Addition phase-transfer-catalyzed

PECH was modified under similar reaction conditions, except that dimethylformamide (DMF) was used as the reaction solvent. In addition, the phase-transfer-catalyzed etherification of the chloromethyl groups of PECH with sodium 4-methoxy -4 -biphenoxide was used to synthesize PECH with direct attachment of the mesogen to the polymer backbone. Similar notations to those used to describe the functionalized PPO are used for functionalized PECH. In this last case, PPO was replaced with PECH. Esterification routes of both PPO and PECH are presented in Scheme I. 

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

The first successful results of the asymmetric Michael addition under phase transfer catalyzed conditions were achieved by use of ingeniously designed chiral crown ethers 13 and 52.1441 The 3-keto ester 49 reacted with methyl vinyl ketone by use of 13 to give the Michael product 50 with excellent enantioselectivity but in moderate yield, as shown in Scheme 18. The Michael addition of methyl 2-phenylpropionate 51 to methyl acrylate afforded the diester 53 by use of another crown ether 52 in good yield with good enantioselectivity.1441 Various chiral crown ethers were studied to... 

One of the most useful classes of metal and phase transfer catalyzed reactions are carbonylation reactions. Cobalt carbonyl is a valuable catalyst for such processes(1 ). When used in conjunction with methyl iodide, acetylcobalt carbonyl [CH3C0Co(C0) ] is generated and can undergo addition to various unsaturated substrates including alkynes and Schiff bases. In addition, one can add this species to styrene oxides to give the enol... 

Preparation of spiro[2.3]hexanes is possible by the addition of methylene or a methylene equivalent to an exocyclic C —C double bond. Generally, the Simmons Smith reaction is used (see Section l.A.1.2.1.), but formation of carbenes from nitroso compounds can also be used.73-75,333 The 1,1-dihalo derivative can be prepared by phase-transfer-catalyzed reactions.48 For example, reaction of a mixture of cis- and trans-1,2-dimethyl-3-methylenecyclobutane with dibromo-carbene gave a mixture of cis- and lrans-1,1-dibromo-4,5-dimcthylspiro[2.3]hexanc (2).48... 

The phase transfer catalyzed alkylation reaction of dodecyl phenyl glycidyl ether (DPGE) with hydroxyethyl cellulose (HEC) was studied as a mechanistic model for the general PTC reaction with cellulose ethers. In this way, the most effective phase transfer catalysts and optimum reaction concentrations could be identified. As a model cellulose ether, CELLOSIZE HEC11 was chosen, and the phase transfer catalysts chosen for evaluation were aqueous solutions of choline hydroxide, tetramethyl-, tetrabutyl-, tetrahexyl-, and benzyltrimethylammonium hydroxides. The molar A/HEC ratio (molar ratio of alkali to HEC) used was 0.50, the diluent to HEC (D/HEC) weight ratio was 7.4, and the reaction diluent was aqueous /-butyl alcohol. Because some of the quaternary ammonium hydroxide charges would be accompanied by large additions of water, the initial water content of the diluent was adjusted so that the final diluent composition would be about 14.4% water in /-butyl alcohol. The results of these experiments are summarized in Table 2. 

It is comparable to crown ethers or R4NBr for phase-transfer catalyzed aliphatic nucleophilic substitutions in addition it catalyzes aromatic nucleophilic substitutions such as the Ullmann synthesis. 

Phase-Transfer-Catalyzed Addition of Anion Supplied as Metal Salt... 

Recently, chiral phase-transfer-catalyzed asymmetric Michael addition has received much attention, and excellent enantioselectivity (up to 99% ee) has been reported using cinchona alkaloid-derived chiral phase-transfer catalysts [40]. Among noncinchona alkaloid-derived chiral phase-transfer catalysts Shibasaki s tartrate derived C2-symmetrical two-center catalyst provided a Michael adduct with up to 82% ee [41]. 

Early examples of intramolecular aryl radical addition reactions to heteroatom containing multiple bonds included cyclizations on N=N and C=S moieties [52, 53]. Recently, cyclizations to imines have been used as part of a new enantio-selective approach to indolines (Scheme 8). In the first step of the sequence, the required ketimines 19 were obtained by phase-transfer catalyzed alkylation of 2-bromobenzyl bromides 20 with glycinyl imines 21 in the presence of a cincho-nidinium salt [54], Due to the favorable substitution pattern on the imine moiety of 19, the tributyltin hydride mediated radical cyclization to 22 occurred exclusively in the 5-exo mode. The indoline synthesis can therefore also be classified as a radical amination. 

