The Reaction of Silylenes with Acetylenes: In ... - ACS Publications

Organometallics 2009, 28, 3573–3586 DOI: 10.1021/om800854a

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The Reaction of Silylenes with Acetylenes: In Search of the Reaction Pathways William H. Atwell 616 Lakeside Circle, Midland, Michigan 48640 Received September 3, 2008

The early generation of silylenes at elevated temperatures commonly used acetylenes as trapping agents. Under these conditions the trapped products were dimeric 1,4-disilacyclohexadienes, rather than the expected silacyclopropenes. Even after the isolation of a number of silacyclopropenes the perplexing pathway responsible for the formation of 1,4-disilacyclohexadiene products remains unsolved. This review examines relevant evidence with the hope of resolving this reaction pathway. I. Introduction

pathways involved in this most interesting and unexpected reaction.

The synthesis and trapping of divalent, dicoordinate organosilylenes began in the 1960s, and the emerging science was reviewed at the end of that decade.1 During the next four decades the chemistry of these highly reactive, short-lived intermediates advanced at a rapid pace and additional reviews were required to remain current.2-5 Once questioned, today the existence of silylenes is well accepted. Numerous properties and characteristics of silylenes have been measured and calculated. The scope of silylene chemistry has been expanded dramatically and has moved into the field of transition-metal chemistry. Singlet silylenes, as well as a rare triplet silylene, have been demonstrated and in recent years persistent silylenes, stable at room temperature, have been isolated. Currently, researchers continue to sort out the distinctions between silylene and silylenoid intermediates. Yet, with all these advances, when silylenes react with acetylenes the perplexing pathways responsible for the formation of 1,4-disilacyclohexadienes (eq 1) apparently remain unsolved: the mechanistic paths are undefined.

My purpose is to review relevant evidence to hopefully revive interest in this chemistry and, if possible, to resolve the (1) Atwell, W. H.; Weyenberg, D. R. Angew. Chem., Int. Ed. Engl. 1969, 8, 469. (2) (a) Gaspar, P. P. In Reactive Intermediates; Jones, M., Jr., Moss, R. A., Eds.; Wiley: New York, 1978, Vol. 1, p 229. (b) Reference 2a, Vol. 2 p 335. (c) Gaspar, P. P.; West, R. In The Chemistry of Organosilicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998, Vol. 2, p 2463. (3) Tokitoh, N.; Ando, W. In Reactive Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones, M., Jr., Eds.; Wiley: New York, 2004, p 651. (4) Gaspar, P. P. In Reactive Intermediates; Jones, M., Jr., Moss, R. A., Eds.; Wiley: New York, 1985, Vol. 3, p 333. (5) Haaf, M.; Schemedake, T. A.; West, R. Acc. Chem. Res. 2000, 33 (10), 704. r 2009 American Chemical Society

II. Our Early Work at Dow Corning In our early work the chemistry of the newly discovered silylenes was studied extensively. For proprietary reasons, certain results were never published in the open literature. In this work I will include previously unpublished results that I feel are relevant to the question of the pathways of the silylene/acetylene reactions. In two of our early studies dealing with the generation of silylenes, acetylenes were used as trapping agents (Scheme 1). These early studies with the 7-silanorbornadiene 1 and dimethoxydisilane 2 were carried out in sealed tubes at temperatures of 200-300 °C in the absence of added solvents. The formation of dimethylsilylene by thermolysis of 1 and 2 was inferred from the isolation of 1,4-disilacyclohexadiene 4. It was proposed6,7 that the disilacyclohexadiene 4 was formed by the dimerization of the initially formed silacyclopropene 3 (Scheme 1). The preparation of the silacyclopropene 3 was first claimed by Vol’pin and co-workers8 (see eq 2), and a bivalent silicon intermediate was postulated. The low yields of 3 (0.5%) obtained in the sodium method (eq 2) improved to 6% when the polysilane polymer (SiMe2)55 was thermolyzed (at 250-260 °C) in the presence of diphenylacetylene.8 Subsequent molecular weight,9a mass spectral,9b and X-ray studies9c showed that the compound Vol’pin had isolated was actually the disilacyclohexadiene 4, a dimer of the proposed silacyclopropene 3. Although the reaction pathway to compound 4 was not defined, Vol’pin’s work formed (6) Gilman, H.; Cottis, S.; Atwell, W. H. J. Am. Chem. Soc. 1964, 86, 1596. (7) See: (a) Atwell, W. H.; Weyenberg, D. R. J. Organomet. Chem. 1966, 5, 594. (b) Chem. Eng. News 1967, Sept 4, 30. (c) J. Am. Chem. Soc. 1968, 90, 3438. (8) Vol’pin, M. E.; Koreshkov, Yu. D.; Dulova, V. G.; Kursanov, D. N. Tetrahedron 1962, 18, 107. (9) (a) West, R.; Bailey, R. E. J. Am. Chem. Soc. 1963, 85, 2871. (b) Johnson, F.; Gohlke, R. S.; Nasutavicus, W. H. J. Organomet. Chem. 1965, 3, 233. (c) Bokii, N. G.; Stuchkov, Yu. J. Struct. Chem. 1965, 6, 543.

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Organometallics, Vol. 28, No. 13, 2009 Scheme 1

Scheme 2

the basis for my choice of diphenylacetylene as a trapping agent in the earlier work on the thermolysis of 7-silanorbornadienes,6 and acetylenes would become the trapping agents of choice for silylenes.

When we heated 1,2-dimethoxydisilane 2 in the inlet chamber of a mass spectrometer, a dimethylsilylene molecular ion (m/z 58) was detected as a short-lived intermediate, lending further support for the intermediacy of dimethylsilylene when the disilane 2 is thermolyzed.7b,7c,9 Initially it was suggested6 that 4 might have been formed by a π-dimerization of silacyclopropene 3 (Scheme 2). Our studies showed that the thermolysis of disilane 2, in the presence of a 1:1 mixture of diphenyl- and dimethylacetylene, afforded 4, 7, and 8 (Scheme 3). Disilacyclohexadiene 6, the product expected if the π-dimerization mechanism was operative, could not be detected. Likewise, reaction of disilane 2 with phenylmethylacetylene gave disilacyclohexadiene 6 and its 2,5-diphenyl isomer 5. Disilacyclohexadiene 7 was not detected. This same paper7c showed that the pathways depicted in Schemes 4 and 5 were also not operative; silacyclopentadiene 9 was not among the products formed when either the 7-silanorbornadiene 1 or the dimethoxydisilane 2 was thermolyzed in the presence of diphenylacetylene, and silacyclopentadiene 9 is not converted to disilacyclohexadiene 4 when present during reactions that generate 4.7c At that time, on the basis of these experiments, we concluded7c “that the formation of the disilacyclohexadiene proceeds by a rather specific dimerization of silacyclopropene intermediates with rupture of the carbon-silicon bond.” (10) Weyenberg, D. R.; Atwell, W. H. Pure Appl. Chem. 1969, 19, 343–351.

