1) and Equation (2).

ene did actually give (E/Z)-3-SMe in high yields, but always

contaminated to a substantial extent with several other un-

identified sulfide by-products (10Ϫ15%, GC). It proved to

be an impossible task to obtain analytically pure (E)- and

(Z)-3-SMe from these coeluting intractable mixtures by pre-

parative GC methods. Therefore a three-step, and more se-

lective, route to (E/Z)-3-SMe was undertaken (Scheme 5),

starting from the available 2,2-dibromo-1-ethenylcyclopro-

pane (11), first reported by Skattebøl.^[28]According to the

protocol of Marino et al.,^[29]compound 11 was easily re-

duced to the monobromide (E/Z)-12 [E/Z ϭ 1.0:2.8] with

tri-n-butyltin hydride. Subsequent lithiation of (E/Z)-12 in

THF/Et₂O (2:1) at Ϫ78 °C with 2 equiv. of tert-butyllithium

and quenching with S-methyl methanethiosulfonate,

smoothly gave (E/Z)-3-SMe in 76% yield [E/Z ϭ 1.0:2.8].

Thus, the pure E/Z mixture was available in large quantities,

and even the minor (E)-isomer was obtained in sufficient

amounts for gas-phase kinetic studies after preparative GC

separation. (Z)-3-SMe was obtained in 97.5% de and (E)-

3-SMe in 98.6% de.

1-SMe: log (k/s^Ϫ1) ϭ

(1)

(13.94 Ϯ 0.35) Ϫ (197.5 Ϯ 4.0 kJ mol^Ϫ1)/RT ln10

1-SiMe₃: log (k/s^Ϫ1) ϭ

(2)

(13.59 Ϯ 0.23) Ϫ (201.4 Ϯ 2.7 kJ mol^Ϫ1)/RT ln10

Table 1. Rate constant variation with temperature for ECP-CP re-

arrangements 1-X Ǟ 2-X

Scheme 5

1-SiMe₃Ǟ 2-SiMe₃

Temp. [°C]

10⁴k [s^{Ϫ1 [a]}

1-SMe Ǟ 2-SMe

Temp. [°C]

10⁴k [s^{Ϫ1 [a]}

]

313.3

323.1

333.3

343.5

352.9

363.1

0.47 Ϯ 0.01

0.88 Ϯ 0.02

1.72 Ϯ 0.05

3.48 Ϯ 0.07

6.48 Ϯ 0.14

11.24 Ϯ 0.27

289.9

299.7

310.2

320.2

330.9

341.0

0.38 Ϯ 0.01

0.91 Ϯ 0.05

1.83 Ϯ 0.04

3.69 Ϯ 0.12

7.21 Ϯ 0.29

13.30 Ϯ 0.49

Kinetics of 1-Trimethylsilyl- and 1-Methylthio-1-

ethenylcyclopropane (1-SiMe₃and 1-SMe)

The gas-phase kinetics of the two 1-substituted ethenyl-

cyclopropanes 1-SiMe₃and 1-SMe were established in an

identical manner to the previously reported kinetic studies

on 1-alkoxy-1-ethenylcyclopropanes.^[20]Both 1-SiMe₃and

[a]

Error limits are standard deviations.

Eur. J. Org. Chem. 2001, 3559Ϫ3573

3561

G. McGaffin, B. Grimm, U. Heinecke, H. Michaelsen, A. de Meijere, R. Walsh

FULL PAPER

from the first-order plot linear regression gave k(343.5

°C) ϭ (3.42 Ϯ 0.12) ϫ 10^Ϫ4^Ϫ1, in good agreement with

s

the value for the unpacked vessel (Table 1). Thus, the ex-

perimental data show no variation in ethenylcyclopropane

conversion other than from reasonable experimental scatter,

and are therefore consistent with a homogeneous reaction,

unperturbed by radical chain processes.

Kinetics of (E)-, (Z)-2-Ethoxy- and (E)-, (Z)-2-Methylthio-

1-ethenylcyclopropane [(E)-, (Z)-3-OEt and (E)-, (Z)-3-

SMe]

General

The complex reaction mechanism for the thermal reac-

tions of 2-substituted ethenylcyclopropanes is indicated in

Scheme 2.^[13a,13b,14]The thermolysis of either (E)- or (Z)-

isomers induces geometrical E Ǟ Z or Z Ǟ E isomerisation

(k₁, k₂), accompanied by the simultaneous ring expansion

with k₃[(E)-isomer] and k₄[(Z)-isomer]. Determination of

the individual rate constants k₁to k₄is therefore a more

sophisticated task than for the simple ECP-CP rearrange-

ments 1-X Ǟ 2-X. We have applied computer-based kinetic

modeling schemes, based on the Gear algorithm,^[30]which

employs a variable-step integration of the differential equa-

tions involved in Scheme 2, or least mean-squares fitting to

the analytical solution^[31]of Scheme 2.

Figure 1. Arrhenius plots for thermal ECP-CP rearrangements of

1-X: A: 1-SiMe₃; B: 1-SMe

Precautionary checks were also carried out at elevated

pressures in order to evaluate the possibility of pressure-

dependent first-order rate constants in the ‘‘fall-off’’ region,

although molecules of the size of 1-X are not usually affec-

ted in this way. Pyrolyses for 10 min of 1-SiMe₃at 363.1

°C and 10, 20 and 75 Torr showed reproducible reactant

conversions of 48.3, 48.1 and 48.3%. Similarly, 30-min. runs

at 322.1 °C at 7, 14 and 57 Torr took 1-SMe to consistent

conversions of 48.3, 49.9 and 49.7%.

On the basis of assumed first-order reactions the Gear

algorithm required an initial set of estimated rate constants

k₁to k₄for which the concentration/time data of each iso-

mer at each reaction temperature were numerically calculat-

ed^[32a]and compared with the experimental time evolution

plots. Calculations were repeated with an appropriately al-

tered set of values for k₁to k₄until the best fit between

calculated and experimental data was achieved (as evalu-

ated by the relative minimum in the sum of the squares of

all deviations, Σx²). The analytical data for the pyrolyses of

both (E)- and (Z)-3-OEt were processed in this way.^[32b]

The computational processing of the kinetic data of com-

plex schemes invariably gives difficulties arising from the

differing sensitivities of individual product channels to-

wards the adjustment of specific k values. Thus, in the pyro-

lyses of neat (Z)-3-X, steps 1 and 4 are primary (Scheme 2)

and therefore affect the decomposition rate directly, while

steps 2 and 3 are secondary and therefore have a lesser af-

fect. The situation is reversed when neat (E)-3-X is consid-

ered. However, the overall quality of fitting does depend on

the absolute magnitude of all four of the rate constants k₁

to k₄. It follows from this that the pyrolysis of (Z)-3-X will

give greater certainty for k₁and k₄(and less for k₂and k₃),

whereas the pyrolysis of (E)-3-X will give greater certainty

for k₂and k₃(and less for k₁and k₄). These considerations

guided us in our procedure for refining the rate constants.

For example, as k₄proved to be the smallest rate constant

for the scheme involving 3-OEt the values were taken from

the processed data for the (Z)-isomer and then incorporated

More detailed attention was given to checks on surface

activity and possible radical chain processes. For this pur-

pose the reaction vessel was removed from the vacuum line

and replaced by an HMDS-conditioned, glass-tube-packed

vessel with an approximate surface-to-volume ratio of 10

cm^Ϫ1(see Exp. Sect.). Pyrolyses were carried out at the

same temperatures as for the unpacked vessel. In further

experiments an olefin was added to serve as an inhibitor

and therefore test for potential radical chain reactions. The

results for these investigations are summarized in Table 2.

Table 2. Product variation in packed and unpacked vessels for

ECP-CP rearrangements 1-X Ǟ 2-X

Reaction

vessel

1-SiMe₃Ǟ 2-SiMe₃

Conversion 1-SiMe₃

(%)

1-SMe Ǟ 2-SMe

Conversion 1-SMe^[b]

(%)

[a]

Unpacked

Packed

48.1

49.0

48.7^[c]

48.6

49.4^[d]

48.2

Packed

49.2^[c]

48.9^[d]

[a]

[b]

10 min pyrolysis at 363.1 °C and 18 Torr. Ϫ 30 min pyrolysis

at 322.1 °C and 14 Torr. Ϫ ^[c]15-fold excess of isobutene added. Ϫ

10-fold excess of cis-2-butene added.

[d]

In addition to the above data a complete set of runs was (frozen) into the data matrix for the (E)-isomer. The data

performed for 1-SiMe₃in the packed vessel at 343.5 °C, and were then reprocessed for (E)-3-OEt and the resulting rate

3562

Eur. J. Org. Chem. 2001, 3559Ϫ3573

Substituent Effects of the Thermal Ethenylcyclopropane-to-Cyclopentene Rearrangement

FULL PAPER

constants taken to be the best extractable values. This is

discussed in more detail below.

