Organometallics 2009, 28, 3571–3572 DOI: 10.1021/om900313e
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Introduction to the Review by W. H. Atwell in This Issue of Organometallics In his review in the present issue of Organometallics, Dr. William H. Atwell has focused on an interesting question: by what pathway is our cover molecule, 1,1,4,4-tetramethyl2,3,5,6-tetraphenyl-1,4-disilacyclohexa-2,5-diene (1), formed in the reaction of dimethylsilylene with diphenylacetylene? This compound was not the expected product and, as Dr. Atwell’s review shows, this question has perplexed organosilicon chemists for some 45 years. In his review Dr. Atwell addresses this question by bringing to bear a considerable amount of relevant, interesting, and important organosilicon chemistry.
To understand why this question has been of special interest, it is necessary to know how it arose. Therefore, this Editor’s Page brings a short historical introduction which, I hope, will enhance the reader’s appreciation of Dr. Atwell’s excellent review. In a seminal paper published in 1962,1 using chemical bonding concepts of the time, Mark Vol’pin and his collaborators at the Institute of Organo-Element Compounds of the USSR Academy of Sciences in Moscow considered the possible existence and aromatic character of unsaturated three-membered-ring compounds that contain as a ring member an atom of the main group 13, 14, and 15 elements. In the case of the group 14 elements, they predicted that the at that time unknown sila-, germa- and stannacyclopropenes, 2, should be sufficiently thermodynamically stable to allow their preparation and isolation. As a possible preparative route to silacyclopropenes, they suggested the addition of a divalent silicon species, a silylene, R2Si, to the CtC triple bond of an acetylene, as shown in eq 1. Two procedures were suggested for the generation of dimethylsilylene: (1) the abstraction of the chlorine substituents from (CH3)2SiCl2 by an alkali metal and (2) the thermolysis of the linear polysilylene [(CH3)2Si]55, which had been prepared earlier by Burkhard at the GE Research Laboratory.2 Since dimethylsilylene was expected to be a highly reactive species (like a carbene), it would have to be generated in situ in the presence of the diphenylacetylene substrate. Both approaches were tried. The action of sodium on (CH3)2SiCl2 in refluxing xylene carried out in the presence of an excess of diphenylacetylene gave a solid product, mp 324-328 °C (without decomposition), in minute (0.5%) yield. In the other approach, a mixture of a [(CH3)2Si]n polysilylene and diphenylacetylene was heated and stirred at 250-260 °C under nitrogen for 7.5 h. The same solid was isolated in 6% yield. On the basis of its elemental analyses, IR (1) Vol’pin, M. E.; Koreshkov, Yu. D.; Dulova, V. G.; Kursanov, D. N. Tetrahedron 1962, 18, 107. (2) Burkhard, C. A. J. Am. Chem. Soc. 1949, 71, 963. r 2009 American Chemical Society
spectrum, and rather strange behavior when cryoscopic molecular weight measurements were attempted, the product was claimed to be the silacyclopropene 3. That the product was very thermally stable and kinetically very stable as well, resistant to air oxidation and to attack by bromine, was cause for suspicion, since a silacyclopropene should be highly strained and hence highly reactive. Within three years other groups, principally on the basis of molecular weight measurements, mass spectral studies, and, especially, an X-ray diffraction study,3 had shown that the product that Vol’pin et al. had isolated actually was a dimer of 3: our cover molecule, 1.
If, as Vol’pin had expected, his two approaches did indeed proceed by attack of dimethylsilylene on diphenylacetylene, then the silylene must have been a very minor side product at best in both reactions.4 However, there was (and still is) doubt that silylene chemistry was involved in these reactions. In particular, Eaborn suggested5 that the (CH3)2SiCl2/Na/ C6H5CtCC6H5 reaction might have involved initial interaction between sodium with diphenylacetylene, instead of with dimethyldichlorosilane, to generate by electron transfer the sodium salt of the diphenylacetylene radical anion, [C6H5CtCC6H5]•-, followed by reaction of the latter with the chlorosilane. Although the well-known dimerization of the radical anion to give the 1,2,3,4-tetraphenylbutadiene dianion probably would be the preferred process, if the radical anion was the initially formed intermediate, a minor extent of interaction between the radical anion and dimethyldichlorosilane would be sufficient to give the extremely low yield of 1. The claimed generation of dimethylsilylene by thermolysis of polydimethylsilylene could not be reproduced by Dr. Atwell, (3) Bokii, N. G.; Struchkov, Yu. T. Zh. Strukt. Khim. 1965, 6, 571. (4) Nefedov and co-workers reported using both procedures to generate dimethylsilylene, but in the examples described, yields of products ascribed to (CH3)2Si trapping processes also were very low. The polydimethylsilylene thermolysis was said to proceed by way of SiSi bond homolysis with subsequent formation of monomeric, dimeric, and oligomeric diradicals: (a) Nefedov, O. M.; Manakov, M. N. Angew. Chem., Int. Ed. Engl. 1964, 3, 226. (b) Nefedov, O. M.; Garza, G.; Szekely, T.; Shiryaev, V. I. Dokl. Akad. Nauk SSSR 1965, 164, 822; Dokl. Chem. 1965, 945 (English translation). (5) Eaborn, C. In Organometallic Compounds of the Group IV Elements; MacDiarmid, A. G., Ed.; Dekker: New York, 1968; Vol. 1, Part I , pp 330-331.