Atwell

Our early studies showed that the formation of silylenes via the thermolysis of 1,2-difunctional disilanes (and R, ω-polysilanes) was general. The functional groups included SiOMe, SiCl, SiF, and SiOSiMe3.11,12 Since our initial work, others have extended the functionality to include SiH,13 SiPh,13 and SiNMe2.14 Silylenes generated using difunctional disilanes did not react with simple olefins: for example, tetramethylethylene. We concluded that these simple olefins could not compete with the starting disilane.7 As I will show later, this was likely a mistaken conclusion on our part. We did not anticipate the thermal lability and chemical behavior of the expected silacyclopropane products. A variety of functional silylenes can be prepared by this method. For example, 1,1,2,2-tetramethoxy-1,2-dimethyldisilane (10) was thermolyzed in the presence of dimethylacetylene to give 11 as a mixture of cis-trans isomers (Scheme 6).15 Cothermolysis of mixtures of the disilanes 2 and 10 with dimethylacetylene (neat in sealed tubes at 225 °C) gave10 the three disilacyclohexadienes 8, 11, and 12 in a 3:4:1 ratio, respectively (Scheme 7).11 Thus, at these temperatures, 1,4-disilacyclohexadiene formation by reaction of a variety of silylenes with disubstituted acetylenes appears to be a general reaction pathway.15 While none of the above results shed light on the silylene/acetylene reaction pathways, they were part of a continuing study to probe this question. Unexpectedly,16 we found that dimethoxysilylene could not be trapped by these acetylenes.7b,7c However, we found that dimethoxysilylene could be readily trapped by methanol (Scheme 8).10,17 Methanol proved to be an effective trapping agent for all of the silylenes generated by methoxydisilane thermolysis.17 Kinetic studies showed that the reactions are first order in disilane and independent of the concentration of methanol.10 Again, the above reactions shed little light on the reaction pathways between silylenes and acetylenes. In further experiments we studied the thermolysis of alkoxydisilanes 2 and 10 in the presence of a 1:1 mixture of dimethylacetylene and methanol18 (Scheme 9). Under these conditions only small amounts of 1,4-disilacyclohexadienes were observed. The major products were the cis-2-butenylsilanes 13 and 14. The same products had been obtained in vapor-phase reactions (vide infra). We believed that these were formed by methanolysis of the intermediate silacyclopropenes, as shown in Scheme 9. Both 13 and 14 were converted to the trimethylsilyl derivative 15 of known stereochemistry.19 These reactions convinced us of the intermediacy of silacyclopropenes and also demonstrated some of their chemistry. We were interested in the synthetic utility of silylenes; therefore, we decided to explore their vapor-phase (400 °C) (11) (a) Atwell, W. H. Unpublished studies at the Dow Corning Corp. 1965-1969. (b) Janzen, E. G.; Pickett, J. B.; Atwell, W. H. J. Am. Chem. Soc. 1968, 90, 2719. (12) Atwell, W. H. (Dow Corning Corp.) U.S. Patent 3,509,191, 1970. (13) (a) Sakurai, H.; Hosomi, A.; Kumada, M. Chem. Commun. 1969, 1, 4. (b) Davidson, I. M. T.; Hughes, K. J.; Ijadi-Maghsoodi, S. Organometallics 1986, 6, 639. (14) Heinicke, J.; Gehrus, B.; Meinel, S. J. Organomet. Chem. 1994, 474, 71. (15) Atwell, W. H. (Dow Corning Corp.) U.S. Patent 3,465,018, 1969. (16) This was unexpected, since dimethoxysilylene, as well as all the silylenes we generated, could be easily trapped by butadienes.10,12 (17) Atwell, W. H. (Dow Corning Corp.) U.S. Patent 3,478,078, 1969. (18) Atwell, W. H.; Weyenberg, D. R. Intra-Sci. Chem. Rep. 1973, 7 (4), 139. For experimental details see: Atwell, W. H. (Dow Corning Corp) U.S. Patent 3,576,024, 1971. (19) Ryan, J. J. Org. Chem. 1966, 31, 2698.

Review

Organometallics, Vol. 28, No. 13, 2009

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Scheme 3

Scheme 4

Scheme 8

Scheme 5

Scheme 9

Scheme 6

Scheme 7

reactions. Our vapor-phase studies were carried out by metering the reactants to a vertical (2.5 cm 62 cm) quartz tube at temperatures of 400-550 °C. The temperature employed depended on the difunctional disilane.20b We also used the vapor-phase studies to continue to probe the silylene/acetylene reaction pathways. The vapor-phase pyrolysis of methoxydisilane 2 was very clean and gave essentially the same polysilane products that had observed in the thermolysis in sealed tubes (Table I).20a (20) (a) Atwell, W. H.; Mahone, L. G.; Hayes, S. F.; Uhlmann, J. G. J. Organomet. Chem. 1969, 18, 69. (b) Atwell, W. H.; Weyenberg, D. R.; Uhlmann, J. G. J. Am. Chem. Soc. 1969, 91, 2025.

Under these vapor-phase pyrolysis conditions these results became the baseline behavior of 2. As mentioned above, the vapor-phase pyrolysis of methoxydisilanes in the presence of a mixture of dimethylacetylene and methanol gave products identical with those obtained in the sealed-tube experiment (Scheme 9). However, we soon encountered some unexpected results in these vapor-phase experiments, results we could not explain. The vapor-phase studies allowed us to explore reactions with a variety of other acetylenes, including acetylene itself. We found that the reaction of dimethoxydisilane 2 with acetylene did not yield the expected 1,4-disilacyclohexadiene 16 (Scheme 10).11 The only product we could isolate was ethynyldimethylsilane

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Atwell

Scheme 10

Scheme 11

Scheme 12

Table I. Percentage of MeO(SiMe2)nOMe Products Observed in the 225 °C Vapor-Phase Pyrolysis of MeO(SiMe2)2OMe (2)

rel %

n=2

n=3

n=4

n=5

n=6

7.5

45.5

30.5

12.8

3.5

18, which we attributed to a hydrogen-shift rearrangement of the expected silacyclopropene intermediate 17. This proposed rearrangement parallels that had been observed by both Skell21 and Nefedov22 for silacyclopropene 17 and was a significant departure from our previously observed reactions between silylenes and disubstituted acetylenes. When 2 was reacted with a 1:1 mixture of dimethylacetylene and methanol, the two products 18 and 19 were formed in an approximate 1:1 ratio (Scheme 10).11 Again, the intermediacy of silacyclopropene 17 was indicated. The pyrolysis of dimethoxydisilane 2 in the presence of bis(trimethylsilyl)acetylene in the vapor phase at 400 °C gave an ethynylsilane (Scheme 11), in this case indicating that a trimethylsilyl-shift rearrangement of the intermediate silacyclopropene had occurred. Surprisingly, however, the vapor-phase pyrolysis of tetramethoxydisilane 10 in the presence of acetylene gave an over 60% yield of a cis-trans mixture of disilacyclohexadiene 20 (Scheme 12). This was the best yield of a 1,4-silacyclohexadiene that we had achieved. We believed that 20 was formed via silacyclopropene intermediate 21. Compound 20 was converted to 16 by reaction with methylmagnesium bromide. The difference in the reaction behavior of the silacyclopropene intermediates 17 and 21 was unexpected. Clearly, the substituents on the carbon and the silicon atoms both have an effect on the stability of the silacyclopropene intermediates. The vapor phase reaction of tetramethoxydisilane 10 with a 1:1 mixture of acetylene and methanol at 400 °C gave the vinylsilane 22, presumably via methanolysis of the intermediate silacyclopropene 21 (Scheme 13).18 Occasionally, small amounts of disilacyclohexadiene 20 were also observed. Thus, silacyclopropene 21 appears to be an intermediate in the route to 20 and 22, yet 21 did not show the rearrangement observed in the case of silacyclopropene intermediate 17. The substitution of a methoxy for a methyl group on silicon had a dramatic effect on the course of the reaction. Surely this had mechanistic implications, implications that we did not understand. (21) Skell, P. S.; Goldstein, E. J. J. Am. Chem. Soc. 1964, 86, 1442. (22) Nefedov, O. M.; Manakov, M. N. Angew. Chem. 1964, 76, 270.

