Silenes 2+2 cycloaddition reactions

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... 

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. 

The temperature at which a cycloaddition reaction of a neopentylsilene takes place (detected by the elimination of LiCl) has turned out to be dependent on the reaction partners added as substrate. This implies that an interaction between the substrate and A or B or the substrate and C occurs somewhere along the reaction pathway depicted above. For the system Cl3SiCH=CH2/LiBut/R2C=NR it was observed that the imine initiates and supports the salt elimination from the species A/B. Based on the knowledge that silenes are stabilized by external donors [1] we conclude that with carbon unsaturated compounds x-donor interactions instead of cr-donor complexes may be possible as well for the lithiated species (D) as for the silene itself (E). 

When the chlorine atoms in silene 2 are substituted by other ir-donors the ability to partake in [2+2] cycloaddition reactions is conserved. This has been proved by studies in our group on the reactivity of amino substituted silenes of the type [11] ... 

In this section we will summarize bimolecular reactions of silenes with alkenes, alkynes and dienes which might be regarded nominally as cycloaddition reactions. 

It is evident that the relative proportions of products from [2 + 2], [4 + 2] and ene reactions depend strongly on the polarity of the Si=C bond, on the polarization of the frontier orbitals and on steric factors both in the silenes and in the reactant. Therefore, we will first discuss cycloaddition reactions of silenes without separating them into subclasses like [2+2] or [2+4] cycloadditions, but we will concentrate on the cycloaddition behaviour of several classes of silenes towards alkenes, alkynes and dienes. We will then discuss some special cycloaddition reactions in subsequent sub-sections. 

Silaacrylate 305 undergoes reactions with ketones165. The initial coordination of the ketones is reminiscent of the donor adducts to silenes. The products 520-522 are formed by an ene or a formal [2 + 2] cycloaddition reaction, depending on the substituents on the ketones (equation 177). 

A siloxetane 526 as an intermediate from a [2 + 2] cycloaddition of silene 241 with acetone has been formulated by lshikawa134. It extrudes a silanone equivalent to give the vinyl ether 527. The second regioisomeric silene 242 generated together with 241 by photolysis of 240 undergoes an ene reaction instead (equation 179)134. 

The synthesis of siladihydrotriazoles 412a is easily achieved by the [2+3] cycloaddition reaction of azides RN3 with the silenes Me2Si=C(SiMe3)2 92 (equation 226). 

Conlin and Bobbitt used 1,2-divinyl-tetramethyldisilane 89 to generate a simple silene 90 used to explore competing [2 + 2] and [2 + 4] cycloaddition reactions with butadiene (see Scheme 13), where the -91 and Z-91 isomers of the [2 + 2] adduct predominated over the [2 + 4] cycloadduct 9245. 

Cycloaddition reactions of acyl silanes appear to be rare, but Brook has shown that a-silyloxy bis(trimethylsilyl)silenes (52), generated photochemically from acyl tris(trimethylsilyl)silanes (vide infra, Section IV.A.4), undergo [2 + 2] and [4 + 2] cycloaddition reactions with ketones, and [4 + 2] cycloaddition reactions with less bulky acyl silanes, as illustrated in Scheme 8717,24 26 72 73,201. They do not, however, react with their parent acyl tris(trimethylsilyl)silanes. 

In a [2+1] cycloaddition reaction the disilacyclopropane 143 is formed from silylene 85 and the ephemeral silene 144 (Equation 11) <2004OM2848>. 

An unexpected und unusual dimerization behaviour is observed for 2-mesityl-1,1-bis(trimethylsilyl)-silene (2c), formed by deprotonation of Ic. The structure of the product, obtained in more than 70 % yield, was revealed as ( )-l,2,3,8a-tetrahydro-l-mesityl-5,7,8a -trimethyl-2,2,3,3-tetrakis(trimethylsilyl)-2,3-disilanaphthalene (5) (Scheme 1). The formation of this unexpected structure is interpreted as the result of a head-to-head-dimerization of the transient silene 2c, which in an unusual [2+4]-cycloaddition reaction is formally acting as the monoene and - involving the aromatic substituent - also as the diene (Eq. 3). 

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). 

They react easily with electrophiles and add nucleophiles at C-6. In cycloaddition reactions they may react as 2jt, 4n, or 6 i compounds. According to frontier orbital considerations they readily react with electron-deficient dienophiles (e.g., silenes) in Diels-Alder reactions this is due to the strong interaction between the fulvene HOMO and dienophile LUMO [9]. Although the n and n orbitals of silenes are generally 1-2.5 eV higher in energy than is the case for the alkene congeners [10] a normal [4+2] cycloaddition behaviour for 3 is observed in earlier works [3-5]. 

Scheme 2. Cycloaddition reactions of silene 3 with fulvene 8...

Furthermore cycloaddition reactions of silene 3 with other pentafulvenes (e.g., 16,17, and 18) [15] lead to similar results, which will be published elsewhere [11]. 

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