Organometallics 1986, 5, 431-435 g, 1.15 mmol) was dissolved in CH2C12and extracted with water containing Na2S.9H20 (1.0 g, 4.16 mmol). Acetophenone (0.27 g, 98%) was the only product contained in the organic phase. When diphenacyl telluride (la) was similarly treated, acetophenone was isolated in 96% yield.
Acknowledgment. Financial support by the Swedish Natural Science Research Council is gratefully acknowledged. Registry No. la, 99766-21-9;lb, 99766-22-0;IC,99766-23-1; Id, 99766-24-2; le, 99766-25-3; If, 99766-26-4; lg, 99766-27-5; lh, 99766-28-6; li, 99766-29-7; lj, 99766-30-0;2a, 36362-83-1;2b, 77017-00-6; 212, 99766-31-1;2d, 99766-32-2; 2e, 66839-13-2; 2f, 99766-33-3; 2g, 77840-07-4;2h, 73537-53-8;2i, 99766-34-4;2j,
431
99766-35-5; 3a, 67096-37-1; 6b, 99766-36-6; 8, 99766-37-7; 9, 3674-77-9;10,99766-38-8;1IC, 99766-39-9;12a, 98797-28-5;12b, 99766-40-2;13 (isomer l),99766-41-3;13 (isomer 2), 99766-42-4; 14a, 99766-43-5;14b, 99766-44-6;17, 99766-45-7;acetophenone, 98-86-2; 4-methylacetophenone, 122-00-9;4-bromoacetophenone, 99-90-1;4-chloroacetophenone,99-91-2;4-methoxyacetophenone, 100-06-1; 1-(1-naphthyl)ethanone, 941-98-0; 1-(2-naphthyl)ethanone, 93-08-3; 1-(2-thienyl)ethanone, 88-15-3; 1-(9anthryl)ethanone, 784-04-3; 1-(2-benzo[b]furanyl)ethanone, 1646-26-0; phenacyl bromide, 70-11-1; 2-(bromomethyl)-2phenyl-l3-dioxolane,341821-1;di-Zthienyl ditelluride, 66697-24-3; cyclooctanone,502-49-8;cyclohexanone,108-94-1;cycloheptanone, 502-42-1;ethyl bromoacetate, 105-36-2;(4-methoxypheny1)tellurium trichloride, 36309-68-9; 2-naphthyltellurium trichloride, 71578-23-9.
Kinetics and Mechanism of Pyrolysis of 1,3-Disilacyclobutane, 1,3-Dlmethyl-l,3-disilacyclobutane, and 1,1,3,3-Tetramethyl-I ,3-disilacyclobutane in the Gas Phase Norbert Auner Anorganisch-Chemisches Znstitut, Westfalische Wlihelms-Universltat, 4400 Miinster, Federal Republic of Germany
Iain M. T. Davidson," Sina Ijadi-Maghsoodi, and F. Timothy Lawrence Department of Chemistry, The University, Leicester LE1 7RH, Great Britain Received June 11, 1985
Kinetic data and product analyses are reported for the pyrolysis of the title compounds. Pyrolysis mechanisms are discussed with reference to recent developments in organosilicon chemistry.