The reactions of 9, 10 and 11 can be carried out under neutral conditions. The decomposition of 13 under phase transfer catalyzed (PTC) conditions usually gives the adducts in yields better than those obtained in the alkoxide-induced decomposition of 12. The reactions of 2,2-disubstituted vinylidenes are rather sensitive to steric hindrance. The stereospecificity of the addition has been demonstrated in the reaction of 12. Intramolecular reactions giving particularly strained methylenecyclopropanes have also been reported " . 

Acid-catalyzed addition of aliphatic, aromatic or heteroaromatic cyanohydrins to ethyl vinyl ether, n-butyl vinyl ether or dihydro-4//-pyran provides base stable, protected cyanohydrin derivatives. Phase transfer catalyzed alkylation of aliphatic cyanohydrins with allylic bromides gave a-substituted a-allyl-oxyacetonitrile. Carbonyl compounds react wiA cyanide under phase transfer catalysis to give cyanohydrin anions, which are trapped by an acyl chloride or ethyl chloroformate to give acyl- or alkoxycarbonyl-protected cyanohydrins respectively. The reduction of the carbonyl group of an acyl cyanide by NaBH4 under phase transfer conditions followed by esterification serves as an alternative route to aldehyde-derived cyanohydrin esters. ... 

LiN(SiMe3)2 to form crixivan intermediate 4 [13]. The reactivity of bases may be dependent on the nature of the alkali metal portion of the base, as seen in the hetero-Michael additions shown in Figure 3.7 [14]. Phase-transfer catalyzed reactions (PTC) are often overlooked [15], and can provide safe, economic, and very productive alternatives to reactions using strong bases (see Figure 2.18). 

It is often difficult to make a comparison between the various results obtained for the same polyenes as different reaction conditions (ratio of reactants, temperature, time) were used in each case. The addition of dichlorocarbene (chloroform/base/phase-transfer catalysis) to straight chain and cyclic unconjugated di- and trienes, carried out under identical conditions but varying the catalysts, showed the peculiar properties of tetramethylammonium chloride. Under precisely tailored conditions, either highly selective mono- or polyaddition of dichlorocarbene to the polyenes is possible tetramethylammonium chloride was the most efficient catalyst for monocyclopropanation. (For the unusual properties of tetramethylammonium salts on the phase-transfer catalyzed reaction of chloroform with electrophilic alkenes see Section 1.2.1.4.2.1.8.2. and likewise for the reaction of bromoform with allylic halides, see Section 1.2.1.4.3.1.5.1.). For example, cyclopropanation of 2 with various phase-transfer catalysts to give mixtures of 3, 4, and 5, ° of 6 to give 7 and 8, ° and of 9 to give 10 and 11. °... 

The asymmetric Michael addition of active methylene or methyne compounds to electron deficient olefins, particularly a,P-unsaturated carbonyl compounds, represents a fundamental and useful approach to construct functionalized carbon frameworks [51]. The first successful, phase-transfer-catalyzed process was based on the use of well-designed chiral crown ethers 69 and 70 as catalyst. In the presence of 69, P-keto ester 65 was added to methyl vinyl ketone (MVK) in moderate yield but with virtually complete stereochemical control. In much the same way, crown 70 was shown to be effective for the reaction of methyl 2-phenylpropionate 67 with methyl acrylate, affording the Michael adduct 68 in 80% yield and 83% ee (Scheme 11.15) [52]. 

A new phase-transfer catalyzed asymmetric glycolate aldol reaction was reported that provides diols in low to good yields, high de, and moderate to good ee as illustrated in Scheme 37. The recystalhzation of diol products could enrich the ee to 95%, additionally, the authors used 128 (R = Ph) to synthesize a known (S TJydiol methyl ester to show utility of this new method <05OL3861>. 

The issue of 1,2- versus 1,4-addition, as shown in Scheme 22, in the phase transfer catalyzed (benzyl-triethylammonium chloride TEBA) version of this cyclopropanation reaction, was addressed in a more recent publication. Simple a-halo esters do not react with acrolein, methyl vinyl ketone or 2-chloroac-rylonitrile under the conditions examined. However several electrophilic alkenes do react with diethyl bromomalonate to give good yields of substituted cyclopropanes (equation 38 Table 13). The only side... 

During the last few years, phase transfer catalyzed reactions between a supercritical phase and a second phase have been investigated in order to find more environmentally friendly solvents than those commonly employed in PTC processes. In addition to the well known advantages deriving from the use of supercritical CO2 (SCCO2) as solvent [53], the accessibility of the critical properties (T]. = 31°C, = 74bar) permits its use when thermally... 

Asymmetric phase-transfer catalyzed cascades initiated by the conjugate addition of heteronucleophiles has been employed for the enantioselective preparation of epoxides and aziridines using a suitable oxygen- or nitrogen-based nucleophile incorporating an appropriate leaving group ready to... 

Scheme 2.85 Initial studies on the phase-transfer catalyzed addition of 1,3-dicarbonyl compounds to enones...

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