Reactions in nonpolar solvents

Under MW irradiation (980 W) in xylene yields of 73-93% were obtained in 10-20 min, whereas under conventional conditions a complex mixture of products was obtained, which was unsuitable for preparative work. Thus it would appear that these reactions not only occur much faster under microwaves but also give cleaner products, indicating greater selectivity. Loupy [44, 45] has speculated that these reactions, among others performed in nonpolar solvents or in the absence of solvent, might be accelerated under MW irradiation due to the involvement of relatively polar 

However, the possibility of the participation of nonthermal effects in MW-assisted reactions in nonpolar solvents is still an open question. Loupy et al. [55] observed an increase in yield and purity of the diazepine 36, in the reaction of ethyl acetoacetate with o-phenylenediamine using monomode MW reactor with focused MW heating, when compared with conventional heating with the same temperature profile. 

Reactions in Homogeneous Media Showing no MW Rate Enhancement 

A number of other reactions in homogeneous media have been shown to occur at the same rate under MW heating and classical heating at the same temperature. 

in a more carefully controlled comparison, Pollington et al. [57] showed that the esterification of acetic acid with 1-propanol also occurred at the same rate under MW and conventional reflux. 

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Carbon dioxide 53 55). If excess C02 and polar solvents are used the carboxylation is quantitative and free of side reactions. In nonpolar solvents association phenomena favor ketone formation 55). An alternate way to get re-carboxylic polymers is to react the living sites with a cyclic anhydride 561. 

Because observed rate enhancements are usually small, or zero, nonthermal effects do not seem to be important in MW heated reactions in homogeneous media, except possibly in some reactions of polymers and reactions in nonpolar solvents. Relatively few studies have been conducted on MW-assisted reactions of polar reactants in nonpolar solvents. Also, since there is some disagreement as to whether or not these reactions are accelerated significantly by MW, in comparison with conventionally heated reactions at the same temperature, more research on the effect of MW irradiation on the rates of these reactions is required. Nonthermal effects may, however, explain the more substantial MW rate enhancements in solvent-free reactions on solid supports [44] (see Chapt. 5) and solid state reactions [68, 69]. 

Silyl Nitronates in the Mannich Reaction The Mannich reaction with SENAs (Scheme 3.194) was also modified (459). This reaction in nonpolar solvents produced previously unavailable 3-nitroamines (331), which are unstable in water and other protic solvents. 

A wide variety of unsaturated carboxylic acids have been allowed to react with HBr.101 Carboxylic acids with remote C—C double bonds react as simple alkenes.138,139 4-Pentenoic acid reacts with HBr neat or in a polar solvent to give exclusively 4-bromopentanoic acid, but the reaction in nonpolar solvents affords only 5-bromopentanoic acid.136 On the other hand, 5-methyl-4-hexenoic acid produces only the S-bromo acid. A similar pattern is followed by 3-butenoic, 3-pentenoic and 4-methyl-3-pentenoic acids. No matter what the substitution pattern, 2-alkenoic acids always favor the 3-bromo acid.113,136,140,141 Addition of HBr to cyclic a,3-unsaturated acids initially forms predominantly the product of trans diaxial addition which upon longer reaction time or higher temperature isomerizes to the trans product (equation 92).57,141,142 Similar observations have been made on bicyclic a.fj-unsaturated acids.141... 

As relatively hard acids, Ln + ions would not be expected to complex readily with soft donor atoms like sulfur. Apart from the thiolates discussed in (see Section Polypyrazolylbo-rates ), several types of complex with chelating ligands such as dithiophosphates and dithiocarbamates have been synthesized by reaction in nonpolar solvents. These fall into two main classes, Ln(S2Q)3 and [Ln(S2Q)4] (Q = R2NC (dithiocar-bamate) or R2P (dithiophosphate)). Several crystal structures... 

Schuster proposed a triplex as a key intermediate for the inter- and intramolecular photo-Diels Alder reaction in nonpolar solvents in the presence of an electron accepting sensitizer [105-107]. This photoreaction can be applied to an enantioselective photo-Diels Alder reaction between 1,3-cyclohexadiene and propenylbenzene by the use of a chiral sensitizer (Scheme 32) [108]. 

In many cases, crown ether-assisted nucleophilic substitution reactions in nonpolar solvents are faster than similar reactions in polar aprotic solvents, even though polar aprotic solvents enhance nucleophilicity by solvating anions poorly. 

Carbonyl groups form complexes or intermediates with Lewis acids like AICI3, BF3, and SnCl4. For example, in the Friedel-Crafts acylation reaction in nonpolar solvents, an aluminum chloride complex of an acid chloride is often the acylating agent. Because of the basicity of ketones, the products of the acylation reaction are also complexes. For more detail on electrophilic aromatic substitution, see Section 7. 