Introduction There has been considerable interest in the kinetics and mechanism of pyrolysis of silacyclobutanes. While 1,ldimethylsilacyclobutane is well-known to undergo pyrolysis cleanly to give ethene and dimethylsilene, Me2Si=CH2, which dimerizes quantitatively to 1,1,3,3-tetramethyl-1,3disilacyclobutane (1),l the pyrolysis mechanisms of 1methylsilacyclobutane2 and silacy~lobutane~ are considerably more complex, with good evidence for the involvement of silylene intermediates. It has now been established that these complications arise partly because hydridosilenes isomerize reversibly to silylenes4aand partly because hydridosilacyclobutanes can decompose by a silylene-forming 1,2-hydrogen shift as well as by silene formation.6 Kinetic data for these hydridosilacyclobutanes have been reported.6 Less attention has been paid to disilacyclobutanes. Pyrolysis of 1 is known to be complex, yielding methane, C2 hydrocarbons, and some high molecular weight produ c t . ~ . ~Pyrolysis ,~ with added benzaldehyde at 700 "C gave (1) Flowers, M. C.; Gusel'nikov, L. E. J. Chem. SOC.B 1968,428,1396. (2) Conlin,.R. T.; Wood, D. L. J. Am. Chem. SOC.1981, 103, 1843. ( 3 ) Conlin, R. T.; Gill, R. S., J. Am. Chem. Soc. 1983, 105, 618. (4) Davidson, I. M. T.; Ijadi-Maghsoodi, S.; Barton, T. J.; Tillman, N. J . Chem. SOC.,Chem. Commun. 1984, 478. (5) Davidson, I. M. T.; Scampton, R. J. J. Organomet. Chem. 1984, 271, 249. (6) Davidson, I. M. T.; Fenton, A.; Ijadi-Maghsoodi, S.; Scampton, R. J.; Auner, N.; Grobe, J.; Tillman, N.; Barton, T. J. Organometallics 1984, 3, 1593. (7) Barton, T. J.; Marquardt, G.; Kilgour, J. A. J. Organomet. Chem. 1975, 85, 317.
evidence for the intermediacy of dimethylsilene,' as did pyrolysis above 600 "C with added methanol, although the product methoxytrimethylsilane might have resulted from direct reaction of 1 with methanol.8 The onset of pyrolysis of 1 was observed a t 600 "C, as opposed to 450 "C for 1,l-dimethylsilacyclobutane;dimethylsilene was not trapped below 600 "C, indicating that the observed thermal stability of 1 was not attributable to an equilibrium between l and dimethylsilene.8 Extensive pyrolysis of l and of 1,l-dimethylsilacyclobutanegave similar product^.^ Analogous results were obtained for the pyrolysis of 1,3dimethyl-1,3-disilacyclobutane(2), 1,3-disilacyclobutane (3), and their monosilacyclobutane c o u n t e r p a r t ~ . ~ J ~ We now report kinetic results and further product analysis for the pyrolysis of these three disilacyclobutanes, enabling some mechanistic conclusions to be drawn in the light of recent developments in organosilicon chemistry.
Experimental Section Product composition after extensive pyrolysis of 2 and 3 was determined at Miinster from pyrolyses in a flow system at ca. mmHg and 850 O C . Products were trapped at low temperature and analyzed by GC, NMR, IR,and GC/MS. Kinetic experiments and product analysis in the early stages of pyrolysis of 1,2, and 3 were carried out at Leicester by low-pressurepyrolysis (LPP)," (8) Nametkin, N. S.; Gusel'nikov, L. E.; Volpina, E. A.; Vdovin, V. M. Dokl. Akad. Nauk SSSR 1975, 220, 386 (English translation, p 81). (9) Auner, N.; Grobe, J. J. Organomet. Chem. 1980, 197, 13. (10) Auner, N.; Grobe, J. 2.Anorg. Allg. Chem. 1979, 459, 15. (11) Davidson, 1. M. T.; Ring, M. A. J. Chem. SOC.,Faraday Trans. 1 1980, 76, 1520.
0276-7333/86/2305-0431$01.50/0 @ 1986 American Chemical Society
432 Organometallics, Vol. 5 , No. 3, 1986
Auner et al.
Table I. Kinetic Data for Decomposition of Disilacyclobutanes reactant 1 2 3
temp I "C 664-746 611-663 516-573
reactant
CH,
1 2
40 30 1
3
log AIS-' 14.4 0.2 13.5 f 0.2 13.3 f 0.3
DressimrnHg 3.0 2.5 2.5
*
E/kJ mol-'
k i w c I s-'
k-1
296 f 3 255 f 3 230 f 4
4.1 x 10-5 2.07 x 10-3 0.05
1 50 1220
Table 11. Relative Yields of Gaseous Products C,H* C,H, MeSiH, 30 13 1
-0
1 1
1
-0
3
Me,SiH,
Me3SiH
1 15 20
2 30 -0
Table 111. Additional Products Detected at 850 O C Pyrolysis of 2
{
SiH,
C4H10
H,C=C=C
H3S'\4
W'\J
Me2lHlS1.
