RESEARCH

Silenes form via alpha elimination Dow Corning chemists detect silenes spectroscopically during thermolysis of methoxy-substituted polysilanes

Two chemists at Dow Corning have found a way to generate silenes that has enabled them to considerably ex­pand the study of these short-lived in­termediates. Dr. William H. Atwell and Dr. Donald R. Weyenberg of the Midland, Mich., company find that these divalent organosilicon com­pounds—for example, dimethylsilylene, (CH : i)2Si:—can be made by thermally decomposing methoxy-substituted pol­ysilanes.

Dr. Weyenberg told conferees at last week's International Symposium on Organometallic Chemistry (spon­sored by the International Union of Pure and Applied Chemistry), in Mu­nich, West Germany, that the reaction involves an alpha elimination to form the silenes. Evidence for this mecha­nism comes from the structure of reac­

tion products, reaction kinetics, and mass spectral studies. In their spec­tral studies, Dr. Atwell, Dr. Weyen­berg, and Dr. Roland S. Gohlke (also at Dow Corning) have actually de­tected formation of the dimethylsilyl­ene intermediate during thermolysis of 1,2-dimethoxytetramethyldisilane in the inlet chamber of a mass spectro­graph.

Dr. Atwell and Dr. Weyenberg find that the silenes react with a variety of acetylenes to give a disilacyclohex-adiene ring system. The reaction route appears to be a rather specific dimerization of silacyclopropene in­termediates. In another reaction with substituted butadiene, the silene inter­mediate gives a silacyclopentene ring system. The product is the same whether the silene is generated from a

disilane or from a 7-silanorbornadiene. Interest in the divalent silicon inter­

mediates has grown steadily in the last five years after research in the Soviet Union suggested their transi­ent existence as products from the re­action of diorganodichlorosilanes with metals or from the thermal decomposi­tion of polysilanes. Since then, a num­ber of specific studies using these two general approaches have been made, notably by Dr. M. E. Vol'pin and Dr. O. M. Nefedov in the U.S.S.R. and Dr. Philip S. Skell at Pennsylvania State University. The Dow Corning research has centered on methoxy-substituted polysilanes, which enable multiple syntheses useful in explaining the role played by these novel divalent species.

Last year, Dr. Atwell and Dr. Wey­enberg found that thermolysis of 1,2-dimethoxytetramethyldisilane proceeds readily at 225° to 250° C. to give di-methyldimethoxysilane and o:,w-dime-thoxypermethyl polysilanes having three, four, and five dimethylsilyl groups. Their data strongly suggest a mechanism for the reaction involving insertion of the silene in unreacted disilane molecules after alpha elimina­tion of the starting compound.

Thermolysis in the presence of a trapping agent such as diphenylacetyl-ene allows interception of the inter­mediate before it inserts in the di­silane. The resulting product is 1,1,-4,4-tetramethyl - 2,3,5,6-tetraphenyl-1,4-disilacy clohexadiene.

Since their first example of silene preparation from 1,2-dimethoxytetra-methyldisilane, the Midland chemists have studied enough related polysilane reactions to suggest that generation of silenes by thermolysis of methoxy-sub­stituted polysilanes is a general reac­tion. The reaction is not limited to disilanes. The tetramethyl tetra-phenyl (TMTP) disila ring compound also forms (in 35% yield) when 1,3-di-methoxyhexamethyltrisilane is heated at 250° to 275° C. with diphenylacety-lene. Moreover, methoxy groups are not essential, Dr. Weyenberg says, since disilanes bearing other groups such as (CH3)3SiO or CI also yield di­valent intermediates.

Further examples of this reaction include thermolysis of tetramethoxydi-methyldisilane. In the presence of di-

Heat decomposes a variety of silicon compounds

CH* CH* i * l 3

C H i - O - S i — S i - O - C H a 0 I l °

CH* CH*

CjH5 C f tHj

H5C6C=CC*H5

Dimethylsilylene C H 3 C H 3

/ri - j Thermolysis of polysilanes, methoxy-substituted

polysilanes, and 7-silanorbornadienes yields di-X M i l valent organosilicon intermediates (silenes).

* c ; " ' These silenes, such as dimethylsilylene, react *CHi I wrth acety ,enes or butadienes to produce di-

silacyciohexadienes or silacyclopentenes through J presumed intermediate silacyclopropenes or

silacyclopropanes

\ ! J > H i CH

H 5 C i ^ j 5 ' i

CH3 C H j

1,1,4,4-Tetramethyl-2,3,5,6-tetra phenyl-1,4-disilacyclohendieiie

30 C&EN SEPT. 4, 1967

RING SYSTEMS. Dr. William H. Atwell (left) and Dr. Donald R. Weyenberg of Dow Corning find that silènes, divalent organosilicon compounds, react with a variety of acetylenes to give disilacyclo-hexadiene ring systems

phenylacetylene, this reaction pro­duces 30 to 35% of the 1,4-dimethoxy-substituted analog of the TMTP disila ring compound. The intermediate in this case is methylmethoxysilylene (CH3SiOCH3) . The product is about a three-to-one mixture of as-yet-unassigned cis and trans isomers.

Dr. Atwell and Dr. Weyenberg have tried extending the methoxy substitu­tion a step further by studying ther­molysis of hexamethoxydisilane. How­ever, no tetramethoxy ring compound resulted from hexamethoxydisilane thermolysis in the presence of diphen-ylacetylene. Instead, they obtained only tetrakis(trimethoxysilyl)silane re­sulting from reaction of the supposed (CH 3 0) 2 Si with the starting disilane. This shows that diphenylacetylene does not compete effectively with hex­amethoxydisilane for this particular in­termediate. The two chemists note that the apparent change in reactivity of the dimethoxysilylene intermediate in this reaction parallels the behavior of dialkoxycarbenes.

