Cross-coupling reactions solvents

Application in organic synthesis of pentacoordinated triorganodifluorosilicate anions, such as [Bu4N][Ph3SiF2] 825, have been extended to palladium(0)-catalyzed cross coupling reactions (solvents DMF, TFIF, dioxane) with arene halides (Scheme 111).825 This method is tolerant to various palladium(O) catalysts and provides excellent yields of mainly heterocoupled products and only small amounts of homocoupled byproducts. 

The preparation of perfluoroalkylzinc compounds has been achieved earlier 111 ethereal solvents [26] However, solvent effects play a significant role in the course of this reaction When a mixture of acetic anhydride and methylene chloride is used, coupled and cross-coupled products can be formed [27, 28] (equations 19 and 20) However, the cross-coupling reaction often gives mixtures, a fact that seriously restricts the synthetic applicability of this reaction [27, 28, 29]... 

A palladium catalyst with a less electron-rich ligand, 2,2-dipyridyl-methylamine-based palladium complexes (4.2), is effective for coupling of aryl iodides or bromides with terminal alkynes in the presence of pyrrolidine and tetrabutylammonium acetate (TBAB) at 100°C in water.37 However, the reactions were shown to be faster in NMP solvent than in water under the reaction conditions. Palladium-phosphinous acid (POPd) was also reported as an effective catalyst for the Sonogashira cross-coupling reaction of aryl alkynes with aryl iodides, bromides, or chlorides in water (Eq. 4.18).38... 

Sterically demanding, water-soluble alkylphosphines 6.10 and 6.11 as ligands have been found to have a high activity for the Suzuki coupling of aryl bromides in aqueous solvents (Eq. 6.35).115 Turnover numbers up to 734,000 mmol/mmol Pd have been achieved under such conditions. Glucosamine-based phosphines were found to be efficient ligands for Suzuki cross-coupling reactions in water.116... 

There are many other examples in the literature where sealed-vessel microwave conditions have been employed to heat water as a reaction solvent well above its boiling point. Examples include transition metal catalyzed transformations such as Suzuki [43], Heck [44], Sonogashira [45], and Stille [46] cross-coupling reactions, in addition to cyanation reactions [47], phenylations [48], heterocycle formation [49], and even solid-phase organic syntheses [50] (see Chapters 6 and 7 for details). In many of these studies, reaction temperatures lower than those normally considered near-critical (Table 4.2) have been employed (100-150 °C). This is due in part to the fact that with single-mode microwave reactors (see Section 3.5) 200-220 °C is the current limit to which water can be safely heated under pressure since these instruments generally have a 20 bar pressure limit. For generating truly near-critical conditions around 280 °C, special microwave reactors able to withstand pressures of up to 80 bar have to be utilized (see Section 3.4.4). 

The cross-coupling reaction of homoallylic alcohols with aldehydes in the presence of bismuth trichloride in solvent-free conditions under microwave irradiation generated 4-chloro-2,6-disubstituted tetrahydropyrans with high c/.v-diastereos-electivity [15]. The reaction of benzaldehyde with l-phenyl-3-buten-l-ol in the presence of bismuth trichloride under microwave irradiation gave the corresponding... 

In 1991, Inanaga achieved Sml2-mediated intermolecular cross-coupling between C=0 (ketones or aldehydes) and C=N by using 0-benzyl formaldoxime as a C=N component7 The reaction requires HMPA as a co-solvent and a suitable proton source such as /-butyl alcohol or ethylene glycol. A cross-coupling reaction between ketones and... 

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