Scheme 13

Thoroughly convinced of their intermediacy, we made some last attempts to isolate a silacyclopropene intermediate. We decided we would have a better chance of success if we returned to the use of disubstituted acetylenes, and we carried out several vapor-phase reactions with dimethoxydisilane 2 and dimethylacetylene (1:2 ratio, respectively) at 400 °C. Surprisingly, under those conditions we had difficulty getting significant yields of the disilacyclohexadiene 8. Instead, we observed the formation and disappearance of a compound (or compounds) that appeared to be very reactive. Finally, we were able to isolate 30% yields of the first 1,2-disilacyclobutene, 24 (Scheme 14).23 We proposed that disilacyclobutene 24 most likely was formed by the insertion of dimethylsilylene into the carbonsilicon bond of the silacyclopropene intermediate 23. The ability of carbenes to insert into the carbon-silicon bond of strained rings had been reported.24 Disilacyclobutene 24 was difficult to isolate and handle because of its high reactivity with oxygen. Merely passing oxygen over an open vial containing a pure sample of 24 resulted in an exotherm and a nearly quantitative conversion to (23) Atwell, W. H.; Uhlmann, J. G. J. Organomet. Chem. 1973, 52, C21. (24) Seyferth, D.; Damrauer, R.; Andrews, S. B.; Washburne, S. S. J. Am. Chem. Soc. 1969, 91, 2025.

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Scheme 14

and 27 play in examining the silylene/acetylene reaction pathways.

hexamethyl-1-oxa-2,5-disilacyclopentene (25). The majority of our subsequent studies became focused on the synthesis and industrial utilization of silylenes. The pathways of the silylene/acetylene reactions remained unresolved.

Later work by Boudjouk and co-workers28 showed that bulky substituents on the silicon atom, such as tert-butyl, increase the thermal stability of siliranes. Although such bulky groups on the silicon atom slow down the silylene extrusion, this approach extended the utility of siliranes. This period also ushered in the low-temperature photochemical generation of silylenes, (eq 6).29

III. The Following Years The initial generation of silylenes, whether in the liquid phase in sealed tubes or in the vapor phase, required relatively high (200-400 °C) temperatures and relatively long residence times, which most likely explains our inability to isolate silacyclopropenes. In 1976 the excellent efforts of Conlin and Gaspar25 succeeded where we had failed. Flash pyrolysis of methoxydisilane 2 in the presence of dimethylacetylene led to the isolation of tetramethylsilacyclopropene 23 (eq 3). Silacyclopropene 23 is stable below 70 °C but, like disilacyclobutene 24, reacts instantly with oxygen. It underwent methanolysis to yield the cis-2-butenylsilane 13, confirming our earlier suggestions11,18 that silacyclopropenes, the initially formed silylene/acetylene products, were being intercepted.

In this same decade, low-temperature silylene routes emerged. The pioneering work of Seyferth and Annarelli26 led to the synthesis of hexamethylsilacyclopropane (26), a silirane whose pyrolysis under mild conditions yielded dimethylsilylene (eq 4). The availability of silacyclopropane 26 led to the synthesis and isolation of a second silacyclopropene, 27, by Seyferth, Annarelli, and Vick (eq 5).27 Silacyclopropene 27 could be distilled at 9294 °C at 30 mmHg. Small samples fumed profusely when exposed to air; larger samples were spontaneously inflammable in air. High yields of 27 were obtained even at a 1:1 ratio of silacyclopropane to acetylene, confirming that silacyclopropenes are the initial reaction products. I will discuss later the important role that silacyclopropenes 23 (25) Conlin, R. T.; Gaspar, P. P J. Am. Chem. Soc. 1976, 98, 3715. (26) (a) Seyferth, D.; Annarelli, D. C. J. Am. Chem. Soc. 1975, 97, 2273. (b) J. Am. Chem. Soc. 1975, 97, 7162. (27) (a) Seyferth, D.; Annarelli, D. C; Vick, S. C. J. Am. Chem. Soc. 1976, 98, 6382. (b) J. Organomet. Chem. 1984, 272, 123.

hν

ðMe2 SiÞ6 f ðMe2 SiÞ5 þ Me2 Si :

ð6Þ

The availability of both high- and low-temperature as well as photochemical routes to a variety of silylenes set the stage for the development of the exciting and often unpredictable silylene story that would be developed further during the next four decades. These developments are summarized in some excellent reviews, whose content I will make no attempt to repeat.2-6 Silylenes were found to react with the π-system of a wide variety of alkenes, dienes, and alkynes, and they insert into Si-H, Si-O, Si-X, Si-N, Si-Si (strained), N-H, O-H, and C-H (intramolecular) bonds. Even today the list of reaction partners continues to grow. Unlike carbenes, silylenes, in the absence of efficient trapping agents, can dimerize to disilenes (R2SidSiR2), and in turn disilenes have been reported to dissociate to silylenes. Silylenes also can be formed by the rearrangement of disilenes as well as by the rearrangement of strained cyclosilaalkanes. The discovery that photochemically generated silylenes could be isolated in low-temperature argon or hydrocarbon matrices enabled the measurement and study of their electronic spectra. The effect of a variety of substituents on their electronic spectra has been defined. The stereospecific additions to carbon-carbon double bonds characteristic of singlet silylenes have been extensively studied. While singlet silylenes are the most prevalent, a triplet silylene has been demonstrated.30 As studies progressed, stable silylenes were identified and have now been isolated.5 The synthesis of silylene-transition-metal complexes involving both terminal and bridging silylenes is under active investigation. (28) Boudjouk, P.; Samaraweera, V.; Soorijakumaran, R.; Chusciel, J.; Anderson, K. R. Angew. Chem., Int. Ed. Engl. 1988, 27, 1355. (29) Ishikawa, M.; Kumada, M. Chem. Commun. 1970, 612. (30) (a) Sekiguchi, A.; Tanaka, T.; Ichioke, M.; Akiyama, K.; TeraKubota, S. J. Am. Chem. Soc. 2003, 125, 4962. (b) Sekiguchi, A.; Tanaka, T.; Ichinohe, M.; Akiyama, K.; Gaspar, P. P. J. Am. Chem. Soc. 2008, 130, 426.

