Alkenes reactions with silenes

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... 

As silenes represent a much more reactive higher energy it system than alkenes, it is not surprising that they show many of the reactions of alkenes as well as some behavior not generally exhibited by alkenes. However, the relative instability of silenes has meant that in most cases little mechanistic information on the observed reactions is available, and it is by no means certain in all cases that the same mechanism prevails for reactions of silenes and alkenes with the same reagent. 

Some remarkable chemistry is observed when silenes react with heteroatom systems, in particular carbonyl compounds (]>C=0) and imines Q>C=N—R). The reaction with ketones was first described by Sommer (203), who postulated formation of an intermediate siloxetane which could not be observed and hence was considered to be unstable even at room temperature, decomposing spontaneously to a silanone (normally isolated as its trimer and other oligomers) and the observed alkene [Eq. (14)]. Many efforts have been made to demonstrate the existence of the siloxetane, but it is only very recently that claims have been advanced for the isolation of this species. In one case (86) an alternative formulation for the product obtained has been advanced (204). In a second case (121) involving reaction of a highly hindered silene with cyclopentadienones,... 

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

When simple alkenes were employed as reaction partners for silenes of the type (Me3Si)2Si=C(OSiMe3)R1, silacyclobutanes were obtained, provided that no allylic hydrogen is present in the alkene. In the reaction with alkenes with allylic hydrogens the ene reaction becomes predominant (see Table 11). Thus, while the reaction with styrene exclusively gives the four-membered ring compound 454, with 1-methylstyrene the ene products 455 were obtained (equation 144). Similarly, from the reaction of 150 with 1-octene only the ene product 456 was isolated (equation 145). 

Results of the photolysis of both a meso and a racemic diastereomer of the 1,2-diphenyldisilane 392 indicated that the reaction leading to the silene 393, followed by its ene reaction with an alkene leading to the adduct 394, was a diastereospecific process206. A concerted mechanism, illustrated in equation 49 for one isomer of the disilane, was proposed to account for the results such results would not be expected of a process involving silyl radical intermediates, which had been proposed earlier as intermediates in the photolysis of disilanes. 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

Ene-additions of alkenes and dienes to silene 6 are considerably slower than [2 + 4]-cycloadditions. cA-Substitution in the ene component of the reaction causes a small acceleration in rate relative to fraws-substitution, as illustrated in Table 2 by the relative rate constants for reaction of 6 with cis- and rraws-2-butene. Reaction with cis, trans-2,4-hexadiene produces only a single adduct (66 equation 51), corresponding to selective ene-reaction with the cA-methyl group in the diene. 

A theoretical comparison of the B=N and B=P x-systems supports earlier findings that the latter bond is stronger, and a genuine p,-p, interaction. Isocyanates have been shown to insert into the P=Si bond of phospha-silenes to form the phospha-alkene systems (316). A macrocyclic system has been isolated from the related reaction with 1,6-di-isocyanohexane. A range of resonance-stabilised monomeric thioxophosphenes (317) has been prepared. Methods for the generation of the phosphenite (318) have also been explored. ... 

The reactions of bare Fe" " ions and related species in the gas phase continue to attract much interest. The remote functionalisation of 1,6-hexanediol by Fe occurs by C-H activation at C(3) and C(4).26 Functionalisation of 3-methyl-2-pentanone at C(4) is diastereoselective, probably because of the conformation of a chair-like intermediate. Reactions of Fe with anisoles and phenols have also been studied.28 Interaction of Fe with silanes gives both silene and silylene species, though the two are not interconvertible. The reactions of Fe(alkene)+ complexes with pentane were found to differ dramatically from those of bare Fe" , and C-H and C-C activation were also observed in reactions of Fe(C2H4) with oxygen. 0,31 interaction of Fe(benzyne)+ with alkyl halides led to C-X or C-C addition followed by p-elimination and loss of HX.32 The gas phase reaction of Fe(NH2)Me" with C2H4 is best explained by insertion into the Fe-C bond followed by P-elimination and loss of propene. The reaction of FeMe with 1-octyne also leads to C-C bond forming processes. 

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. 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

The double bond in silenes is strongly polarized. They react with phosphorus ylides, as shown by Brook and MacMillan,45 like alkenes with the strongly polar C=C bond. Therefore, it is reasonable to suggest that the reaction also occur through the betaine intermediate (12) (Scheme 6). 

These studies proved that the reactions of 22 with butadienes and propenes take place both regioselectively and stereoselectively and are accelerated by electron-donating groups on propenes and butadienes (e.g., 2-methylpropene in relation to propene) and retarded by increasing bulkiness of substituents in 1,4- or 1,3-positions. As in the case of alkenes and silenes, the reactions of 22 occur in a concerted way and are HOMO (dienes or enes)-LUMO (dienophiles or enophiles) controlled.31 However, some small differences are observed between germene 22 and the analogous... 

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

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]. 

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