Silenes intramolecularly coordinated

It is an intriguing idea to stabilize low-valent silicon species, such as silyl cations, silylenes, silenes, and disilenes using intramolecularly coordinating ligands. Corriu et al. succeeded in the preparation of the first hypervalent silyl cation [(8-Me2NCioH6)2SiH]+l/2[l8]z 782 by the reaction of the hexacoordinated diorganosilane... 

Intramolecularly coordinated silenes have been prepared by Oehme et al.868-871 Adopting their general procedure for the synthesis of silenes (Scheme 33), 8-dimethylaminonaphth-l-yl lithium has been reacted with (Me3Si)3SiCHCl2 yielding the disilene (Me3Si)RSiC(SiMe3)2 929 via the intermediates 930-933 (R = 8-dimethyl-aminonaphth-l-yl) (Scheme 131). 

In a similar way, 2-(A, A -dimethylaminomethyl)phenyllithium and 2,6-bis(iV,iV-dimethylaminomethyl)-phenyllithium (Figure 4) have been used to prepare the intramolecularly coordinated silenes 934 and 935 (Figure 7). 

The reactivity of the intramolecularly coordinated silenes 929, 934, and 935 has been investigated. The hydrolysis and methanolysis of 929 proceed via formation of Si-O bonds and yield the products 936 and 937 (Scheme 132). The reaction of 929 with benzaldehyde can be regarded as sila-Wittig reaction, and proceeds via the initial formation of the cycloaddition product 938, which rearranges into the cyclodisiloxane [R(Me3Si)SiO]2 939 (R = 8-dimethylamino-naphthyl-l-yl) and l,l-bis(trimethylsilyl)-2-phenylethene 940 (Scheme 132). Methyl iodide catalyzes the isomerization of 929, which proceeds via addition to the Si-C... 

Silenes (compounds with an Si=C double bond) are known to be extremely reactive compounds. Till now, only a few silenes, stabilized kinetically by bulky substituents, could be isolated under normal conditions and were characterized by X-ray structural analyses [1]. An effective stabilization of Si=C systems is also achieved by coordination of a base, such as amines, THF, or fluoride ions, to the electrophilic silene silicon atom [2], and recendy we succeeded in synthesizing thermally unexpectedly stable intramolecularly donor-stabilized silenes [3]. In this paper, the synthesis and, in particular, the behavior of the l-[2,6-bis(dimethylaminomethyl)phenyl]silenes 2a-d are described. 

The intramolecularly donor-stabilized silenes 2a-d are thermally stable compounds. 2a, 2c and 2d are colorless solids 2b is an oil. X-ray structural analyses unambiguously revealed that in the solid state only one dimethylamino group of the ligand is coordinated to the silene silicon atom. Important structural parameters, e.g., the Si=C bond lengths and the Si-N distances, are in good agreement with data obtained for other intramolecularly amine-coordinated silenes [3]. 

Donor-acceptor interactions between the electrophilic Si atoms of three-coordinate silicon derivatives and suitable bases lead to an effective stabilization of these usually extremely reactive systems. Thus, for example, silenes and silanimines have been shown to be stabilized by tertiary amines [1]. Extraordinary stabilizations have been achieved by intramolecular Si-N coordinations, making possible the isolation of stable silanethiones [2] and silylium salts [3]. Intramolecularly donor-stabilized silenes have been unknown till now, possibly because of the lack of suitable synthetic methods. 

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