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IV. Thoughts on Reaction Pathways A significant complication in defining the reaction pathways of the silylene/acetylene reaction lies in the broad range of conditions employed in the generation of silylenes and their reaction with acetylenes. Few controlled studies are available, and seemingly related studies often employed different conditions and substituents on the silylene and the acetylene. My initial focus will center on the reactions of silylenes and acetylenes carried out in sealed tubes and under related thermal conditions. Experiments using temperatures in the 200-300 °C range that had been used in our experiments will be considered. Sealed-tube reactions are not a requirement, since we reported7a that heating 1,12,2-tetramethoxy-1,2-dimethyldisilane (10) and diphenylacetylene in a flask (at 185-190 °C) connected to a spinning band column allowed the continuous removal of methyltrimethoxysilane and gave a 50% yield of disilacyclohexadiene 11 (Scheme 6). Typically, the yields of disilacyclohexadienes ranged from 30 to 50%, an observation that in itself suggests that a complex mechanism is operative. At least seven 1,4-disilacyclohexadienes, 4-8, 11, and 12, have been prepared under essentially similar conditions.7 Assuming one operative reaction pathway, any one selected must be consistent with the formation of all these compounds. In section II of this review it was shown that the formation of disilacyclohexadienes did not involve the π-dimerization of the intermediate silacyclopropenes, and I will not examine this mechanism further. A literature search shows that four other possible pathways have been suggested to account for the formation of 1,4-disilacyclohexadienes. An analysis of each follows. A. Reaction Pathway I: Silylene to Disilene to Disilacyclobutene. In 1974, Barton and Kilgour31a reexamined the silylene/acetylene puzzle. They proposed that 1,4-disilacyclohexadienes might be formed by the reaction of 1,2disilacyclobutenes with acetylenes, possibly through a Diels-Alder reaction involving the reversible formation of a 1,4-disilabutadiene (Scheme 15). This was indeed a bold suggestion. Disilene precursors had been reported previously,32 and indeed Barton and Kilgour found that reaction of 28 with dimethylacetylene in a sealed tube at 260 °C for 18 h gave disilacyclohexadiene 8, presumably via 24 (Scheme 16). The authors, in a later paper31b on the synthesis and isolation of 24 (see Scheme 17), reported that reasonable amounts of 8 were also formed in this reaction. Apparently in our excitement with the initial isolation of 2423 we failed to observe the formation of 8. Barton and Kilgour’s reaction of 24 with diethylacetylene also gave the disilacyclohexadiene 29 (Scheme 17).31 This work clearly demonstrated a 1,2-disilacyclobutene/ 1,4-disilacyclohexadiene link. Further, it provided a perhaps critical link between the Atwell, Barton, and Gaspar works, where one or more of these silacyclopropene, disilacyclobutene, and disilacyclohexadiene ring systems were all obtained from the reaction of 2 and acetylenes (although under different thermal conditions). However, the reaction sequence leading to the formation of 24 remained unresolved. One suggestion31a involved the (31) (a) Barton, T. J.; Kilgour, J. A. J. Am. Chem. Soc. 1974, 96, 7150. (b) J. Am. Chem. Soc. 1976, 98, 7746. (32) Roarke, D. N.; Peddle, G. J. D. J. Am. Chem. Soc. 1972, 94, 5837.

Atwell Scheme 15

Scheme 16

Scheme 17

Scheme 18

Scheme 19

dimerization of dimethylsilylene to tetramethyldisilene (30) and the reaction of the latter with acetylene (Scheme 18). It has been reported2 that the dimerization of a silylene to a disilene is the preferred route in the absence of a suitable trapping agent. However, in the case of dimethoxydisilane 2, the disilane itself is known to be an effective silylene trapping agent7c and the polysilane trapped products (also known trapping agents) are stable under the conditions employed by Barton and Kilgour. Thus, the involvement of a disilene in these sealed-tube experiments seems unlikely. In a later study31b Barton and Kilgour provided convincing evidence that under their static conditions there is no significant interconversion of silylene to disilene. Later work by two Japanese groups33,34 show that some disilacyclobutenes, for example 31 and 32, are thermally (33) Ishikawa, M.; Sugisawa, H.; Kumada, M.; Kawakami, H.; Yamabe, T. Organometallics 1983, 2, 974. (34) Sakurai, H.; Kobayashi, T.; Nakadaira, Y. J. Organomet. Chem. 1978, 162, C43.

Review


Scheme 21

stable and do not react with acetylenes under conditions even more stringent than those used by Barton and Kilgour (Scheme 19). From these studies one must conclude that disilacyclobutene 32 is not an intermediate in the formation of 4 from the reaction of dimethylsilylene and diphenylacetylene, a reaction which we know7c occurs under these conditions (Scheme 20). Since 32 is not an intermediate en route to disilacyclohexadiene 4, then we must conclude that 1,2-disilacyclobutenes are not the common intermediates for the large number of 1,4-disilacyclohexadienes that have been obtained in these reactions. The Barton and Kilgour result must be a rare coincidence in the elusive unfolding of silylene chemistry. However, the above results raise another interesting pathway issue. Compound 24 reacts with both dimethyl- and diethylacetylene to give the disilacyclohexadienes 8 and 29 (Scheme 21), while the phenyl- and trimethylsilyl-substituted disilacyclobutenes 32 and 31 do not undergo this reaction. What then is the reaction path of 24 with acetylenes, and does the pathway really involve a 1,4-disilabutadiene intermediate as shown in Scheme 21? It would appear that the enhanced kinetic stability of 31 and 32 may be due to the larger substituents on the ring carbon atoms, perhaps preventing the required ring-opening isomerization to the respective 1,4-disilabutadiene (Scheme 19). This is an issue that requires more study. These results also remind us again that, as substituents change, we must be cautious with generalizations concerning these small-ring compounds. This issue will reoccur later in this review. The now available body of evidence shows convincingly that, in the thermal reactions noted above, silacyclopropenes are the primary adducts formed in the reaction of the silylenes with acetylenes. Thus, understanding the chemistry of silacyclopropenes is the key to understanding the formation of the 1,4-disilacyclohexadiene ring system. We know that a variety of 1,4-disilacyclohexadienes are formed when silylenes, acetylenes, and silacyclopropenes are present at elevated temperatures. Is the simultaneous presence of all three of these species necessary? In the case of silacyclopropenes, which sequence or set of reactions leads to the 1,4-disilacyclohexadiene ring system? The remaining three proposed mechanisms will be explored in an attempt to answer this question.