(E)- and (Z)-3-OEt

(E)-3-OEt was pyrolyzed between 18 and 22 Torr in a

temperature range of 221.0Ϫ278.1 °C. Similarly, pyrolyses

of (Z)-3-OEt were performed between 18 and 25 Torr and

at 220.8Ϫ277.6 °C (for further information on conditions

see Table 11, Exp. Sect.). (E)-3-OEt was 100.0% iso-

merically pure but (Z)-3-OEt was contaminated with 3.0%

of the (E)-isomer (due to incomplete separation during

isolation). This isomeric impurity was taken into account

in the data processing. The collecting of analytical time

evolution data was carried out similarly to the method de-

scribed for ethenylcyclopropanes 1-X (see also Exp. Sect.).

In order to achieve optimal k values a greater amount of

analytical data was collected for each temperature (10 runs

at least), covering total (E)- and (Z)-reactant conversions

between 7.6 and 94.3% (see Table 11). Table 3 gives an ex-

ample of the product time evolution for the pyrolysis of (E)-

3-OEt at 266.8 °C. This is shown graphically in Figure 2

together with that for (Z)-3-OEt at the same temperature.

Table 3. Product time evolution for the thermal reactions of (E)-3-

OEt at 266.8 °C

Time

[min]

(E)-3-OEt^[a]

(%)

(Z)-3-OEt^[a]

(%)

4-OEt^[a]

(%)

0.0

2.5

5.0

100.00

82.96

71.61

61.73

53.72

41.15

32.75

24.96

21.49

14.74

10.41

7.47

0.00

6.37

9.26

11.55

12.73

13.67

13.07

11.47

10.54

8.02

0.00

10.67

19.13

26.72

33.56

45.18

54.18

63.57

67.97

77.24

83.82

88.35

Figure 2. Product distribution for thermal rearrangements of (E)-

and (Z)-3-OEt at 266.8 °C: A: (E)-3-OEt; B: (Z)-3-OEt; * ϭ (E)-

3-OEt; ϫ ϭ (Z)-3-OEt; ∆ ϭ 4-OEt

7.5

10.0

15.0

20.0

26.0

30.0

40.0

50.0

60.0

Table 4. Individual variation of rate constants k₁to k₄with temper-

ature for thermal isomerizations of both (E)- and (Z)-3-OEt deter-

mined by the Gear algorithm

5.77

4.18

Temp. [°C]

221.0 234.0 248.1 257.6 266.8 278.3

[a]

Gas-phase composition at time t.

[a]

[b]

[c]

2.63

2.76

3.04

6.81

8.28

9.00

28.1

27.3

28.1

59.9

54.8

61.7

117

112

130

284

260

303

10⁵k₁

[s^Ϫ1

]

Data for (E)- and (Z)-3-OEt were individually processed

by the Gear algorithm. Additionally, the k₄values from the

optimal (Z)-3-OEt product fit were incorporated as fixed

parameters into the data matrix for the (E)-isomer product

fitting and k₁to k₄values processed again. Table 4 contains

the k₁to k₄values for these three fitting exercises.

[a]

[b]

[c]

1.12

1.38

1.05

3.29

3.54

3.22

10.8

12.3

10.1

21.7

19.1

20.8

45.8

44.3

106

101

10⁵k₂

[s^Ϫ1

]

[a]

[b]

[c]

2.73

2.81

2.85

7.28

7.58

7.70

21.0

21.3

21.5

41.8

43.6

42.7

74.5

78.7

78.1

156

166

164

10⁵k₃

[s^Ϫ1

]

[a]

[b]

[c]

The data fit to the (E)- and (Z)-3-OEt product distribu-

1.09

0.26

2.95

0.83

5.20

2.61

7.85

3.47

23.2

7.50

45.0

10.8

tions using the Gear algorithm shows tolerable consistency 10⁵k₄

[s^Ϫ1

]

between the k₁to k₃values starting from either isomer. The

values of k₄, however, appear to be about four times greater Σx²

when starting from (E)- than from (Z)-OEt. We believe that

this probably arises from a small (unidentified) systematic

[a]

[b]

[c]

1.84

30.2

6.09

4.96

6.40

10.1

4.58

17.9

8.49

5.36

52.9

7.90

3.21

10.33 19.9

9.49 11.7

2.80

Σx²

error. However, because k₄is the smallest rate constant, the

only data points showing sensitivity to its value will be

[a]

[b]

[c]

(E) isomer product fit. Ϫ

(Z) isomer product fit. Ϫ

(E)

isomer product fit with frozen k₄values from (Z) isomer product

those at early reaction times starting from the (Z)-isomer. fit.

Eur. J. Org. Chem. 2001, 3559Ϫ3573

3563

G. McGaffin, B. Grimm, U. Heinecke, H. Michaelsen, A. de Meijere, R. Walsh

FULL PAPER

Thus, we believe that a better data analysis is obtained from sample contained 2.5% of the (E)-isomer]. The stereoiso-

the (E)-isomer with the k₄value fixed. An examination of meric impurities were accounted for in the data fitting.

the table shows that the k₁to k₃values thus derived are not Table 5 contains sample experimental data for the product

substantially different and in most instances within a few time evolution from pyrolysis of (Z)-3-SMe at 259.0 °C and

percent of those from the unconstrained Z and E fits. The Figure 4 gives a graphical impression of the product distri-

quality of fit (Σx²) is not seriously worsened and the

Table 5. Product time evolution for the thermal reactions of (Z)-3-

SMe at 259.0 °C

graphical plots show a good fit of experiment to the

modeled product-time curves. A test (not shown) with k₄ϭ

0 produced significantly poorer fits, thus showing that, in

Time

[min]

(E)-3-SMe^[a]

(Z)-3-SMe^[a]

4-SMe^[a]

spite of its small contribution, step 4 cannot be neglected.

The Arrhenius equations for reaction steps 1Ϫ4 [Equa-

tion (3) to Equation (6)] were derived from the Gear-pro-

cessed and -correlated data for the (E)-isomer (Figure 3).

Minor sources of error are due to the slightly different tem-

peratures (a few tenths of K at most) for (E)- and (Z)-3-

OEt, but calculations show these to cause less than 5% devi-

ation in determined k values, thus being well within experi-

mental error.

(%)

0.0

4.0

10.0

15.0

20.0

30.0

40.0

60.0

80.0

100.0

120.0

150.0

180.0

2.46

41.51

62.94

66.31

68.60

67.53

64.87

59.51

53.95

50.15

45.94

40.51

36.30

97.54

56.37

32.54

25.52

22.33

19.78

18.99

17.30

16.19

14.31

13.56

11.61

10.85

0.00

2.11

4.52

8.17

9.08

12.68

16.14

23.19

29.86

34.26

40.50

47.87

52.85

log (k₁/s^Ϫ1) ϭ (14.80 Ϯ 0.18) Ϫ

(3)

(182.9 Ϯ 1.8 kJ mol^Ϫ1)/RT ln10

log (k₂/s^Ϫ1) ϭ (14.13 Ϯ 0.12) Ϫ

(4)

(180.8 Ϯ 1.2 kJ mol^Ϫ1)/RT ln10

[a]

Gas-phase composition at time t.

log (k₃/s^Ϫ1) ϭ (12.44 Ϯ 0.04) Ϫ

(5)

(160.7 Ϯ 0.4 kJ mol^Ϫ1)/RT ln10

log (k₄/s^Ϫ1) ϭ (10.22 Ϯ 0.95) Ϫ

(6)

(148.8 Ϯ 9.6 kJ mol^Ϫ1)/RT ln10

Figure 3. Arrhenius plots for thermal rearrangements of (E)-3-OEt

(based on correlated data and the Gear algorithm).

(E)- and (Z)-3-SMe

The kinetic data for (E)- and (Z)-3-SMe were collected

in an identical manner to the procedure described for (E)-

and (Z)-3-OEt. Both ethenylcyclopropanes were pyrolyzed

in a common pressure range between 14 and 21 Torr and a

temperature interval of 238.3Ϫ289.2 °C [the upper range

limit for (E)-3-SMe is insignificantly different at 289.1 °C,

see Table 11]. Both isomers were chemically pure, although

‘‘stereochemically’’ marginally impure [the (E)-isomer

Figure 4. Product distribution for thermal rearrangements of (E)-

and (Z)-3-SMe at 259.0 °C: A: (E)-3-SMe; B: (Z)-3-SMe; * ϭ (E)-

sample contained 1.4% of the (Z)-isomer; the (Z)-isomer 3-SMe; ϫ ϭ (Z)-3-SMe; ∆ ϭ 4-SMe

3564

Eur. J. Org. Chem. 2001, 3559Ϫ3573

Substituent Effects of the Thermal Ethenylcyclopropane-to-Cyclopentene Rearrangement

FULL PAPER

butions during pyrolyses of the individual (E)- and (Z)-iso-

mers at this temperature.

