Ultrasound zinc reagents

The effects of ultrasound on zinc reagents has been explored in some detail as well. Ishikawa first examined the use of Zn for trifluoromethyla-tion of carbonyl compounds [Eq. (45)] (229). In this case the choice of... 

Despite the lower reactivity of solvated perfluoroalkylzinc reagents, perfluoroalkyl iodides undergo synthetically useful zinc-mediated reactions under Barbier conditions which often employ ultrasound and co-catalysts. Under these conditions, the zinc reagents are not well characterized and radical intermediates and SET mechanisms are proposed in some cases. Representative examples are presented below and include the ultrasound-promoted, zinc-mediated perfluoroalkylation of various substrates as reported by Ishikawa and coworkers (Scheme 10)123 126. Yields of carbinols could be improved by use of Ti(II) co-catalyst. Ultrasound promoted the coupling of perfluoroalkyl iodides with vinyl and allyl halides in the presence of Pd co-catalysts. 

Review.1 Ultrasound irradiatx with many organometallic reagent -zinc reagents (64 references). 

Review.1 Ultrasound irradiation improves the rate and the yields of reactions with many organometallic reagents, particularly of organocopper, -lithium, and -zinc reagents (64 references). 

Activation. Erdik1 has reviewed the methods used since 1970 for activation of zinc and of organozinc reagents. Although chemical activation is still useful, ultrasound activation is being used increasingly. Thus sonic activation allows use of ordinary zinc for cyclopropanation of alkenes with CH2I2 in 67-97% yield and for Reformatsky-type reactions at room temperatures. 

A novel approach to perfluoroarylzinc reagents was recently reported by Miller, Platonov and coworkers. This route involves direct insertion of zinc into carbon-fluorine bonds in the presence of metal salts, such as SnCl2, CuCl2 and ZnBr2. These reactions are accelerated by ultrasound (equation 75)71. The reaction rate decreased in THF or if ZnBr2 or CuCl2 were used. 

The preparation of alkylzinc iodides is facilitated if the reaction is performed in THF and NMP mixtures. Such solutions of Me02C(CH2)4ZnI add to benzaldehyde in the presence of TMSCI (2 equiv.) in 70% yield.33 The use of ultrasound also promotes the formation of organozinc compounds.34>34a 34e This procedure proved to be especially useful for the preparation of Jackson reagent 6 derived from serine. In the presence of a copper(l) or palladium(O) catalyst, this zinc derivative reacts with various electrophiles (Scheme 1) 3S,3Sa-3Sj 0f z nc reagent 7 leading to methyl but-3-enoate and the zinc amide... 

Reductions. Conversion of aromatic esters to benzyUc alcohols by zinc borohydride is catalyzed by alkenes. Quinolines are reduced to the 1,2,3,4-tetrahydro derivatives with ultrasound assistance. Reductions with the reagent on solid supports have been shown in the case of imines on silica geP and of epoxides and enones on Y-zeolite. ... 

It seems likely that many simple a-bromo esters can be reacted with zinc to produce the Reformatsky reagent nearly quantitatively, and that the reagent forms condensed ester primarily in a subsequent step. A successful synthesis of a Reformatsky reagent should then depend strongly on the time and temperature required for the preparation. Recently, a number of methods, including ultrasound promotion and the use of highly active forms of zinc, have allowed completion of the Reformatsky sequence in... 

Preparation of the zinc is critical to good yields in the Simmons-Smith reaction. Both Zn-Cu and Zn-Ag couples have been used, with both iodine and lithium.323 The zinc must be activated or the yield of cyclopropane product would be very poor. A useful development is the use of ultrasound to activate the zinc. Treatment of (-)-a-pinene (405) with the usual Simmons-Smith reagent gave only 12% of 406.324 When the zinc was activated by ultrasound, however, a 67% yield of 406 was obtained.325... 

Cyclopropanation of alkenes, in both inter- and intramolecular modes, constitutes an attractive route to polycycloalkanes. These cyclopropanations of alkenes are carried out through car bene or carbenoid intermediates . For the direct cyclopropanation of alkenes, the methylene iodide zinc-copper couple (Simmons-Smith reaction)reagent is commonly used. Several modifications of this procedure, including acceleration with ultrasound, are known °°. A somewhat less frequently used procedure for cyclopropanation is through methylene addition from diazomethane, which can be carried out either thermally or photochemically or in the presence of metal salts, e.g. Pd(OAc)2 or Rh2(OAc)4. In Table some examples of the preparation of cyclopropane bearing... 

Certain other reagents like samarium iodide in aqueous THF ", sodium dithionite in aqueous DMF , sodium sulfide in presence of polyethylene glycol " and metallic zinc along with nickel chloride. " Using the latter reagent (Zn/NiClj), a,P-unsaturated carbonyl compounds can be very readily reduced under ultrasound conditions (Scheme 105). 

The potential for introduction of chiral centres using a Diels-Alder approach makes this a particularly attractive route to benzo-fused cyclohexanes. However, the chemistry of orthoxylylenes is relatively unexplored as previous routes to this elusive intermediate involved the use of reagents such as disodium tetracarbonylferrate. This reagent is extremely intolerant of other functional groups, mainly as a result of its high basicity and the yields obtained are only a fraction of those available using the zinc/ultrasound route. 

---