H2
HCzCCH, HC-CSiH3
Lt
HC=CSiMe3
I
a
Pyrolysis of 3
SIH, (HC=CH
H ~ ( M ~ )4S I1
(H2SiCH&
C3H6
H&=C=CH,
Trace quantities in braces
with analysis by quadrupole mass spectrometry, and by pyrolysis in a stirred-flow apparatus (SFR),'*with analysis by GC and by GC/MS.
Results LPP of 1. Preliminary experiment^.'^ These pyrolyses were carried out between 662 and 782 "C, with an initial pressure of 1 of 0.14 mmHg. The main products detected were hydrogen, methane, and ethene. In the presence of equal pressures of hydrogen chloride, trimethylchlorosilane was also formed, in approximately equal amount to the methane; in these experiments the formation of hydrogen and ethene was significantly reduced. Likewise, added methoxytrimethylsilane gave the silene insertion product.I4 1 was found to react slowly with hydrogen chloride in the vacuum line at room temperature, but not in the reaction vessel below pyrolysis temperature. The internal surface of the vacuum line would have more adsorbed water than the reaction vessel; also, the surface of the latter was passivated by pyrolytic deposition. Detailed kinetic measurements were not made, but the half-life for decomposition of 1 a t 677 "c was 66 s, increased to 70 s in the presence of hydrogen chloride. Kinetics of Pyrolysis of 1-3. The kinetics of the first-order decomposition of 1,2, and 3 were measured by LPP, with the results given in Table I. Product Formation. Pyrolysis of 1 in the SFR at 740 "C gave methane, ethene, trimethylsilane, propene, and dimethylsilane, in the approximate ratios of 40:30:2:1:1. Pyrolysis of 2 in the SFR at 676 "C gave trimethylsilane, methane, dimethylsilane, ethene, propene, and methylsilane in the approximate ratios of 3030:15:13:1:1. In LPP experiments, significant quantities of hydrogen (undetectable by our GC) was observed. Pyrolysis of 3 in the SFR at 571 "C gave dimethylsilane, methylsilane, methane, and ethene in the approximate ratios of 20:5:1:1. Hydrogen was again observed in significant quantity by LPP, as were several products of high molecular weight. The foregoing results are summarized in Table 11. (12) Baldwin, A. C.; Davidson, I. M. T.; Howard, A. V. J . C h e n . Soc., Faraday Trans. 1 1975, 71, 972. (13) Lawrence, F. T. Ph.D. Thesis, University of Leicester, 1983. (14) Davidson, I. M. T.; Wood, I. T. J . Chem. Soc.. Chern. Curnmun. 1982, 550.
In all of the above pyrolyses there were also small quantities of other unidentified products. Many of these were identified in Munster in the flow pyrolyses there of 2 and 3 to higher conversion at 850 "C. The new products identified in these experiments are listed in Table 111. For simplicity, the products identified in the SFR experiments (Table 11) are omitted from Table I11 but were, of course, also detected in the flow pyrolyses at 850 "C, in the same order of importance.
