AUyl transfer

AUyl transfer reactions, 73, 1 Allylic alcohols, synthesis from epoxides, 29, 3 by Wittig rearrangement, 46, 2 Allylic and benzylic carbanions, heteroatom-substituted, 27, 1 Allylic hydroperoxides, in... 

Enholm, E J, Gallagher, M E, Jiang, S, Batson, W A, Free radical aUyl transfers utilizing soluble non-cross-linked polystyrene and carbohydrate scaffold supports, Org. Lett., 2, 3355-3357, 2000. 

Scheme 21.4 AUyl-transfer reaction LA = Lewis acid.

Similarly to aUylsilanes, 1-allysilatrane (76) reacts with carbonyl compounds such as aldehydes, ketones or esters. Thus, treatment of 76 with aldehydes in the presence of TiCLj followed by hydrolysis afforded y,(5-unsaturated alcohols (equation 127). AUyl transfer is also catalyzed by other Lewis acids, such as AICI3 or BF3—OEt2, but the... 

The epoxidation of enones using chiral phase transfer catalysis (PTC) is an emerging technology that does not use transition metal catalysts. Lygo and To described the use of anthracenylmethyl derivatives of a cinchona alkaloid that are capable of catalyzing the epoxidation of enones with remarkable levels of asymmetric control and a one pot method for oxidation of the aUyl alcohol directly into... 

A combination of radical and electron transfer steps mediated by manganese triacetate has been used in the synthesis of 5-acetoxyfuranones 21 through carbox-ymethyl radical addition to mono- and disubstituted alkynes 20, followed by oxidative cyclization of the resulting vinyl radicals 22 (Scheme 2.4). The cyclic intermediate 24 is transformed into the furanone 21 through stepwise one-electron oxidation and capture of the resulting aUyl cation 26 by acetate. 

AUyl Sulfoxide Rearrangements of Compounds Without a Stereogenic Center at C-l (Chirality Transfer from Sulfur)... 

AUyl-based handles 19 are very useful systems since cleavage of the protected peptide from the resin can be achieved under practically neutral conditions by palladium(0)-catalyzed allyl transfer to weakly basic nucleophiles.These conditions are, in principle, compatible with both of the major SPPS strategies. 

Several air-stable, storable, and non-carbanionic reagents including allyhc silanes and stannanes have been used for C-C bond formation with carbonyl compounds, usually under nonbasic conditions. Unfortunately, however, transferable groups are hmited to aUyl, alkyne, and enol groups. TetraaUcyUead compounds, on the other hand, reacted smoothly with aldehydes in the presence of TiCU to produce the corresponding alcohols in moderate to good yield, with exceUent diastereoselectivity (Scheme 13.26) [49]. 

When vinyl and aUyl monomers undergo chain transfer with solvents, so-caUed telomers may form, generaUy of rather low molecular weight (1). 

Because of the low reactivity and tendency to undergo chain transfer, smaU additions of most aUyl compounds retard polymerization of typical vinyl monomers in free-radical systems (1,3) and may be useful in controlling molecular weight and stmcture in polymers. 

The hydrogenation of trons-1-phenyl-1,3-butadiene has been discussed" in terms of the mechanism originally proposed," and 77 -aUyl species were invoked. NMR studies on the hydrogenation of butadiene have shown that r/ -but-2-enyl and T/ -l-methylallyl complexes are present in the reaction mixture. With isoprene, an t7 -methylbut-2-enyl complex was observed. By using a two-phase system and a phase-transfer catalyst, the turnover numbers attainable with conjugated dienes can be increased to about 30. ... 

