Dienes reaction with silanes

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

The axially chiral (allenylmethyl) silanes 110 were also prepared in optically active form using chiral Pd catalysts [98]. For the asymmetric synthesis of 110, a Pd/(R)-segphos system was much better in terms of enantioselectivity than the Pd/(R)-binap catalyst. Under the optimized conditions, 110m and llOt were obtained in 79% ee (57% yield) and 87% ee (63% yield), respectively (Scheme 3.56). The enantio-merically enriched (allenylmethyl) silanes 110 served for Lewis acid-promoted SE reaction with tBuCH(OMe)2 to give conjugated dienes 111 with a newly formed chiral carbon center (Scheme 3.56). During the SE reaction, the allenic axial chirality was transferred to the carbon central chirality with up to 88% transfer efficiency. 

The intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

Treatment of siloxycyclopropane 162 with equimolar amounts of iodotrimethyl-silane and triethylamine generates the electronrich diene 163 in high yield 88). Many further applications of compounds like 163 — cycloadditions, for instance, or reactions with electrophiles — should be manageable. 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

Reactions alkylations, reactions with epoxides and aldehydes, conjugate additions Heterocyclic synthesis with allyl silanes Reactions with Co-stabilised cations An Allyl Dianion The Role of Tin in Anion Formation Halide Exchange with Chelation Indium Allyls Allyl Anions by Deprotonation The synthesis ofall-trans dienes The synthesis ofall-trans retinol... 

Palladium-catalyzed hydrosilylation of 1,3-dienes is one of the important synthetic methods for allylic silanes, and considerable attention has been directed to the asymmetric synthesis of the latter by catalytic methods [9]. Optically active allyhc silanes have been used as chiral allylating reagents in S reactions with electrophiles, typically aldehydes [38,39]. In the presence of Pd catalysts the reaction with hydrosilanes containing electron-withdrawing atoms or substituents on sihcon usually proceeds in a 1,4-fashion giving allyHc silanes [40,41]. Asymmetric hydrosilylation of cyclopentadiene (29) forming optically active 3-silylcyclopentene (30) has been most extensively studied (Scheme 13). In the first report, hydrosilylation of cyclopentadiene (29) with methyldichlorosilane in the presence of 0.01 mol % of palladium-(l )-(S)-PPFA (15a) as a catalyst gave... 

Allylation. y-Selective allylation of aldehydes using chiral reagents formed in situ from tartrate esters and allyldiisopropoxyboranes shows 37-85% ee. On the other hand, the reaction with allylic silanes follows a pathway leading to hydroxyallylation of the double bond, and primary alcohols are obtained from 5-silyl-l,3-dienes. ... 

There are a number of remarkable features in this very facile linear dimerization reaction with trapping by disilanes, (i) In contrast to silanes with which substituted 1,3-dienes (e.g., isoprene, piperylene) give predominantly to exclusively simple hydrosilylation prodncts rather than linear dimerization products, disilanes afford only disilylated linear dimers—no disilylated monomer, (ii) Only the head-to-head dimerization products... 

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