Published on Web 05/20/2009
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Organometallics, Vol. 28, No. 13, 2009
and it was pointed out6 that the formation of dimethylsilylene in Vol’pin’s experiment could have been due to, or facilitated by, the presence of siloxane impurities in the polymer or to the intervention of homolytic Si-Si bond fission rather than direct silylene extrusion. Therefore, the provenance of 1,4-disilacyclohexadiene 1 as of 1969 was uncertain. In retrospect, we now know that Vol’pin’s innovative ideas and predictions concerning the thermal stability of silacyclopropenes of type 2 (M = Si) were perfectly sound. In the intervening years, various silacyclopropenes have been isolated as thermally stable, albeit hyperreactive, compounds using exactly the route proposed by Vol’pin et al.: the addition of a silylene to an acetylene. If Vol’pin and his co-workers would have had available a more reliable silylene precursor, their quest for a silacyclopropene would have been successful. And now Dr. Atwell, the author of the review which follows, enters the story. A compound, 2,3-benzo-7,7-dimethyl-1,4,5,6tetraphenyl-7-silanorbornadiene (4), whose thermolysis at 300 °C without doubt results in the extrusion of dimethylsilylene, was prepared by Dr. Atwell a short time later when he was a graduate student at Iowa State University. Thermolysis of 4 in the absence of a silylene trapping agent gave 1,2,3,4tetraphenylnaphthalene and an amorphous, high-melting polydimethylsilylene. When, following Vol’pin’s lead, the thermolysis of 4 was carried out in the presence of diphenylacetylene, 1 was obtained in 67% yield.7 The possibility that 1 might have been formed by a [2 + 2] cyclodimerization, 5, of initially formed silacyclopropene 3 was suggested, but there was no experimental support for such a process. This was Dr. Atwell’s first encounter with the (CH3)2Si/C6H5CtCC6H5 system. It provided the impetus for his further experimental work and long-term interest in the silylene/acetylene reactions.
William Atwell, a native of Milwaukee, obtained a B.S. in chemistry (1958) and an M.S. degree in Organic Chemistry (1960) from Marquette University in Milwaukee. He continued his graduate studies at Iowa State University, where, under (6) Atwell, W. H.; Weyenberg, D. R. Angew. Chem., Int. Ed. Engl. 1969, 8, 469 (cf. footnote 41, p 471). (7) Gilman, H.; Cottis, S.; Atwell, W. H. J. Am. Chem. Soc. 1964, 86, 1596.
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the guidance of Professor Henry Gilman, he carried out research for his dissertation in several areas of organosilicon chemistry. These included the synthesis of sterically encumbered silicon hydrides such as (PhMe2Si)3SiH and studies of perphenylated linear and cyclic polysilanes and silacyclic compounds such as silacyclobutenes, silacyclopentenes, and 1,3-disilacyclobutanes, in addition to the preparation of 4 and related compounds by Diels-Alder reactions of 1-silacyclopentadienes. After the award of his Ph.D. degree in 1965, Dr. Atwell joined the Dow Corning Corporation in Midland, Michigan, where he had a distinguished and productive career, initially in research and later in research and business administration. In collaboration with Donald R. Weyenberg, he carried out outstanding research on new and useful silylene precursors and silylene reaction chemistry, including the silylene/acetylene reactions which are covered in some detail in his review. Also reported in papers and patents were aspects of disilane and polysilane reactivity not related to silylene chemistry as well as other molecular organosilicon chemistry. He became heavily involved in the ceramics program when this was started at Dow Corning, with extensive studies on the synthesis of polycarbosilanes, polysiloxanes, and polysilanes whose high-temperature pyrolysis produced silicon-containing ceramics such as silicon carbide and silicon oxycarbide. At one time Dr. Atwell served as Director of Basic Silane and Semiconductor Research and Development and for the eight years prior to his retirement in 1997 he was Director of Ceramics, responsible for both the business aspects and research and development. Since his retirement, Dr. Atwell has retained an active interest in organosilicon chemistry. In particular, his interests have been focused on the unanswered question of how in some cases;by what pathways;1,4-disilacyclohexadienes are produced when a silylene precursor is thermolyzed in the presence of an acetylene. A lot of very interesting organosilicon chemistry relevant to this question has been published since Dr. Atwell first encountered this reaction in 1964, work that he carried out at Dow Corning and the work of many others, and he has done a fine job of extracting the results that are important from these many papers. The result is an interesting story. My thanks to Professor Arnold L. Rheingold for providing the figure of the cover molecule.
Dietmar Seyferth Editor Received April 23, 2009