Because the data set from (Z)-3-SMe was marginally

more extensive than for (E)-3-SMe, it was preferred as the

In view of the reasonable modeling outcome for (E)- and basis for working out the Arrhenius equations [Equation (7)

(Z)-3-OEt (Table 4) the analytical data for (E)- and (Z)-3- to Equation (10)]. Nevertheless, the alternative set from (E)-

SMe were processed in a similar way. In this case we had 3-SMe is, within error limits (single standard deviations),

available a program for minimization to an analytical the same.

fit.^[31a]The problem with fitting the analytical data here was

that for this reaction system both k₃and k₄were signific-

log (k₁/s^Ϫ1) ϭ (14.54 Ϯ 0.32) Ϫ

antly smaller than k₁and k₂. Thus, in this case, under the

operating conditions, only at the lowest three temperatures

(239Ϫ259 °C) was the equilibration between (E)- and (Z)-

3-SMe slow enough to permit the determination of all four

rate constants. At 269 °C the data fitting could only be

achieved for the (Z)-isomer, whereas for the (E)-isomer at

this temperature and both isomers at the higher temper-

atures, fitting was achieved only by keeping k₁/k₂fixed with

values taken by extrapolation from lower temperatures. In

this way, the values for k₃and k₄could be obtained. Since

the magnitudes of k₃and k₄were comparable there was

no special problem with discrepancies between them arising

from fitting to each isomer. Indeed the values derived

(Table 6, Figure 5) show a good consistency of the data

from each isomer at those temperatures where unique

values were obtained.

(7)

(8)

(174.6 Ϯ 3.2 kJ mol^Ϫ1)/RT ln10

log (k₂/s^Ϫ1) ϭ (14.92 Ϯ 0.62) Ϫ

(183.9 Ϯ 6.3 kJ mol^Ϫ1)/RT ln10

log (k₃/s^Ϫ1) ϭ (12.91 Ϯ 0.11) Ϫ

(173.8 Ϯ 1.1 kJ mol^Ϫ1)/RT ln10

(9)

log (k₄/s^Ϫ1) ϭ (13.52 Ϯ 0.13) Ϫ

(180.2 Ϯ 1.1 kJ mol^Ϫ1)/RT 1n10

(10)

Additional experimental checks addressing possible pres-

sure effects and surface catalysis were performed represent-

atively for (E)-3-OEt and (Z)-3-SMe. Identical reactant

conversions were found in all pressure checks. In the surface

catalysis test, a 10-min. pyrolysis of (E)-3-OEt in a packed

vessel at 266.9 °C gave 54.07% of (E)-3-OEt, 12.54% of (Z)-

3-OEt and 33.39% of 4-OEt in comparison with 53.72%

of (E)-3-OEt, 12.73% of (Z)-3-OEt and 33.56% of 4-OEt

obtained from an identical run in the unpacked vessel. Sim-

ilarly, a 15-min. pyrolysis of (Z)-3-SMe in the same packed

vessel at 289.2 °C revealed a product distribution of 44.40%

of (E)-3-SMe, 14.26% of (Z)-3-SMe and 41.34% of 4-SMe,

compared with a reaction mixture composition of 44.63%

of (E)-3-SMe, 13.93% of (Z)-3-SMe and 41.45% of 4-SMe

in the unpacked vessel under otherwise identical conditions.

Checks on radical chain components in the presence of ex-

cess amounts of cis-2-butene [(E)-3-OEt] and isobutene

[(Z)-3-SMe] revealed no deviations in product distributions

outside experimental scatter. Thus, these complex systems

represent true homogeneous unimolecular processes.

Table 6. Individual variation of rate constants k₁to k₄with temper-

ature for thermal isomerizations of both (E)- and (Z)-3-SMe deter-

mined by least-squares analytical fitting

Temp. [°C]

10⁵k₁/s^Ϫ1

10⁵k₂/s^Ϫ1

10⁵k₃/s^Ϫ1

10⁵k₄/s^Ϫ1

239.3 249.1 259.0 269.4^[a]279.0^[a]289.1^[a]

[b]

[c]

[b]

[c]

[b]

[c]

[b]

[c]

54.4

14.9

1.57

1.48

123.5 246.8 (538)

120.7 242.8 536.6

(1062) (2072)

(1091) (2035)

34.1

35.9

3.34

3.47

3.28

3.08

67.5

71.0

6.73

7.08

6.92

6.90

(150)

168.8

15.0

15.3

(282)

(354)

28.8

30.2

32.7

30.9

(565)

(682)

57.5

58.5

67.6

60.7

[a]

Figures in parentheses used for fitting purposes only (not

[b]

[c]

Arrhenius parameters); see text. Ϫ

(E)-isomer fit. Ϫ (Z)-iso-

mer fit.

Discussion

General

Table 7 and Table 8 contain the Arrhenius parameters

[see Equations (1), (2), (5), (6), (9) and (10)] for the gas-

phase ECP-CP reactions studied in the present work. Also

included are the known values for other examples of 1-sub-

stituted (1-X) and 2-substituted [(E)- and (Z)-3-X] ethenyl-

cyclopropanes. To facilitate discussion, the rate-accelerating

factors due to substitution [given by the ratio of rate con-

stants k_X/k_Hat 300 °C for substituted (X) and unsubsti-

Figure 5. Arrhenius plots for thermal rearrangements of (E)-3-SMe tuted (X ϭ H) ethenylcyclopropanes] are also given.

Eur. J. Org. Chem. 2001, 3559Ϫ3573

3565

G. McGaffin, B. Grimm, U. Heinecke, H. Michaelsen, A. de Meijere, R. Walsh

FULL PAPER

X series (Table 7), the values of A are about an order of

magnitude greater than for the 3-X series (Table 8). This

signifies a general tightening of the transition state for the

2-substituted examples, suggesting a restriction of motion

probably brought about by the more demanding environ-

ment of the bulky substituent. While this may be indicative

of a greater degree of concert in the rearrangements of the

ethenylcyclopropanes 3-X compared with 1-X, it could also

arise from a biradical mechanism in which ring closure to

the cyclopentene (i.e. the second step) was rate determining.

In this latter case the bulky substituent would interfere

sterically in the transition state for this step. We are aware

of the vexed question of the mechanism of this reaction,^[17]

and these questions are addressed further in the discussion

of the individual molecules. It should be noted that the very

low A factor [log (A/s^Ϫ1) ϭ 10.22] for (Z)-3-OEt is probably

an error arising from the fact that the rate constants were

small and hard to measure reliably.

Table 7. Arrhenius parameters for the thermal ECP-CP rearrange-

ments of various 1-substituted ethenylcyclopropanes 1-X

1-Substituted Ethenylcyclopropanes (1-X)

Table 7 reveals that both 1-trimethylsilyl and 1-methyl-

thio substitution cause modest rate acceleration factors,

arising mainly from activation energy reductions of 6.3 and

10.2 kJ mol^Ϫ1relative to parent ethenylcyclopropane. These

are not as large as the ca. 19 kJ mol^Ϫ1reduction caused by

1-methoxy and 1-ethoxy substitution.^[20]

[a]

Calculated for T ϭ 300 °C.

Table 8. Arrhenius parameters for the thermal ECP-CP rearrange-

ments of various 2-substituted ethenylcyclopropanes (E)- or (Z)-

3-X

For 1-SiMe₃, the result is consistent with the reported

activating effect of trimethylsilyl substitution on the cyclo-

propane rearrangement although comparisons have to be

made with care. Conlin and Kwak^[21a]have studied the iso-

merization of trimethylsilylcyclopropane (15) for which the

major product is allyltrimethylsilane (14). However, this is

not the correct comparison since 14 is believed to form via

a β-stabilized biradical 13 (Scheme 6). Only the minor prod-

ucts, (Z)- and (E)-1-trimethylsilylpropene [(E)- and (Z)-17]

formed via the α-stabilized biradical 16, provide the correct

comparison. Nevertheless, from the reported rate constants

for this latter pathway there is still a rate-accelerating factor

of ca. 3 compared with cyclopropane itself.^[21b]The α- and

β-stabilizing effects of trimethylsilyl substitution have been

nicely rationalized by bond dissociation energy measure-

ments.^[34]

[a]

Calculated for T ϭ 300 °C.

Scheme 6

In the case of 1-SiMe₃, the stabilizing effect of the ethenyl

The values for the Arrhenius A factors are generally reas- group is so dominating that adjacent bond breaking of the

onable, corresponding to small entropies of activation cyclopropane ring overwhelms any tendency to remote

(either positive or negative). It is noteworthy that for the 1- bond breaking caused by the SiMe₃β-stabilization effect.

3566

Eur. J. Org. Chem. 2001, 3559Ϫ3573

Substituent Effects of the Thermal Ethenylcyclopropane-to-Cyclopentene Rearrangement

FULL PAPER

Thus, only the modest α-stabilization effect is possible. The

fact that the trimethylsilyl group activating effect is of the

correct magnitude supports the idea that stabilizing effects

are additive and provides further evidence that the energy

considerations appropriate to biradical mechanisms apply

in these ECP-CP rearrangements in spite of evidence of

concerted effects^[17e,17h](in other cases).

