Olefins silylene reactions

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

One novel and interesting method of generating a silacarbonyl ylide occurred through the addition of a carbonyl species with a silylene formed under photolytic conditions. Komatsu and co-workers (177) found that photolysis of trisilane (315) in solution with a bulky carbonyl species led initially to the formation of a silacarbonyl ylide followed by a dipolar cycloaddition of an olefinic or carbonyl substrate. Reaction of simple, nonbulky aldehydes led to only moderate yields of cycloadduct, the siladioxolane. One lone ketone example was given, but the cycloadduct from the reaction was prepared in very low yield (Scheme 4.89). 

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of f-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (/-Pr3Si)2Si (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 °C in 2,2,4-trimethyl-pentane (Scheme 14.11). Reactions of 15 include the precedented insertion into an Si H bond, and additions to the ti bonds of olefins, alkynes, and dienes. 

Although the silylformylation of aldehydes is catalyzed by [Rh(COD)Cl]2 or [Rh(CO)2Cl]2, no secondary silylformylation of /i-silylenals (316-318) takes place, probably due to the electronic nature of the aldehyde functionality conjugated to olefin moiety (vide supra). Direct comparison of the reactivity of acetylene and aldehyde functionalities is performed using alkynals328. The reactions of 5-hexyn-l-al, 6-heptyn-l-al and 7-octyn-l-al with different hydrosilanes catalyzed by Rh or Rh—Co complexes at... 

Siliranes are also formed by the reaction of the cyclotrisilane [2-(Me2NCH2)C6H4]6Si3 with terminal and strained internal olefins under mild thermal conditions. The products obtained from the thermolysis of the siliranes thus prepared suggest a thermal equilibrium of the silirane with the cyclotrisilane and the corresponding alkene. This observation provides evidence for an equilibrium between the silylene and the cyclotrisilane and, moreover, proves that free silylenes are involved in the silylene transfer reaction48. 

In addition to ruthenium, Tilley and coworkers also reported that cationic iridium silylenoid complexes were efficient olefin hydrosilation catalysts [reaction (7.6)].56 This silylene complex catalyzes the hydrosilation of unhindered mono- or disubsti-tuted olefins with primary silanes to produce secondary silanes with anti Markovni-kov selectivity. Iridium catalyst 32 exhibited reactivity patterns similar to those of ruthenium 30 only primary silanes were allowed as substrates. In contrast to 30, cationic iridium 32 catalyzed the redistribution of silanes. Exposing phenylsilane to 5 mol% of 32 in the absence of olefin produced diphenylsilane, phenylsilane, and silane. 

Insight into the mechanism of silver-catalyzed silylene transfer from cyclohexene silacyclopropane to an olefin was obtained using bistriphenylphosphine silver triflate as a catalyst.83 Woerpel and coworkers chose to employ ancillary ligands on silver to address the both the poor solubility of silver triflate as well as its propensity to decompose to afford a silver(0) mirror or precipitate. The addition of triphenylpho-sphine, however, attenuated the reactivity of the silver catalyst. For example, the reaction temperature needed to be raised from —27°C to 10°C to obtain a moderate rate of reaction (Scheme 7.15). 

The relative reactivities of various silacyclopropanes were compared to gain insight into the reversibility of the reaction (Scheme 7.16).83 Woerpel and coworkers observed that the reactivity of the silacyclopropane toward the silver catalyst depended on the ring size, and that cyclohexene silacyclopropane was the most reactive. Notably, benzyl-substituted silacyclopropane 63d did not react when exposed to the silver complex. This lack of reactivity was interpreted as evidence in support of the irreversibility of silylene transfer to monosubstituted olefins. 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

In the silylative coupling reactions of olefins and dienes with vinylsubsti-tuted silanes, ruthenium catalysts, containing initially or generating in situ Ru-H/Ru-Si bonds, catalyze polycondensation of divinylsubstituted silicon compounds to yield unsaturated silylene (siloxylene, silazanylene)-vinyl-ene-alkenylene (arylene) products (Eq. 112). For recent results see Refs. [177, 178] and for reviews see Refs. [6,7,117,118]. 