Discussion The kinetic data in Table I relate to the overall decomposition of the three disilacyclobutanes. In view of the obvious complexity of these pyrolyses, the Arrhenius parameters should not be identified with primary processes; their main use is to demonstrate the substantial differences in thermal stability between I, 2, and 3, as indicated by the relative rate constants in the final column of Table I. Although these pyrolyses were done at the highest pressure possible in our LPP apparatus, there may still have been some unimolecular fall-off;if so, this would have been most pronounced for 3, with 12 atoms in the molecule, and least for 1, with 24 atoms. Consequently, the differences in thermal stability between 1,2, and 3 may be even greater than indicated in Table I. A basic reason for these differences is that 2 and 3 can undergo the 1,2-hydrogen shift from silicon to carbon with concomitant, formation of a silylene, whereas 1 cannot. This reaction, already shown to be important in the pyrolysis of hydridomonosilacyclobutanes,6was first observed as a minor reaction in the pyrolysis of methylsilane:" MeSiH, :SiH2 + CH,
-
Arrhenius parameters have now been measured or estimated for this type of reaction in the pyrolysis of methylsilane, dimethylsilane, and trimethylsilane. The activation energy increases with increasing methyl substitution at silicon, from 279 kJ mol-l for methylsilane, through 301 for dimethylsilane, to 326 for trimethyl~ilane.'~ The last two methylsilanes are simple, acyclic analogies for 3 and 2, respectively. Alternative primary processes would involve silicon-carbon bond rupture to form radi(15) Ring, M. A,; O'Neal, H. E.; Rickborn, S. F.; Sawrey, B. A. Organometallics 1983, 2, 1891.
Organometallics, Vol. 5, No. 3, 1986 433
Disilacyclobutanes T a b l e IV. A r r h e n i u s P a r a m e t e r s for Ring Opening in the Disilacyclopropane, 4
--
reactn type
H shift, Si H shift, Si
Si C
-
Me shift, Si Si Si Si bond rupture
log A
E/kJ mol-'
kWK/S-l
14.0 14.3 13.3 15.5
209-EO3 301-EO3 282-EO3 337-EO3
3.65 X 10" 3.33 X lo6 4.22 X lo6 4.30 X lo5
" T h e values of KSWK (at the mid temperature of the experiments) are based on a n estimatez0 of 167 kJ mol-' for E03.
cals, the silicon-carbon bond in the ring being weaker than silicon-methyl because of ring strain. Recent estimates of the silicon-methyl bond dissociation energy, which is not strongly sensitive to substituent effects,16vary between 355 and 370 kJ mol-'; from recent work on the pyrolysis of trimethyl~hlorosilane,'~ we now favor 366 kJ mol-'. Hence, the activation energy for the 1,2-hydrogen shift in 3 would be 301-EO4 kJ mol-', where E 0 4 is the element of ring strain released on opening the disilacyclobutane ring. The corresponding figure for 2 would be 326-EO4 kJ mol-', while for silicon-carbon bond breaking in the ring to form a biradical (the main primary route open to l),it would be 366-EO4 kJ mol-'. As E 0 4 is unknown, absolute values are speculative, but the activation energy would be lowest for 3, 25 kJ mol-' higher for 2, and a further 40 kJ mol-' higher for 1. These are almost exactly the differences that were observed experimentally (Table I). While this trend is interesting, we stress our earlier comment that the Arrhenius parameters in Table I should not be identified with the primary reactions, which would undoubtedly have higher A factors and activation energies. In the pyrolysis of 2 and 3, the primary 1,2-hydrogen shift would form silylenes that could insert into the silicon-hydrogen bond of the parent disilacyclobutane but could also cyclize to form disilacyclopropane intermediates, as illustrated below for the pyrolysis of 2.