Table 9. Methyl substituent effects on the ECP-CP rearrangement

R¹R²log (A/s^Ϫ1

)

E_a[kJ mol^Ϫ1

]

10⁴k [s^Ϫ1

]

Rel. rate Ref.

A seemingly conflicting finding of rate retardation by a

1-trimethylsilyl group on the ECP-CP rearrangement has

been reported by Paquette et al.^[16c][16d]Upon pyrolysis, 1-

cyclopropyl-1-(1Ј-trimethylsilylcyclopropyl)ethene (20) was

[2a]

H

Me

H

13.50

14.11

207.7

206.8

213.0

211.3

0.04

0.18

0.03

0.08

1.0

4.5

0.8

2.0

[2d]

[2b]

Me 13.89

[2h]

Me Me 14.14

quantitatively

converted

into

2-trimethylsilylbicy-

clo[3.3.0]oct-1-ene (22). It was proposed that in this intra-

molecular competition situation the ECP-CP rearrange-

ment of the unsubstituted unit in 20 going to 21 wins over

the one of the substituted side going to 18 which would

have led to 5-trimethylsilylbicyclo[3.3.0]oct-1-ene (19)

(Scheme 7). From this observation it was concluded that 1-

trimethylsilyl substitution must exert a retarding effect on

the ECP-CP rearrangement.

For 1-SMe, the modest activating effect of the methylthio

group is consistent with unpublished findings^[35]for the

pyrolysis of methylthiocyclopropane, which show a rate ac-

celeration of a factor of ca. 30 compared with cyclopropane

itself, but not as much as the factor of ca. 75 for methoxycy-

clopropane.^[36]We have previously discussed^[20]the magni-

tude of the methoxy stabilizing effect at a radical center of

ca. 20 kJ mol^Ϫ1. The equivalent figure for the methylthio

group may be represented by the bond dissociation energy

difference, D(CH₃CH₂CH₂ϪH) Ϫ D(CH₃SCH₂ϪH) which

has the magnitude of ca. 8 kJ mol .

^{Ϫ1 [37]}The substituent ef-

fects on the parent cyclopropane suggest a slightly larger

figure (ca. 12 kJ mol^Ϫ1), which is well within the margin of

error. Thus, as Table 7 shows for 1-SMe, just as for 1-

SiMe₃, the energetic effect of the SMe substituent is consist-

ent with the additivity of the SMe and ethenyl stabilizing

effects. It is worth noting that the methylthio substituent

effect lies between those of methyl and methoxy.

Scheme 7

2-Substituted Ethenylcyclopropanes (3-X)

Table 8 reveals that 2-methoxy, 2-ethoxy and 2-methyl-

thio substitution cause quite large rate-acceleration factors,

significantly greater than those for their 1-substituted coun-

terparts. The effects are variable dependent on the (E) or

(Z) starting configuration. For the (E)-isomers, the rate en-

hancements correspond to activation energy reductions of

47 kJ mol^Ϫ1(3-OEt) and 34 kJ mol^Ϫ1(3-SMe) relative to

the parent ethenylcyclopropane. If the 1-substituted ethen-

ylcyclopropane kinetics can be rationalized with the ener-

getic arguments for biradical-type mechanisms, these fig-

ures are much more difficult to accommodate within this

mechanistic framework. However, rate measurements do re-

veal some clear-cut features.

Taking our kinetic results for 1-SiMe₃into account, we

believe that the likely explanation for the selective re-

arrangement of 20 to 22 lies in steric effects. Formation of

18 from 20 requires the incorporation of an extremely

crowded substituent at the double bond, involving a cis in-

teraction between trimethylsilyl and cyclopropyl groups;

this sterically unfavorable situation already exists in the in-

termediate biradical precursor to 18. Thus, the weak elec-

tronic effect of the trimethylsilyl group will be totally offset

by the steric disadvantage of forming 18 rather than 21 (and

hence the observed product 22).

Some indication that such steric effects might be import-

For (Z)-3-OEt we have established from the modeling

ant can be gleaned from a comparison of methyl-substi- calculations the unequivocal existence of the direct pathway

tuted ethenylcyclopropanes, as shown in Table 9. The effects (k₄) to 4-OEt, even though the rate constants are small and

are not large but clearly the double methyl substitution have significant uncertainties. Since the 2-ethoxy substitu-

(R¹ϭ R²ϭ Me) produces less than half the rate enhance- ent is certainly larger than the 2-methoxy group, this study

ment of single methyl substitution (R¹ϭ Me). A more sig- improves on the study of 3-OMe by Simpson and Rich-

nificant test would be to place the methyl group in the po- ey,^[13a]who could not detect this pathway and assumed that

tentially sterically crowded site in 1-SiMe₃. This test is it was negligible. They rationalized this by pointing out that

planned.

the (Z)-1-ethenyl-2-methoxycyclopropane could not reach

Eur. J. Org. Chem. 2001, 3559Ϫ3573

3567

G. McGaffin, B. Grimm, U. Heinecke, H. Michaelsen, A. de Meijere, R. Walsh

FULL PAPER

the transition state for formation of the necessary cisoid in- methylthio activation energy for 3-SMe of ca. 34 kJ mol^Ϫ1

termediate due to steric interactions between the methoxy is very significantly larger than that already indicated in this

and ethenyl groups.^[38]Whether or not this is an important paper (vide supra) of ca. 8Ϫ12 kJ mol^Ϫ1for the stabilizing

consideration, our findings show that the pathway exists. In effect of the methylthio group at a radical center. Even if

our view the striking finding for the 3-OEt system is not this quantity is regarded as still subject to further confirma-

the low value for k₄but rather the high value for k₃, the tion, there remains the contrast between the 3-X examples

rate constant for the (E)-isomer. For all other substituents (Table 8) in which X ϭ SMe has a higher rate constant than

in these systems, viz. Me,^[2f]SMe, and NMe₂,^[13b]the (E) Ǟ X ϭ OEt [(Z)-isomers] and the 1-X examples (Table 7)

(Z) isomerization is faster than ring expansion (i.e. k₂Ͼ k₃) where X ϭ SMe has a lower rate constant than X ϭ OEt.

by at least an order of magnitude. The same is true for the

Acknowledging that our values for k₄are subject to some

nearly equivalent systems with slightly more encumbered error, a perhaps better illustration of the reversal of sub-

alkenyl substitution.^{[14,17a,17b,17d]}Only in the 2-methoxy^[13a]stituent effects from 1- to 2-substitution is in the (E) Ǟ (Z)

and the 2-ethoxy cases is this not so. This argues strongly isomerization process. Table 10 shows some rate and equi-

that step 3 has a large degree of concertedness. This is per- librium information on this.

haps not altogether surprising since some of the earliest ex-

It can be seen that for X ϭ SMe compared with X ϭ

amples of concerted 1,3-sigmatropic shifts involved an alk- OEt the rate constants are greater in both directions show-

oxy substituent at the migrating carbon atom.^[39]The re- ing clearly the greater activating effect of the methylthio

maining rate constants in the 3-OEt system, k₁, k₂and k₄substituent. These data also indicate how marginal is the

are also quite high, and for them to be accommodated steric effect in the reactant ethenylcyclopropanes 3-X. Al-

within a biradical energetic framework would require the though the (Z)-compound is disfavored in both cases, the

ethoxy group to stabilize a radical center by at least a fur- differences between them are very small. Thus, the steric

ther 10 kJ mol^Ϫ1more than we have already suggested (i.e. bulk of SMe does not appear to be significantly larger than

30 versus 20 kJ mol^Ϫ1).^[40]

that of OEt. In summary, to explain the rate enhancements

For 3-SMe, there are some noteworthy features although of the methylthio group and to a lesser extent the ethoxy

a complete mechanistic understanding remains elusive. The group, on the 2-substituted ethenylcyclopropanes we are

values for k₃and k₄are much more nearly comparable than forced to invoke concerted processes with differing degrees

those for 3-OEt and both are significantly smaller than k₁of transition-state stabilization. These must be to some ex-

and k₂. This makes the argument about the importance of tent concertedness, based on the energy criterion. It may be

the synclinal conformation of the ethenyl group as a critical that sulfur, with its greater capacity for expansion of its

rate-determining influence on the ECP-CP rearrangement coordination (increase of its valency), is more capable of

highly questionable, since the steric bulk of the methylthio such stabilization than oxygen.

group ought thereby to render the rate for the (Z)-isomer

even more marginal in this case. Indeed it seems that this

Experimental Section

argument can have very little force since in their study of

(E)- and (Z)-2-cyano-1-isopropenylcyclopropanes, Doering

and Sachdev^[14]found that ring expansion to the cyclopen-

tene product proceeded, remarkably, three times faster for

the (Z)- than the (E)-isomer.

