Transition metal carbonyls and ultrasound

In 1981, the first report on the sonochemistry of discrete organometallic complexes demonstrated the effect of ultrasound on iron carbonyls in alkane solutions (174). The transition metal carbonyls were chosen for these initial studies because their thermal and photochemical reactivities have been well characterized. The comparison among the thermal, photochemical, and sonochemical reactions of Fe(CO)5 provides an excellent example of the unique chemistry which homogeneous cavitation can... 

Besides using sonochemistry as a means of promoting the pyrolysis of metal carbonyl compounds, ultrasound has been effective in the preparation of supramolecular organobismuth-transition metal carbonyls.The compound [ -BuBiFe(CO)4]oo has been formed by the reaction of -BuBr and [Et4N]3[Bi Fe(CO)4l4 in an ultrasonic cleaning bath in MeCN, followed by acidification. The reaction has been postulated to involve an ultrasound-generated radical mechanism. 

Since power ultrasound is capable of generating extremely high pressures and temperatures within microbubbles in a liquid medium the possibility arises that sonochemistry might replace conventional (and expensive) high pressure reactors. Power ultrasound has been found to dramatically reduce the temperatures and pressures required for the preparation of early transition metal carbonyl anions from the direct reaction of the corresponding metal chlorides with carbon monoxide (Scheme 10.6) [18]. In this particular case the yield of V(CO)6 (35%) can be obtained at 4.4 atmos pressure and 10°C whereas conventional methodology requires 200 atmos and 160°C. 

Suslick has made an extensive study of the sonochemistry of Fe(CO)5 which he has used as a probe to explore the chemical effects of high intensity ultrasound. Suslick and Johnson [280] have also shown that sonication greatly facilitates the preparation of early transition metal carbonyl anions. Hence, sonication of vanadium trichloride and sodium sand in THF solution gave a 35 % yield of NaV(CO)g under 4.4 atmospheres of carbon monoxide at 10 °(i. The equivalent thermal reaction requires the reaction to be caried out at 160 °C under 200 atmospheres of carbon monoxide. That is, the temperatures and pressures produced by cavitation are comparable to the bomb conditions normally required for the preparation of these compounds [281]. Suslick s review [3] presents further evidence in support of this original observation however, no further details of this work have appeared to date. 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

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