Silenes cycloaddition

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. 

In conclusion, it is clear that the photochemical decomposition of bis(silyldiazomethyl) oligosilanes produces one silene moiety (43c) in the molecule, followed by a [2 + 3] silene-diazo cycloaddition, while thermal decomposition gives two silene moieties (37c) at the same molecule which undergo a [2 + 2] silene-silene cycloaddition in a head-to-tail (not head-to-head) fashion. 

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). 

It was also well established that silenes could take part as the dienophile in Diels-Alder reactions. In many cases, particularly with unsymmetric dienes such as isoprene, the reactions were not clean because, in addition to formation of the [2+4] cycloadduct 61, the possibility exists for the formation of it regioisomer 62, products of an ene reaction 63, and conceivably the [2+2] cycloaddition product 64, as shown in Eq. (23). Wiberg... 

Wiberg had also described a number of [2+3] cycloadditions of his silenes with 1,3-dipolarophiles such as N20, diazo compounds, and azides.98... 

Dimerization is a special case of [2+2] cycloaddition with silenes it has been observed to occur in both a head-to-tail and in a head-to-head manner, yielding 1,3- or 1,2-disilacyclobutanes. These two cases will be discussed separately below. 

Recent study of silenes has clearly established their ability to act as dienophiles in [2 + 4] Diels-Alder-type reactions involving 1,3-dienes. However, it has also been clearly demonstrated that products of the ene reaction commonly accompany the [2 + 4] cycloadducts and may on occasion be the major products. In addition, unlike those in carbon chemistry, [2 + 2] cycloadditions are often observed to occur in competition with the above processes—not only from reactions of silenes with dienes... 

Reactions favoring [2 + 2] cycloaddition tended to be those that had strongly electronegative groups on the sp2-hybridized silicon but only H and the neopentyl group on the sp2-hybridized carbon atom. Butadiene and cyclohexadiene generally favored [2 + 2] cycloaddition with these silenes. The [2 + 2] adducts with cyclohexadiene appear to be kinetic products, since they cleanly isomerized to the Diels-Alder adducts over time.182... 

Ene reactions tended to occur as alternative reaction pathways to [2 + 4] cycloaddition, especially when sterically bulky silenes had substituents on the sp2-hybridized carbon atom and dimethylbutadiene served as the diene component. In the ene reactions studied the silene acted as the enophile as often as it acted as the ene. 

There are several known examples of silenes participating in [2 + 2 + 2] cycloadditions, including reactions with norbornadiene and with quadricyclane. Examples of these reactions with norbornadiene are shown in Eq. (30), where the silene bridges the 2,6-positions.38 5, 53 ,7U85 ,88 This reaction has only been investigated using some of the Auner-type silenes. In many cases mixtures of stereoisomers were obtained, and in some cases products of an ene reaction were also observed. 

The reactions with quadricyclane, shown in Eq. (31), gave products identical to those formed by the same silene reacting in a [2 + 2] manner with norbornene. Mixtures of exo endo isomers were frequently observed. Again, only silenes of the Auner type have been studied with this reagent,51-53,185,188 so it is not known whether the Wiberg- or Brook-type silenes will undergo this mode of cycloaddition. 

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... 

Fig. 1. Relative energies of silene + formaldehyde cycloaddition and fragmentation.

Silenes derived from aromatic di- or polysilanes have been characterized in particular by the ene-type reactions they undergo when treated with isobutene or acetone. Recently, Leigh80 observed the first reported case of one of these silenes undergoing [2+2] cycloaddition (21% yield) with acetone. The ene product, the only product previously detected from the reaction of such silenes, was formed in 41% yield, as shown in Eq. (33). 

Whether the [2+2] or [2+4] cycloaddition product is formed in the first reaction depends on the reaction conditions however, one product is convertible to the other via the silene intermediate. In the other reactions shown, the identity of the product formed, [2+2] or [2+4], seemed to be a function of the structures of both the silene and imine. In the last case, the [2+4] product appeared to be the kinetic isomer since conversion to the [2+2] isomer slowly occurred on standing in the dark, or faster if photolyzed, even at room temperature. 

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] ... 

Dichloroneopentylsilene is formed in situ by reaction of trichlorovinylsilane with LirBu [1], The [2+2] cycloaddition to imines yields Si-dichloro functionalized 2-silaazetidines in a preparative scale [2], When aldimines are used as trapping agents for the silene, the resulting SiN-four membered ring compounds are isolated as syn/anti-isomers (syn/anti 2/1). 

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