Allyl ammonium enolates

High-boiling products found in this procedure and in similar experiments involving cyclohex-2-enone derivatives5 probably result from bimolecular reduction processes.15 3-Methylcyclohexanone, which arises by protonation rather than alkylation of the enolate (and which made up ca. 12% of the volatile products), is probably the result of reaction of allyl bromide with liquid ammonia to form the acidic species allyl ammonium bromide.5 10... 

The third mechanism starts with addition of the AT-allylamine 103 to the cumulated acceptor system of an allene carbonester 108 (Acc=CHC02Me) to form an intermediate iV-allyl ammonium amide enolate 109 (allene carbonester Claisen rearrangement). The anion stabilizing group is exclusively placed... 

Schiff base 52 in one-pot under mild phase-transfer conditions. For example, the initial treatment of a toluene solution of 52 and (S,S)-32e (1 mol%) with allyl bromide (1 equiv.) and CsOHH20 at —10 °C, and the subsequent reaction with benzyl bromide (1.2 equiv.) at 0 °C, resulted in formation of the double alkylation product 53 in 80% yield with 98% ee after hydrolysis. Notably, in the double alkylation of 52 by the addition of the halides in reverse order, the absolute configuration of the product 53 was confirmed to be opposite, indicating intervention of the chiral ammonium enolate 54 at the second alkylation stage (Scheme 4.17) [50]. 

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. 

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). 

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]. 

In 2000, Tanino and his co-workers developed the novel [5- -2]-cycloaddition reaction of a propargyiic cation equivalent bearing allylic silane 17 with enol silane 18 to give the corresponding cycloheptyne complexes 19 in good yields with an excellent diastereoselectivity (Scheme 3). While ceric ammonium nitrate (CAN) is generally used to... 

Allyl ethers of enols and phenols undergo rearrangement to C-allyl derivatives when heated to sufficiently high temperatures. The reaction, named after its discoverer (Claisen, 1912), was first observed when ethyl O-allylacetoacetate was subjected to distillation at atmospheric pressure in the presence of ammonium chloride.1 2... 

We also observed similar phenomena in the reaction of silyl enol ethers with cation radicals derived from allylic sulfides. For example, oxidation of allyl phenyl sulfide (3) with ammonium hexanitratocerate (CAN) in the presence of silyl enol ether 4 gave a-phenylthio-Y,5-un-saturated ketone 5. In this reaction, silyl enol ether 4 reacts with cation radical of allyl phenyl sulfide CR3 to give sulfonium intermediate C3, and successive deprotonation and [2,3]-Wittig rearrangement affords a-phenylthio-Y,6-unsaturated ketone 5 (Scheme 2). Direct carbon-carbon bond formation is so difficult that nucleophiles attack the heteroatom of the cation radicals. 

Use of trimethylsilyl triflate to bring about Piunmeier rearrangement requires the presence of a base such as a tertiary amine (vide supra equations 15 and 26). In some instances, involving attempts to alkylate Pummerer intermediates with silyl enol ethers under such conditions, the base has been found to compete as a nucleophile. In the absence of the silyl enol ether, amine addition can be very efficient. For example, treatment of methallyl phenyl sulfoxide with diisopropylethylamine and trimethylsilyl inflate in dichloromethane (equation 29) at 0 C yields the ammonium triflate indicated in 91% yield. Other tertiary amines which undergo this reaction include niethylamine and Af,Af-diethyltrimethylsiI-amine. In the latter case with allyl phenyl sulfoxide as the substrate and a mildly acidic wotk-up, the Mannich derivative shown in equation (30) can be obtained in 90% yield. ... 

In all three cases the starting materials are already chiral. If the allylic alcohol or the cycx. ammonium salt were prepared, by resolution or by asymmetric synthesis, as a single enantiomer, then A too would be a single enantiomer. However, the ketone used in the first route, though aJK chiral, has an achiral enolate so asymmetry could be introduced at that point. It isn t possible to u-se the chiral enolates described in the chapter (p. 1230). Probably the easiest approach is to make thf aUylic alcohol by asymmetric reduction, say by the CBS reducing agent (p. 1234). 

Conjugate addition to vinylthionium ions is observed in the reaction of allylic sulfoxide (120) with enol silyl ethers in the presence of trimethylsilyl triflate and diisopropylethylamine (Scheme 27). However, in certain cases moderate yields were obtained. This has been traced to a competing reaction of the intermediate (121) with the tertiary amine base to give the quaternary ammonium salt (122). 

Even simple enols have substantial lifetimes, provided that bases or acids are completely excluded173. Thus, an aromatic enol 4 is prepared in situ by Norrish-type fragmentation of 2. If (-)-ephedrine is present in the reaction mixture, the enol reverts enantioselectively to (/ )-2-rnethy 1 -1 -indanone (3). With as little as 0.01 mol % catalyst, 45% ee is obtained176. The crucial enol 4 has also been generated from either the benzyl enol ester 5 by palladium on charcoal and hydrogen or from the allyl ester 6 by palladium acetate, triphenylphosphine and ammonium formate. In the presence of a chiral 1.2-hydroxyamine, e.g., ephedrine, substantial stereogenic induction in 2-methylindanone 3 was observed175. 

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