3579

B. Reaction Pathway II: Silylene σ-Insertion into Silacyclopropenes. In an earlier study23 we suggested that disilacyclobutene 24 could be formed by insertion of dimethylsilylene into the C-Si bond of silacyclopropene 23. Coupling this step with the observations of Barton and Kilgour would provide yet another route to 8 (Scheme 22). The insertion of carbenes into the C-Si bond of strained-ring silacyclobutanes has been reported.24 What is known about the reactivity and insertion reactions of silacyclopropenes? In section II it was noted that Seyferth, Annerelli, and Vick27 thermolyzed hexamethylsilacyclopropane (26) in the presence of bis(trimethylsilyl) acetylene and obtained 1,1-dimethyl-2,3-bis(trimethylsilyl)1-silacyclopropene (27). Silacyclopropene 27 could be distilled without decomposition and was isolated as an analytically pure liquid. The preparation and thermal properties of tetramethylsilacyclopropene (23)25 and silacyclopropene 2727 showed that silacyclopropenes were more thermally stable than silacyclopropanes. The synthesis of additional silacylopropenes confirmed this observation. In all cases, their enhanced stability has been noted.27b,35,36 The source of this enhanced stability has been attributed to a “quasiaromatic delocalization” of π-electrons into the silicon d orbitals and such d-π to p-π overlap has been claimed to have experimental support. For example, the NMR spectra of 1,1-diphenylsilacyclopropene show that the 1H and 13C chemical shifts of the silacyclopropene ring occur at an unusually low field.36b This was claimed to provide evidence for electron delocalization in the SiC2 ring and π-bonding involving participation of the vacant (virtual) silicon 3d orbitals. Jones used CNDO semiempirical calculations to predict pseudoaromatic delocalization in silacyclopropenes.37a Other researchers have attributed the pseudoaromatic character to, not the vacant silicon 3d orbitals but, rather, the antibonding σ orbitals.37b However, an analysis of overlap populations by French workers indicates that there is little pseudoaromatic character in these rings.38 While the source of the stability of silacyclopropenes remains controversial, they are more thermally stable than silacyclopropanes and their C-Si bonds show high reactivity, as has been discussed previously.2-5 After our initial isolation of 24, a number of 1,2-disilacyclobutenes have been prepared. In many cases a silylene σ insertion into the SiC2 ring has been the favored proposed reaction pathway. For example, reaction of silacyclopropane 26 and the silacyclopropene 27, under very mild conditions, gave 34 and 35 via insertion of 26derived dimethylsilylene into the SiC2 rings of 26 and 27 (Scheme 23).39 Like 1,2-disilacyclobutenes 31 and 32, 35 was thermally stable, being unchanged when heated at 175 °C for 20 h. One would have hoped that the isolation of stable silylenes could provide insight into the silylene/acetylene reaction pathways. However, the N-heterocyclic silylene (35) Jiang, P.; Trieber, D.; Gaspar, P. P. Organometallics 2003, 22 (11), 2233. (36) (a) Seyferth, D. J. Organomet. Chem. 1975, 100, 237. (b) Zielinski, M.; Trommer, M.; Sander, W. Organometallics 1999, 18, 2791. (c) Belzner, J.; Ihmels, H. Tetrahedron Lett. 1993, 34, 6541. (37) (a) Jones, P. R.; White, D. D. J. Organomet. Chem. 1978, 154, C33. (b) Goller, A.; Heydt, H.; Clark, T. J. Org. Chem. 1996, 61, 5840. (38) Barthelat, J.-C.; Trinquier, G.; Bertrand, G. J. Am. Chem. Soc. 1979, 101, 3785. (39) Seyferth, D.; Vick, S. C. J. Organomet. Chem. 1977, 125, C11.

3580


Atwell Scheme 22

Scheme 23

36 and phenyl(trimethylsilyl)acetylene also gave a 1,2-disilacyclobutene, 37, presumably through a silacyclopropene (eq 7).40

The more sterically hindered silylenes, such as 38, are kinetically stabilized and do not react with acetylenes.5 Thus, it is surprising that silylene 39 is proposed to insert into the intermediate silacyclopropene 40 to give the disilacyclobutene 41 (Scheme 24).41 Compound 41 is the first 1,2-disilacyclobutene with a pentacoordinate silicon atom. No disilacyclohexadienes have been obtained when these stable silylenes were reacted with acetylenes.

One of the more interesting studies on silylene insertion into silacyclopropenes involved the reaction of silylene precursor 42, a trisilacyclopropane, with acetylenes. Under mild conditions the initial product was the silacyclopropene 43. Monitoring the reaction progress by 1 H NMR spectroscopy showed that 43 then reacted with silylene to give the 1,2-disilacyclobutene 44. However, formation of 44 did not begin until all the acetylene reactant was transformed into the silacyclopropene 43 (Scheme 25)!42 These results appear to provide another example of 1,2-disilacyclobutene formation by a two-step silylene addition-insertion sequence.43 In this case the reaction of the silylene with the acetylene is faster than the silylene insertion into the silacyclopropenes 4, possibly due to steric hindrance in the insertion reaction. The isolation and reactions of these intramolecularly coordinated silylenes raises a reactivity issue. Most silylenes (40) Gehrhus, B.; Lappert, M. P. Polyhedron 1998, 17, 999. (41) Tamao, K.; Nagata, K.; Asahara, M.; Akawachi, Y. Ito; Shiro, M. J. Am. Chem. Soc. 1995, 117, 11592. (42) J. Belzner, H. Ihmels, B. O. Kneisel and R. Herbst-Irmer, J. Chem. Soc. Chem. Commun. 1994, 1989.

have a singlet ground state2b in which the silicon atom has a free electron pair in a σ orbital and an energetically lowlying, unoccupied orbital of π symmetry. As a result, silylenes may potentially react toward substrates either as electrophiles or as nucleophiles. Calculations show that reactions of dimethylsilylene with carbon-carbon multiple bonds proceed via an electrophilic interaction of its LUMO with the π electrons of the alkene or alkyne.44 In the case of stable silylenes such as 36 and 38 and the nitrogen-coordinated (chelated) silylene 39 the nucleophilic behavior appears to be enhanced. This was elegantly demonstrated in the reaction of the silylene from precursor 42 with a series of para-substituted diarylacetylenes.45 The reactions of such silylenes do not appear relevant to the present study involving more electrophilic silylenes, such as dimethylsilylene. Kira and co-workers have reported46 the synthesis and isolation of the dialkylsilylene 45, which is believed to be stabilized sterically from dimerization but is less perturbed electronically than the nitrogen-coordinated silylenes. Silylene 45 was reported to form a silacyclopropene on reaction with bis(trimethylsilyl)acetylene, although no details were reported.

On the basis of the above discussions on σ-insertion reactions, it seems reasonable to expect dimethylsilylene to insert into 23 to give 24.43 It has been shown that 24 reacts with dimethylacetylene to give 8, thus providing a silyleneacetylene route to the 1,4-disilacyclohexadiene ring system (Scheme 26). In the section reaction pathway I it was shown that 1,2disilacyclobutenes cannot be the common intermediates to all of the aforementioned disilacyclohexadienes: for example, compound 4. Disilacyclobutenes 31 and 32 with bulky carbon substituents have been shown to be stable toward acetylenes, supporting this conclusion. An interesting additional example involves the thermolysis of silacyclopropene 46 to give disilacyclobutene 47 (eq 8).33 The authors suggested that 46 decomposes to phenyl(tertbutyldimethylsilyl)acetylene and dimethylsilylene and the latter then inserts into a C-Si bond of as yet undecomposed (43) The reaction of 23 with dimethylacetylene to give 24 has been demonstrated in the vapor phase, but not under the static conditions being considered in this study. (44) (a) Anwari, F.; Gordon, M. S. Isr. J. Chem. 1983, 23, 129. (b) Baggot, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. J. Chem. Soc. Faraday Trans. 2 1988, 515. (45) Belzner, J.; Dehnert, U.; Ihmels, H. Tetrahedron 2001, 57, 511. (46) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999, 121, 9723.

Review


3581

Scheme 24

Scheme 25

Scheme 27

Scheme 26

46 to give 47. Clearly disilacyclobutene 47, even at high temperatures, is unreactive toward the acetylene byproduct.