Mechanism. Dr. Atwell and 'Dr. Weyenberg suggest that the divalent species is formed by an alpha-elimina­tion route from the original polysilane material. This is consistent with the formation of the disila ring compound during thermolysis of variously substi­tuted disilanes in the presence of di­phenylacetylene.

New evidence of silène formation now comes from mass spectral studies showing the silènes as directly ob­served, short-lived intermediates. This elimination mechanism is also indi­cated in the kinetics of thermolysis. Distribution of thermolysis products remains constant upon dilution with

either dioxane or diphenyl ether. In addition, the rate of disilane consump­tion is not accelerated by the presence of trapping agents such as acetylenes.

Although the mechanism for the re­action of silènes with acetylenes is not completely worked out, Dr. Atwell and Dr. Weyenberg have shown that the disilacyclohexadienes seem to be formed from the silènes by a rather specific dimerization of silacyclopro-pene intermediates. Thus, they pro­pose the novel small-ring intermediate structure originally considered by the U.S.S.R. scientists as the structure of the TMTP disila ring compound.

This selectivity in forming products is shown by the absence of products which might be expected from simple reaction paths such as pi-dimerization (linkage of two silacyclopropene rings to form a six-membered compound such as the TMTP disila ring com­pound). For example, research at Midland on thermolysis of 1,2-dime-thoxytetramethyldisilane in the pres­ence of a mixture of diphenylacety­lene and dimethylacetylene shows no resulting 3,5-diphenyl-substituted di-silacyclohexadiene compound. This effectively eliminates pi-dimerization as a possible mechanism.

Dr. Atwell and Dr. Weyenberg also rule out formation of disilacyclohexa­dienes by dimerization of an initial diradical in which the silène is bound to one carbon of the acetylenic com­pound. This does not seem likely be­cause the low-concentration diradical should reasonably react with excess acetylene to give a substituted silacy-clopentadi^ne. In fact, no such Rve-membered ring has been detected de­spite repeated efforts.

The two chemists have also turned to other unsaturated trapping agents besides acetylenes. Heating 1,2-di-methoxytetramethyldisilane with 2,3-dimethylbutadiene gives the five-mem-bered ring product 1,1,3,4-tetramethyl-l-silacyclopent-3-ene (14%) . The same product also results (40%) when a 7-silanorbornadiene is used as the source of the dimethylsilylene. The lower yield in the first instance is a consequence of the competing inser­tion reaction of the ( CH3 ) 2Si interme­diate with the disilane starting mate­rial.

Dr. Atwell and Dr. Weyenberg fore­see future work on thermolysis of other substituted polysilanes. Cur­rent work indicates that thermolysis in the vapor phase proceeds along very similar lines. Vapor phase reactions are especially interesting because of the possibility they offer for detecting the so-far elusive silacyclopropene in­termediates. These intermediates may be in a more isolated state in the vapor phase and could perhaps be detected prior to dimerization.

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SEPT. 4, 1967 C&EN 31

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B12 coenzyme bonds measured precisely

Bond distances within the molecule of vitamin B12 coenzyme (5'-deoxyaden-osylcobalamin) have been measured precisely in work at Vanderbilt Uni­versity's department of physics. Dr. P. Galen Lenhert, continuing x-ray dif­fraction studies of the coenzyme begun seven years ago at Oxford University, described his results at the 7th Inter­national Congress of Biochemistry, in Tokyo.

The 2.24-A. cobalt-benzimidazole bond in the coenzyme is about 0.2 A. longer than that in vitamin B12 (cyano-cobalamin). The distances between the coenzyme's cobalt atom and the four nitrogen atoms in the corrin nu­cleus surrounding it are apparently slightly longer than are those for vita­min B12. The cobalt-carbon bond, connecting the adenine nucleoside to the cobinamide portion of the mole­cule, is surrounded by two methyl and two methylene groups, all of which project 2 A. above the cobalt atom. Along with the adenosine itself, these groups protect the organometallic bond from the approach of reagents. Judging from the distances separating the corrin nucleus from some atoms in the adenosine portion, Dr. Lenhert points out, free rotation of adenosine about the cobalt-carbon bond in solu­tion doesn't seem likely.

While at Oxford in 1960-61, Dr. Lenhert was associated with Dr. Dorothy Crowfoot Hodgkin in the crystallographic study that solved the coenzyme's three-dimensional molecu­lar structure. In the vitamin B l 2 co­enzyme work, crystals grown from acetone-water solution were photo­graphed wet. Four coenzyme mole­cules and some 68 water molecules make up the unit cell. Visual estima­tion of the intensities of 3068 bragg re­flections yielded atomic positions with a standard deviation of about 0.04 A. after mathematical refinement.

Dr. Lenhert continued to refine the coenzyme's crystal structure using the Oxford data until about 18 months ago, when more accurate data from diffractometer measurements began to be gathered at Vanderbilt.

RESEARCH IN BRIEF

The antibiotic Myxin decomposes vio­lently on combustion, according to Dr. A. I. Rachlin of Hoffmann-La Roche research laboratories, Nutley, N.J. Dr. Rachlin cautions that "utmost care" should be used if large quantities of Myxin are heat dried. Pure Myxin, heated at 20° C. per minute, is strongly exothermic at 149° C.

32 C&EN SEPT. 4, 1967