Preparation of Compounds

General: IR: PerkinϪElmer 399 spectrophotometer. Abbreviations

for signal assignments: Cp-H ϭ cyclopropyl. Ϫ H NMR: Bruker

1

AM-250 (250 MHz), AW-250 (250 MHz), WH-270 (270 MHz) and

Varian VXR 200 (200 MHz), VXR 500 S (500 MHz) spectrometers

at ambient temperature. Chemical shifts are referenced to internal

tetramethylsilane or to the solvent resonance employed as the in-

ternal standard (chloroform at δ ϭ 7.24), multiplicity [d ϭ doublet,

nd ϭ n-fold doublet (n ϭ 2, 3, 4, 5), t ϭ triplet, dt ϭ double triplet,

pt ϭ pseudo triplet, q ϭ quadruplet, dq ϭ double quadruplet, pt ϭ

pseudo triplet, m ϭ multiplet, m_cϭ symmetrical multiplet, s ϭ

singlet], coupling constant(s) [Hz], integration and assignment. All

coupling constants were determined according to the rules for first-

If the steric effects of the 2-substituents are not very im-

portant for the ECP-CP rearrangement it is tempting to

suggest that initial cyclopropane ring-opening is not rate-

determining and the mechanism is two-step, i.e. biradical in

nature.^[41]This idea would indeed fit in with the generally

rapid (Z) Ǟ (E) isomerization compared with ring expan-

sion. However, in view of the well-documented evidence for

concertedness in the ECP-CP process^[17]this would be a

rash proposition. In this case, additionally, there are the ar-

guments about the energy criterion. The lowering of the order spectra. Ϫ ¹³C NMR: Bruker WP-80 (20.17 MHz), AM-250

Table 10. Kinetic and thermodynamic data for cis/trans isomerizations of (E/Z)-3-OEt and (E/Z)-3-SMe at 300 °C

ECP

10⁴k₁

10⁴k₂

K ϭ k₁/k₂

∆_RH°^[a]

∆_RS°^[a]

∆_RG°

[kJ mol^Ϫ1

]

[J K^Ϫ1mol^Ϫ1

]

[kJ mol^Ϫ1

]

(E/Z)-3-SMe

(E/Z)-3-OEt

506

136

133

45

3.82

3.10

Ϫ0.24

2.03

10.72

12.83

Ϫ6.39

Ϫ5.39

[a]

Temperature dependence was assumed to be negligible.^[42]

3568

Eur. J. Org. Chem. 2001, 3559Ϫ3573

Substituent Effects of the Thermal Ethenylcyclopropane-to-Cyclopentene Rearrangement

(62.9 MHz), AW-250 (62.9 MHz) and Varian VXR 200 (50.3 MHz)

FULL PAPER

(20.17 MHz, CDCl₃): δ ϭ 0.1 (q), 15.2 (t), 16.0 (s), 19.4 (m), 82.1

spectrometers at ambient temperature; δ ϭ 77.0 for CDCl₃. Signal (s), 108.2 (s). Ϫ GC/EIMS (70 eV): m/z (%) ϭ 184 (100) [M^ϩ], 169

multiplicity was determined by the DEPT method and is given as [M^ϩϪ CH₃], 141 [M^ϩϪ SiCH₃], 129.

follows: ϩ ϭ primary or tertiary, Ϫ ϭ secondary and C_quatϭ qua-

1-Ethynyl-1-methylthiocyclopropane (6-SMe): Potassium carbonate

ternary carbon atoms; * designates interchangeable assignments. Ϫ

5.0 g (36 mmol) was added to a solution of 1-methylthio-1-(trime-

thylsilylethynyl)cyclopropane (3.5 g, 19 mmol) in 30 mL of meth-

MS: Varian MAT CH-7 and MAT 311 A (high resolution) spectro-

meter. Ϫ Analytical GC: Siemens Sichromat 4 (25 m capillary col-

anol, and the mixture stirred at room temp. for 4 days. The progress

umn with CP-Sil-5-CB, carrier gas H₂). Preparative GC: Varian

of the reaction was monitored by gas chromatography. The mixture

Aerograph 920 with 3/8" Teflon columns [1.5 m, 10 (15) % DC-

710 on Chromosorb W-AW-DMCS (60Ϫ80 mesh), carrier gas H₂].

Retention times (R_t) [min]. Ϫ Combustion analyses: Beller Micro-

was diluted with 20 mL of water and then extracted with five por-

tions of pentane (10 mL each), the combined extracts were washed

with saturated NaCl solution (20 mL), and dried with MgSO₄. The

solvent was removed by distillation through a 40-cm column

analytical Laboratory, Universität Göttingen. Starting materials 1-

ethenyl-1-trimethylsilylcyclopropane (1-SiMe₃),^[22a]1-ethynyl-1-

packed with glass helices and the residue distilled through a 20-cm

methylthiocyclopropane (6-SMe),^[22g](E)-2-ethoxy-1-ethynylcyclo-

Vigreux column to yield 1.7 g (79%) of 1-ethynyl-1-methylthiocy-

propane (E)-10,^[25]3-diazopropene (7)^[41]and (E/Z)-2-bromo-1-

clopropane as a colorless liquid; bp 86Ϫ88 °C. Ϫ ¹H NMR

ethenylcyclopropane [(E/Z)-12]^[29]were prepared according to

(270 MHz, CDCl₃): δ ϭ 1.05Ϫ1.12 (m, 2 H), 1.27Ϫ1.33 (m, 2 H),

known literature procedures and were freshly distilled prior to use

(except for 7). All reactions were performed in anhydrous solvents

2.10 (s, 1 H), 2.31 (s, 3 H). Ϫ ¹³C NMR (67.9 MHz, CDCl₃): δ ϭ

15.2 (s and q), 19.0 (m), 65.8 (d), 86.4 (s).

and under nitrogen.

1-Ethenyl-1-methylthiocyclopropane (1-SMe): A solution of 1-ethy-

1-Chloro-1-(trimethylsilylethynyl)cyclopropane:^[42]To a solution of

nyl-1-methylthiocyclopropane (6-SMe) (3.00 g, 26.7 mmol) in

1-chloro-1-(trichloroethenyl)cyclopropane (5)^[22f](45.0 g, 0.22 mol)

200 mL of n-pentane was placed in a Parr hydrogenator (volume

in 300 mL of dry diethyl ether was added dropwise with stirring at

0.5 L) and a slow flow of nitrogen passed through the solution for

Ϫ78 °C a 1.3  methyllithium solution in diethyl ether (390 mL,

5 min. Lindlar catalyst (444 mg, 0.77 mol-% Pd) and quinoline (104

0.5 mol) at such a rate that the temperature did not exceed Ϫ60

µL, 3 mol-%) were then added, the hydrogenator flushed several

°C. After 1 h, the dark mixture was warmed to room temp. and

times with H₂and the H₂pressure raised to 3.5 bar at 20 °C (con-

stirred for an additional 1 h, then cooled to 0 °C, and trimethylsilyl

tinuous H₂feed). Several checks on the slow conversion were car-

chloride (freshly distilled from calcium hydride; 70 g, 0.65 mol) was

ried out during the reaction (GC). After complete conversion

added slowly. After the mixture had been stirred for an additional

(144 h), the reaction was immediately stopped and the mixture fil-

12 h at room temp., the light brown suspension was cooled to 0

tered through a short Celite pad. The solvent was distilled through

°C and hydrolyzed with 250 mL of saturated NH₄Cl solution. The

a 50-cm packed column and the residue trap-to-trap-distilled in

organic phase was separated, and the aqueous phase extracted with

vacuo yielding 2.99 g (97%) of 1-SMe as a colorless liquid. A small

three portions of diethyl ether (100 mL each). The combined or-

sample was separated from solvent traces by preparative GC for

ganic phases were dried with MgSO₄. The solvent was removed by

Ϫ1

˜

analytical characterization. Ϫ IR (film): ν ϭ 3084 cm (CϭCH),

distillation through a 40-cm column packed with glass helices, and

the black residue was bulb-to-bulb-distilled at 0.1 Torr. The color-

less liquid was redistilled under reduced pressure through a 20-cm

Vigreux column to yield 28 g (74%) of 1-chloro-1-(trimethylsilyl-

ethynyl)cyclopropane; bp 64 °C/20 Torr. Ϫ ¹H NMR (270 MHz,

CDCl₃): δ ϭ 0.45 (s, 9 H), 1.32 (m, 4 H). Ϫ ¹³C NMR (20.15 MHz,

CDCl₃): δ ϭ 0.2, 20.4, 29.3, 86.5, 105.6. Ϫ IR (film): ν˜ ϭ 3090

cm^Ϫ1, 3010, 2170, 1240, 860, 840. Ϫ EIMS (70 eV): m/z (%) ϭ 174/

3003, 2918 (CH), 1633 (CϭC), 1419, 1250, 1171, 1024, 987, 909,

860. Ϫ ¹H NMR (250 MHz, CDCl₃): δ ϭ 0.95 [m_c, AAЈ part of

an AAЈBBЈ system, 2 H, 2(3)-H_A], 1.05 [m_c, BBЈ part of an AAЈBBЈ

system, 2 H, 2(3)-H_B], 2.07 (s, 3 H, SCH₃), 5.04 (dd, ²J ϭ 1.6,

3

³J_2Ј(E),1Јϭ 10.0 Hz, 1 H, 2Ј-H_E), 5.25 (dd, ²J ϭ 1.6, J_1Ј,2Ј(Z)

ϭ

16.8 Hz, 1 H, 2Ј-H_Z), 5.57 (dd, ³J_1Ј,2Ј(E)ϭ 10.0, ³J_1Ј,2Ј(Z)ϭ 16.8 Hz,

1 H, 1Ј-H). Ϫ ¹³C NMR (62.9 MHz, CDCl₃): δ ϭ 14.5 (ϩ, SCH₃),

17.1 [Ϫ, C-2(3)], 27.9 (C_quat, C-1), 113.4 (Ϫ, C-2Ј), 140.3 (ϩ, C-1Ј).

