Conjugate addition allylic silanes

Conjugate additions of allylic silanes to enones are also catalyzed by InCl3-TMSC1.134... 

Some examples of conjugate addition reactions of allylic silanes are given in Scheme 9.5. Entries 1 to 3 illustrate the synthesis of several (3-allyl ketones. Note that Entry 2 involves the creation of a quaternary carbon. Entry 4 was used in the synthesis of a terpenoid ketone, (+)-nootkatone. Entry 5 illustrates fluoride-mediated addition using tetrabutylammonium fluoride. These conditions were found to be especially effective for unsaturated esters. In Entry 6, the addition is from the convex face of the ring system. Entry 7 illustrates a ring closure by intramolecular conjugate addition. 

Scheme 3.7. Diastereoselective formation of /S-silyl ( )- or (Z)-ester enolates by silylcuprate conjugate addition followed by alkylation with aldehydes [49]. Stereoselective synthesis of ( )-and (Z)-allyl silanes [50].

Intramolecular conjugate addition of allylic silanes can also be used to construct new rings, as illustrated by entry 6 in Scheme 9.5. 

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. 

The Lewis acid catalyzed conjugate addition of allylsilanes (140) to (142) and allylstannanes (154) and (155) to ot,0-enones, described by Sakurai,68a,68b is highly efficient and experimentally simple in contrast to the allylcuprate additions. Various substituents can be incorporated into the allylsilanes (allylstannanes), e.g. alkoxy, alkoxycarbonyl and halogen, some of which are incompatible with cuprate reagents 69 In addition, Heathcock and Yamamoto report that diastereoselectivity is correlated to the alkene geometry of both the allylmetals and the acceptor units for example, allylation of ( )-enones (143) and (146) affords predominantly the syn adducts (144) and (147), while (Z)-enone (149) gives predominantly the anti adduct (150 Scheme 25).680 On the other hand, with cyclohexen-2-one the (Z)-silane (141) affords predominantly the threo adduct (152), while (142) affords erythro adduct (ISS).686 The more reactive allylstannanes (154) and (155) also afford similar diastereoselectivity.68e,f... 

The reaction with EtAlCb is a Lewis-acid-catalysed conjugate addition of the allyl silane on to the enone. Conjugate addition is preferred because the nucleophile (allyl silane) is tethered to the electrophile (enone) and the five-membered ring is easier to form than the alternative seven-membered ring. 

In the presence of titanium(IV) chloride or aluminum chloride, carbonyl compounds such as aldehydes, ketones, acetals, and acid halides are allylated with allylic silanes (eq (12)) [9]. The conjugate addition to enones is also reported (eq (13)) [10],... 

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... 

C—C Bond Formation using Organosilanes. Under catalysis by TiCU, allylsilanes transfer the allyl group, with rearrangement, to ketones, a/3-unsaturated ketones (conjugate addition), and quinones (initially giving the carbonyl addition product which then further rearranges to the 2-allyl-l,4-hydroquinone). But-3-enyl-silanes react with acid chlorides to yield cyclopropylmethyl ketones [reaction (3)) under similar conditions. 

Although allylic silanes fail to react with a,/3-ethylenic esters, conjugate addition to a,/3-ethylenic acyl cyanides occurs quantitatively under the influence of titanium tetrachloride to give 5,e-ethylenic acyl cyanides, acids, or esters, depending on the work-up procedure (e.g. Scheme 68)/ Allenic silanes react in an analogous way to give the corresponding 5,e-acetylenic compounds. 

Stable precursors, such as boranes and silanes, were also shown to display good reactivity in NHC-Cu-catalyzed conjugated additions. The groups of Ohmiya and Sawamura reported the copper-catalyzed 1,4-addition of alkyl-boranes to enones, respectively in a racemic and enantioselective version. Organosilanes, te. RSiFj or RSi(OR )3, can also be used with NHC-Cu catalysts in conjugate addition to enones and allylic epoxides. ... 

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