Anybody would expect that the silicon and germanium analogues of all the species listed above should be quite reactive indeed, and they'd be right. What you might not expect is that some of the relatively stable organic functional groups, such as olefinic C=C bonds and carbonyl groups, become extremely "hot" when this simple atomic substitution is made. In fact, multiple bonds involving silicon or germanium are so thermodynamically unstable that up until about 30 years ago, there were no known examples of molecules containing them. We now know a few hundred examples of molecules which contain Si=C (silenes) or Si=Si (disilenes) bonds. Several are stable enough that they can be synthesized and placed in a bottle. Most, however, are highly reactive species with lifetimes on the order of microseconds or less in the gas phase or in solution. Understanding their chemistry is important from the fundamental perspective of bonding and reactivity in main group chemistry, and also because they are involved as reactive intermediates in a wide variety of thermal and photochemical reactions in organosilicon chemistry, many of which are important technologically.

Our group employs nanosecond laser flash photolysis techniques to generate various M=C, M=M and R₂M: species photochemically, detect them directly, and study their rich reactivity. The main thrust of our work so far has been the elucidation of the mechanisms of "classic" silene and disilene trapping reactions (such as the addition of alcohols, amines, carbonyl compounds, alkenes, etc.) through product- and kinetic-studies of photochemically-generated, transient derivatives. Similar work is now being carried out on the germanium analogs, germenes (containing a Ge=C bond) and digermenes (containing a Ge=Ge bond)..

Two examples of stable molecules which yield reactive silenes upon photolysis are aryldisilanes (1) and silacyclobutanes (3). Photolysis of 1 yields the (1-sila)hexatriene derivative 2, which has a lifetime of only a few microseconds in solution at room temperature.

Photolysis of diphenylsilacyclobutane (3) and related compounds yields 1,1-diphenylsilene (4), whose lifetime is shorter than that of 2 under the same conditions. Both these silenes react rapidly with alcohols, carbonyl compounds, carboxylic acids, oxygen, and halocarbons to give a variety of interesting and unusual products. Many of these reactions are new and are continuing to be studied by our group.

Former students Nick Toltl and Tracy Morkin carried out studies of the reactivity of the germanium analogues of some of these compounds, such as 1,1-diphenylgermene (5) and the (1-germa)hexatriene derivative 6, which are prepared in the same ways as the corresponding silicon compounds 4 and 2. The results indicate that all else being equal, Ge=C bonds are orders of magnitude less reactive than Si=C bonds in their reactions with nucleophiles, although the mechanisms for reaction are similar.  On the other hand, 4 and 5 dimerize to the corresponding 1,3-dimetallacyclobutane derivatives with equal facility in solution, at the near-diffusion-controlled rate of ~1 x 10¹⁰ M^-1s^-1. 

The photolysis of silacyclobutanes affords a versatile method for synthesizing a wide variety of reactive silenes with different substituents at silicon, for study by flash photolysis methods.  Former post-doc Corinna Kerst examined the reactivity of more than 10 such silenes toward alcohols, in an effort to learn how the inductive, resonance, and steric character of substituents at silicon affect Si=C reactivity as electrophiles.  The synthesis of reactive silenes varying systematically in the substituents at carbon cannot be done easily using silacyclobutanes as precursors, but we have put together a fairly comprehensive series of silenes using other photoactive silicon-containing compounds such as silacyclobutenes, vinyldisilanes, 1,3-disilacyclobutanes, and a-silyl ketenes and diazo compounds.  The work on a-silyl ketenes (7; XY = CO) was part of a collaborative project with T.T. Tidwell and his group at the University of Toronto; photolysis of these compounds yields the corresponding a-silyl carbene (8), which undergoes rapid rearrangement to a transient silene (9). a-Silyl diazo compounds behave similarly and are more versatile than the ketenes in that they also allow us to study the kinetics of the a-silyl carbene rearrangement.  The bulk of former PhD student Tracy Morkin's work was directed at synthesizing several compounds of this type in order to study their photochemistry and the chemistry of their carbenes and silenes.

More recent students have taken this work in exciting new directions.  PhD graduate Tom Owens studied the reactions of stabilized silenes in solution, and those of various simple disilenes in solution and the gas phase. His work on the stabilized silene 11 and the disilene 12, which are obtained together upon photolysis of the novel trisilacyclobutane 10, was carried out collaboratively with Y. Apeloig and his group at the Technion in Israel, and involves both conventional laser flash photolysis and stopped-flow kinetic techniques. Cam Harrington, and Greg Potter were both interested mainly in germanium reactive intermediates; Greg's MSc thesis continued our studies of diarylgermene (Ar₂Ge=CH₂) reactivity and diarylgermacyclobutane photochemistry, while Cam's PhD thesis concerns the reactivity of diaryl- and dialkylgermylenes formed by photolysis of 1,1-disubstituted 1-germacyclopent-3-ene derivatives (13) and other appropriately-structured organogermanium compounds. Cam also recently studied the chemistry of the divalent tin species SnMe₂ in solution, generated by photolysis of the organotin analogue of 13 (with R1=R2=Me).

The current group is further exploring the photochemistry of germacyclopent-3-enes and the chemistry of transient germylenes in solution. PhD student Farah Lollmahomed has recently completed her first paper on the chemistry of GeMe₂ in solution and is now looking at the chemistry of various other substituted germylenes, while MSc student Lawrence Huck is unravelling the mechanisms of various germylene reactions through the study of substituent effects in ring-substituted diphenylgermylenes. Post-doc Andrey Moiseev is examining the chemistry of transient silylenes such as SiMe₂ and SiPh₂ in solution, while senior undergrad Paul Billone is working on a project in silene chemistry.

Selected Publications - Group 14 Reactive Intermediates

30dec05; wjl