Ϫ MS (EI, 70 eV): m/z (%) ϭ 114 (23) [M^ϩ], 99 (22) [M^ϩϪ Me],

71 (38), 67 (100) [M^ϩϪ SMe], 65 (41), 61 (15), 53 (10), 47 (8)

[SMe^ϩ], 45 (31), 41 (39). Ϫ C₆H₁₀S (114.21): calcd. C 63.10, H

8.83, S 28.08; found C 63.25, H 8.92, S 27.96.

172 (1.2/4.5) [M^ϩ], 159/157 (76) [M^ϩϪ CH₃], 136 (1.2) [M^ϩ

Ϫ

HCl], 119/117 (100), 95/93 (82), 81/79 (27).

1-Methylthio-1-(trimethylsilylethynyl)cyclopropane:^[22g]A 1.4  n-

butyllithium solution in n-hexane (22 mL, 31 mmol) in 70 mL of

dry diethyl ether was added with stirring at Ϫ78 °C to a solution

of 1-chloro-1-(trimethylsilylethynyl)cyclopropane (5.0 g, 29 mmol).

The mixture was kept at Ϫ78 °C for 45 min, then warmed to room

temp., and after another 45 min cooled to Ϫ78 °C again, where-

upon methyl methanethiosulfonate (3.6 mL, 35 mmol) was added.

The reaction mixture was stirred at room temp. for an additional

1 h, then hydrolyzed with 100 mL of saturated NH₄Cl solution.

The organic phase was separated, the aqueous layer extracted with

three portions of diethyl ether (50 mL each) and the combined or-

ganic phases were dried with MgSO₄. The solvents were removed

by distillation through a 40-cm column packed with glass helices

and the crude product was distilled under reduced pressure to yield

4.8 g (90%) of 1-methylthio-1-(trimethylsilylethynyl)cyclopropane

General Procedure for the Hydrogenation of Ethynylcyclopropanes

6-SiMe₃and 10:^[23a]In a hydrogenator (equipped with a burette for

the determination of hydrogen consumption) a mixture of 1 mmol

of ethynylcyclopropane in 10 mL of n-pentane, the specified

amount of Lindlar catalyst and quinoline was first flushed with

nitrogen and subsequently with hydrogen (continuous H₂feed). Ini-

tiation of the reaction was performed by vigorous agitation at 20

°C and immediately stopped after total ethynylcyclopropane con-

version (consumption of the equivalent volume-measured amount

of H₂). The mixture was filtered through a short Celite pad and

the solvent distilled through a 50-cm packed column. Trap-to-trap

distillation of the residue in vacuo yielded the products as colorless

liquids. A small sample was separated from solvent traces by pre-

parative GC for analytical characterization.

˜

as a colorless liquid; bp 90Ϫ92 °C/12 Torr. Ϫ IR (film): ν ϭ

3080 cm^Ϫ1, 2160 (CϵC), 1440 (SiCH₃), 1270, 1250, 1160, 935, 870,

1

840, 760, 655. Ϫ H NMR (270 MHz, CDCl₃): δ ϭ 0.13 (s, 9 H), 1-Ethenyl-1-trimethylsilylcyclopropane (1-SiMe₃): 1-Ethynyl-1-tri-

1.02Ϫ1.08 (m, 2 H), 1.24Ϫ1.30 (m, 2 H), 2.28 (s, 3 H). Ϫ ¹³C NMR methylsilylcyclopropane (6-SiMe₃; 2.33 g, 16.8 mmol),^[22a]Lindlar

Eur. J. Org. Chem. 2001, 3559Ϫ3573

3569

G. McGaffin, B. Grimm, U. Heinecke, H. Michaelsen, A. de Meijere, R. Walsh

FULL PAPER

catalyst (233 mg, 0.65 mol-% Pd) and quinoline (15 µL, 0.70 mol- 15 min, S-methyl methanethiosulfonate (8.6 g, 68 mmol) was added

%) were allowed to react according to the General Procedure for and the mixture left to warm to room temp. The mixture was then

20 min, yielding 2.00 g (85%) of 1-SiMe₃(consumption of 377 mL

poured into 300 mL of satd. NH₄Cl solution, diluted with 100 mL

of H₂). Ϫ IR (film): ν˜ ϭ 3069 cm^Ϫ1(CϭCH), 2995, 2957 (CH), of ether and the phases were separated. The organic phase was

1

1626 (CϭC), 1249, 1189, 1022, 991, 836, 748, 689, 663, 637. Ϫ H washed twice with 80 mL of satd. NaCl solution and dried with

NMR (250 MHz, CDCl₃): δ ϭ Ϫ0.04 [s, 9 H, Si(CH₃)₃], 0.55 [s,

MgSO₄. The solvent was distilled through a 50-cm packed column

A₄system, 4 H, 2(3)-H], 4.87 (m, 1 H, 2Ј-H_E), 4.92 (m, 1 H, 2Ј- and the residue trap-to-trap-distilled to yield 5.9 g (76%) of pure

H_Z), 5.91 (m, 1 H, 1Ј-H). Ϫ ¹³C NMR (62.9 MHz, CDCl₃) δ ϭ

Ϫ3.3 [ϩ, Si(CH₃)₃], 9.6 [Ϫ, C-2(3)], 11.4 (C_quat, C-1), 112.9 (Ϫ, C-

(E/Z)-3-SMe [E/Z ϭ 1.0:2.8, NMR, GC] as a colorless liquid. A

sample of the diastereomeric mixture was separated by preparative

2Ј), 142.9 (ϩ, C-1Ј). Ϫ MS (EI, 70 eV): m/z (%) ϭ 140 (2) [M^ϩ], GC (80 °C) to give pure (E)-3-SMe, de 98.6% and pure (Z)-3-SMe,

125 (3) [M^ϩϪ Me], 73 (100) [SiMe₃^ϩ], 59 (18), 45 (17), 43 (18). Ϫ de 97.5%.

C₈H₁₆Si (140.30): calcd. C 68.49, H 11.50; found C 68.44, H 11.44.

(E)-3-SMe: Fraction I, t_Rϭ 9. Ϫ ¹H NMR (250 MHz, CDCl₃):

(E)-1-Ethenyl-2-ethoxycyclopropane [(E)-3-OEt]: Diastereomer-

δ ϭ 0.95 (m, 2 H, 3-H_Zand 3-H_E), 1.60 (m, 1 H, 2-H), 1.89 (m,

4

ically pure (E)-2-ethoxy-1-ethynylcyclopropane^[25][(E)-10] (5.46 g, ³J_1,1Јϭ 7.9, J_1,2Ј(E)ϭ 0.4, J_1,2Ј(Z)ϭ 0.6 Hz, 1 H, 1-H), 2.15 (s, 3

3

2

4

49.5 mmol), Lindlar catalyst (300 mg, 0.28 mol-% Pd) and quinol-

H, SCH₃), 4.91 (3d, J_2Ј(E),1Јϭ 10.2, J ϭ 2.0, J_2Ј(E),1ϭ 0.4 Hz, 1

ine (32 µL, 0.50 mol-%) were allowed to react according to the H, 2Ј-H_E), 5.09 (3d, ³J_2Ј(Z),1Јϭ 17.5, J ϭ 2.0, J_2Ј(Z),1ϭ 0.6 Hz, 1

2

4

3

General Procedure for 1 h, yielding 4.72 g (85%) of pure (E)-3-OEt, H, 2Ј-H_Z), 5.43 (3d, J_1Ј,2Ј(Z)ϭ 17.5, J_1Ј,2Ј(E)ϭ 10.2, J_1Ј,1

ϭ

de 100.0% (consumption of 1109 mL of H₂). Ϫ IR (film): ν ϭ 3084 7.9 Hz, 1 H, 1Ј-H). Ϫ ¹³C NMR (62.9 MHz, CDCl₃): δ ϭ 16.4 (ϩ,

cm^Ϫ1(Cp-H), 2977, 2931, 2872 (CH), 1637 (CϭC), 1446, 1374, C-2), 16.5 (Ϫ, C-3), 23.1 (ϩ, C-1), 26.2 (ϩ, SCH₃), 113.3 (Ϫ, C-

1351, 1300, 1257, 1190, 1121, 1087, 898, 818. Ϫ ¹H NMR 2Ј), 139.4 (ϩ, C-1Ј).