Okuno, Y. Theoretical investigation of the mechanism of the Baeyer-Villiger reaction in nonpolar solvents. Chem.— Eur. J. 1997, 3, 212-218. 

Uses. Thallous ethoxide is soluble in most organic solvents, including heptane, and thus possesses the considerable advantage over sodium ethoxide of effecting homogeneous base-catalyzed reactions in nonpolar solvents. Thallous ethoxide converts most /3-dicarbonyl compounds into stable, crystalline, light-insensitive, colorless, 1 1 salts (2) which may be subsequently mono-C-alky lated in almost quantitative yield by heating with alkyl iodides.3 The salts are readily prepared by addition... 

Evidently there is not much difference in the overall conversions of styrene in polymerizations using rcrt-butyl chloride and 3-chloro-1 -butene coinitiators under similar conditions (Figs. 5 and 6). This is expected because of the similar reactivities of the tertiary and substituted allylic chlorines in these coinitiator molecules. In addition, since rates of the various individual competing reactions in nonpolar solvents (i.e. methylcyclohexane and n-heptane) are expected to be higher at higher temperatures (20° C) than at lower temperatures (—50° C), and if the propagation or initiation rate overrides that for termination, it would not be unexpected to find that yields increase with increasing temperature. Also, it is... 

Acid and base pairs are known to activate molecules simultaneously and efficiently via dual coordination to promote reactions [1, 2, 43]. Acid and base sites on amorphous SA-supported aminopropyl groups (SA-NEt2), prepared by the treatment of SA with 3-(diethoxyamino)propyltrimethoxysilane, promoted the cyano-ethoxycarbonylation reaction in nonpolar solvents such as toluene and diethyl ether (Scheme 6.17) [121]. A proposed mechanism involves the dual activation of donor and acceptor substrates at the amine base site and the neighboring Bronsted acid site on the SA surface, respectively. 

On the other hand, true ligand acceleration (type 3 processes) shows preference for solvents of low polarity and lower Lewis basicity (toluene, dioxane and THF) with soluble tertiary amines as bases. In this respect, these Mizoroki-Heck reactions resemble cross-coupling processes, which also display strong preference for these solvents. Reactions in nonpolar solvents (toluene or xylene) have been known since Heck s seminal articles [8]. The halide remains a crucial subject of concern in reactions catalysed by phosphine complexes of palladium, aryl iodides prefer triarylphosphines and polar solvents, whereas reactions of aryl bromides and chlorides indeed prefer electron-rich trialkylphosphines and nonpolar solvents [63-65]. 

Moyano, Rios, and co-workers [38] have shown that the beneficial effect of hydrogen-bond donors in proline-catalyzed aldol reactions in nonpolar solvents [39] is due both to the facilitation of proline solubilization by formation of an oxazolidinone with the ketone and to the stabilization of the iminium carboxylate zwitterionic form that is the direct precursor of the reactive enamine intermediate,... 

SCHEME 2.7. Thiourea-assisted oxazolidinone formation and aldol reaction in nonpolar solvents. 

As mentioned in Chapter 2.1, the formation of radicals requires the homolytic cleavage of a covalent bond. Energetically such homolytic cleavage is particularly favorable in gas-phase reactions and for liquid-phase reactions in nonpolar solvents. In polar solvents, however, the energy contribution from the solvation of ionic species formed in heterolytic cleavage reverses the picture and heterolytic cleavage becomes more favorable. 

Different cocatalysts such as RjATCU or R4A12C12 produce a comparable catalyst with a somewhat decreased activity. The high rates of reaction in nonpolar solvents make the presence of ionic intermediates unlikely. Use of other solvents such as benzene or chlorobenzene do not produce any major change in the product composition. The following active species are formed by the reaction of the monomer with a coordinated nickel hydride ... 

The process of inversion (racemization) without exchange is called isoinversion. This involves an ion-pair mechanism. Indeed, in the presence of a crown ether, to separate the ions, the percentage of racemate increases (307). In fact, the stereochemical course of many metal-alkoxide catalyzed reactions in nonpolar solvents can be drastically modified by addition of catalytic amounts of crown ethers to the medium. For this reason, ion pairs, in low dielectric media, play a remarkable role as intermediates in reactions in which the negative ion is a carbanion. For example. Cram studied the rate of exchange (ke) and racemization kj as a function of the substituents present with deuterium-labeled 9-methylfluorene (309). The most interesting case was kjk < 0.5, racemization without exchange. This happens for X = di-... 

The reaction of the electron with CO2 [Eq. (16)] is an equilibrium reaction in nonpolar solvents at ordinary pressure (Holroyd et al.,... 

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