Synthesis with Ultrasonic Waves Philip Bwdjouk North Dakota State University, Fargo, ND 58105 The pioneering work on the chemical applications of ultrasound ( I ) was conducted in the 1920's by Richards and Loomis in their classic survey of the effects of high-frequency sound waves (>280 kHz) on a variety of solutions, solids, and pure liquids (2). These studies showed that ultrasound accelerates a broad ranee of transformations such as the dispersion of mercury, th;: degassing of liquids, the explosion of nitrogen triiodide, the flocculation of silver chloride, the depression of the boiling temperatures of liquids, the hydrolvsis of dimethvlsulfate, and the iodine clock reaction. - In spite of the diversity of the chemical effects of ultrasonic waves discovered by Richards and Loomis, research since then has been sparse and uneven. For the most part, the emphases have heen on inorganic reactions, in particular aqueous solutions, and in nearly all cases the systems were homogeneous. Also, the sonicators used by the various research erouns were often auite different in confieuration and delivered differenr frequencies, intensities,and wattages.To comolirate the matter further. little was known of the roleoi thesk variables in affecting reaction rates. As a result, there are few generalizations upon which a new investigator can rely, and sonochemistry, in particular preparative sonochemistry, must he considered an area that is in its earliest stages and in need of much further study. One of the points on which there is general agreement is that, in order to produce a chemical effect in liquids using ultrasonic waves, sufficient energy must he imparted to the liquid to cause cavitation, i.e., the formation and collapse of bubbles in the solvent medium and the consequent release of enerw. When ultrasonic waves are oassed through a medium, i i e particles experience oscillati'ons that l e a l t o regions of compression and rarefaction. The negative pressure in the rarefaction region gives rise to the formation of bubbles, that mav he filled with a eas. the vapor of the liauid, or may he almost empty depenbing on the pressure a n d the forces holdina the liauid together. strictly defined, cavitation refers only t o the completely evacuated bubble or cavitv, a true void, hut, since dissolved gases are present unless Special steps are taken to remove themand, since thevapor of the liquid can also penetrate the cavity, the term cavitation most often encompasses the three kinds of bubbles. The collapse of these bubbles, caused by the compression region of the ultrasonic wave, produces powerful shock waves that are resoonsible for well-known processes such as cleaning, dispersion, and the erosion of metals. The energy output in the reaion of the collapsine . - bubble is considerable, with estimatesbf 2-3000°C and pressures in the kiloha; range for time Deriods in the nanosecond region (3). In summary, the cavitation process generates a transitory highenergy environment. These forces are responsible for the well-known commercial processes of ultrasonic cleaning, dispersion, and pasteurization as well as the efficiency of ultrasonic drills and the corrosion of ship propeller blades. Although the early experiments with ultrasound were
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' Ullrasonic waves are usually defined as those sound waves with a frequency of 20 kHz or higher. The human ear Is most sensitive to z with upper and lower limits of 0.3 frequencies in the 1-5 k ~ range and20 kHz, respectively. For general treatments of the theory and applications of ultrasound see ref. 1.
promising and the commercial applications of ultrasonic waves have been very successful, little evidence accrued in the 50 years following Loomis' studies to suggest that sound would he a form of energy useful to synthetic chemists. However, there were some indications. In 1938, Porter and Young described the sonically induced Curtius rearrangement of henzazide,
noting rate enhancements of more than an order of magnitude ( 4 ) . A decade later Mivaeawa r e ~ o r t e dthat the hvdrolysis &acetates, the s a p o n i k k i o n of'fats, and the nitration of m-xvlene were accelerated simificantlv in the oresence of high f;equency sound waves (5).In thesame paper i t was also reported that sake and whiskey mellowed more rapidly in the presence of ultrasound. Later, Zechmeister (6)showed that the cleavage of halides of benzene and thiophene by suspensions of silver nitrate was accelerated by ultrasonic waves, and Weissler (7) demonstrated that ultrasound promoted the decomposition of acetonitrile to give nitrogen, hydrogen, and methane. A verv useful communication anoeared in 1966 in which ~jbberidescriheda convenient of stable solutions of the dimsvl anion from sodium hvdroxide and dimethylsulfoxide using ultrasonic waves a t room temperature (8).
Ultrasound eliminated the need for heating and reduced the extent of decomposition, two distinct advantages over the conventional preparation. In recent years, synthetic applications of ultrasound have attracted widespread attention, and there have been some notable successes. One of the main reasons for the resureence has been the discovery that the common ultrasonic iahoratory cleaner is often gobd enough to do the job. T o my knowledge, Fry (9) was the first to report the use of an ultrasonic cleaner to accelerate a reacGon, in this case, a partial reduction of some dihromoketones by mercury. The report by Luche and coworkers that ultrasonic waves aid the formation of organolithium and Grignard reagents and also improve the Barhier reaction spurred much of the current interest in the synthetic applications of ultrasound (10) R-Br
+ Li
,I)
RLi (61-9570)
Ih
where R = Pr, n-Bu, Ph, and
where R = alkyl, aryl, henzyl, allyl, vinyl, and R'&O = ketones and aldehydes. The ultrasound-promoted Barbier reaction found recent application by Trost and Coppola (11) as the method of choice for preparing complex cyclopeutanones. Ultrasound also proved useful in improving the synthesis of lithio-organocuprates and in preparing aldehydes by the Bouveault formylation (12). Volume 63 Number 5 May 1986
427
Our entrv into the field of sonochemistrv was sourred hv our need for high-yield preparations of symmetrical organics ) lithium wire and bimetallics. Our first efforts ( 1 3 ~with were satisfactory hut have since been greatly improved through the use of lithium dispersion (13b) 2 R,MCl+ Li-
111
>I h
using ultrasound led us to attempt t o generate West's novel compound, tetramesityldisilene (22).the first example of a stable species with a silicon-silicon double hond. We prepared this species in one step (23) MesnSiClz+ Li
R,M-MR, (85-95%)
where R = alkyl, aryl, and M = C, Si, Sn. Reactions involvina lithium appear t o be ~articularlvgood candidates for ultras&ic accel&ation. WLhave founhthat lithium promoted de~rotonations,reductions to anions and dianions ( l 4 ) ,and reductions of metal halides to active metal powders (15)are enhanced in rate and yield when carried out in the presence of ultrasonic waves. Operating under the hypothesis that ultrasonic irradiation would be p .titularly useful for reactionsinvolving metals we extendeu our studies to zinc and soon found three reactions that were noticeahlv"imnroved . when conducted in the presence of ultrasonic waves: the Simmons-Smiti reaction (15).the Reformatskv reaction (16). . . . and the aeneration of o-xyl;lene (17). We felt the Reformatskv reaction was a worthwhile target because it is the most generally applicable method for converting aldehvdes and ketones to B-hvdroxvesters (18).The impro;emen
