Kinetics, silene reactions

The focus of the section on silene reaction kinetics is mainly on studies of bimolecular reactions of transient silene derivatives, because little absolute kinetic data exist for the reactions of stable derivatives and there have been few quantitative studies of the kinetics of unimolecular isomerizations such as ,Z-isomerization and pericyclic rearrangements, although a number of examples of such reactions are of course well known. In contrast, most of the studies of disilene reaction kinetics that have been reported have employed kinetically stable derivatives, and E,Z-isomerization has thus been fairly well characterized. The paucity of absolute rate data for unimolecular isomerizations of transient silenes and disilenes is most likely due to the fact that it is comparatively difficult to obtain reliable data of this type for transient species whose bimolecular reactions (including dimerization) are so characteristically rapid, unless the unimolecular process is itself relatively facile. Such instances are rare, at least for transient silenes and disilenes at ambient temperatures. 

Over the past ten years, absolute rate data have been reported on the kinetics of several bimolecular silene reactions in solution, including both head-to-tail and head-to-head dimerization the [l,2]-addition reactions of nucleophilic reagents such as water, aliphatic alcohols, alkoxysilanes, carboxylic acids and amines and the ene-addition, [2 + 2]-cycloaddition and/or [4 + 2]-cycloaddition of ketones, aldehydes, esters, alkenes, dienes and oxygen. The normal outcomes of these reactions are summarized in Scheme 1. 

Reactions favoring [2 + 2] cycloaddition tended to be those that had strongly electronegative groups on the sp2-hybridized silicon but only H and the neopentyl group on the sp2-hybridized carbon atom. Butadiene and cyclohexadiene generally favored [2 + 2] cycloaddition with these silenes. The [2 + 2] adducts with cyclohexadiene appear to be kinetic products, since they cleanly isomerized to the Diels-Alder adducts over time.182... 

Whether the [2+2] or [2+4] cycloaddition product is formed in the first reaction depends on the reaction conditions however, one product is convertible to the other via the silene intermediate. In the other reactions shown, the identity of the product formed, [2+2] or [2+4], seemed to be a function of the structures of both the silene and imine. In the last case, the [2+4] product appeared to be the kinetic isomer since conversion to the [2+2] isomer slowly occurred on standing in the dark, or faster if photolyzed, even at room temperature. 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

The effect of ring substituents on the rate constants, deuterium kinetic isotope effects and Arrhenius parameters for ene-additions of acetone to 1,1-diphenylsilane have been explained in terms of a mechanism involving fast, reversible formation of a zwitterionic silene-ketone complex, followed by a rate-limiting proton transfer between the a-carbonyl and silenic carbon. A study of the thermal and Lewis acid-catalysed intramolecular ene reactions of allenylsilanes with a variety of... 

Silene (27) can undergo a 1,2-shift to give either methylsilylene (28) or, less favorably, to silylcarbene (29). The thermochemistry and the kinetics of these reactions have been points of major disparity between theory and experiments... 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

Silenes of the family Me3SiR1Si=C(OSiMe3)Ad-l 137 undergo a complex silene-to-silene photoisomerization reaction90,94,96. When silenes 137 are generated by photolysis of acylsilanes 138, the isomeric silenes 139 and 140 are formed in a subsequent reaction. The reaction was followed by UV and NMR spectroscopy. The disappearance of 138 cleanly follows first-order kinetics and the overall kinetics were consistent with the transformation 138 -> 137 -> 139. 137 as well as 139 were characterized by NMR spectroscopy and, in addition, the structure of 137 was established by trapping with methanol. The identity of 139 and 140 was confirmed by the isolation of their head-to-tail dimers from which crystals, suitable for X-ray analyses, were isolated (equation 34)90. 

In recent years the application of laser techniques has become an important tool in the study of the kinetics of relatively simple reactive organosilicon species (silylenes, silenes etc.). Gaspar and coworkers have reviewed the use of laser techniques to study the generation and reactions of silylenes14. 