˜

(500 MHz, CDCl₃): δ ϭ 0.71 (3d, ²J ϭ 5.8, ³J_3(E),1ϭ 6.2, ³J_2(E),2ϭ

(Z)-3-SMe: Fraction II, t_Rϭ 12. Ϫ IR (film): ν ϭ 3080 cm^Ϫ1(Cp-

˜

3

6.2 Hz, 1 H, 3-H_E), 1.01 (3d, ²J ϭ 5.8, J_3(Z),1ϭ 9.9, J_3(Z),2

ϭ

H), 2999, 2917 (CH), 1634 (CϭC), 1435, 1280, 1211, 1038, 992,

3.5 Hz, 1 H, 3-H_Z), 1.20 (t, 3 H, OCH₂CH₃), 1.58 (6d, ³J_1,2ϭ 2.4,

1

3

942, 895. Ϫ H NMR (250 MHz, CDCl₃): δ ϭ 0.66 (3d, J_3(Z),1

ϭ

3

4

³J_1,3(E)ϭ 6.2, J_1,3(Z)ϭ 9.9, J_1,1Јϭ 8.1, J_1,2Ј(E)ϭ 0.4, J_1,2Ј(Z)

ϭ

5.8, ³J_3(Z),2ϭ 5.6, ²J ϭ 5.2 Hz, 1 H, 3-H_Z), 1.22 (3d, ³J_3(E),2ϭ 8.2,

3

0.8 Hz, 1 H, 1-H), 3.15 (3d, J_2,1ϭ 2.4, J_2,3(E)ϭ 6.2, J_2,3(Z)

ϭ

³J_3(E),1ϭ 8.2, ²J ϭ 5.2, 1 H, 3-H_E), 1.77 (5d, ³J_1,1Јϭ 8.9, ³J_1,3(E)ϭ

3.5 Hz, 1 H, 2-H), 3.55 (q, 2 H, OCH₂CH₃), 4.88 (dd, ²J ϭ 1.6,

3

4

8.2, J_1,2ϭ 7.0, J_1,3(Z)ϭ 5.8, J_1,2Ј(Z)ϭ 0.4 Hz, 1 H, 1-H), 2.11

(s, 3 H, SCH₃), 2.17 (3d, ³J_2,3(E)ϭ 8.2, ³J_2,1ϭ 7.0, ³J_2,3(Z)ϭ 5.6 Hz,

1 H, 2-H), 5.07 (dd, ³J_2Ј(E),1Јϭ 10.3, ²J ϭ 1.9 Hz, 1 H, 2Ј-H_E), 5.21

³J_2Ј(E),1Јϭ 10.3 Hz, 1 H, 2Ј-H_E), 4.99 (dd, ²J ϭ 1.6, J_2Ј(Z),1Ј

ϭ

3

17.1 Hz, 1 H, 2Ј-H_Z), 5.51 (3d, J_1Ј,1ϭ 8.1, J_1Ј,2Ј(E)ϭ 10.3,

³J_1Ј,2Ј(Z)ϭ 17.1 Hz, 1 H, 1Ј-H). Ϫ ¹³C NMR (62.9 MHz, CDCl₃):

δ ϭ 14.1 (Ϫ, C-3), 15.2 (ϩ, OCH₂CH₃), 22.5 (ϩ, C-1), 60.4 (ϩ, C-

2), 66.0 (Ϫ, OCH₂CH₃), 112.5 (Ϫ, C-2Ј), 138.4 (ϩ, C-1Ј). Ϫ MS

(EI, 70 eV): m/z (%) ϭ 85 (5) [M^ϩϪ C₂H₃], 84 (22), 83 (18) [M^ϩ

Ϫ Et], 67 (10) [M^ϩϪ OEt], 55 (100) [M^ϩϩ 1 Ϫ CHOEt], 41 (50).

(3d, ³J_2Ј(Z),1Јϭ 17.1, J ϭ 1.9, J_2Ј(Z),1ϭ 0.4 Hz, 1 H, 2Ј-H_Z), 5.81

2

4

3

(3d, J_1Ј,2Ј(Z)ϭ 17.1, J_1Ј,2Ј(E)ϭ 10.3, J_1Ј,1ϭ 8.9 Hz, 1 H, 1Ј-H).

Ϫ

¹³C NMR (62.9 MHz, CDCl₃): δ ϭ 15.1 (Ϫ, C-3), 18.2 (ϩ, C-

2), 22.3 (ϩ, C-1), 22.6 (ϩ, SCH₃), 115.1 (Ϫ, C-2Ј), 137.4 (ϩ, C-1Ј).

Ϫ MS (EI, 70 eV): m/z (%) ϭ 114 (6) [M^ϩ], 99 (37) [M^ϩϪ Me],

(Z)-1-Ethenyl-2-ethoxycyclopropane [(Z)-3-OEt]:^[24]A precooled 75 (44), 67 (78) [M^ϩϪ SMe], 66 (100), 65 (49), 47 (11) [SMe^ϩ], 45

(Ϫ30 °C) 0.4



solution of 3-diazopropene (7; 37.8 mL,

(42), 41 (59). Ϫ HRMS: calcd. for C₆H₁₀S [M^ϩ] 114.0503, found

15.1 mmol)^[41]in ether at 0 °C was slowly added (7 h) to a vigor-

ously stirred suspension of Rh₂(OAc)₄(6.7 mg, 0.1 mol-%) in

ethenyl ethyl ether (8; 33.50 g, 465 mmol). The mixture was stirred

for 8 h. The excess alkene was trap-to-trap-distilled in vacuo at 0

°C and 100 mL of n-pentane was added to the residue. After filtra-

tion of the mixture through a short Celite pad, the solvent was

distilled through a 50-cm packed column and the residue slowly

trap-to-trap-distilled in vacuo yielding 1.05 g (62%) of (E/Z)-3-OEt

[E/Z ϭ 1:3.5, NMR, GC] as a colorless liquid. A sample of the

diastereomeric mixture was separated by preparative GC (25 °C) to

114.0503.

Kinetic Measurements

General: Gas-phase thermolyses were performed in a hexamethyl-

disilazane (HMDS)-conditioned (24 h) static reaction vessel con-

nected to a vacuum line similar to the previously described appar-

atus.^[20]Conditioning was repeated after approx. 70Ϫ100 thermo-

lytic runs. Prior to all GC runs with the thermolyzed reaction mix-

tures, reactant composition stability checks and mass recovery GC

checks were carried out by injecting and analyzing a sample of the

reactant mixture [ϭ master mixture, containing defined amounts

of the reactant, cyclohexane (CH) as an internal standard and di-

luted with nitrogen, see Table 11]. For checks on possible surface

catalysis and on radical chain components the reaction vessel was

exchanged for a spherical glass-tube-packed reaction vessel with an

approximate surface (S) to volume (V) ratio S/V of 10 cm^Ϫ1. All

reactant and reaction mixture samples were pressurized to

100Ϫ340 Torr with nitrogen before GC analysis.

1

yield pure (Z)-3-OEt, de 98.0% (t_Rϭ 15). Ϫ H NMR (500 MHz,

2

3

CDCl₃): δ ϭ 0.71 (3d, J ϭ 6.2, J_3(Z),2ϭ 3.7, J_3(Z),1ϭ 6.2 Hz, 1

2

3

H, 3-H_Z), 0.94 (3d, J ϭ 6.2, J_3(E),2ϭ 6.3, J_3(E),1ϭ 9.2 Hz, 1 H,

3-H_E), 1.20 (t, 3 H, OCH₂CH₃), 1.49 (4d, J_1,2ϭ 6.2, J_1,3(Z)

6.2, J_1,3(E)ϭ 9.2, J_1,1Јϭ 9.4 Hz, 1 H, 1-H), 3.40 (3d, J_2,3(Z)

3

ϭ

3

ϭ

3

3.7, J_2,3(E)ϭ 6.3, J_2,1ϭ 6.2 Hz, 1 H, 2-H), 3.52 (q, 2 H,

OCH₂CH₃), 5.00 (dd, ²J ϭ 2.0, J_2Ј(E),1Јϭ 10.4 Hz, 1 H, 2Ј-H_E),

3

5.18 (dd, ²J ϭ 2.0, J_2(Z),1Јϭ 17.2 Hz, 1 H, 2Ј-H_Z), 5.62 (3d,

3

³J_1Ј,2Ј(Z)ϭ 17.2, ³J_1Ј,1ϭ 9.4, ³J_1Ј,2Ј(E)ϭ 10.4 Hz, 1 H, 1Ј-H). Ϫ ¹³C

NMR (62.9 MHz, CDCl₃): δ ϭ 13.6 (Ϫ, C-3), 15.1 (ϩ, OCH₂CH₃),

21.9 (ϩ, C-1), 57.8 (ϩ, C-2), 66.2 (Ϫ, OCH₂CH₃), 113.8 (Ϫ, C-2Ј),

136.6 (ϩ, C-1Ј).

