RESEARCH
Aliphatic alcohols split Si-Si bond Reactions catalyzed by palladium chloride form variety of silane products Organosilicon chemists at Kyoto University, Kyoto, Japan, have worked out several syntheses of unusual compounds, giving insight into several little-worked areas of silicon chemistry. Highlighting the work under way in the laboratory of Dr. Makoto Kumada, this year's F. S. Kipping Award winner (see page 7 5 ) , are these recent findings: • 1,1' - Bis ( silicon-substituted )ferrocenes rearrange on heating, one silicon side group shifting from the ferrocenyl group to the other to form a disubstituted group. • Vinyl-substituted disilanes undergo cleavage at the Si-Si bond when treated with aliphatic alcohols and a catalytic amount of palladium chloride. The reaction, which is accompanied by hydrogénation, yields a variety of silane products. • Cyclic polysilanes such as decamethylcyclopentasilane and dodecamethylcyclohexasilane form when hexaphenyltrisilane is treated with small amounts of triphenylsilyllithium in tetrahydrofuran. • Heterocyclic compounds such as 1, l,2,2-tetramethyl-l,2-disilacycloalkanes containing three to six methylene groups in the ring can be formed by reacting α,ω-bis ( chlorodimethylsilyl)alkanes with sodium-potassium alloy or by reacting 1,2-dichlorotetramethyldisilane with di-Grignard re agents from α,ω-dibromoalkanes. The Japanese work establishes guideposts in what has been largely uncharted territory. The ferrocene rearrangement, for example, appears to be a general reaction, Dr. Kumada says. Thus, both l,l'-bis(pentamethyldisilanyl) ferrocene and l,l'-bis(trimethylsilyl) ferrocene isomerize on heating to about 300° C. for about 60 hours, one side group migrating from one ring to the other. The reaction is relatively sensitive to temperature. Below about 280° C , the reaction pro ceeds very slowly. But between 290° and 310° C. it goes quite smoothly. The reaction also seems to be rever sible. Using gas chromatographic analysis, the Japanese chemists detected starting and end materials in a 1/1 ratio after heating at 300° C. for about 60 hours. They separated the two fractions by preparative GC, then determined structure by infrared and 46 C&EN MARCH 13, 1967
nuclear magnetic resonance measure ments. So far, they haven't been able to pin down relative positions of the side groups. Dr. Kumada feels, how ever, that these side groups probably occupy the 1,3 rather than the 1,2 po sitions. While the mechanism of the reac tion is not clearly understood, Dr. Ku mada and his group have what they feel are some pretty good ideas. They think that interaction of the nonbonding orbital of iron and the d-orbital of a silicon atom is involved. But much more work is needed, the award win ner says. "We must work with other compounds such as feri-butyl ferro cene," he notes. "Here, the carbon atoms have no d-orbitals. If our ex planation is correct, this compound should not rearrange. We are now checking this out. We are also work ing with 1, l'-bis ( dimethylphosphino ) ferrocene." In compounds such as hexamethyldisilane, the Si-Si bond normally resists cleavage when the compounds are treated with alkaline or acidic reagents under normal conditions. Recent work at Kyoto, though, shows that vinylpentamethyldisilane, (CH 3 ) 3 Si-Si(CH 3 ) 2 CH=CH 2 , does split at the Si-Si bond when reacted with an equivalent amount of ethanol or other aliphatic alcohol in the presence of catalytic amounts (5 to 10 mole%) of palladium chloride at 0° C. Without palladium chloride, there is no cleav age. The reaction yields a number of compounds. Among them are trimethylethoxysilane, dimethylvinylethoxysilane, dimethylethylethoxysilane,
and pentamethylethyldisilane (an uncleaved product). The concentration of alcohol has a marked effect on the reaction. With equimolar amounts of alcohol and vinylpentamethyldisilane, for instance, the reaction takes about 45 minutes to complete. With 50% excess alcohol, however, the reaction starts instantly and produces a full range of products. The Japanese chemists have devel oped a tentative but "most probable" reaction model. In this model, oxy gen from the alcohol attacks the sili con atom that is attached to the vinyl group. At the same time, a hydrogen atom from the alcohol bonds with the other silicon to form the trimethylsilane intermediate. This way, alcohol facilitates cleavage of the Si-Si bond. The trimethylsilane intermediate then reacts with more alcohol to form (CH 3 ) 3 SiOR and a complex of hydro gen and palladium. Meanwhile, palla dium has also complexed with the other fragment of the original disilane molecule, the part containing the vinyl group. Finally, the complexed hydro gen is transferred to the vinyl group, forming a range of saturated products. This hydrogénation, which the Japanese chemists find always accompanies cleavage of this type, is perhaps the most significant and potentially useful aspect of the system. In fact, an analogous system consisting of silicon hydride, alcohol, and catalytic amounts of palladium chloride gives a general route for hydrogenating unsaturates. This way, olefins can be hydrogenated to the corresponding aliphatic compounds. To name a few, 1-hep-
REARRANGEMENT. Dr. Makoto Kumada traces the silicon side group shift which forms a disubstituted group when l,l'-bis(trimethylsilyl)ferrocene is heated
Nucleophilic cleavage of vinyl-substituted disilanes yields variety of silane products 100 90 80
(CH3)3Si-Si(CH3)2CH-CH2+C2H50H (equimolar) PdCI2 (10 mole %) , 0° C. products
70
