Tetramer yields

Controlled O2 oxidation of the MoIII,ni IV,lv tetramer yields a product which titrates for MoIV and from an incomplete X-ray crystal study appears to be basically the same tetrameric structure, except that all the bridging ligands are now fi-oxo. This Mo tetramer is also an intense red colour, Figure 10. The reduction potential for the couple involving the two tetrainers is +0.07 V. Rapid reactions of both tetramers with 02 are of interest in that there is no site for binding unless the coordination number increases. Mild oxidants such as [Co(NH3)5Cl]2+ react rapidly (fi/2 < 1 min) with both tetramers to give the Mo)1 dimer. 

In 1956 Rose (19) reported that oxetane and 3,3-dimethyl oxetane, when treated with typical cationic initiators, form mixtures of polymer and cyclic tetramer (16-membered rings). Increase of the temperature increases the proportion of tetramer in the reaction products. Oxetane for example gives 4% of tetramer at —80 °C but at 50 °C 66% of the reaction products was tetramer (total tetramer yield based on monomer 42%). It is not an equilibrium effect, for the final conversion of monomer to polymer and tetra-... 

LDH (M.W. 134,000) oceurs as five tetrameric isoenzymes composed of two different types of subunits. Subunits M (for muscle) and H (for heart) are encoded by loci in chromosomes 11 and 12, respectively. Two subunits used in the formation of a tetramer yield five combinations H4(LDH-1), H3M(LDH-2), H2M2(LDH-3), HM3(LDH-4), and M4(LDH-5). The tissue distributiont of LDH isoenzymes is variable. For example, LDH-1 and LDH-2 are the principal isoenzymes in heart, kidney, brain, and erythrocytes LDH-3 and LDH-4 predominate in endocrine glands (e.g., thyroid, adrenal, pancreas), lymph nodes, thymus, spleen, leukocytes, platelets, and nongravid uterine muscle and LDH-4 and LDH-5 preponderate in liver and skeletal muscle. In tissue injury or insult, the appropriate tissue isoenzymes appear in plasma (Chapter 8) thus, determination of LDH isoenzyme composition has diagnostic value. 

An alternative route to purines is via the self-condensation of dilute solution (>0.01 M) of HCN at pH 9 to a tetramer of HCN. Photolysis of the HCN tetramer yields 4-aminoimidazole-5-carbonitrile (9,10), This aminoimidazole reacts with HCN to generate adenine. Other purine bases are also formed by the reaction of the aminoimidazole carbonitrile with other one or two carbon compounds (5). 

Controlled O2 oxidation of the tetramer yields a product which titrates for Mo ... 

Acryhc esters dimerize to give the 2-methylene glutaric acid esters catalyzed by tertiary organic phosphines (37) or organic phosphorous triamides, phosphonous diamides, or phosphinous amides (38). Yields of 75—80% dimer, together with 15—20% trimer, are obtained. Reaction conditions can be varied to obtain high yields of trimer, tetramer, and other polymers. 

Chemicals responsible for odor in some PUR foams were synthesised by polymerisation of PO in CH2CI2 with Bp2(C2H )20 catalyst (114). The yield was 25% volatile material and 75% polymeric material. The 25% fraction consisted of dimethyldioxane isomers, dioxolane isomers, DPG, TPG, crown ethers, tetramers, pentamers, etc, and 2-ethy1-4,7-dimethyl-1,3,6-trioxacane (acetal of DPG and propionaldehyde). The latter compound is mainly responsible for the musty odor found in some PUR foams. This material is not formed under basic conditions but probably arises during the workup when acidic clays are used for catalyst removal. 

The ratio of cycHc to linear oligomers, as well as the chain length of the linear sdoxanes, 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 dim ethyl dichi oro sil a n e is performed by either batch or a continuous process (62). In the typical industrial operation, the dimethyl dichi orosilane is mixed with 22% a2eotropic 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 cycHc oligomers is between 35 and 50%. The mixture of cycHc oligomers consists mainly of tetramer and pentamer. Only a small amount of cycHc trimer is formed. 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

Pyrroles react with the conjugate acids of aldehydes and ketones to give carbinols (e.g. 67) which cannot normally be isolated but which undergo proton-catalyzed loss of water to give reactive electrophiles (e.g. 68). Subsequent reaction may lead to polymeric products, but in the case of reaction of pyrrole and acetone a cyclic tetramer (69) is formed in high yield. 

In a very recent study, it has been demonstrated116 that zinc 5,15-bis(3,5-di-tert-butylphenyl)-porphyrin (13) without any activating halogen atoms at the chromophore can be directly linked in a very simple oxidative coupling reaction with silver(I) hexafluorophosphate to a mixture of porphyrin dimers, trimers and tetramers. The separation of the product mixture was achieved by gel-permeation chromatography based on the molecular weights of the oligomers. The dimer when re-exposed to the same reaction conditions yielded 25% of the tetramer.116... 

Time dependence of the reaction products can be seen more clearly in the time-yield curves of oligomerization in methylene chloride at —40° (Fig. 4). The yield of mixture of the cyclic tetramer and hexamer (mostly the latter), passed through a maximum value of about 40% and then decreased to nearly zero after 48 hours. On the other hand, the yield of the cyclic dimer increased rather sigmoidly with reaction time. 

Temp., °C Time, hr Cyclic dimer Yield 5, % Cyclic tetramer Polymer + cyclic hexamer Mnc... 

Fig. 4. Time-yield curves of the oligomerization of 6,8-dh>xabicyclo[3.2.1 Joctan-7-one. Monomer, 4 g CH2Cl2 4 ml BF3Et20, l mol% to monomer temp., -40 °C , cyclic dimer o, cyclic tetramer plus cyclic hexamer45)...

Due to an interest in studying their unusual reactivity (vide infra), several attempts were made to maximize yields of the strained dimers 74. Lengthening reaction times and decreasing substrate concentrations in the cyclooligomerization experiments proved fruitless. In response to this situation, a stepwise synthesis of the tetrahexyl-substituted dimer was developed as shown in Scheme 18. Surprisingly, Hay coupling of 77 resulted in an improved yield of the tetramer (45 vs 8 %) and a substantial decrease in the yield of dimer (13 vs 30%). This product distribution was unexpected, since intramolecular reactions are typically much faster than intermolecular reactions. 

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