Diimid precipitation

Silicon nitride has found many applications in which structural features play an important role. Conventionally derived silicon nitride is fairly difficult to sinter, and pressureless sintering requires large amounts of sintering aids (e.g., 8% AI2O3 and 2% Y2O3). The first chemical route , which has been applied for the formation of silicon nitride at comparably low temperature, is diimid precipitation, already reported in 1830 [36]. 

Equation (9) shows that high quantities of ammonium chloride are formed during the reaction that must be evaporated during calcining. In addition, a highly exothermic reaction is much easier to control if a solvent, such as n-hexane [38], is used. The required formation temperature of only 500°C is fairly low. Until recently, diimid precipitation has been the focused of numerous papers. 

A reactor was charged with the Step 1 product (0.011 mol), A,A -dicyclohexylcarbo-diimide (0.0176 mol), and A-hydroxysuccinimide (0.0176 mol) dissolved in 150 ml of CH2CI2 and then stirred overnight at ambient temperature. A cloudy heterogeneous white formed, which was removed by filtration and the filtrate concentrated. The residue was precipitated in cold diethyl ether and the product isolated after recrystallization with ethanol. 

The dialkylsulfur diimide 1 or 3 (1.0 mmol) was added in portions to a suspension of NaH (0.2 g, 8.3 mmol) in anhyd DMSO (15 mL) and the mixture stirred for 1 h under argon. Then MeOH (5 mL) was added with external cooling. The solvent was evaporated in vacuo and H20 (30 mL) was added to the residue. The resultant precipitate was filtered and recrystallized (EtOH). 

During the reaction of silicon tetrachloride with liquid ammonia, the intermediate, Si(NH2)4 was presumed. A precipitation of different polymeric products [e.g., the diimid, Si(NH)2] subsequently occurs, which finally yields silicon nitride. This reaction involves a multiple-step mechanism. Simplified chemical reactions are shown in Eqs. (9) to (11) (for a more detailed description, see, e.g.. Refs. 37-40). 

As already mentioned, one of the acylating species formed by activation with dicyclohexylcarbodiimide has been known for a long time to be the symmetric anhydride of the carboxylic component. Its formation can be favored by admixture of two equivalents of the N-protected amino acid with one of the diimide. Since the major portion of the dicyclohexylurea is precipitated, if dichloromethane is used as solvent, the byproduct can be filtered off before mixing of the anhydride solution with the polymer phase [ 141 ]. In this way, in the actual peptide synthesis the chance for side reactions is diminished, since only the dissolved part of the urea by-product, small amounts of the 0-acyl lactim, and traces of the diimide initially are still present. Though in most cases the peptides obtained by this procedure obviously are purer than those from the original method of activation with equivalent amounts of the components, the in situ formation of symmetric anhydrides with dicyclohexylcarbodiimide does not entirely overcome diimide dependent side reactions and the wash-out problem of the dicyclohexylurea. 

The third reaction stage is the most important and complex in the sequence, and is an alkaline fusion reaction. This is the synthesis stage in which the perylene chromophore is chemically formed, as well as the step in which many of the important physical properties of perylene tetracarboxylic acid diimide (PTCI) are determined. In the fusion reaction, (4) is fused with itself via a bimolecular nucleophilic substitution in the presence of molten alkali at temperatures in excess of 200 C. The resulting alkaline mass is then precipitated by slowly mixing into water. The subsequent slurry contains perylene in its reduced (leuco) form or salt. One of the several possible isomers of the lenco form is represented with the general formula (5). 

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