acterized by a ring containing two adjacent silicon atoms and three, four, five, or six methylene groups. (Dr. Gilman and Dr. S. Inoue previously prepared a heterocyclic compound which contains five silicon atoms and three methylene groups in the ring.) The Kyoto group has developed two routes to these 1,1,2,2-tetramethyl1,2-disilacycloalkanes :
((%) 2 Si—Si (CH 3 \
50 100 Time, minutes
tene gives η-heptane; styrene gives ethylbenzene; cyclohexene gives cyclohexane; isoprene gives 2-methylbutene-2 and two other isomers; phenylacetylene gives styrene and ethylbenzene; and acetophenone gives aphenethylalcohol. In similar studies, Dr. John L. Speier and his coworkers at Dow Corning in Midland, Mich., have studied reactions of olefins with trichlorosilane, catalyzed with either platinum or palladium to form corre sponding aliphatic compounds. This type of hydrogénation gives a relatively easy route to otherwise hardto-make compounds. Chloromethyldimethylisopropylsilane [ ClCH 2 Si( C H 3 ) 2 C H ( C H 3 ) 2 ] , for instance, can be formed this way. Thus, chloromethyldimethylchlorosilane reacts with isopropenylmagnesium bromide, yielding chloromethyldimethylisopropenylsilane as an intermediate in yields exceeding 7 0 % . In the following step, this intermediate is hydrogenated at room temperature with triethoxysilane in the presence of palladium chloride to produce the desired chloromethyldimethylisopropylsilane end product, a waterlike liquid at room temperature. The chloromethyl group remains intact throughout the reaction sequence, Dr. Kumada notes. In contrast, direct synthesis using the sterically hindered and relatively unreactive Grignard reagent isopropyl magnesium halide and chloromethyldimethylchlorosilane is not "smooth/* Hydrogénation of this type will be extended to a wide range of compounds and will possibly have commercial application in the future, Dr. Kumada says. He notes that this study ties in with other work by Dr. C. Eaborn, Sussex University, England,
who suggested in the patent literature that transition metal salts, such as palladium acetate, should be active hydrogénation catalysts when treated with silicon hydride compounds. In still other work, the Kyoto chemists have staked out a new route to cyclic polysilanes. They start with a synthesis previously worked out by Dr. Henry Gilman of Iowa State University. In this synthesis, triphenylsilyllithium is reacted with dimethyldichlorosilane in tetrahydrofuran to give an intermediate product, 1,3-hexaphenyl-2-dimethyltrisilane. This intermediate substituted trisilane is the starting point for the Japanese synthesis. The trisilane is treated with a small amount of triphenylsilyllithium in T H F to form both decamethylcyclopentasilane and dodecamethylcyclohexasilane. About three times as much pentamer as hexamer forms. (The hexamer was characterized previously by Dr. Gilman and his coworkers.) In contrast with the Kyoto findings, Dr. Robert West and his coworkers at the University of Wisconsin find that the reaction yields more hexamer than pentamer when dimethyldichlorosilane reacts with sodium-potassium alloy in THF. Furthermore, this synthesis produces some heptamer in small yield. The isolated pentamer, a crystalline solid melting between 188° and 190° C., should help in studies of oxidation. The pentamer changes in air to a liquid in a few days as it oxidizes to a compound whose structure is not known. The Japanese chemists have also prepared a new series of heterocyclic organosilicon compounds that are char-
In one system, an α,ω-bis ( chlorodimethylsilyl)alkane is reacted with so dium-potassium alloy in benzene-nheptane. In the other, 1,2-dichlorotetramethyldisilane is reacted with the di-Grignard reagent from an α,ω-dibromoalkane in T H F . These reactions were carried out mainly at reflux tem perature. The disilacyclopentane is rather sen sitive to air oxidation, the Japanese find. On oxidizing, it forms a ring compound in which the oxygen is in serted between the two silicon atoms. This reaction may help elucidate the oxidation of the cyclic pentasilane. The Japanese work on heterocyclic compounds opens a whole new area of silicon chemistry, Dr. Kumada says. "In carbon chemistry, heterocyclic compounds have been extensively studied. But in silicon chemistry, this type of compound is new," he adds. The Kyoto chemists have already ex tended their original study, and have synthesized a new series of l,2-dimethyl-l,2-diphenyl-l,2-disilacycloalkanes, where the methylene groups number four, five, or six. The route to these compounds involves treatment of tetrachlorodimethyldisilane with phenylmagnesium chloride in T H F to give l,2-dimethyl-l,2-dichloro-l,2-diphenyldisilane as an in termediate. This intermediate is then reacted with a bis(bromomagnesio)alkane, BrMg(CH 2 ) n MgBr, having four, five, or six methylene groups to form the desired ring compound. Since the intermediate phenyl-substituted disilane contains two asym metric silicon atoms, it exists as a mix ture of d,l and meso isomers. These can be separated by fractional distilla tion and crystallization. From each of the isomers, it is possible to form geo metric isomers having the heterocyclic ring configuration. The Kyoto chemists are now study ing the stereochemistry of this class of compounds. One reaction being in vestigated is chlorodephenylation of the l,2-dimethyl-l,2-diphenyl-l,2-disilacycloalkanes at 10° C.; sulfuric acid and ammonium chloride give the analogous chlorine-substituted di methyl disilacycloalkanes. MARCH 13, 1967 C&EN 47