Time, temperature and pressure dependencies were investigated for

all ethenylcyclopropanes. Up to sixteen individual pyrolyses were

performed for each measured temperature and approximately equi-

distant temperatures were selected over a range of 50 °C (ca. 10 °C

intervals). Kinetics were based on reactant disappearance and good

linear first-order graphs (least mean-squares procedure) were ob-

tained for all ethenylcyclopropanes. For the straightforward decom-

(E)- and (Z)-1-Ethenyl-2-methylthiocyclopropane [(E)-, (Z)-3-SMe]:

A 1.5  solution of tBuLi in pentane (91.3 mL, 137 mmol) was

added to a stirred and cooled (Ϫ78 °C) solution of (E/Z)-2-bromo-

1-ethenylcyclopropane [(E/Z)-12]^[29][10.0 g, 68 mmol; (E)/(Z) ϭ position reactions of 1-X (X ϭ SiMe₃, SMe) individual k values

1.0:2.8, NMR, GC] in 100 mL of THF and 50 mL of ether. After were derived from the slopes of the first-order graphs. From these,

3570

Eur. J. Org. Chem. 2001, 3559Ϫ3573

Substituent Effects of the Thermal Ethenylcyclopropane-to-Cyclopentene Rearrangement

FULL PAPER

Table 11. Physical properties, operational parameters and gas-handling conditions in the ECP-CP rearrangements of ethenylcyclopropanes

1-X and (E)-, (Z)-3-X

ECP

Initial conc. of

Vapor pressure

[Torr]^[b]

Reaction pressure

[Torr]^[c]

Conversion of

ECP (%)^[c]

Temp. [°C]

ECP (CH) (%)^[a]

1-SMe

0.20 (0.50)

1.03 (0.65)

0.66 (0.80)

0.80 (0.80)^[d]

0.58 (0.19)^[d]

0.53 (0.16)^[d]

7

12Ϫ15

15Ϫ25

18Ϫ22

18Ϫ25

14Ϫ21

7.4Ϫ92.8

6.5Ϫ89.9

16.3Ϫ94.0

7.6Ϫ94.3

7.6Ϫ84.7

24.4Ϫ94.0

289.9Ϫ341.0

313.3Ϫ363.1

221.0Ϫ278.1

220.8Ϫ277.6

239.3Ϫ289.1

239.3Ϫ289.2

1-SiMe₃

12

15

10

4

(E)-3-OEt

(Z)-3-OEt

(E)-3-SMe

(Z)-3-SMe

3

[a]

[b]

[c]

Averaged over all master mixtures. Ϫ

At room temp. Ϫ

Maximum and minimum limits given over all measured time and

[d]

temperature intervals. Ϫ Diastereomeric impurities neglected.

Arrhenius plots (Figure 1) were then produced and the activation

at 395.8 °C). Ϫ ¹H NMR (200 MHz, CDCl₃): δ ϭ 0.57 [s, 9 H,

parameters determined. For (E)- and (Z)-3-X (X ϭ OEt, SMe) a Si(CH₃)₃], 1.80 (m, 2 H, 4-H), 2.38 [m, 4 H, 3(5)-H], 5.97 (m, 1 H,

more sophisticated computer-based refinement proved to be neces-

1-H). Ϫ ¹³C NMR (50.3 MHz, CDCl₃): δ ϭ Ϫ0.3 [ϩ, Si(CH₃)₃],

sary in terms of determining k values, employing Gear^[30,32]and 25.4 (Ϫ, C-4), 37.2 (Ϫ, C-5*), 38.4 (Ϫ, C-3*), 141.7 (ϩ, C-2), 145.9

Marquardt^[31,33]algorithms. Each temperature-dependent set of

analytical data for the two individually pyrolyzed isomers of both

compounds was examined and the four k values (k₁to k₄) varied

until a minimum of least-squares differences (Σx², see Table 4) be-

tween observed and calculated data were achieved. The Arrhenius

parameters for all four steps of each reaction were obtained as

mentioned above. All reactions proved to be pressure- and surface-

independent. Table 11 contains practical conditions of operation

for each individually investigated compound.

(C_quat, C-1).

4-Ethoxycyclo-1-pentene (4-OEt): Four independent quantitative

pyrolyses were performed with (Z)-3-OEt (20-min runs at 298.4

°C). Ϫ ¹H NMR (200 MHz, CDCl₃): δ ϭ 1.20 (t, 3 H, OCH₂CH₃),

2.48 [m, ²J ϭ 17.0 Hz, 4 H, 3(5)-H], 3.47 (q, 2 H, OCH₂CH₃), 4.20

(m_c, 1 H, 4-H), 5.68 [s, 2 H, 1(2)-H]. Ϫ ¹³C NMR (50.3 MHz,

CDCl₃): δ ϭ 15.5 (ϩ, OCH₂CH₃), 39.3 [Ϫ, C-3(5)], 64.1 (Ϫ,

OCH₂CH₃), 79.0 (ϩ, C-4), 128.4 [ϩ, C-1(2)].

4-Methylthiocyclo-1-pentene (4-SMe): Five independent quantitat-

ive pyrolyses were performed with (Z)-3-SMe (30-min runs at 315.3

°C). Ϫ ¹H NMR (200 MHz, CDCl₃): δ ϭ 2.12 (s, 3 H, SCH₃), 2.36

[m_c, 2 H, 3(5)-H], 2.79 [m_c, 2 H, 3(5)-H], 3.39 (m_c, 1 H, 4-H), 5.71

[s, 2 H, 1(2)-H)]. Ϫ ¹³C NMR (50.3 MHz, CDCl₃): δ ϭ 14.3 (ϩ,

SCH₃), 40.2 [Ϫ, C-3(5)], 42.4 (ϩ, C-4), 129.2 [ϩ, C-1(2)].

Analysis: GC analyses were performed with PerkinϪElmer 8310 [1-

SMe, (E)- and (Z)-3-OEt] and PerkinϪElmer F 33 [1-SiMe₃, (E)-

and (Z)-3-SMe] gas chromatographs with FID, and separations

were carried out on 4 m ϫ 3 mm [(E)-3-OEt, 1-SMe] and 7 m ϫ

3 mm [(Z)-3-OEt, 1-SiMe₃, (E)- and (Z)-3-SMe] silicon oil columns

(15% MS 550 on 60/80 Chromosorb P). In all cases well-defined

peaks and excellent baseline separations were achieved. Retention

times (t_R) [min]. Column temperatures were 50 °C for analyses of

(E)-3-OEt [t_Rϭ 4.35, CH; 15.06, (E)-3-OEt; 17.88, (Z)-3-OEt;

21.37, 4-OEt], 95 °C for 1-SMe (t_Rϭ 1.73, CH; 5.32, 1-SMe; 14.73,

2-SMe), 80 °C for (Z)-3-OEt [t_Rϭ 6.10, CH; 15.21, (E)-3-OEt;

17.40, (Z)-3-OEt; 20.53, 4-OEt], 110 °C for 1-SiMe₃(t_Rϭ 4.39,

CH; 7.64, 1-SiMe₃; 10.83, 2-SiMe₃) and 120 °C for (E)- and (Z)-

3-SMe [t_Rϭ 4.37, CH; 11.31, (E)-3-SMe; 13.76, (Z)-3-SMe; 16.44,

4-SMe]. Constant gas pressures were 123 kPa (nitrogen carrier gas),

133 kPa (hydrogen) and 81 kPa (air) for studies of 1-SiMe₃, (Z)-3-

OEt, (E)- and (Z)-3-SMe and 166 kPa (nitrogen), 174 kPa (hydro-

gen) and 152 kPa (air) for 1-SMe and (E)-3-OEt.

Acknowledgments

This work was supported by the Deutsche Forschungsgemeinschaft

(SFB 357, Project A14), the Fonds der Chemischen Industrie and

by a grant within the Academic Research Collaboration (ARC)

Programme administered by the British Council and the Deutscher

Akademischer Austauschdienst (DAAD). The authors are indebted

to Dr. B. Knieriem, Göttingen, for his careful reading of the final

manuscript.

The columns described above were conditioned with HMDS (10 h,

150 °C) approximately after every 20 h of performance. All isomeric

compounds were assumed to possess identical FID response fac-

tors. The characterization of all products under reaction conditions

was achieved by freezing thermolyzed and totally converted react-

ant samples (without internal standard) from the line and recording

the corresponding NMR spectra.

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Substituent Effects of the Thermal Ethenylcyclopropane-to-Cyclopentene Rearrangement

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1988.

Received March 12, 2001

[O01120]

Eur. J. Org. Chem. 2001, 3559Ϫ3573

3573

Article Doi

On the substituent effects of the thermal ethenylcyclopropane-to-cyclopentene rearrangement: Gas-phase kinetics of ethoxy-, methylthio-and trimethylsilyl-substituted ethenylcyclopropanes

DOI: 10.1002/1099-0690(200109)2001:18<3559::AID-EJOC3559>3.0.CO;2-V

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