Silenes addition reactions

Summary The formation, reactivity, and cycloaddition behavior of neopentylsilenes towards suitable reaction partners is described. Especially l,l-dichloro-2-neopentylsilene. Cl2Si=CHCH2Bu (2) - easily obtained from vinyltrichlorosilane and LiBu - is a useful building block for the synthesis of SiC four membered ring compounds. These can be converted into the isomeric Diels-Alder and retro ene products upon thermolysis reactions. The mode of the silenes cycloaddition reactions ([4+2] vs [2+2] addition) can be directed by either the substitution pattern at the Si=C moiety, the choice of reaction partners or the conditions. Furthermore the products resulting from cycloaddition reactions open up a wide variety of following reactions, which possibly will lead to new organosilicon materials or pharmaceutical compounds. 

Mechanism and structures in alcohol addition reactions of disilenes and silenes... 

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. 

Nevertheless, mechanistic investigations on the addition reaction to disilenes and silenes advanced considerably in recent years. In this chapter, the author tries to survey the progress achieved recently. The author will not try to review all aspects of the chemistry of silicon unsaturated species since many extensive reviews on this topic are already available6-12. 

The addition reaction of alcohols to silenes is a strictly regiospecific process the OR group in the product is always attached to the silicon. The reaction is of special importance since silenes are easily trapped by reagents like methanol and t-butanol, and isolation of the adducts is normally taken as evidence for an intermediate silene. Furthermore, the alcohol addition to silenes is the prototypical reaction for the 1,2-addition of polar bonds across the Si=C double bond. A complete account on the alcohol addition to silenes, including the most recent mechanistic implications, is given in the chapter by Sakurai. We will therefore give only a brief survey of the reactivity of several families of silenes towards alcohols. 

Addition reactions of lithium organyls have been investigated for 9763-201. The reaction is not a nucleophilic substitution of Br by R in 97-LiBr, but rather a two-step mechanism with an initial dissociation via 97 (equation 124). This was proven by the addition of the very efficient silene trap t-Bu2MeSiN3, that competes with the lithium organyl for the silene. 

An interesting addition reaction to silenes was recently described by Oehme and coworkers. They found that at high concentrations of LiBr the dimerization of the transient silene 371 yields 33% of the head-to-head dimer 373 and the head-to-tail dimer 407 in a 1 5.6 relative ratio108. The formation of the unexpected dimer 407 was rationalized by the addition of LiBr to the Si=C bond and intermolecular cyclization of the w-lithiosilyl bromide 408 or its reaction with the transient silene 371 with subsequent cyclization to the 1,3-disilacyclobutane 407 (equation 126)108. 

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. 

SILENE-TYPE SPECIES IN ELIMINATION-ADDITION REACTIONS... 

But till now only methods involving an in situ formation of deprotonated 1 have been described. In this paper we present a procedure leading to pure, isolated 1-hydroxyalkyl-polysilanes 1. In presence of base fliey are easily converted into transient silenes, which were characterized by various dimerization and addition reactions. The availability of isolated 1 in the synthesis of silenes according to the Peterson concept offers the possibility of a free choice of the solvent and the base used to initiate the silanolate elimination. With respect to the significance of the reaction medium for the silene generation and its subsequent reactions, this is of particular importance. 

Summary The reaction between in situ formed l,l-dichloro-2-neopentylsilene, Cl2Si=CHCH2tBu (3), and 1,1-dimethylpentaftilvene (8) leads to the formation of exo/endo-isomeric [4+2] cycloadducts 9 and [2+2] stereoisomers 10 in good yields. 2D-NMR spectroscopic investigations on the product mixture prove die different mode of the silene cycloaddition reactions ([4+2] vs [2+2] addition). 

Stable silenes and germenes also react with various reagents as shown in Scheme 24." Addition reactions of metallenes with MeOH lead to the formation of R2M(OMe)-C(H)R2. The reactions of a,p-unsaturated carbonyl compounds afford the corresponding [4 - - 2] cycloadducts regiospecifically, reflecting the M +=c - polarity. 

In an early study, a series of chiral alcohols was added to prochiral silenes generated by 254 nm photolysis of silacyclobutanes and asymmetric induction in the addition reactions was observed, compatible with a planar structure of the silenes156,157. The authors pointed out the possibility that the reaction is concerted and involves the oxygen lone pair in a 4 + 2 ([Pg.1087]

Ishikawa and coworkers studied addition reactions to silenes with substituted acetylenes which give [2 + 2]-addition products frequently followed by a complex ring-opening chemistry (Scheme 20) [187, 188]. 

Scheme 27 Addition reactions of nitrogen nucleophiles to silenes...

Addition reactions to unsaturated silicon compounds (silenes, disilenes, disilynes) offer opportunities to form aminooligosilanes. An example is the reaction of Sekiguchi s disilyne with pyrrolidine, which gave the silicon version of an enamine. Further addition gave a geminal diaminotetrasilane (Scheme 26) [274],... 

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