Nucleophilic silica-supported

Nucleophilic substitution of the chlorine atom of 1.1 with potassium diphenylphosphide (KP(C6H5)2) gives 3.1 [7, 8]. The reaction of 3.1 with Rh(I) and Pt(II) transition metal complexes (Eq. 1-3) yields compounds which can be used as models for silica supported catalysts [8],... 

Ionic liquids are proposed as designer solvents for nucleophilic aromatic substitution. Coating with a layer of ionic liquid onto silica-supported sulfonic acid improves its utility (such as acetalization) boasting selectivity in aqueous media. ... 

The actual chemical nature of the support material may be and often is of direct importance to its usefulness as a support material.15 Silicas can react with small nucleophiles such as F-, OH- and CN. Thus, silica-supported fluorides are inactive, both as nucleophilic fluorinating agents and as bases. Similarly, silicas are not effective support materials for cyanides due to the formation of strong Si-CN bonds. For different reasons, an acidic clay would not be a suitable support for cyanides, due to the possible formation of toxic HCN. Charcoal is the most effective support material for stabilising Cu(I), probably due to its aromatic character.16 For many chemisorbed supported reagent catalysts, silicas are preferred since they give relatively strong surface bonds. However, Si-O-C bonds are hydrolytically vunerable and direct Si-C bonds are preferred.17... 

In a less frequently used technique, a silica surface is first chlorinated with, for instance, thionyl chloride. Next the Cl is displaced by a strong nucleophile such as benzyl- [21] or phenyllithium [22] (Figure le). Thus a new Si-C bond is formed containing an Si atom that initially was part of the silica support. This approach precludes the formation of surface-bound Si oligomers [12]. Eventual sulfonation of the aromatic rings leads to a useful catalyst. 

The PS-TBD catalyst has been shown to be effective for epoxide ring opening reactions with several nucleophiles such as thiols under solvent free conditions [37,78] (Scheme 6.21). In this case, the reusabihty of the catalyst was also established without a significant loss of reactivity and selectivity. As a related work, the utility of mesoporous silica-supported TBD catalysts was demonstrated in the reaction of propylene oxide with carbon dioxide to prepare the corresponding carbonate derivative under the ultrasonic activation [79]. 

Sn2 nucleophilic substitutions take place in scC02. The presence of silica-supported onium salts 42 as a phase-transfer catalyst enhances the reaction (Scheme 72). Esters formation from carboxylic acids and alcohols via dehydration also proceeds in scC02. The conversion increases as the CO2 pressure increases. A single homogeneous phase is obtained around the critical point, at which the conversion is maximized. 

DeSimone. J. Selva. M. Tundo. P. Nucleophilic displacements in supercritical carbon dioxide using silica-supported phase-transfer agents. J. Org. Chem. 2001. 66. 4047-4049. 

Due to the ability of imidazolium compounds to form metallic Af-heterocyclic carbene complexes, imidazolium-based ionosilicas have widely been studied for the formation of silica-supported NHC species and found wide applications in organometallic catalysis. However, ionic species recently found to promote a large variety of reactions due to their ionic nature. Cooperative nucleophilic-electrophilic activation is a widely accepted concept in catalysis, and due to their ionic nature, ionic liquid should be considered as bifunctional catalysts. 

Key to this strategy was removal of the isocoumarin F ring in the spirocyclization precursor, thereby increasing the nucleophilicity of the E ring phenol such that cyclization could occur under mildly acidic conditions. Thus, treatment of precursor 59 with silica-supported sodium hydrogen sulfate afforded the bis(benzannulated) spiroacetal 60 in high yield. 

Other examples of nucleophilic attack on the oxirane ring include the formation of (3-halohydrins with silica-gel supported lithium halides <96TL1845>, the addition of amines catalyzed by lithium triflate, an ersatz for lithium perchlorate <96TL7715>, and the addition of pyrroles, indoles and imidazoles under high pressure i.e., 91 —> 93) <96JOC984>. 

Few examples have been described of nucleophilic cleavage of carbonate- or carbamate-linked alcohols from insoluble supports. A serine-based linker for phenols releases the phenol upon fluoride-induced intramolecular nucleophilic cleavage of an aryl carbamate (Entry 2, Table 3.36). A linker for oligonucleotides has been described, in which the carbohydrate is bound as a carbonate to resin-bound 2-(2-nitrophen-yl)ethanol, and which is cleaved by base-induced 3-elimination (Entry 3, Table 3.36). Trichloroethyl carbonates, which are susceptible to cleavage by reducing agents such as zinc or phosphines, have been successfully used to link aliphatic alcohols to silica gel (Entry 4, Table 3.36). These carbonates can also be cleaved by acidolysis (Table 3.22). 

In the case of nonsymmetrical epoxides, the regioselectivity is determined by the particular reaction conditions. Thus, styrene oxide (6) undergoes methanolysis in the presence of the Lewis acid catalyst copper(II) tetrafluoroborate to give the hydroxy ether 60, derived from attack of the nucleophile at the more substituted oxiranyl carbon. Similar outcomes have been observed in the solvolysis of 6 with the assistance of aminopropyl silica gel (APSG) supported iodine in catalytic quantities <02SL1251>. This selectivity appears to be much less decisive, however, in the case of monoalkyl epoxides, as illustrated in the corresponding reaction of 1-octene oxide (61), which yields an almost 1 1-mixture of isomers under the same conditions <02OL2817>. 

Two approaches have been employed to prepare supported guanidines - a ringopening of the glycidyl-functionalised silica by 1,5,9-triazabicyclodecane (TBD)[15], and the nucleophilic displacement of a chloro substituent again by TBD [16]... 

As for many other nucleophiles, the nitrite anion undergoes addition to the iodonium ion generated by the reaction of alkenes and 1,3-alkadienes with electrophilic iodine reagents. Two procedures have been described bis(pyridine)iodine(I) tetrafluoroborate136,137 [prepared from mcrcury(II) oxide and tctrafluoroboric acid supported on silica gel and pyridine on dichloromethane] and copper(II) tetrafluoroborate [prepared from copper(II) oxide and te-trafluoroboric acid] and iodine138 139. trans Addition would be expected for all products from mechanistic considerations, however, only the cyclohexene adduct 1 has been shown to have trans configuration ( H-NMR spectroscopy)139. 

Soai et al. [62a] first reported the use of sihca gel or alumina as a heterogeneous support for chiral catalysts in the enantioselective addition of dialkylzincs to aldehydes. Chiral N-alkyhrorephedrines (R = Me, Et, n-Pr) were immobilized covalently on (3-chloropropyl)silyl-functionalized alumina or silica gel via a nucleophilic substitution. However, the catalytic activities and enantioselectivities were only moderate (24—59% ee) in comparison with those of homogeneous and polymer-... 

Tetramethylguanidine (TMG) 84 was supported on mesoporous silica according to the methodology depicted in Scheme 3.24, involving the nucleophilic substitution of the chloropropyl moiety anchored to silica (83) by TMG (84) in the presence of the strong basic TBD that was necessary to remove the formally produced hydrochloric acid. 

Silica gel has been used as reagent and absorbent as well as catalyst for many nucleophilic processes. Kotsuki and Shimanouchi have demonstrated an elegant epoxide ring opening using LiX supported silica gel (Scheme 5.35) in the absence of any solvent. 

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