Coupling with Reagent Schemes

Lower original potential in grinding is beneficial to flotation selectivity of galena. Thus, collectors should be added into the mill directly. Table 10.12 is the results of flotation separation of Beishan concentrator of PbS-ZnS ore for collector addition in different places. It shows that the recovery and grade of galena concentrate are improved obviously when collectors are added in mill compared to in agitator. 

---

Indium enolates, prepared conveniently by transmetalation of hfhium enolates with IriCl j, react wifh aldehydes to give fhe corresponding -hydroxy esters [80]. Ultrasound irradiation promotes fhe Reformatsky reaction of aldehydes and ethyl bromoacetate wifh indium [81]. Indium-mediated Reformatsky reaction of phenyl a-bromoalkanoates wifh ketones or aldehydes gives di-, tri-, and tetrasubstituted -lactones (Scheme 8.57) [82]. Indium-mediated reaction of imines with ethyl bromoacetate gives 3-unsubstituted -lactams (Scheme 8.58) [83]. An indium-Refor-matsky reagent prepared from 2-(chlorodifluoroacetyl)furan couples with aldehydes (Scheme 8.59) [84]. 

Hydroxythiophene also exists mainly in ketonic forms. Electrophilic reagents react either at oxygen or at C-5. O-Methyl and O-acetyl derivatives are obtained in alkaline solution, probably through intermediacy of the anion. In acidic solution, coupling with benzenediazonium ion, a characteristic phenolic reaction, is found to take place (Scheme 72). 

Weik and Rademann have described the use of phosphoranes as polymer-bound acylation equivalents [65]. The authors chose a norstatine isostere as a synthetic target and employed classical polymer-bound triphenylphosphine in their studies (Scheme 7.54). Initial alkylation of the polymer-supported reagent was achieved with bromoacetonitrile under microwave irradiation. Simple treatment with triethyl-amine transformed the polymer-bound phosphonium salt into the corresponding stable phosphorane, which could be efficiently coupled with various protected amino acids. In this acylation step, the exclusion of water was crucial. 

Sodium cyanoborohydride [123], sodium triacetoxyborohydride [124] or NaBH4 coupled with sulfuric acid [125] are common agents used for the reductive amination of carbonyl compounds. These reagents either generate waste or involve the use of corrosive acids. The environmentally friendlier procedures developed by Varma and coworkers have been extended to a solvent-free reductive amination protocol for carbonyl compounds using moist montmorillonite K 10 day supported sodium borohydride that is facilitated by microwave irradiation (Scheme 6.39) [126]. 

---