Multiple bonding silenes

This fundamental discovery dramatically affected the whole chemistry of main-group elements. Subsequently, a series of new compounds with silicon element multiple bonds has been introduced. Within only a few years, stable silenes (silaethenes with a Si = C double bond) [8-11], silaimines Si = N [12-14], and silaphosphenes Si = P [15] were synthesized. As a pacemaker, silicon chemistry has exerted a strong influence on further areas of main-group chemistry a variety of stable molecules with Ge = Ge [16], P = P [17], As = As [18], P = C and P = C [19-22] bonds were subsequently isolated, and systems with cumulated double bonds P = C = P [23-25] are also known today. 

Many silenes react cleanly in a [2 + 2] manner with carbon-carbon multiple bonds, and several examples of such behavior have been given above. Some additional examples are listed in Scheme 16, there now being too many examples known to allow a full listing. For the cases listed,33,65 185189 each of which involves a polarized carbon-carbon double bond, the products were isolated and well characterized and the main products were those of a [2 + 2] reaction. Wherever regioisomerism was possible, only a single regioisomer has been observed this includes cases in which unsymmetric alkynes were involved. 

In addition to undergoing cycloaddition reactions with alkenes and al-kynes, silenes readily undergo cycloaddition reactions with heteroatom multiple bonds such as C=0 and C=N, most commonly when the trapping reagent for the silene is either an aldehyde, ketone, or imine. In many... 

Despite the differing levels of calculations, the same general conclusions were reached. The silicon-carbon double bonds in 1-silaallene (1.69 A) and 2-silaallene (1.70 A) are shorter than in isolated silenes at the same level of theory. This trend is also observed in the analogous carbon series. 1-Silaallene is thermodynamically more stable than 2-silaallene by 21 kcal/mol (22). Intuitively, this is what would have been expected, realizing the low ability of silicon to participate in multiple bonds. As may be expected from simpler systems (i.e., H2Si=CH2)(i97), silylene isomers (for example, structures 8 and 9) are considerably more stable (approximately IS kcal/mol) than their silaallene counterparts. 

Silaethylenes (silenes) were extensively reviewed in 1979 and the first example stable enough to be bottled was prepared two years later. It results from the photolysis of the ketone (56) and is stable in the absence of air and other reagents. It melts at 92-95 °C and shows a ai NMR spectrum with the two Me3Si groups on the Si=C multiple bond nonequivalent, indicating restricted rotation. The IR absorption at 1135 cm-1 typifies a silaethylene (Scheme 85) (81CC191). 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

Experimentalists have been particularly well served by numerous reviews from some of the leading workers in the field. For example, Brook and Baines (76) have reviewed silenes, Wiberg (77) discussed M=C and M=N double bonds (M = Si and Ge), Cowley and Norman (78) have reviewed M=M (M, M = group 14 and 15 elements) double bonds, Raabe and Michl (79,80) have provided complementary reviews of multiple bonds to silicon, West (81) has reviewed disilene chemistry, Masamune (82) has reviewed the work of his group on Si=Si and Ge=Ge compounds, and most recently Barrau et al. (83) have summarized multiple bonds to germanium. Studies from the reactive intermediates era of this field are beautifully summarized by Gusel nikov and Nametkin (84). 

Brook, Adrian G., and Brook, Michael A., The Chemistry of Silenes Brook, Michael A., see Brook, Adrian G. Brothers, Penelope J and Power, Philip P., Multiple Bonding Involving 39 71... 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

Since several excellent reviews on the chemistry of multiple bonds to silicon [see the chapter written by G. Raabe and J. Michl in a previous volume of this series (1989) ] and also the reviews in Advances in Organometallic Chemistry, Vol. 39 (1995) (e.g. The Chemistry of Silenes by A. G. Brook and M. A. Brook , IminosUanes and Related Compounds—Synthesis and Reactions by I. Hemme and U. Klingebiel, Silicon-Phosphorus and Silicon-Arsenic Multiple Bonds by M. Driess and Chemistry of Stable Disilenes by R. Okazaki and R. West ) have appeared in recent years and as some parts of this chapter will be discussed in detail in related chapters of this book (e.g. Silenes and Iminosilanes by N. Auner and coworkers and Recent Advances in the Chemistry of Disilenes by H. Sakurai ), we will concentrate our attention in this chapter on the chemistry of silicon-chalcogen doubly-bonded compounds. 

Precursor of Silene. 1,1-Dimethylsilacyclobutane is pjTO-lyzed at ca. 600 °C to generate 1,1-dtmethylsilene. The highly reactive silene rapidly undergoes dimerization to provide l,3-disila-l,l,3,3-tetramethylbutane.4 C-C4 and C-O multiple bonds are able to trap the silene. The copyrolysis... 

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