Summary New silacyclopropanes were synthesized quantitatively under mild thermal conditions by reaction of olefins with cyclotrisilane (cyclo-(Ar2Si)3, Ar = Me2NCH2QH4) 1, which transfers all of its three silylene subunits to terminal and strained internal olefins. Thermolysis of silacyclopropanes 3a und 3b indicated these compounds to be in a thermal equilibrium with cyclotrisilane 1 and die corresponding olefin. Silaindane 13 was synthesized by reaction of 1 with styrene via initially formed 2-phenyl-1-silacyclopropane 3d. Reaction of 1 with conjugated dienes such as 2,3-dimethyl-l,3-butadiene, 1,3-cyclohexadiene or anthracene resulted in the formation of the expected 1,4-cycloaddition products in high yield. 

The reaction is initiated by extrusion of a silylene 2 from 3a or 3b, thus paralleling the well known equilibrium between hexamethylsilacyclopropane and dimethylsilylene [7]. In the absence of a silylene trapping reagent [8] dimerization of 2 to disilene 5 takes place. Addition of a third silylene to the Si=Si double bond eventually yields cyclotrisilane 1 [9]. The reversibility of the cyclotrisilane formation from 3a and 3b provides evidence, that the reverse reaction of 1 with olefins includes free silylenes 2 as reactive species as well. 

Compound 1 did not react with unstimned internal olefins such as tetramethylethylene, /ra s-3-hexene, tram-stilbene, cyclooctene, cyclohexene, or cyclopentene. But imposing strain to the olefinic moiety resulted in a clean silylene transfer to the double bond Norbomene formed with 1 the tricyclic silacyclopropane 6. Whereas 2 did not add to the double bond of 7, methylene cyclopropane 8 could be transformed into spiro[2.2]pentane 9 by reaction with 1. Addition of 2 to bicyclopropylidene allowed the convenient synthesis of dispiro[2.0.2.1]heptane 10 in a quantitative manner. 

Stirring 1 for 12 h at 40 °C with excess styrene led quantitatively to silaindane 13 [12], The silacyclopropane 3d was identified as an intermediate in this reaction by its Si NMR shift (5 = -82.5 ppm) [6]. Thus, 13 appears to be formed by initial formation of 3d, which rearranges to intermediate 12. Rearomatization eventually yields 13 (Scheme 2). This pathway resembles the well known mechanism of the reaction of silylenes with conjugated olefins via initial formation of vinylsilacyclopropanes [3]. 

Silylenes only undergo rearrangements at elevated temperatures, and the high barriers for their intramolecular reactions have made possible the convenient study of their intermolecular chemistry. Alkylcarbenes undergo rapid H-shifts, converting them to olefins even at room temperature, and thus the intermolecular insertions and additions of alkyl carbenes have remained rather obscure. 

It has been theoretically and experimentally well established that silylenes have a singlet ground state [1]. Such species posses a free electron pair in a o-orbital and an empty orbital of Jt-symmetry therefore, they are a priori ambiphilic compounds, which can react either as an electrophile or as a nucleophile towards appropriate substrates. However, most silylenes have revealed a distinctive "electrophilic character". Dimethylsilyene, e g., adds to olefins and alkynes in the gas phase via a rate-controlling step that is accelerated by electron-donating substituents [2] these experimental results are in good agreement with a theoretical study of the reaction of SiH2 with ethylene, which shows that this cycloaddition proceeds via an initial electrophilic phase in which the silylene LUMO interacts with the 7t-electron system of the double bond [3]. Up to now, only some stable silylenes, such as recently described 1 [4] or silicocene 2 [5] have shown nucleophilic reactivity. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

Oxidative additions are a special class of insertion reactions. In addition to the categories mentioned in Section 10, which covers this topic, insertions of alkylidenes, silylenes, etc., into M-H bonds fall into an ambiguous domain they are insertion reactions of the unsaturated species into the M-H bond, yet oxidative additions at the C, Si, etc., atom. A similar ambiguity exists regarding the reverse reactions, namely /i-hydride and a-hydride eliminations from element-alkyls compounds to yield hy-drido-olefin and hydrido-alkylidene complexes, respectively. The former reaction is a reverse insertion if the product is viewed as an olefin complex, but an oxidative addition if it is viewed as a three-membered metallocycle. The latter reaction is a reverse insertion if the alkylidene is viewed as neutral, but an oxidative addition of a C-H bond to the metal centre. The tautomerization of phosphorous acid and of dialkylphosphites ... 

For 1, the expected cyclic product was not obtained, instead the product appeared to be polymeric. This led us to try the reaction of 1 with simple olefins. To our surprise, the reaction of 1 with 1-hexene also yielded a polymer, poly(l-hexene). Moreover, experiments showed that the process is a catalytic one, in which only a small amount of silylene is needed. 

---