These results suggest that the reaction of silylenes with acetylenes may indeed form 1,4-disilacyclohexadienes by more than one mechanism, with acetylenes with smaller substituents giving silacyclopropenes, followed by silylene σ insertion into the latter, giving 1,2-disilacyclobutenes, in turn followed by reaction of the disilacyclobutene with the acetylene to give the 1,4-disilacyclohexadiene (Scheme 26). Whether phenylmethylacetylene would follow Scheme 26 to give the disilacyclohexadienes 5 and 6 is not clear, but diphenylacetylene and other acetylenes with bulky substituents do not follow Scheme 26. C. Reaction Pathway III: Silylene π-Insertion into Silacyclopropene. This interesting possibility was first mentioned by Barton and Kilgour,31 although it appears to have received little further consideration. Attack of dimethylsilylene at the CdC bond of the initially formed silacyclopropene 23 would give the bicyclo[1.1.0] butane 48 (Scheme 27). It was proposed that 48 could rearrange to the 1,4-disilabutadiene 49. Although it has been demonstrated (reaction pathway I) that a 1,2-disilacyclobutene cannot be the common intermediate en route to a 1,4-disilacyclohexadiene, it is possible that 1,4-disilabutadienes could play this role. For example, direct reaction of 49 with dimethylacetylene would give 8, while the collapse of 49 would then lead to the byproduct disilacyclobutene 24 (see Scheme 27). The formation of species 48 and 49 has precedent in carbene

chemistry. Chesick found that 1,3-dimethylcyclo[1.1.0] butane isomerizes quantitatively to 2,3-dimethylbutadiene (eq 9). 47

We have reported that the thermolysis of 50 gives methoxycarbene (eq 10).48 Because of the similarity to methylmethoxysilylene we later explored the thermolysis of 50 in the presence of bis(trimethylsilyl)acetylene and obtained the cyclopropene 51 (eq 11).49 We found that 51

reacts with additional methoxycarbene to give a mixture of two isomeric compounds, 52 and 53 (Scheme 28). The stereochemistry of 52 and 53 was not determined. Heating the mixture of isomers at 150 °C gave a quantitative conversion of 52 to 53, and the reaction followed first-order (47) Chesick, J. P. J. Phys. Chem. 1964, 68, 2033. (48) Atwell, W. H.; Weyenberg, D. R.; Uhlmann, J. G. J. Am. Chem. Soc. 1969, 91, 2035. (49) Atwell, W. H.; Uhlmann, J. G. Upublished studies at Dow Corning Corp. 1970.

3582


Atwell Scheme 28

kinetics (k = 2.75 10-4 s-1; half-life 42 min at 150 °C). Thus, it seems entirely reasonable that silylenes and silacyclopropenes could follow a similar mechanistic path. With the silicon analogues one would expect the 1,4disilabutadiene to be extremely reactive, going on to form other products. This mechanism could explain how silacyclopropenes, 1,2-disilacyclobutenes, and 1,4-disilacyclohexadienes could all be formed from the reaction of silylenes with acetylenes. A similar pathway would explain the formation of related silicon heterocycles, such as Belzner’s 54 (Scheme 29).50 In chemistry relevant to the present discussion the preparation and structure of a 1,3-disilabicyclo[1.1.0]butane, 55, has been reported by Fritz and co-workers (Scheme 30).51 The specific reaction pathway to 55 was not defined. While highly reactive toward moisture and oxygen, 55 was found to be thermally stable. It was unchanged when heated at 130 °C for 20 h. Even after 50 h at 150 °C only 60% of 55 had decomposed. Unfortunately, no decomposition products were identified. It was noted earlier (reaction pathway II) that Seyferth and Vick39 prepared the 1,2-disilacyclobutene 34 in a reaction carried out at 70 °C. If 55 was a common intermediate to 34, it would have to isomerize (presumably through a 1,4-disilabutadiene) at temperatures of 70 °C (Scheme 31). Clearly, this is not the case. Also, a number of silylenes have been reacted with a variety of acetylenes,1-5 yet in none of these studies has the formation of 1,3-disilabicyclo[1.1.0]butanes been reported. All the available evidence suggest that the extreme reactivity of the silacyclopropene C-Si bond is greater than the reactivity of its π system. Gaspar and Conlin reported that tetramethylsilacyclopropene 23 is not an active dienophile, being inert to cyclopentadiene at room temperature for 1 week.25 All reported reactions of silacyclopropenes appear to involve the C-Si bond rather than the CdC bond. There may be one exception. The thermolysis of bis(silacyclopropene) 56 to give disila-dewarbenzene 57 and disilabenzvalene 58 was suggested by Ando and co-workers52 to proceed by way of silylene 59, as shown in Scheme 32. While other mechanisms are possible, the authors invoked the involvement of silylene 59 because they observed no isomerization between 57 and 58. If silylene 59 is involved in this reaction, the intramolecular insertion must be greatly preferred, since silylene 59 could not be trapped with external reagents such as methanol and butadiene. (50) Belzner, J.; Ihmels, H.; Noltemeyer, M. N. Tetrahedron Lett. 1995, 36, 8187. The authors prefer reaction pathway II to explain the formation of 51. (51) Fritz, G.; Wartanessian, S.; Matern, E. Z. Anorg. Allg. Chem. 1981, 475, 87. (52) (a) Ando, W.; Shiba, T.; Hidao, T.; Morihashi, K.; Kikuchi, O. J. Am. Chem. Soc. 1997, 119, 3629. (b) Kabe, Y.; Ohkubo, K.; Ishikawa, H.; Ando, W. J. Am. Chem. Soc. 2000, 122, 3775.

Scheme 29

Scheme 30

Scheme 31

Scheme 32

The synthesis of disilabicyclo[1.1.0]butane 55 by Fritz51 and the NMR experiments by Belzner42 (reaction pathway II) seem to provide strong evidence that, in the reaction of silylenes with acetylenes, 1,2-disilacyclobutenes are formed by a σ insertion of the silylene into the initially formed silacyclopropene. Thus, in the reaction of silylenes with

Review


acetylenes, reaction pathway III does not play a role en route to the formation of 1,4-disilacyclohexadienes. Reaction Pathway IV: σ-Dimerization of Silacyclopropenes. In our early studies we concluded that “the formation of disilacyclohexdienes proceeds by a rather specific dimerization of silacyclopropene intermediates with rupture of the carbon-silicon bond” 8c (see Scheme 33). At that time we considered this to be a highly unusual process, especially for such reactive silacyclopropenes that had been formed at high temperatures in the presence of both silylenes and acetylenes. Yet we were forced to this conclusion by the available evidence. It now appears that the above conclusion is actually valid for some silacyclopropenes, although it cannot be universally applied. As was stated previously, understanding the chemistry of silacyclopropenes is one of the keys to understanding the formation of 1,4-disilacyclohexadienes. I will selectively explore this subject. There are a number of reports dealing with the photolysis of silacyclopropenes. The decomposition mechanism generally results in the extrusion of a silylene and the formation of an acetylene (Scheme 34).2b,5,53-55 In some cases 1,2-disilacyclobutenes may be formed by insertion of the extruded silylene into the as yet undecomposed starting silacyclopropene56 (Scheme 34). Ring-opening rearrangement is often observed59 (eq 12). In all of these studies of the photolysis of silacyclopropenes, the formation of disilacyclohexadienes was not reported.