MeHS1vSIMez Me A M e H H S I V S ' 'IMe2
\ /
Me SiASl
SIH~ H VMeMe
9
\
It H MeSi-SiMen V
4
IMeSipSiMe; j H V H
;
f
H MeS'i SiMez
Hij'i
v
'L _ _ _ _2_ _ _ J'
SiMe3
v
==
H2SiASiMe2
v
5
0
A Me A Me H SI SI S I ~ S I M ~ ~MeglS1Me3 H "MeH
\
H CHZ=SISIM~~
r
A Me H
A
Me
S i v S i S i SiMe3 MeH
Me3SiH t
Me H
AMeMe SivSi Si:
As regards the product composition in the pyrolysis of 2, formation of trimethylsilane and dimethylsilane (Table 11) is particularly significant mechanistically. The range of products obtained in flow pyrolyses at 850 "C (Table Me ,A Me Me H SI SI + :SI SiMe2 MeSi-SiMe2 111) is certainly interesting but not surprising in view of H V H V H v the extensive pyrolysis that took place under these con2 4 ditions. However, it is noteworthy that both 2 and 3 gave silane, SiH4, as a product. Silane would have been unDisilacyclopropanes are well established as key interdetectable by the analytical methods used in the kinetic mediates in intramolecular rearrangements of silylenes,18 experiments. and we have successfully modelled a number of rearrangements involving them by numerical i n t e g r a t i ~ n , ~ , ' ~ The foregoing considerations (Tables I1 and IV) lead to the outline mechanism for the early stages of the pyrolysis including the specific case of 4,5 which has been very of 2 given in Scheme I. reasonably suggested18 as an intermediate in the pyrolytic The silylenes 6, 7, and 8 and the biradical 9 are the formation of 2 and its unsymmetrical isomer, 5, from respective products of the ring-opening reactions of disiMe4Si2 Recently,a we have refiied our earlier estimates5J9 lacyclopropane 4 in the order listed in Table IV. As the of Arrhenius parameters for reactions involving disilacyrapid ring opening of 4 to 6 is balanced by equally rapid clopropanes by applying the methods of thermochemical ring c l ~ s i n g , 4~ and J ~ 6 are essentially equilibrated. Conkinetics.21 The resulting rate parameters for silylenesequently, the reactions forming 7 , 8 , and 9 are not unimforming ring-opening reactions of 4 are in Table IV, top ~ r t a n t . While ~ ~ all three silylenes insert into the siligether with our estimates for bond rupture to form a bicon-hydrogen bond in 2, silylenes 6 and 8 only do so reradical. For the latter, the A factor was taken to be the same as for cyclopropane and 1,1,2,2-tetramethylcyclo- versibly, while the insertion product of 7 into 2 may also decompose to give trimethylsilane, which therefore emerges propane,22while the activation energy is derived from as a major gaseous product (Table 11). Once biradical9 D(Me,Si-SiMe,) in he~amethyldisilane.~~ has been formed, it will dissociate to a silene and a silylene, aided by the silicon-carbon *-bond energy in the silene (16) Walsh, R. Acc. Chem. Res. 1981, 14, 246. and the silylene stabilization energy.I6 These simpler si(17) Davidson, I. M. T.; Dean, C. E., to be submitted for publication. lylenes and silenes provide a further source of methylDean, C. E. Ph.D. Thesis, University of Leicester, 1983. (18) Wulff, W. D.; Goure, W. F.; Barton, T. J. J. Am. Chem. SOC.1978,
100.6236. ~ >.- -. --
(19) Davidson, I. M. T.; Hughes, K. J.; Scampton, R. J. J. Organomet. Chem. 1984, 272, 11. (20) Davidson, I. M. T.; O'Neal, H. E., unpublished work. (21) Benson, S. W. 'Thermochemical Kinetics", 2nd ed.: Wilev: New York, 1976. (22) Robinson, P. J.; Holbrook, K. A. "Unimolecular Reactions"; Wiley: London, 1972; p 189.
(23) Davidson, I. M. T.; Howard, A. V. J . Chem. SOC., Faraday Trans. 1 1975, 71, 69.
(24) A reviewer has pointed out that Scheme I includes reactions such as the methyl shift converting 4 to 8, previously dismissed5 as being insignificantly slow; there is no inconsistency in that because in the previous case5 there was another route to the silylene 7. In the absence of that route, as in Scheme I, conversion of 4 to 8 cannot be ignored.
434 Organometallics, Vol. 5, No. 3, 1986
A u n e r et al.