This chapter reviews the body of literature (to August 2000) that deals specifically with kinetic studies of the reactions of silenes and disilenes and the mechanistic information that has been derived from them. The spectroscopic properties, structures, methods of synthesis and qualitative aspects of the reactivity of silenes and disilenes have been covered comprehensively in the preceding two volumes in this series and elsewhere, and so will not be treated extensively here. 

III. KINETICS AND MECHANISMS OF THE REACTIONS OF SILENES A. Unimolecular Reactions... 

As mentioned earlier, few absolute kinetic data exist for unimolecular reactions of silenes, largely because most of the known silene rearrangements occur under high temperature pyrolytic conditions and are difficult to measure directly8,10,12. 

Diphenylsilene (19a), produced by photolysis of 1,1-diphenyl- or 1,1,2-triphenylsilacyclobutane (17a and 18, respectively equation 11), has been particularly well studied, and absolute rate constants have been reported for a wide variety of silene trapping reactions in various solvents at room temperature (see Table 3)40-46. Not all of these have been accompanied by product studies, unfortunately. A number of other transient silenes have been characterized as well with solution-phase kinetic data for a range of bimolecular silene trapping reactions, though much less extensively than 19a. These include the cyclic l,3,5-(l-sila)hexatriene derivatives 21a-c (formed by photolysis... 

Absolute rate constants for reaction of MeOH with a wide variety of other transient silenes have also been reported, but with the main goal of quantitatively defining the effects of substituents on the kinetic and/or thermodynamic stability of the Si=C bond in various structural situations. These data are discussed in Section III.C of this chapter. 

TABLE 6. Absolute rate constants and deuterium kinetic isotope effects for reaction of 1,1-diaryl-silenes (XC6H4)2Si=CH2 (19a-e) with acetic acid (AcOH) in acetonitrile solution at 23 °C (in units... 

The absolute rate constants for ene-addition of acetone to the substituted 1,1-diphenyl-silenes 19a-e at 23 °C (affording the silyl enol ethers 53 equation 46) correlate with Hammett substituent parameters, leading to p-values of +1.5 and +1.1 in hexane and acetonitrile solution, respectively41. Table 8 lists the absolute rate constants reported for the reactions in isooctane solution, along with k /k -, values calculated as the ratio of the rate constants for reaction of acetone and acctonc-rff,. In acetonitrile the kinetic isotope effects range in magnitude from k /k y = 3.1 (i.e. 1.21 per deuterium) for the least reactive member of the series (19b) to A hA D = 1.3 (i.e. 1.04 per deuterium) for the most reactive (19e)41. Arrhenius plots for the reactions of 19a and 19e with acetone in the two solvents are shown in Figure 9, and were analysed in terms of the mechanism of equation 46. 

The kinetics of acetone addition have also been studied for several aryldisilane-derived (l-sila)hexatrienes, including 21a-c47,51, 4651 and 62 (from photolysis of 61 equation 47), where the reaction follows a different course than that of simpler silenes such as 1952. In these cases, the reaction proceeds via two competing pathways, formal [2 2]-cycloaddition and ene-addition. Unlike the case with the simpler silenes, however, the ketone rather than the silene acts as the enophile in the reaction, presumably because this alternative has the formation of an aromatic ring as an added driving force. 

Kinetic studies have recently been reported in an effort to assess systematically the effects of substituents at silicon and carbon on silene reactivity, and put the earlier theoretical work on a firm experimental basis111,117. Absolute rate constants for addition of methanol served as the diagnostic indicator of silene reactivity in these studies, since methanol addition is mechanistically the best understood reaction of transient silenes (vide infra) and can be studied under the widest possible range of conditions in photochemical experiments. Both studies were carried out in hydrocarbon solution at 23 °C, so as to provide a standard set of experimental conditions for the analysis and to minimize the effects of solvation on Si=C reactivity. 

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