In contrast to the photolysis reactions, the thermolysis of silacyclopropenes, although similar in many aspects, is considerably more complex. The isolation of tetramethylsilacyclopropene (23)25 provided the first opportunity to test the σ-dimerization reaction pathway. However, it was found (53) Ishikawa, M.; Fuchikami, T.; Kumada, M. J. Am. Chem. Soc. 1977, 99, 245. (54) Sakurai, H.; Kamiyama, Y.; Nakadaira, Y. J. Am. Chem. Soc. 1977, 99, 3879. (55) (a) Gaspar, P. P.; Beatty, A. M.; Chen, T.; Haile, T.; Lei, D.; Winchester, W. R.; Braddock-Wilking, J.; Rath, N. P.; Klooster, W. T.; Koetzle, T. F.; Mason, S. A.; Albinati, A. Organometallics 1999, 18, 3921. (b) Hawari, J. A.; Griller, D.; Weber, W. P.; Gaspar, P. P. J. Organomet. Chem. 1987, 326, 335. (56) Ishikawa, M.; Nishimura, K.; Segisawa, H.; Kumada, M. J. Organomet. Chem. 1980, 194, 147. (57) Seyferth, D.; Annarelli, D. C. J. Am. Chem. Soc. 1975, 97, 2273. (58) Ishikawa, M.; Fuchikami, T.; Kumada, M. J. Chem. Soc., Chem. Commun. 1977, 352. (59) Sakurai, H.; Kamiyama, Y.; Nakadaira, Y. J. Am. Chem. Soc. 1977, 99, 3879.

3583

Scheme 34

Scheme 35

that heating a solution of 23 above 75 °C gave only polymer and none of the expected disilacyclohexadiene 8 (Scheme 35). In this study the structure of the polymer was not determined. A decomposition path involving extrusion of dimethylsilylene is unlikely in this case, since this silylene is known57 to insert into dimethyldimethoxysilane (at temperatures of 75 °C) to give dimethoxytetramethyldisilane (2), which is stable under the reaction conditions employed. Neither 2 nor dimethylacetylene were reported. It is also unlikely that the dimethyldimethoxysilane diluent inhibited dimerization, since this silane is a byproduct when 2 reacts with dimethylacetylene to give disilacyclohexadiene 8.7 A second opportunity to test the σ-dimerization proposal came with isolation of the previously described silacyclopropene 27.27 However, 27 seemed to decompose by silylene extrusion, and bis(trimethylsilyl)acetylene was the major product identified. None of the dimeric disilacyclohexadiene was observed. While silacyclopropenes were found to readily dimerize in the presence of palladium salts (eq 13),58-60 these reaction conditions are outside the scope of this review.

It was the studies of Kumada and co-workers33,61 that provided the first experimental confirmation of the σ dimerization of a silacyclopropene. Heating silacyclopropene 60 in a sealed tube at 250 °C gave an 80% yield of the expected 1,4-disilacyclohexadienes 61a,b as well as a (60) Ishikawa, M.; Sugisawa, H.; Kumada, M.; Higuchi, T.; Matsui, K.; Hirotsu, K. Organometallics 1982, 1, 1473. (61) Ishikawa, M.; Fuchikami, T.; Kumada, M. J. Organomet. Chem. 1977, 142, C45.

3584


Atwell Scheme 36

Scheme 37

20% yield of the 1,2-disilacyclobutene 31 and small amounts of silacyclopentadiene 62 (Scheme 36). These results suggested that the primary thermolysis path involved σ dimerization, with silylene extrusion being a secondary mode and two-atom insertion of the acetylene into the silacyclopropene ring a minor mode. Numerous two-atom insertions into silacyclopropene 27 had been reported earlier.62 In a significant experiment it was found that heating silacyclopropene 60 at 250 °C in the presence of a 3-fold excess of diphenylacetylene gave a 62% yield of the isomers 61a,b and none of the crossover product 63, again confirming (reaction pathway II) that 1,2-disilacyclobutenes are not the common intermediates en route to 1,4-disilacyclohexadienes33 (Scheme 37). These results clearly support a σ-dimerization mechanism for the conversion of 60 to 61a,b! Further, heating 60 in the presence of isopropylbenzene (a free-radical trapping agent) again gave the isomers 61a,b (75% yield) and 31 (15% yield). No homolytic scission, hydrogen abstraction products were observed, suggesting a concerted pathway for all involved mechanisms.33 In their studies, Kumada and co-workers33 provided initial molecular orbital calculations to further support their proposed concerted [2σ + 2σ] dimerization. While more detailed studies were promised, I am unaware of their completion. These researchers also studied the effect of different substituents on the ring carbon and silicon atoms in an attempt to unravel the complicated chemistry of these silacyclopropenes. From their results I see these general trends. With dimethylsilicon groups and smaller carbon substituents, for example silacyclopropene 60, the primary decomposition mechanism (80%) is σ dimerization. Silylene extrusion is a secondary (20%) route, and two-atom insertion of the acetylene into the silacyclopropene ring is a minor (62) (a) Seyferth, D.; Duncan, D. P.; Vick, S. C. J. Organomet. Chem. 1977, 125, C5. (b) Seyferth, D.; Vick, S. C.; Shannon, M. L. Organometallics 1984, 3, 1897.

Scheme 38

(5%) pathway. Because of the secondary mechanism, 1, 2-disilacyclobutenes are formed via silylene insertion into the silacyclopropene ring. These latter disilacyclobutenes do not react with the generated acetylene to give disilacyclohexadienes. When the bulk of the substituents on the ring carbon atoms is significantly increased, for example in silacyclopropene 46, the σ-dimerization is completely suppressed and thermally stable disilacyclobutenes are obtained as the major product. Two-atom insertion of the acetylene into the silacyclopropene ring does not occur, presumably due to steric considerations. With phenyl substituents on carbon, silylene extrusion continues to dominate, and the major side reaction is two-atom insertion of the resulting acetylene into the as yet unreacted silacyclopropene. Silylene insertion into the initial silacyclopropene (leading to disilacyclobutene formation) is now a minor pathway. With phenyl substitution on silicon, silylene extrusion is the major path and two-atom insertion of the resulting acetylene into the starting silacyclopropene becomes an increasingly important path. The minor path is now σ dimerization to disilacyclohexadienes, and no silylene insertion into the starting silacyclopropene is observed. Finally, with two phenyl groups on silicon the primary path is silylene extrusion followed by two-atom insertion of the resulting acetylene into the as yet unreacted silacyclopropene. In this case, neither disilacyclohexadiene nor disilacyclobutene formation is observed. Previous studies had shown56 that in silacyclopropenes with bulky substituents on the silicon atom, e.g. 64, trimethylsilyl migration to a ring-opened ethynylsilane (65) is the process that is favored (eq 14). However, a related phenyl-substituted silacyclopropene (66) was remarkably stable and on thermal decomposition gave phenyl (trimethylsilyl)acetylene as the only identified product (Scheme 38). The effect of substituents on the kinetic,

Review


thermal, and oxidative stability is striking. For example silacyclopropene 67 with only hydrogen (Scheme 39) on ring carbons is highly unstable with a half-life of 46 min at -50 °C.36b Larger substituents on the ring carbons seem to be a requirement for kinetic stabilization. With increasing size of the substituents on the ring carbon atoms, silacyclopropene 60 is stable to at least 130 °C.53 Although most silacyclopropenes are very reactive to oxygen, water, and methanol, two bulky groups on the silicon atom of silacyclopropene 66 cause it to be resistant to these reagents and to have thermal stability greater than 200 °C.56

Clearly, the thermal reactions of silacyclopropenes are quite complex and the reaction pathways are substituent dependent. However, the available evidence shown in this section supports the conclusion that, indeed, certain silacyclopropenes dimerize by a specific, concerted σ-dimerization mechanism.

