Tin alkoxides

The obtaining of tin(IV) alkoxides was first reported in a well-known publication by Meerwein and Bersin [1101] devoted to bimetallic alkoxides. At the end of the 1950s Bradley [222] and Make [1049] practically simultaneously devoted the synthetic approaches to and described the properties of nearly all major representatives of the Sn(OR)4homologous series. During the last 10 to 20 years interest in these compounds was renewed due to the prospect of their application in the synthesis of optically transparent and conducting films based on Sn02, and also of related ceramic materials. The alkoxides of Sn(IV) were considered in detail in a review by Hampden-Smith etal. [702], 

The synthesis of the alkoxides of tin(II) was first reported at the beginning ofthe 1960s when the preparation ofthe cyclic derivatives of diatomic phenols (Zuckermann et al.) [1814] and also that of their precursor methoxide was described. Recently it has also been reported that the synthesis and structural characterization of Sn(OBu )2, derivatives of sterically hindered phenols, and also ofthe bimetallic complexes (mainly with alkaline and alkaline earth metals) (Veith) [1682-1685, 1688, 1692-1695]. 

All the above mentioned approaches have been used in preparation ofthe derivatives of Sn(II) (the most convenient for the preparation ofthe methoxide being the reaction of SnCl2 with MeOH and Py or Et3N [766]) and the universal approach being provided by alcoholysis of Sn[N(SiMe3)2]2 [311, 568] or 

SnCp2 [710, 766, 554]. The electrochemical synthesis of Sn(OMe)2 has been patented in Japan [828], 

The pyrolysis of Sn(OR)2, R = Pr , Bu as well as that of their Ge and Pb analogs results in the formation of metal and not of oxides, which is of primary interest for their application in the CVD technique [1563,1684]. The thermolysis of bimetallic tin (II) derivatives results in two phases containing Sn° and SnIV, respectively. For example, that of [BaSn2(OBu )6] provides small spheres of Sn, which are then covered by a new layer forming a fractal structure (that resembles a blackberry), whose surface is covered by BaSn03 [1684]. 

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Tin, nitratodiphenyltris(dimethy) sulfoxide)-structure, 1,77 Tin, nitratotris(triphenyltin)-structure, 1, 47 Tin,tetrakis(acetato)-stereochemistry, 1,94 Tin, tetrakis(diethyldithiocarbamato)-angular parameters, 1, 57 Tin, tetrakis(ethyldithiocarbamato)-angular parameters, 1, 57 Tin, tetranitrato-stereochemistry, 1, 94 Tin, tri-n-butylmethoxy-, 3, 208 Tin alkoxides physical properties, 2, 346 Tin bromide, 3, 194 Tin bromide hydrate, 3,195 Tin carboxylates, 3, 222 mixed valence, 3, 222 Tin chloride, 3, 194 hydroformylation platinum complexes, 6, 263 Tin chloride dihydrate, 3,195 Tin complexes, 3, 183-223 acetyl ace tone... 

The first palladium-catalyzed formation of aryl alkyl ethers in an intermolecular fashion occurred between activated aryl halides and alkoxides (Equation (28)), and the first formation of vinyl ethers occurred between activated vinyl halides and tin alkoxides (Equation (29)). Reactions of activated chloro- and bromoarenes with NaO-Z-Bu to form /-butyl aryl ethers occurred in the presence of palladium and DPPF as catalyst,107 while reactions of activated aryl halides with alcohols that could undergo /3-hydrogen elimination occurred in the presence of palladium and BINAP as catalyst.110 Reactions of NaO-/-Bu with unactivated aryl halides gave only modest yields of ether when catalyzed by aromatic bisphosphines.110 Similar chemistry occurred in the presence of nickel catalysts. In fact, nickel catalysts produced higher yields of silyl aryl ethers than palladium catalysts.108 The formation of diaryl ethers from activated aryl halides in the presence of palladium catalysts bearing DPPF or a CF3-subsituted DPPF was also reported 109... 

