Silenes with amines

Following studies of our research group [16-19] and later of other work groups [20], silenes with polar double bonds form adducts with donors D, which like halogenides, ethers, or amines possess atoms with a free, that is a nonbonding electron pair (Eq. 2) ... 

The formation of w-complexes of silenes with donors like ethers or amines is understandable within the scope of effective HOMO/LUMO overlapping (Scheme 8). 

The relative rates of reaction of the silene Me2Si=C(SiMe3)2 with a series of amines, alcohols, phenols, thiophenols, dienes, and alkenes were obtained174 and are reported in Table VIII Section IV.C. 

Wiberg determined the relative rates of addition of various alcohols and amines to silene 6, the results of which have been summarized above in Table 26. The fastest rates of addition were observed with aliphatic alcohols and amines, leading to the hypothesis that the first step of the reaction involves complexation of the neutral nucleophile at silicon, followed by proton transfer to the silenic carbon. Subsequent reports of the X-ray crystal... 

Several examples were discussed earlier of the use of substituent effects for the elucidation of the mechanisms of silene reactions with nucleophilic reagents. For example, the trends in the rate constants for reaction of the series of 1,1 -diarylsilenes 19a-e with alcohols, acetic acid, amines, methoxytrimethylsilane and acetone all indicate that inductive electron-withdrawing substituents at silicon enhance the reactivity of the Si=C bond, and are consistent with a common reaction mechanism in which reaction is initiated by the formation of an intermediate complex between the silene and the nucleophile. 

Silene Insertion into CH Bonds By thermolyzing complexes of silaneimines with ethyldimethyl amine in solvents, one surprisingly observes the formation of insertion products of the silenes into a-C-H bonds of the donor [3]. For example, the silaneimine tBu2Si=NSiChBu2 slowly transforms in benzene even at room temperature into the mentioned insertion product (Scheme 9) [12]. 

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]. 

By far the best source for 3a is (trimethylsilyl)diazomethane (19). It has already been mentioned that gas-phase pyrolysis of 19 alone yields products which are derived from intramolecular carbene reactions such as 1,3-C,H insertion and silylcarbene-to-silene rearrangement (see equation 20). Also, copyrolysis of 19 with alcohols or benzaldehyde allowed one to trap the silene but not the carbene 33 (see equation 5). Furthermore, solution photolysis of 19 in the presence of alcohols or amines did not give the X,H insertion products of the carbene but rather trapping products of the silene . On the other hand, photochemically generated carbene 3a did undergo some typical intermolec-ular carbene reactions such as cyclopropanation of alkenes (ethylene, frani-but-2-ene, but not 2,3-dimethylbut-2-ene, tetrafluoroethene and hexafluoropropene), and insertion into Si—H and methyl-C—H bonds (equation 39). The formal carbene dimer, trans-1,2-bis(trimethylsilyl)ethene, was a by-product in all photolyses in the presence of alkenes it is generally assumed that such carbene dimers result from reaction of the carbene with excess diazo compound. 

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