Table V. Kinetic Data for Pyrolysis of Tetramethylsilane log Ais-' ElkJ mol-' k,,*cls-'
temp/'C
conditns
537-705 567-677 567-677
static system unpacked SFR packed SFR
14.3 f 0.2 11.2 i 0.1 14.4 i 0.2
283 i 3 239 k 2 296 f 3
ref
0.128 0.023 0.025
Scheme I1
29 27
27
Scheme I11
ti R H t MeSiASi6Hp H V H
":SiCSi:
+
t
71
SiH4
Me. I
P
Hzl
15 \\
CH4 MeSiHs
SiH4
..
r-------?
: H 2 S C S i H 2 \ r HSi !
I
L-
___ S
I
H Me SiH2 t H z S i e i M e V
= H3Si vSiMe 12
10
11
--J'
lHp Me2SiHp
//
silanes by reacting with the significant quantities of hydrogen present. While it is well-known that silylenes insert rapidly into hydrogen at high temperature, we have done a simple experiment to show that silenes do so as well; when dimethylsilene was produced by pyrolysis of 1,ldimethyl-1-silacyclobutanein the presence of hydrogen, trimethylsilane was formed instead of 1. Dimethylsilylene and methylsilylene would also insert reversibly into 2, but the latter insertion reaction could also give hydrogen, thus contributing to the low yield of methylsilane (Table 11). For simplicity, silene + silylene isomerizations4 that do not lead to new products are omitted from the scheme. So too are secondary decomposition reactions of the products of silylene insertion into 2; such reactions would undoubtedly have occurred, producing a variety of higher molecular weight products that would not have been identified in the kinetic experiments, even if they were thermally stable under these conditions, but on the other hand would have decomposed in the flow experiments at 850 "C to the observed smaller molecules (Table 111) or to polymer. While biradical9 can dissociate as shown in Scheme I, its dissociation is less favourable energetically than the corresponding dissociation in 1,l-dimethylsilacyclobutane;' 9 may therefore undergo some radical reactions, such as hydrogen abstraction from 2 or addition to silenes, which is known to be a rapid reaction.% There may also be some minor heterogeneous pathways forming radicals (we and othersz6 have found it difficult to obtain reproducible kinetic results for the pyrolysis of hydridosilacyclobutanes and disilacyclobutanes). Once radicals had been generated, they would undoubtedly undergo the reactions shown in Scheme 11, by analogy with the radical reactions known to occur in the pyrolysis of tetramethylsilane and trimethyl~ilane.~~ Secondary decomposition of the resulting polymer is probably the main source of hydrogen, methane, ethene, and propene. While there are obvious similarities between the pyrolysis mechanisms of 2 and 3 that need not be labored, there
This process may be envisaged as proceeding by direct elimination of :SiH2 or by the intermediate formation of a disilene, which dissociates quite easily to two silylenes.28
(25) Bastian, E.; Potzinger, P.; Ritter, A.; Schuchmann, H.-P.; von Sonntag, C.; Weddle, G. Ber. Bunsen-Ges. Phys. Chern. 1980, 84, 56. (26) Walsh, R. personal communication. (27) Baldwin, A. C.; Davidson, I. M. T.; Reed, M. D. J . Chem. Soc., Faraday Trans. 1 1978, 74, 2171.
(28) A reviewer suggested that this point required amplification; for the specific case of Me2Si=SiMe2 2Me2Si:,thermochemical estimatesM of enthalpies of formation gave A H 268 kJ mol-'. In terms of bond energies, that may be thought of as corresponding to -339 (u-bond) + -1% (*-bond) - 2 X -113 (silylene stabilization).
H MeSiS!=CH2 H2
MeSIgMe H2
HfiSiMe2 H
14
13
Scheme IV
/ \ M ~ ~ S tI Hg 1~H 2
M e 2 5 1 t SiH4
IH
1
SiH4
HZ
Me2SiH2
are significant differences on which attention should be focussed. Initially, the pyrolysis of 3 would proceed as in Scheme 111, analogous to the first part of Scheme I. However, a significant difference between 2 and 3 is that the latter has two hydrogen atoms attached to silicon; insertion of the simple silylenes :SiMeH and :SiHz into 3 thus leads to monosilanes, as shown in Scheme 111, while insertion of silylsilylene 13 into 3 produces a trisilane that has more decomposition routes by alternative 1,Zhydrogen shifts open to it than has the trisilane resulting from insertion of 7 into 2 (Scheme I), as shown in Scheme IV. In particular, one of these routes leads to a silylsilylene which can propagate silylene chains, thus
-
Organometallics, Vol. 5, No. 3, 1986 435
Disilacyclobutanes Scheme V A .
