Aqueous conditions allylic substitution

A polyethylene glycol-polystyrene graft copolymer palladium catalyst has been used in allylic substitution reactions of allyl acetates with various nucleophiles in aqueous media.58 Another polymer-bound palladium catalyst 40 was developed and used in a Heck coupling of allylic alcohols with hypervalent iodonium salts to afford the substituted allylic alcohols as the sole products under mild conditions with high catalytic efficiency.59 The same polymer-bound palladium catalyst has also been used for Suzuki cross-coupling reactions.60... 

Stereoselective 1,4-asymmeric stereoinduction under aqueous conditions is realized by use of oxygen-substituted allylic indium reagents (Tab. 8.5) [30]. O-Silylated allylindium shows moderate anti selectivity, via the FeUdn-Anh transition state, whereas hydroxy-bearing allylindium exhibits syn selectivity by dual coordination of indium intramolecularly to the hydroxy group and intermolecularly to the aldehyde. 

Substitutions of Sn2 type are frequently used for carbon-carbon or carbon-heteroatom bond formation. However, little attention has been devoted to the development of such reactions in water. This is likely due to concerns about competitive hydrolysis of the electrophile in water and SN2-type reactions being slower in aqueous conditions than in aprotic polar solvents due to the higher cost of desolvation of nucleophiles. We shall discuss the ring opening of epoxides and aziridines, palladium-catalyzed allylic substitutions, as well as acylations and sulfonylations of amines and alcohols. 

Treatment of ethacrylate esters 1 with nitronium tetrafluoroborate in acetonitrile has been shown to give cyclopropanes 2 and the products of allylic nitration 3. Formation of 2 was postulated to proceed via an a-carbonyl cation. In an attempt to obtain evidence for the possible intermediacy of a-carbonyl cations in these reactions in terms of Wagner-Meerwein derived products, the more highly substituted substrates 4a, b were subjected to the same reaction conditions of NC>2BF4/MeCN followed by aqueous work-up. This gave 5a, b and 6a, b as shown. 

Conversion of phenols into their methyl or ethyl ethers by reaction with the corresponding alkyl sulphates in the presence of aqueous sodium hydroxide affords a method which avoids the use of the more expensive alkyl halides (e.g. the synthesis of methyl 2-naphthyl ether and veratraldehyde, Expt 6.111). Also included in Expt 6.111 is a general procedure for the alkylation of phenols under PTC conditions.38,39 The method is suitable for 2,6-dialkylphenols, naphthols and various functionally substituted phenols. The alkylating agents include dimethyl sulphate, diethyl sulphate, methyl iodide, allyl bromide, epichlorohy-drin, butyl bromide and benzyl chloride. 

The effectiveness in carbonylations of Ni(CO)4 is well documented, as well as its toxicity. Substitutes for this catalyst are therefore of much interest, and [Ni(CN)(CO)J], generated in situ from Ni(CN)2, CO and aqueous NaOH under phase transfer conditions, fulfills this role in many cases394. Under these conditions (1 atm CO), several types of organic halides are carbonylated, including allyl halides394, benzyl chlorides (with lanthanide salts)395, aryl iodides396, vinyl bromides397 and dibromocyclopropanes (equation 199)398. 

To test the validity of their assumption, Mootoo and Fraser-Reid prepared NPGs 12-18 and treated them with NBS in 1% aqueous acetonitrile [16]. Their results, summarized in O Table 1, showed that differently substituted NPGs could be chemoselectively liberated at the anomeric center to yield hemiacetals 19-24. Furthermore, benzylidene, silyl, p-methoxybenzyl (PMB), ethoxyethyl, and allyl protecting groups proved to be compatible with the conditions employed in the deprotection of the anomeric pent-4-enyl group. Diol 18, however, furnished a complex reaction mixture probably related to competing glycosylation processes, vide infra. 

The outer-sphere OAc anions can be replaced by other anions. For instance, the and PF anions readily substitute for OAc anions in an aqueous solution containing KPFft, affording the giant cluster with the idealized formula [Pdsei LeoOeoKPFeleo [Ik 16, 17]. The Pd-561 clusters exhibit a high catalytic activity in alkene acetoxylation in an AcOH solution under mild conditions (20-60 °C at 0.1 MPa). Besides reaction (1), the clusters provide the oxidative acetoxylation of propylene to allyl acetate (eq. (6)) or of toluene to benzyl acetate (eq. (7)). 

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