Allylic substitutions silyl enolates

Scheme 5.26 Enantioselective palladium-catalyzed allylic alkylation of fluoro-substituted silyl enol ethers 80.

Lewis acids are also effective to induce the nucleophilic substitution of allylic nitro compounds. These compounds react with allyltrimethylsilane,28 silyl enolates,28 or cy-anotrimethylsilane29 in the presence of SnCl4 to give substitution products, respectively (see Eqs. 7.24-7.26). 

Only a few examples exist for the intermolecular trapping of allyl radicals with alkenes68,69. The reaction of a-carbonyl allyl radical 28 with silyl enol ether 29 occurs exclusively at the less substituted allylic terminus to form, after oxidation with ceric ammonium nitrate (CAN) and desilylation of the adduct radical, product 30 (equation 14). Formation of terminal addition products with /ram-con figuration has been observed for reaction of 28 with other enol ethers as well. 

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

Ceric ammonium nitrate promoted oxidative addition of silyl enol ethers to 1,3-butadiene affords 1 1 mixtures of 4-(/J-oxoalkyl)-substituted 3-nitroxy-l-butene and l-nitroxy-2-butene27. Palladium(0)-catalyzed alkylation of the nitroxy isomeric mixture takes place through a common ij3 palladium complex which undergoes nucleophilic attack almost exclusively at the less substituted allylic carbon. Thus, oxidative addition of the silyl enol ether of 1-indanone to 1,3-butadiene followed by palladium-catalyzed substitution with sodium dimethyl malonate afforded 42% of a 19 1 mixture of methyl ( )-2-(methoxycarbonyl)-6-(l-oxo-2-indanyl)-4-hexenoate (5) and methyl 2-(methoxycarbonyl)-4-(l-oxo-2-indanyl)-3-vinylbutanoate (6), respectively (equation 12). 

Recendy, we found that A -allyl-o-vii rlaniline 44 gave 1,2-dihydroquinoline 45 by normal RCM and developed silyl enol ether-ene metathesis for the novel synthesis of 4-siloxy-1,2-dihydroquinoline and demonstrated a convenient entry to quinolines and 1,2,3,4-tetrahydroquinoline [13], We also have found a novel selective isomerization of terminal olefin to give the corresponding enamide 46 using rathenium carbene catalyst [Ru] and silyl enol ether [14], which represented a new synthetic route to a series of substituted indoles 47 [12], We also succeeded an unambiguous characterization of mthenium hydride complex [RuH] with ACheterocyclic carbene... 

Use of TMSCl in combination with HMPA, DMAP, or TMEDA all favored 1,2-addition over 1,4-addition. Sequential a-alkoxyalkylcuprate conjugate addition, enolate trapping with TMSCl, and silyl enol ether alkylation provides a one-pot synthesis of tetrahydrofurans (Scheme 3.35) [129]. Cyclic enones afford as-fused tetrahydrofurans, while acyclic systems give complex mixtures of diastereomers. a-Alkoxyalkylcopper reagents also participate in allylic substitution reactions with ammonium salts [127]. 

Unstabilized enolates react with allylic carbonates in the presence of metalacyclic iridium-phosphoramidite catalysts. Although ketones and aldehydes have not yet been used directly as pronucleophiles with this catalyst system, silyl enol ethers [80] and enamines [81] react with linear allylic carbonates to form, after workup, p-branched, y-8 unsaturated ketones (Scheme 13). Both methods form products in high yield, branched selectivity, and enantioselectivity for a range of cinnamyl and alkyl-substituted allylic carbonates. However, the silyl enol ethers derived from aliphatic ketones reacted in lower yields than enamines derived from the same ketones. 

Allyl cyanides can be added across alkynes in the presence of a nickel catalyst prepared from (COD)2Ni and (4-CF3CeH4)3P in situ to give functionalized di- or tri-substituted acrylonitriles in a highly stereoselective manner, presumably via n-allylnickel intermediates. a-Siloxyallyl cyanides also react at the y -position of a cyano group with both internal and terminal alkynes to give silyl enol ethers, which can be converted into the corresponding aldehydes or ketones upon hydrolysis.70... 

Mikami et al. also investigated the addition of ketene silyl acetals. They found that addition of the silyl enol ether of acetone and allylic silanes did not result in the synthesis of substituted l,2-dihydro[60]fullerenes [218a,220], In 1997, Mikami et al. [221] reported the photoaddition of allylic stannanes that leads to monoallylation of C6o (Scheme 13). 

The functionalized silyl enol ethers 156 are useful synthetic intermediates since electrophiles can now be introduced either directly in the P-position by known methodology 55) or in the opposition after deprotonation with LDA to an allyl anion (Eq. 70)61>. Both pathways should enormously widen the scope of specifically substituted y-oxoesters and their derivatives obtained via siloxycyclopropanes. 

Treatment of 1,3-dicarbonyl compounds with DBP in a methoxide/methanol system affords 2-alkyl-4-[(phenylsulfonyl)methyl]furans, where reaction proceeds by Initial addition-elimination on the vinyl sulfone moiety. In contrast, silyl enol ethers in the presence of silver tetrafluoroborate resulted in products derived from Sn2 displacement at the allylic site.11 Anions derived from 1,3-dicarbonyls substituted at the C-2 position are found to induce a complete reversal in the mode of ring closure.12 The major products obtained are 3-[(phenylsulfonyl)methyl]-substituted cyclopentenones. The internal displacement reaction leading to the furan ring apparently encounters an unfavorable Ai -interaction in the transition state when a substituent group is present at the 2-position ol the dicarbonyl compound. This steric Interaction is not present in the transition state leading to the cyclopentenone ring. 

The coupling of an allyl or acyl moiety onto carbon atoms is achieved by anodic oxidation of a-heteroatom substituted organostannanes or Oj -acetals in the presence of allylsilanes or silyl enol ethers. The reaction probably involves carbocations as intermediates that undergo electrophilic addition to the double bond [245c]. 

Palladium(0)-catalyzed a-allylations of TMS enol ethers can be carried out cleanly with allylic carbonates. These reactions are highly regioselective, e.g. the mtne- and less-substituted TMS enol derivative of 2-methylcyclohexanone cf. Scheme 37) gave 2-allyl-2-methylcyclohexanone and 2-methyl-6-allylcyclohexanone, respectively. Allylations of aldehyde silyl enol ers occur similarly. Allylations of enol acetates occur with allyl carbonates in the presence of catalytic amounts of palla-dium(0) complexes and tri-n-butyltin methoxide. ... 

Activation of allylic ethers. An allylic ether y to an electron-withdrawing group is activated by forming the Fe(CO)4 complex. On acid treatment ionization occurs to generate the allyl cation (still complexed to iron), which is reactive towards nucleophiles such as silyl enol ethers, malonate ester enolates, etc. The substitution is stereoselective. ... 

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