Tin precursors

On the other hand, the corresponding tin precursor (63) undergoes smooth cycloaddition with a wide variety of aldehydes to produce the desired methylene-tetrahydrofnran in good yields [32, 33]. Thus prenylaldehyde reacts with (63) to give cleanly the cycloadduct (64), whereas the reaction with the silyl precursor (1) yields only decomposition products (Scheme 2.20) [31]. This smooth cycloaddition is attributed to the improved reactivity of the stannyl ether (65) towards the 7t-allyl ligand. Although the reactions of (63) with aldehydes are quite robust, the use of a tin reagent as precursor for TMM presents drawbacks such as cost, stability, toxicity, and difficult purification of products. 

The synthesis of carbon templated mesoporous tin MFI catalysts with different Si/Sn was carried out using microwave and in typical synthesis methodology TEOS, TPAOH, [Sn(C5H70)2]2]Cl2, ethanol and water were employed where the molar composition of the reaction mixture was 0.06 TPAOH 0.67 H20 0.028 TEOS 1.3 g EtOH X mg of tin precursor (X = 85, 63, 42, 21 mg). This synthesis mixture was stirred for 90 min at room temperature and then Black pearl 2000 carbon (10% wt. of TEOS) was added and again stirred for 4 h vigorously. The crystallization of C-meso-Sn-Silicalite was performed in a Teflon cup placed in a microwave oven (MARS-5, CEM, maximum power of 1200 W). 

We present results for two standard tin oxide precursors, DMTC and MBTC, as well as for tin tetrachloride. The latter compoimd is included in the analysis to provide perspective on the thermal stabihty of the inorganic system relative to the organometalhc ones. All chemical equihbrium calculations were performed with the EQUIL-code from the CHEMKIN-suite [ 100], using the thermochemical data discussed in the previous sections. The temperature range selected was 298-1023 K, the concentration of tin precursor was kept at 2 mol %, while the concentrations of oxygen and water were held at 20 mol % and 5 mol %, respectively. The total pressure was 1 atm. These conditions are similar to those used in commercial tin oxide CVD processes. Note that in the following discussion of reaction mechanisms, all heats of reaction (AHg) are given at 298 K. 

Experimental investigations have already shown that addition of water to the reaction mixture dramatically increases the growth rate. The ab initio calculations presented here show that water easily forms complexes with both SnCl2 and SnCls. Some highly exothermic pathways exist by which these complexes can form tin hydroxides. However, direct reaction between the tin precursor and water at temperatures considerably below those required to break Sn - C bonds cannot be ruled out. 

It has been demonstrated that skeletal nickel catalysts can be modified with tin by using CSRs taking place between tin alkyls and hydrogen adsorbed on nickel. Upon applying this type of modification the selectivity pattern of the catalysts in the reductive ami-nation of acetone can be tailored. Selective poisoning of sites responsible for the formation of isopropanol could be achieved by using tin dibenzyl (or diethyl] dichloride as tin precursor compound. ... 

In a similar context, 6-18F-3,4-dihydroxyphenylalanine (6-18F-DOPA) was synthesized by direct fluorination of L-3-(3-hydroxy-4-pivaloxyphenyl)alanine (ra-P-DOPA) with Ac018F in acetic acid resulting in the 2- and 5-18F isomers. Hydrolysis of the reaction mixture in hydrochloric acid followed by HPLC separation gave 6-18F-DOPA (equation 23)44. Another application of Ac018F was reported in the synthesis of a trimethyl tin precursor of 2-oxoquazepam, 7-chloro-1 -(2,2,2-trifluoroethyl)-1,3--dihydro-5-(2-fluorophenyl)-2//-l,4-benzodiazepin-2-one, a benzodiazepine agonist, and its conversion to [18F]-2-oxoquazepam by reaction with AcQ18F (equation 24)45. 

A solution of a pertinent amount of ruthenium chloride (RuCb.x H2O (x < 1), Aldrich) and tin precursor (SnCb, Aldrich) was stirred for 30 minutes at 333 K in 1,2-ethanediol (p.a). Added molar amount of 1,2-ethanediol was 2/1 compared to carrier precursor (Si(OC2Hs)4) molar amount. Tetraethoxysilane (98%, Aldrich) was inserted to the cooled solution of metal precursors under stirring at a room temperature. Acquired mixture was heated to 343 K and stirred at this temperature for 3 hours. Then, a stoichiometric excess of distilled water (90 ml) was added to the solution and the solution was further stirred at 343 K until a gel formed. 

In addition to the necessary properties of all CVD sources, TiN-precursors should fulfill the following requirements low decomposition temperature in order to obtain TiN deposition below 450 °C and compatibility with interconnects such as Al. 

The lithiation-substitution of M-Boc-M-(p-methoxyphenyl)-benzylamine (4) in the presence of (-)-sparteine (5) is illustrative of an asymmetric deprotonation (Scheme 3). Lithiation of 4 with -BuLi in the presence of (-)-sparteine followed by electrophilic incorporation with methyl triflate provides (S)-6 with an enantiomeric ratio of 97 3. Generation of the epimeric organolithium intermediates by tin-lithium exchange from (S)-7 in the presence of (-)-sparteine provides the epimeric product R)-6 with a 95 5 er. Additional proof that the enantioselectivity is not established in the methylation is the demonstration that generation of the racemic organolithium species in the presence of (-)-sparteine by tin-lithium exchange of the racemic tin precursor followed by methyl triflate provides a racemic product [8]. 

Nakai and coworkers have reported that a mixture of diastereomeric tin precursors 28 can be used to provide highly enantio enriched products, as shown for the conversion of 28 to 29 (Scheme 8). Furthermore, reduction of 29 is diastere-oselective to afford (following chiral auxiliary removal) enantio enriched (3-amino alcohols, 30 [22]. In another report, Nakai has described the conversion of the diastereomeric organolithium intermediates from tin-lithium exchange of 28 to copper species which can be used for 1,4 additions to a, 3-unsaturated aldehydes... 

A chiral auxiliary approach to control a carbolithiation from a tin precursor has been used by Coldham and coworkers [24-26]. The Hthiation-cyclization of 33 produced 34 with a dr of 79 21 in the presence of (-)-sparteine, and a ratio of 74 26 in the absence of the ligand, Eq. (2). 

This reaction showed high enantioselectivity even when (-)-sparteine was added after hthiation of the carboxamide. Also, the racemic lithium carbanion derived from the racemic tin precursor via metal exchange reaction, gave products with high enantioselectivity [Eq. (41)], whereas when the carbanion prepared from the corresponding chiral tin compound was reacted with an electrophile such as TMSCl without (-)-sparteine it yielded racemic product. These results indicate that the reaction of the lithiated 3-phenylpropionamide proceeds through an asymmetric substitution pathway. Furthermore, a warm-cool procedure and then reaction with a substoichiometric amount of an electrophile confirmed a dynamic thermodynamic resolution pathway for this reaction. 

G. Vaidyanathan, D. J. Affleck, M. Schottelius, H. Wester, H. S. Friedman, and M. R. Zalutsky, Synthesis and evaluation of glycosylated octreotate analogues labeled with radioiodine and At via a tin precursor, Bioconjug. Chem., 17 (2006) 195-203. 

The tin precursor, chosen to deposit Sn02, is an organometalhc compound tin dibutyldiacetate. In ambient air it is a colorless and viscous liquid compound and does not react with air. 

N° Tin precursor compound Initial concentration rrmol.dm Surface number of R reacted (x) reaction (53 rate of tin anchoring mol.dm min x10... 

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