Neat reagent

The first report appeared in 1999 when Banks described the basic reaction of 2,5-hexanedione and aniline, which was successfully carried out mixing the neat reagents and irradiating for 30 s in a domestic microwave oven. Several 2,5 dimethypyrroles 9 were prepared starting from the same 1,4 diketone [30] (Scheme 3). 

A pyridyl bis-N-heterocydic carbene (NHC) ligand has been prepared by Steel and Teasdale based on nudeophilic aromatic substitution of dichloroisonicotinic amides with N-methylimidazole (Scheme 6.122) [250]. Microwave heating of the neat reagents at 140 °C for 10 min provided a 91% yield of the corresponding bis-... 

MW heated reactions in homogeneous media, using either neat reagents or in the presence of solvents, may also be performed at atmospheric pressure. This approach has been used particularly by Bose et al. [17]. (MORE Chemistry), who reported, for example, the rapid synthesis of heterocycles [18] in open vessels. Another approach, which avoids hazards due to the flammability of solvents, is to perform the reactions under reflux in a MW oven, which is modified to allow the reaction vessel to be attached to a reflux condenser outside the MW oven [7, 19]. It should be pointed out, however, that most of the available evidence shows that rate enhancements of MW heated reactions in homogeneous media at atmospheric pressure are small or nonexistent [19], This will be discussed in more detail later in this review (see also Chapt. 5 of this book). 

When the reactions are performed by adsorption of the reagents on to Ki0 mon-tmorillonite clay the aminoester is formed but when the reaction is performed by mixing the neat reagents with KF and 18-crown-6 and irradiation in a closed vessel /(-lactams were isolated in moderate to good yields [67]. 

Methyl hypofluorite (McOF) and tert-butyl hypofluorite (f-BuOF) have been prepared by the reaction of elemental fluorine with methanol and tert-butyl alcohol, respectively, in acetonitrile or propionitrile at — 78 °C. Methyl hypofluorite can be removed from the reaction mixture in a stream of nitrogen and purified by fractional distillation the liquid compound has a freezing point of — 142 C, is moderately long-lived, but explodes upon rapid warming.is tert-Butyl hypofluorite melts around — 94 °C and has an extrapolated boiling point of about 40 C the neat reagent must be regarded as a substance the hazards of which have not been fully defined.16... 

A mixture of methanesulfonic acid and P2Os used either neat or diluted with sulfolane or CH2C12 is a strongly acidic system. It has been used to control the rcgiosclcctivity in cyclization of unsymmetrical ketones. Use of the neat reagent favours reaction into the less substituted branch whereas diluted solutions favour the more substituted branch[3]. 

Reaction with neat reagent proceeds mainly with inversion. A marked solvent effect is observed in the reaction of 1 with DAST. 

Enantioselective reductions. The neat reagent (1), prepared from ( + )-< -pinene, reduces aryl a-halomethyl ketones slowly but in high chemical yield to (R)-halohydrins in 90-96% ee, but optical induction is mediocre in the case of aliphatic a-halo ketones (35-66% ee). The chiral halohydrins are useful precursors to chiral epoxides. 

Reduction of a,p-ynones. a,/J-Acetylenic ketones are reduced by 1 to (S> propargyl alcohols in 86-96% enantioselective purity. It is thus comparable to B-3-pinanyl-9-BBN in enantiomeric yields, although the reductions require 1-2 days and an excess of neat reagent for complete reaction. 

Phase-transfer catalytic (PTC) conditions, which are specific for anionic reactions (and anionic activation) are perfectly well tailored for microwave activation, because after ion exchange between a substrate and catalyst, the resulting nucleophilic ion pair is a highly polar species especially prone to interaction with microwaves [32]. Eventually, the mixture of neat reagents in an open vessel can lead to a reaction under microwave conditions provided that one of the reagents is liquid or a low melting solid that couples well with microwaves. On the other hand, even a small amount of a good microwave absorber (e.g,. H20, DMF for example, see Table 1.3) added to reaction mixtures that consist of substrates that do not absorb microwaves in the solid state can initiate an increase of reaction mixture temperature and then chemical reaction. 

Tris[(trimethylsilyl)methyl]aluminum, [(CH3)3SiCH2]3Al (1). This reagent is generated in situ by reaction of A1C13 with [(CH3)3SiCH2]Li. The neat reagent is pyrophoric. 

An important and extensively studied case involves reactions carried out in heterogeneous media between a solid and a liquid as neat reagents. The reaction can occur as an interfacial procedure allowed by either a partial solubilization of the solid in the liquid or by adsorption of the liquid on the surface of the solid, essentially owing to its high specific area [34,55]. 

A convenient method to prepare short-chain dialkyl tellurium diiodides uses tellurium and alkyl iodides as starting materials. The neat reagents are heated in a closed tube for several days at temperatures not exceeding 85°. An excess or the stoichiometrically required amount of alkyl iodide may be used. The yield diminishes rapidly with increasing length of the alkyl chain and is only 5% for dipentyl tellurium diiodide1. At temperatures above 80 to 100° the dialkyl tellurium diiodides decompose to the starting materials. 

The reactions of 4-alkoxyphenyl 4-dimethylaminophenyl tellurium dichlorides with hydrobromic acid were carried out with the neat reagents. The reaction mixtures were diluted with water and partially neutralized with dilute aqueous ammonium hydroxide to precipitate the tellurium dibromide6. 

Unlike alkyl halides, phenyl bromide and iodide reacted only sluggishly with diaryl telluriums. The neat reagents had to be heated at 150° for 40 h to obtain moderate yields of triaryl telluronium halides3. 

Solvent-free reactions without support or catalyst (neat reagents)... 

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