Porous ultrafine tin oxide ethanol gas sensors92 in the form of a thin film have been prepared from tin alkoxide by the sol-gel process. The microstructural evolution of the tin oxide films, which affected the ethanol gas-sensing properties of the films, was investigated as a function of firing temperature and solution concentration. Theoretically, it was expected that ethanol gas sensitivity would increase monotonically with decreasing film thickness, but experimental results showed a maximum sensitivity at about 70 nm. The sudden decrease of the sensitivity below the thickness of 70 nm seemed to be due to the sudden decrease of film porosity, i.e., the sudden decrease of the number of the available sites for the oxidation reaction of ethanol molecules. Thus, it seemed that below the thickness of 70 nm, the sensitivity was governed by microstructure rather than by film thickness. 

The mechanism for cyclic formation via depolymerization is the same type of transesterification which occurs on polymerization, as outlined in Scheme 3.3. Metal alkoxides such as tetraalkyl titanates or dibutyl tin alkoxides have proven... 

The mechanism of the tin(II) bis-(2-ethylhexanoate)-mediated ROP of lactones remained a matter of controversy for many years, and many different mechanisms were proposed. Indeed, tin(II) bis-(2-ethylhexanoate) is not made up of alkoxides but of carboxylates, known as poor initiators for the ROP of lactones. In 1998, Penczek and coworkers made a major contribution in this field. They reported that, if the polymerization is carried out in THF at 80 °C, then tin(II) bis-(2-ethylhexanoate) is converted in situ into a new tin alkoxide by the reaction with either an alcohol, purposely added in the reaction medium, or with any other protic impurity present in the polymerization medium (Fig. 14) [37]. Tin alkoxides formed in situ are the real initiators of the polymerization, which takes place according the usual... 

The polymerization is not carried out under strictly anhydrous conditions as is the case when aluminum and tin alkoxides are used as initiators... 

Fig. 30 Protected functionalized lactones polymerizable by aluminum and tin alkoxides [15, 114, 136-147]...

To this end, a very widely used approach is ROP initiated by polyols (at least triols) in the presence of tin(II) bis-(2-ethyUiexanoate) [155,156]. By implementing this technique, alcohols are dormant species and have to be activated by reaction with tin(II) bis-(2-ethylhexanoate) into tin alkoxides to initiate or to propagate the polymerization. The alcohols are thus not activated at the same time and no side-reactions between them are observed. Besides, it is more appropriate to initiate... 

Again several alkyls add—molybdenum, chromium, iron, cobalt, nickel, the alkali metal alkyls and aluminum alkyls react. A tin alkoxide has recently been studied by Russian workers and found to add to acetylenes. Mercury chloride, of course, adds and two cobalt—cobalt bonded compounds add to acetylene. The second is questionable because it dissociates in solution and the reaction may be a radical reaction, one cobalt adding to each end of the triple bond. 

Cobalt hydrocarbonyl, diborane, and aluminum hydrides add, I think, to all of these carbonyl compounds. Of course, there is the well known Grignard reagent and the alkyllithium additions to carbonyl compounds. Aluminum alkyls add, and we could have listed all the other alkali metal alkyls. Recent work has shown that the tin alkoxides add readily to all these derivatives, and similarly, a tin amide adds to most of these carbonyl compounds. 

In 1976, Ueno and Okawara highlighted the fact that no oxidation of primary saturated alcohols to aldehydes via tin alkoxides had been reported in the literature and published a procedure for the selective oxidation of secondary alcohols.25 Interestingly, rather than performing the oxidation on pre-formed tin alkoxides, these researchers subjected a mixture of the diol and (Bu3Sn)20 in CH2C12 to the action of Br2. Regardless of the fact that no complete formation of tin alkoxides is secured and no HBr quencher is added, this method may provide useful yields of hydroxyketones during the selective oxidation of diols.26... 

The selective oxidation of the secondary alcohol is performed by dropping a bromine solution on a mixture of (Bu3Sn)20 and the diol in CH2CI2. Although, no complete formation of bis-tin alkoxide is secured and the generated HBr—that may cause the hydrolysis of tin alkoxides—is not quenched, a useful yield of hydroxyketone is obtained. 

