Mixed cyclic-trimer

Collum and co-workers investigated the aggregation structure of LDA with LiX. At high LiCl concentrations the mixed-dimer structure MD-1 is the major LDA species at lower concentrations two distinctive structures are possible, the mixed-cyclic trimer MCT-1 and the ladder-trimer LT-1 (Fig. 4) [47]. Rigorous establishment of the presence of LTMP-LiX aggregates has not been forthcoming, because the various LTMP... 

The mixed (heterogeneous) complexes of a lithium amide (LDA or LiTMP) and a ketone lithium enolate (acetone, cyclohexanone or diisopropyl ketone) have been examined by semiempirical methods (MNDO) by Romesberg and Collum48. If the stabilization associated with these mixed complexes was not determined, the solvation (by THF and HMPA) of the mixed cyclic dimers and trimers was calculated to be generally exothermic (but decreasingly with the steric demand of the enolate) and led to disolvated entities. A set of solvated dimers, trimers and tetramers, cyclic or not, has thus been identified... 

Uranyl carbonate complexes have attracted considerable interest in recent years as they are intermediates in the processing of mixed oxide reactor fuels and in extraction of uranium from certain ores using carbonate leaching more topically they can be formed when uranyl ores react with carbonate or bicarbonate ions underground, and can be present in relatively high amounts in groundwaters. The main complex formed in carbonate leaching of uranyl ores is 8 coordinate [1102(003)3], but around pH 6 a cyclic trimer [(002)3(003)6] has been identified. 

Table 5.19 Binding energies (kcal/mol) of cyclic trimers of mixed water (W) and aleohols, eomputed at SCF/6-31G level. Data computed at SCF/6-31G level .

Polymers with different groups attached to the phosphorus atom can be obtained from phosphazene cyclic trimers or tetramers with mixed halogen and alkyl or aryl side groups by a similar procedure. Further attachment of side groups, such as fluoroalcoxy, can be achieved by treatment of the polymer with excess of sodium fluoroalkoxides [2], By this procedure are obtained, for example, poiy fluoroaikoxyphosphazenes) with the structure shown below ... 

The ratio of cyclic to linear oligomers, as well as the chain length of the linear siloxanes, is controlled by the conditions of hydrolysis, such as the ratio of chlorosilane to water, temperature, contact time, and solvents (60,61). Commercially, hydrolysis of dimethyldichlorosilane is performed by either batch or a continuous process (62). In the typical industrial operation, the dimethyldichlorosilane is mixed with 22% azeotropic aqueous hydrochloric acid in a continuous reactor. The mixture of hydrolysate and 32% concentrated acid is separated in a decanter. After separation, the anhydrous hydrogen chloride is converted to methyl chloride, which is then reused in the direct process. The hydrolysate is washed for removal of residual acid, neutralized, dried, and filtered (63). The typical yield of cyclic oligomers is between 35 and 50%. The mixture of cyclic oligomers consists mainly of tetramer and pentamer. Only a small amount of cyclic trimer is formed. 

As a structural model of the zinc(II)-OH species of carbonic anhydrase. the first mononuclear OH -bound zinc(TI) complex 6 was prepared by mixing zinc(II) ion. the ligand, and KOH. and was characterized by x-ray crystal analysis.The zinc(II)-OH complex lb is a cyclic trimer linked by three hydrogen bonds between each zinc(II)-bound hydroxide group, as shown by x-ray crystal studyP However, in aqueous solution, this trimer dissociates into monomeric zinc(II)-OH species lb. The zinc(II) complex 6 in CHCI3 reacted immediately with CO2, possibly to form a HCOa" complex 7 that ultimately and irreversibly gave a bridging carbonate complex 8. However, quantitative determination of the value for the zinc (II)-bound H2O or of the nucleophilicity of 6 was not reported. The HCOs complex 7 was characterized by IR spectroscopy [1675 and 1302 cm for zinc(n)-bound bicarbonate]. The formation of the bicarbonate com-... 

The o-chlorophenol/chloroform (25 75) solvent system was initially developed for PET by Screaton and Cullen (29) on a Waters 200 chromatograph and later reported in more detail by Jabarin and Balduff (30). The polymer solution was prepared by adding o-chlorophenol (OCP) to PET at 110°C with agitation until the polymer dissolved (10-30 minutes). The solution was then cooled to room temperature, at which time the chloroform was then added to make the final solution. SEC was run at ambient conditions with the mixed solvent. Using this system, Jabarin and Balduff were able to quantify the amount of cyclic trimer in PET. 

The phosphorus NMR chemical shifts of a few selected polyphosphazenes are given in Table 3.7. Usually slightly broad signals are seen in the NMR spectrum of the polymers because of the inerease in viscosity when polymers are dissolved in solution. The P-NMR chemical shifts of polyphosphazenes indicate that these are about 20-30 ppm upfield shifted with respect to the cyclic trimers. The cyclic tetramers have intermediate chemical shifts (entries 1, 2, 5, 8, 11 in Table 3.7). In polyphosphazenes containing mixed substituents the P-NMR chemical shifts are diagnostic... 

An intensive investigation has been made of the thermal degradation of poly(dimethylsiloxane) and poly(methylphenylsiloxane). " The main products are a mix of all the possible stereoisomeric cyclic trimers and tetramers together with small amounts of pentamer and cyclic oligomers. In the degradation of poly(dimethylsiloxane) fluids, however, significant amounts of linear products are... 

Excess C03 " converts U02 to stable complexes such as, at pH=6, the cyclic trimer [ U02(C03)2 3] . In general, high ionic strength and high An02 concentrations especially favor [ An02(C03)2 3] ", as well as such mixed ions as [(U02)2(Np,Pu)02(C03)6]"-. 

Figure 3.4 HPLC trace showing the formation of 5 % PET/PBT cyclic oligomers from the depolymerization of 5% PET/PBT. Cyclic dimers are indicated at 3.0 and 4.0 min, with trimers appearing at 5.0 and 6.0min, etc. the small peaks represent mixed co-cyclics...

The trimeric compounds have been cleaved with a variety of neutral ligands [Eq. (41)] (154). With PPh3 the reaction proceeds smoothly, the rate and extent of reaction being dependent on the nature of R (the process occurs more readily when R is aromatic), but with isonitriles species of indeterminate constitution are obtained in addition to lAu C(OR )=NR (CNR")]. The cyclic complexes also underwent stepwise oxidative addition of bromine or iodine to yield (155) mixed gold(I)-gold(III) species, and finally the analogous gold(III) trimers. 

It is clear that the ideas of combinatorial chemistry could potentially be applied to identifying excellent matches of "hosts" and "guests", and that to achieve this requires the synthesis of libraries of macrocycles. As noted above, when cyclic oligomers are prepared under thermodynamic conditions the family of oligomers formed can equilibrate, Reactions 3 in Scheme 1, i.e. dimers can interconvert with trimers and tetramers etc. Use can be made of this to establish equilibria between cyclic oligomers of different families, i.e. to "mix" the different families, and in this way a soluble macrocyclic library may be prepared. 

The same cyclic pentamer was prepared much more efficiently by the cross-coupling of a preformed trimer, 45, with a preformed dimer, 47 [22, 31]. This was accomplished by separately converting the trimer to the corresponding bis-cuprate, 46, and the dimer to the bis-bromoalkyne, 48, and then mixing the two in pyridine under dilute conditions (Fig. 9-13). In this way, the five-sided macrocycle, 44, could be isolated in the greatly improved yield of 54%. 

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