Organic synthesis, sonication

In organic synthesis, sonication has a long tradition. Ultrasound in electrochemistry has been introduced as a means to enhance convection in coulometric cells [286, 287]. Most important application became Anal Chem [288, 289]. Very popular was electrochemical stripping analysis [290], since in this case the action of ultrasound can be restricted to the electrochemical trace accumulation, where no interaction between power sound and sensitive signal measurement has to be suspected. 

These sonically prepared Rieke powders show enhanced reactivity in organic synthesis involving metals e.g. in the preparation of organosUicon compounds (Scheme 3.11) [81]. 

Ultrasound assisted organic synthesis gives excellent yields compared to other reactions. It can dramatically effect the rates of chemical reactions and is helpful for a large number of organic transformations. In fact, a combination of sonication with other techniques, e.g., phase transfer techniques, reactions in aqueous media etc. give best results. Sonication has also been shown to stimulate microbiological reactions. 

In many syntheses activation is not effected by sonochemical preparation of the metal alone but rather by sonication of a mixture of the metal and an organic reagent(s). The first example was published many years ago by Renaud, who reported the beneficial role of sonication in the preparation of organo-lithium, magnesium, and mercury compounds [86]. For many years, these important findings were not followed up but nowadays this approach is very common in sonochemistry. In another early example an ultrasonic probe (25 kHz) was used to accelerate the preparation of radical anions [87]. Unusually for this synthesis of benzoquinoline sodium species (5) the metal was used in the form of a cube attached to the horn and preparation times in diethyl ether were reduced from 48 h (reflux using sodium wire) to 45 min using ultrasound. 

Indeed, the reaction with phenyl- or 1-naphthylisocyanate is sufficiently general to be recommended in traditional schemes for qualitative organic analysis (e.g. Ref [1]), as a general method for characterizing organic halides. However, it has been surprisingly little exploited in synthesis, although a Barbier adaptation promoted by sonication has been reported [2]. 

Recently, ultrasound has been very successfully applied to different organ-ometallic and catalytic reactions (15). Rates are strongly accelerated, and reactions are often much more selective than those carried out under typical thermal conditions. Sonication of sterically hindered dichlorosilanes in the presence of lithium yields disilenes and cyclotrisilanes 16, 17). We have applied ultrasound in the synthesis of poly(phenylmethylsilylene) in the presence of dispersed sodium, which was prepared directly from small pieces of sodium by sonication. 

The synthesis of the silica/organic sol follows a formulation previously reported to prepare a pure silica sol. In the present study, 40.0 ml of tetraethylorthosilicate (TEOS), 106.6 ml ethanol, 11.4 ml distilled water, and 0.2 ml of HCl (2.6 M) were placed into a 250 ml Erlenmeyer flask and vigorously stirred for 1 hour. Ten grams of Plmonic P123 (BASF) triblock copolymer paste was then added to the resultant partially hydrolyzed sol and the mixture was stirred for an additional 4 hours until the sol was visually clear. To aliquots of this sol, C481 (alone as well as in combination with TSPP) was added, with sonication used to promote dissolution. The concentration (in mg/ml of sol) of C481 used was 1.5 mg/ml, while that of TSPP was 0.04 mg/ml. 

Elimination. Sometimes zinc dust alone is inadequate for achieving organic reactions. Surface modification can have dramatic effects. The Zn-Cu couple is useful for the synthesis of chiral allylic alcohols from epoxy tosylates and of ally-lamines from 2-(bromomethyl)aziridines (with sonication). 

Vinylic monomers such as acrylonitrile (AN) or methacrylonitrile (MAN) undergo an electropolymerization when submitted to electroreduction at metallic cathodes in an anhydrous organic medium [1,2], This synthesis leads to two different kinds of products (i) a physisorbed polymer which can be removed by rinsing with an appropriate solvent and (ii) a so-called grafted polymer, which is not removed with a solvent, even under sonication, and bearing carbon/metal interface chemical bonds which have been identified by X-ray Photoelectron Spectroscopy [2] and EXES [3], The former can be up to several micrometers thick, whereas the latter has a thickness which never exceeds a few hundreds Angstroms. 

In the case of miniemidsion polymerization, aqueous droplets are generated by sonication of the two-phase mixture and become stabilized in organic solvent by oil-soluble surfactants. The kinetically stable emulsion is formed usually at surfaaant concentration below or near its CMC. The inverse miniemulsion approach has been successfully used for the preparation of hoUow PNIPAM miaosphaes or PNIPAM miaogels functionalized by AAc. The synthesis of miaogels based on AAc, AAm, and hydroxy ethyl methacrylate (HEMA) was reported. The size of the miaogels prepared in invase miniemulsion systems typically varies between 150 and 300 nm. 

One of the most important aspects inherent in sonochemistry concerns synthesis and treatment of organic and inorganic materials. Effects of ultrasound on chemical transformations were studied in three directions sonochemistry in homogeneous liquid system, sonochemistry in heterogeneous liquid-liquid or liquid-solid several times as well as sonocatalysis. Thus, the cavitation concentrates sound energy to affect the synthesis from soluble precursors. Chemical reactions are usually not observed in sonicated solid-solid and solid or gas systems. 

The essential interest in sonication consists in making the reaction possible in short times at room temperature in organic solvents. Interestingly, the authors explain that sonication can diminish the zwitterionic character of the initial unprotected amino acid, therefore allowing a smooth reaction in media where the solubility is generally poor. Extensions to peptidic synthesis were explored. 

On the other hand, the work of Atobe and co-workers was probably the first modem example investigating electropolymerization under sonication in a complete series of papers at low frequencies. Starting from electro-organic reactions under ultrasonic fields [12], polymerization of aniline was studied both in electrochemical [13] and chemical route [14, 15] as well as synthesis of nanoparticle synthesis [16, 17]. 

---