Subsequent researchers introduced substantial improvements on the Ueno and Okawara s protocol of selective oxidations via tin alkoxides and broadened considerably the scope of its application.223 24b,c Thus, it was established that good yields in the selective oxidation of diols—and even triols and tetrols can be achieved in two steps i) pre-formation of a tin alkoxide, by reaction with either (Bu3Sn)20 or Bu2SnO with elimination of water by molecular sieves or azeotropic distillation of water ii) treatment of the tin alkoxide with Br2 or NBS in the presence of a HBr quencher. 

General Procedure for Selective Oxidation of Secondary Alcohols in Presence of Primary Alcohols by Treatment of Intermediate Tin Alkoxides with Bromine or Af-Bromosuccinimide... 

Failure to add a HBr quencher may lead to the partial hydrolysis of the tin alkoxide and a lower yield in the selective oxidation. Excess of molecular sieves or stannylating agent employed in the formation of the tin alkoxide may operate as HBr quenchers during the tin alkoxide oxidation. 

Although hydrolysis of tin alkoxides and alkoxochlorides, especially in the mixture with indium alkoxides, is used extensively, such as forpreparation of conductive ITO (solid solution of indium and tin oxides) layers, only few fundamental works on hydrolysis of Sn(OR)4 are known, and they have been summarized in a comprehensive review [702], Bradley proved the formation of oligomeric molecules as first hydrolysis products of [Sn(OPri)4 PrOH]2 by... 

Scheme 6. The main ROP mechanism proposals with Sn(Oct)2 as catalyst, a) complexation of a monomer and alcohol prior to ROP and b) formation of a tin-alkoxide before ROP of...

Monotin alkoxides, tin dialkoxides and cyclic tin alkoxides have been utilized as initiators in the ROP of cyclic esters. The tin alkoxides are known to form cyclic species during synthesis and the dibutyltin alkoxides are known to exist as monomers and dimers [74]. The cyclic tin alkoxides were originally studied because of their resistance towards hydrolysis [75]. The tin alkoxides have been reported to be effective transesterification catalysts initiating polymerization at moderate temperatures [76]. 

The polymerization of lactones with tin alkoxides is thought to follow the co-ordination-insertion mechanism[77a]. The ring-opening of the monomer proceeds through acyl-oxygen cleavage with retention of the configuration. Tin(IV) complexes have been used to produce predominantly syndiotactic poly((3-hy-droxybutyrate) [78,79],macrocyclic poly((3-hydroxybutyrate) [80],poly(e-CL), and polylactide [77,76,81]. 

The cyclic tin alkoxides have the additional advantage of offering a convenient synthetic pathway for the synthesis of macromers, triblock, and multiblock copolymers [81,82]. Macromers from l-LA [83],e-CL [84], and l,5-dioxepan-2-one (DXO) [85] have been synthesized as well as triblock poly(L-LA-b-DXO-b-L-LA) [86] and multiblock copoly(ether-ester) from poly(THF) and e-CL [87]. The polymerization proceeds by ring expansion and the cyclic structure is preserved until the polymerization is quenched by precipitation. 

In a kinetic and mechanistic study on the polymerization of L-LA with a cyclic tin alkoxide [81] the number-average molecular weight increased linearly with increasing conversion and the MWD remained narrow (<1.15) throughout the polymerization reaction, Fig. 2. 

The linearity of the plot of number-average molecular weight versus percentage conversion indicates that the amount of transfer reactions was low throughout the reaction. The increase in molecular weight was proportional to the degree of monomer conversion. The same characteristics have been observed for ROP of L-LA initiated by other cyclic tin alkoxides [83]. 

The large difference in reactivity ratio between L-LA and DXO makes it difficult to synthesize a tri-block copolymer, but the task can be carried out by using a cyclic tin alkoxide as initiator. The DXO macrocycle can initiate polymerization of l-LA and by ring expansion polymerization the two side blocks of l-LA are formed simultaneously. Scheme 7 shows the reaction pathway for the synthesis of tri-block copolymers from l-LA and DXO. 

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