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radicaTT ation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radicaT ation to form the r-butyl carbocation and an unreactive aUyl-type radical ... 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-hutene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carhocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic pol3fmerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbo-cation formed by proton transfer is more stable than the aUyl carhocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Group transfer polymerization allows the synthesis of block copolymers of different methacrylate or acrylate monomers, such as methyl methacrylate and aUyl methacrylate [Herder, 1996 Webster and Sogah, 1989]. The synthesis of mixed methacrylate acrylate block copolymers requires that the less reactive monomer (methacrylate) be polymerized first. The silyl dialkylketene acetal propagating center from methacrylate polymerization is more reactive for initiation of acrylate polymerization than the silyl monoalkylketene acetal propagating center from acrylate polymerization is for initiation of methacrylate polymerization. Bifunctional initiators such as l,4-bis(methoxytrimethylsiloxymethylene)cyclohexane (XXXin) are useful for synthesizing ABA block copolymers where the middle block is methacrylate [Steinbrecht and Bandermann, 1989 Yu et al., 1988]. 

The transfer of electrons is not susceptible to steric hindrance so substituted alkenes pose no problem. In the next example, the enolate reacts with allyl bromide to give a single diastereo-isomer of the product (the aUyl bromide attacks from the face opposite the methyl group). Naturally, only one regioisomer is formed as well. 

From the structural analogies of the aUyl group and in view of the a-substituent bearing 77- or unshared electrons, benzyl, hydroxymethyl, methoxymethyl, cyanomethyl, ethoxycarbonylmethyl, and acetonyl groups can be selectively transferred from the corresponding trialky Itins. ... 

The process from the FMC company involves as the pivotal step an intramolecular stereoselective [2 + 1 [-cycloaddition. In a Prins reaction [94] of chloral and isobutene, followed by an isomerisation, a racemic, trichloromethyl-substituted aUyl alcohol is obtained. Reaction with the isocyanate from (R)-naphthylethyl-amine enables separation ofthe diastereomers by crystallisation. The carbamate is cleaved by trichlorosilane/triethylamine, thus permitting the recycling of the chiral auxiliary. The optically pure (R)-aUyl alcohol is reacted with diketene, to produce the / -keto-ester. After diazo transfer and basic cleavage, the diazoacetate is obtained catalysed by a copper salt, this is converted in a [2 + 1 ]-cyclo-addition into a bicyclic lactone. The Boord reaction (discovered by Cecil E. Boord in 1930) [95] finally gives (IR)-cis-permethric acid. [96]... 

A few years ago, a research group in Israel published a fundamentally revised version of the procedure. [97] Significant improvements were found in the synthesis of the optically pure aUyl alcohol, which was prepared by an enzymatic kinetic resolution, and the diazo transfer is replaced by diazotisation of the corresponding glycine ester. 

Monoaddition of 1,3-butadiene followed by instantaneous halide transfer from the counter anion and selective formation of the trans-l,4-adduct (PIB-AUyl-X) was observed in hexanes/MeCl 60/40 (v/v) solvent mixtures at —80 °C at [1,3-butadiene] <0.05 mol 1 ([1,3-butadiene]/[chain end] <12). Simple nucleophilic substitution reactions on these chloro or bromoallyl functional PIBs allowed the syntheses of end functional PIBs including hydroxy, amino, carboxy, azide, propargyl, methoxy, and thymine end groups [131]. 

In general, a higher concentration of the intermediate product (tetra-n-butylammonium alkoxide, or the active catalyst, ArOQ) in the aqueous phase enhances the reaction rate. This is due to a large concentration gradient across the interface in transferring the species from the aqueous phase to the organic phase. For the reaction of aUyl bromide and 2,4-dibromophenol in synthesizing 2,4-dibromophenyl allyl ether in an... 

The enantioselective phase-transfer catalyzed Michael addition of A-(diphenyl-methylene)glycine fert-butyl ester to several Michael acceptors such as methyl acrylate, cyclohex-2-enone and ethyl vinyl ketone was initially studied by Corey et al. employing 0(9)-aUyl-Af-9-anlhraceny]melhylcinchonidimum bromide (173) (Fig. 2.24) as catalyst and cesium hydroxide as base [272]. Different studies followed this pioneering woik, presenting diverse modifications over the standard procedure such as the employment of non-ionic bases [273], variations of the nucleophile functionality [274], and using new chiral phase-transfer catalysts, the most attention paid to this latter feature. For instance, catalyst 173 was successfully employed in the enantioselective synthesis of any of the isotopomers of different natural and unnatural amino acids... 

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