Silicon & Germanium Reactive Intermediates

To organic chemists, the term "reactive intermediate" brings to mind such things as carbenes, free radicals, carbenium ions, strained double bonds, and many others. How does the extraordinary - and often fascinating - reactivity of such molecules change when we replace one or more of the carbon atoms with another Group 14 element, like silicon for example? What happens when the "active" carbon is replaced, as for example in going from R₂C: (carbene) to R₂Si: (silylene)? What happens when the switch is made at a neighbouring or even more remote site? How do reaction mechanisms change? Does this knowledge help us predict what might happen when we go a step further, as in replacing silicon with germanium? These are just some of the questions that our research in Silicon & Germanium Reactive Intermediates is directed at answering.

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, in particular germenes.

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.

Nick Toltl, Tracy Morkin, and Cam Harrington are examining 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. 

Former post-doc Corinna Kerst pioneered the use of the argon fluoride excimer laser (193-nm) as an excitation source for solution phase laser flash photolysis experiments. With this technique and the aid of Rabah Boukherroub's synthetic expertise, she has studied the chemistry of several very simple silenes - including Me₂Si=CH₂ and MeHSi=CH₂ - in hydrocarbon solvents. These silenes, and a number of others of the type MeRSi=CH₂, are obtained from 193-nm flash photolysis of the corresponding 1-Me-1-R-silacyclobutane derivatives. The manner in which the absolute rate constants for reaction of these silenes with alcohols varies with R defines how the inductive, resonance, and steric effects of substituents at silicon affect the electrophilic reactivity of the Si=C bond.

The effects of substituents at carbon are being probed using silene precursors of a number of different types, such as silacyclobutenes, vinyldisilanes, 1,3-disilacyclobutanes, and a-silyl ketenes and diazo compounds.  The work on a-silyl ketenes (7; XY = CO) is 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.  Tracy Morkin is synthesizing several compounds of this type in order to study their photochemistry and the chemistry of their carbenes and silenes.

Tom Owens is studying 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, is being 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.

Postdoctoral fellow Xiaojing Li is also employing stopped-flow kinetics in his work on the kinetics of chlorosilane hydrolysis and other substitution reactions in various solvents, in a project sponsored by Dow-Corning Inc.

Selected Publications - Organosilicon Photochemistry

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