* *\
Me2SiVSiMe2
'
Me2SivSiMe2
\ Me2Si;SiMe
A
Me. t Me2SiVSiMe2 Me2SikSiCH2 A .* Me
= 2Me2Si=CH2
-
-
t Me-
A
CH4 t M e 2 S i V S i e H 2 Me
MezSiCSi=CHz
t Me-
polymrr
H2
R.
t CH4 t C2H4
is any r a d i c a l
A very similar silylene chain branching reaction, forming :SiMeH instead of :SiHz, would result from reaction of silylsilylene 14 with 3. These features, combined with the lower activation energy for the initial 1,Zhydrogen shift,15 account for the substantially higher rate of decomposition of 3 (Table I). Formation and secondary decomposition of polymer would also occur with 3, giving mainly hydrogen rather than hydrocarbons. Again, there would be gaseous products of higher molecular weight resulting from further secondary reactions. Such products were observed in LPP experiments and in the flow experiments at 850 "C (Table 111)* We have noted above that while the Arrhenius parameters in Table I should not be identified with primary reactions, the trend in activation energies is consistent with these primary reactions being the 1,2-hydrogen shift in 2 and 3 and rupture of the silicon-carbon ring bond in 1 to form a biradical, which then dissociates to give two molecules of dimethylsilene. In the pyrolysis of l we had (29) Clifford, R. P.; Gowenlock, B. G.; Johnson, C. A. F.;Stevenson, J. J . Organornet. Chern. 1972, 34, 53.
evidence for the formation of dimethylsilene from trapping experiments, as had earlier ~ o r k e r s . ~However, J the Arrhenius parameters for the decomposition of 1 do not rule out some contribution from silicon-methyl bond rupture. While that activation energy would be higher than for silicon-carbon bond rupture in the ring by an amount equal t o E04,the A factor would also be higher, ca. 1017 s-l instead of ca. 1015.5 s-l. A good analogy is the pyrolysis of tetramethylsilane, which proceeds by a short-chain reaction with kinetic behavior sensitive to experimental condition^.^^ Because the chain is short, a chain length approaching unity can be achieved by raising the temperature, giving Arrhenius parameters approaching those for dissociation of the silicon-methyl bond.27 At lower temperatures, the Arrhenius parameters for the chain reaction depend on conditions, as shown in Table v. The observed Arrhenius parameters for the decomposition of 1are very similar to some of those for the pyrolysis of tetramethylsilane in Table V. The rate constant for the decomposition of 1 at 700 "C would be 0.032 s-l. Consequently, a chain reaction initiated by silicon-methyl bond rupture, exactly as in the pyrolysis of tetramethyl~ilane,~~ cannot be excluded. We therefore agree with earlier sugg e s t i o n ~that ~ , ~both modes of silicon-carbon bond breaking may occur in the pyrolysis of 1. Our suggestions are embodied in Scheme V. Our observation that added hydrogen chloride had a small effect on the half-life for decomposition of l , but a greater inhibitory effect on the formation of hydrogen and ethene,13 confirms that the decomposition of polymer is an important secondary process in the pyrolysis of 1.
Acknowledgment. We warmly thank Professors J. Grobe (Westfdische Wilhelms-Universitat, Munster) and T. J. Barton (Iowa State University) for stimulating discussions and encouragement. The financial support of the SERC is gratefully acknowledged. We are also grateful to the reviewers for their constructive comments and suggestions. Registry No. 1, 1627-98-1; 2, 68060-11-7; 3, 287-55-8; 4, 1628-01-9.
