Sn-based compounds

Base-stabilized stannylenes have been known to form complexes with transition metals before stable stannylenes were detected. They are synthesized by a reaction similar to process (17) or by reduction of Sn(IV) compounds according to Eq (23) 146) ... 

The two most characteristic properties of the stannylenes are their behaviour as Lewis acids and Lewis bases, and their ready oxidative addition to give Sn(IV) compounds. The amphoteric nature provides the model for the structure of the dimers, the distannenes, and for the oligomerisation that occurs with the less hindered examples two dative bonds are assumed to be formed between the doubly-occupied sp2 orbital and the vacant p orbital, affording a trans non-planar distannene (equation 21-9). These structures are discussed in more detail below. 

We beheve that the initial growth step in the polymerization is the formation of the -O-Sn-0- based on the following reaction with antibiotics, in particular enro-floxacin, occurs rapidly and in good yield. Here, only the enrofloxacin-Sn-enrofloxacin compound can form (13). 

We must mention that intensive research is on-going in the design of metal-free catalysts for the REX ROP of lactide. By comparison with metal-based catalysts like Sn(Oct)2, the advantages of these metal-free catalysts are that they are less harmful for biomedical applications and readily removed during, e.g., extrusion processes. Among the simplest ones, some selected enzymes, tertiary amines, urea- and iodine-based compounds, phosphines and carbenes have proven efficient metal-free catalysts. However, most of these investigations have shown that these metal-free catalysts are less active and may yield intensive side-reactions, such as epimerization, when applied during bulk REX ROP of LA. ... 

Acyl carbonate 949 is formed as an intermediate in the reaction of anhydride 948 with carbonate 885. It decomposes to afford the corresponding ester 950. This exchange reaction between anhydrides and carbonate offers a new, solvent-free method for the synthesis of esters in the presence of Ti- or Sn-based catalysts [672]. Various esters have been synthesized by this method, typically in yields of 80-90%, although the reaction requires high temperatures of about 300 °C, and is thus incompatible with sensitive compounds. 

Some reports about the effect of the Sn-based catalyst on PLLA thermal degradation have been published. Noda [40] evaluated the activity of mono-, di-, tetraalkyl and aryl tin(IV) compounds as intramolecular transesterification catalysts and found that monobutyltin trichloride was the most active catalyst with some other compounds indicating almost the same depolymerization activity as Sn(Oct)2. Degee et al. [41] also reported that PLLA prepared with Sn(Oct)2 was less stable thermally than that prepared with Al alk-oxides based on TGA data. However, Cam and Mamed [33] found a contrary effect on thermal decomposition onset temperature in the order of Al > Zn > Sn. 

The homopolymerization of the most popular six-membered cyclic carbonates (TMC and neopentyl carbonate) using nucleophilic initiators based on Li-, K-, Mg-, A1-, Zn-, and Sn-containing compounds were also studied. The systems containing alkali metals. Mg and Sn, were found to afford during polymerization, besides linear, also cyclic oligomers. This shows that for this group of initiators the... 

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