V. Relevant Reaction Pathways During this extensive review of the evidence, I found myself wondering about the relevance of the thermal results on individual silacyclopropenes to the reactions of our early work involving methoxydisilane 2 and acetylenes.7 While reaction temperatures were similar, in the thermolysis of individual silacyclopropenes their concentrations are high, decomposition of the silacyclopropene is easily detected through acetylene or 1,2-disilacyclobutene formation, and acetylene concentrations are generally low. In contrast, in the case of the thermolysis of 2 in the presence of acetylenes, acetylene concentration is high, silylene concentrations are low, and silacyclopropene concentrations are likely quite low and their decomposition is nondetectable. Our early work involving methoxydisilane 2 and acetylenes showed that at least seven disilacyclohexadienes (Scheme 40) were formed under essentially identical reaction conditions. What role might these differing reaction conditions (i.e., heating silacyclopropenes vs heating methoxydisilane 2 with acetylenes) play in the formation of 1,4-disilacyclohexadienes? Earlier I stated that “assuming one operative mechanism, any selected reaction pathway must be consistent with the formation of these seven disilacyclohexadienes.” The available evidence strongly suggests, although not conclusively, that more than one mechanism is involved in the formation of the above seven disilacyclohexadienes. Indeed,

3585

Scheme 40

the suggestion that more than one mechanism may be involved had been made earlier.31,63 All available evidence on tetramethylsilacyclopropene 23 indicates that it does not undergo such thermal dimerization. However, 1,4-disilacyclohexadiene 8 is a major product in the reaction of disilane 2 with dimethylacetylene.7a What are the reaction pathway alternatives for the formation of 8 under these reaction conditions? The evidence suggests only two alternatives. The first involves the formation of 8 via the thermal ring-chain equilibrium of the polymer shown in Scheme 35. While ring-chain equilibration is common to many polymer systems, a catalyst is normally required. I find this mechanism less likely. Only reaction pathway II, silylene σ-insertion into silacyclopropenes, seems available. In this pathway dimethylsilylene reacts with dimethylacetylene to give 23, into which dimethylsilylene inserts, giving 24. Finally 24 reacts with dimethylacetylene to give 8 (see Scheme 22). While the formation of 24 has not been demonstrated in sealed-tube experiments,43 its formation seems reasonable. With the known, limited stability of 23, the formation of 24 requires that silylene insertion into 23 be more rapid than its reaction with dimethylacetylene. Just the opposite was demonstrated in the work of Belzner42 (see reaction pathway II). However, it seems that dimethylsilylene and tetramethylsilacyclopropene should both be more reactive than the compounds of the Belzner study. The thermal sealed-tube reaction of 24 with dimethylacetylene has been demonstrated,31 making this reaction pathway alternative more acceptable. As mentioned earlier, the pathway involved in the reaction of 24 with dimethylacetylene has not been defined but may indeed involve electrocyclic ring opening of 24 to give the previously suggested disilabutadiene intermediate (Scheme 15).31 In view of their structural similarity to 8, I am inclined to believe that disilacyclohexadienes 11 and 12 are formed by the same mechanism. With regard to disilacyclohexadiene 4, the evidence shows that it cannot be formed as shown in Scheme 21, as the required 1,2-disilacyclobutene 32 is thermally stable in the presence of acetylenes.34 Thus, I must conclude that 4 is formed by mechanism IV, the σ dimerization of silacyclopropene 3. In the case of disilacyclohexadienes 5-7 I can find no additional studies between silylenes and phenylmethylacetylene and it is not possible to reach a mechanistic conclusion concerning their reaction pathways with silylenes. (63) Halevi, E. A.; West, R. J. Organomet. Chem. 1982, 240, 129.

3586


VI. Summary and Conclusions The evidence indicates that the reaction of silylenes with acetylenes is rather straightforward and 2-fold; they react with acetylenes to give initially silacyclopropenes and, in addition, they can insert into silacyclopropenes to give 1, 2-disilacyclobutenes. The solution to the perplexing disilacyclohexadiene problem appears to lie mainly, although not solely, with the chameleon-like behavior of silacyclopropenes, a behavior that seems to be due to the electronic as well as the steric effect of its substituents. The substituents have a dramatic effect on the course of the thermal chemistry of silacyclopropenes, a chemistry dominated by reactions at the carbon-silicon bonds. This study has described five substituent-dependent pathways for the thermal reactions of silacyclopropenes: (a) σ-dimerization to 1,4-disilacyclohexadienes, (b) silylene insertion to give 1,2-disilacyclobutenes, (c) decomposition to silylenes and acetylenes, (d) two-atom insertion of acetylenes to give silacyclopentadienes, and (e) group (H- and Me 3Si-) migration with ring opening to isomeric ethynylsilanes. While the mechanisms of these reactions are not well understood, the evidence provided by Kumada and coworkers suggest that routes a-d may well be concerted, as no homolytic scission or hydrogen abstraction products are found when silacyclopropene 60 is heated in the presence of isopropylbenzene.34 This suggests an unprecedented silacyclopropene/disilacyclohexadiene reaction pathway. In spite of the excellent work by numerous researchers, the course of the thermal reaction of silacyclopropenes remains largely unpredictable.

Atwell

With regard to the reaction pathway to the aforementioned seven disilacyclohexadienes (Scheme 40), evidence on tetramethylsilacyclopropene 23 indicates that it does not undergo σ dimerization, yet 1,4-disilacyclohexadiene 8 is a major product in the reaction of methoxydisilane 2 with dimethylacetylene. On the basis of the available evidence, reaction pathway II (Scheme 22) remains as the only pathway alternative. In the case of 1,4-disilacyclohexadiene 4 the evidence shows that it does not follow reaction pathway II, leaving reaction pathway IV as the only available alternative. Thus, I must conclude the reaction pathways II and IV are both involved in the formation of 1,4-disilacyclohexadienes when silylenes react with acetylenes in sealed tubes at 200275 °C. My personal opinion is that we are still missing some critical piece of evidence required for a resolution of this problem. Thus, despite the efforts of the past 40 years, aspects of this perplexing problem remain unresolved at this time. It is my hope that the alliance between mechanistic experiments and theoretical calculations will continue to accelerate and hopefully focus on this issue. Finally, while it is with some disappointment that I leave aspects of this problem unsolved, I find that often the journey is more enjoyable than the destination.

Acknowledgment. This review is dedicated to the memory of Dr. Donald R. Weyenberg, my mentor and lifelong friend. I am indebted to Dr. Peter Gaspar for his invaluable assistance and encouragement during my preparation of this paper. The fine editing efforts of Dr. Dietmar Seyferth and the efforts of Sophia White for the preparation of a publication-ready document are also gratefully acknowledged.

The Reaction of Silylenes with Acetylenes: In ... - ACS Publications

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