Silaimines

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. 

Unfortunately, the chemistry of the deamlnation/condensation process is poorly understood thus, little can be said about the mechanisms and kinetics of the amine elimination step, the nature of the "silaimine" (7,8) intermediate or the condensation step. It seems reasonable to predict that if one could learn to control the relative rates for these two steps, more control could be exerted over the condensation process and the properties of the precursor polymer. 

The mechanism of the dehydrocyclization reaction is not completely understood. Two alternatives were proposed (11), one which proceeds via a silaimine mechanism, exemplified by reaction (25), and one wherein ring closure occurs by nucleophilic displacement of hydrogen, reaction (26). 

Modeling studies where R = R = Me provide evidence in favor of a silaimine intermediate however, efforts to trap the silaimine species were unsuccessful. Thus, the support for a specific ring closure mechanism remains inconclusive. 

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). 

The above work follows earlier and more comprehensive efforts by the Seyferth group25 on the use of base catalyzed crosslinking of cyclotetrasilazane, —[MeHSiNH]4—, to increase its ceramic yield and processability. The currently proposed mechanism is suggested to involve silaimine formation followed by an addition reaction across the reactive double bond, as illustrated in equation 1325,26. 

Silylene-azide complexes are probable intermediates in the formation of silaimines from silylenes and covalent azides (equation 66)147,148. 

Summary Treatment of geminal silyldiamines with monohalo silanes leads to the formation of 1-amino-1,3-disilazanes. Mono-NH-SiF-functional cyclodisilazanes can be isolated by ring closure of their lithium derivatives in the reaction with trifluorosilanes. Lithium salts of these cyclodisilazanes react with H-acidic compounds like alcohols as amides and with unsaturated compounds like aldehydes as silaimines. 

Their reactivity toward unsaturated compounds like aldehydes is comparable to silaimines, e.g. ... 

The reaction proceeds in 65% yield. When a less sterically encumbered azide (NsSiMcs) is used, a different product derived from an intermediate silaimine, results (71%) ... 

Subsequent studies showed that silyl azides, where the silicon atom was located at the bridgehead of either bicyclo[2.2.2] or [2.2.1] ring systems, also led to silaimines as a result of ring expansion, the imine being trapped by (trimethoxy)methylsilane present in the photolysis solution136 (equation 87). 

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