Dimeric general structure

For equilibration processes, one must synthesize both oligomers and what are termed dimers, or disiloxanes. Our primary interest is in the utilization of these functional oligomers for the synthesis of both linear block or segmented copolymers, and also surface modified, oughened networks such as the epoxy and imide systems (3-27). The generalized structure of the oligomers of interest is shown in Scheme 1. 

The great majority of platinum(I) complexes are binuclear with monofunctional or bifunctional bridging groups. However, there is also a series of unsupported dimers with the general structure shown in (12). These are generally stabilized by phosphine, carbonyl, and isocyanide ligands.17 Dimeric hydride complexes can have terminal or bridging hydrides and these are discussed above in Section 6.5.2.1.4. 

As described earlier, the C=N double bond of the azaquinone structures is readily hydrated to provide the corresponding hydroxy derivatives, a principal step involved in their dimerization. Generally the chemis-... 

Binding of C02 takes place in aqueous medium by the carboxylation reaction of ribulose-diphosphate (RuDP) with the formation of 3-phospho-glycerine acid (PGA) - table 5. Water molecule and radical C=0 at the distances of molecular interaction have quite similar values of PE-parameters for forming the general structural grouping of dimeric composite type. Total PE-parameter of water molecule and radical C=0 hearly equals PE-parameter of C02 and therefore the molecules of C02 and H20 join RuBP with the formation of two radicals COOH b PGA (table 5). In ferment RuDP- carboxylase, Mg atoms and 0" ions (5.4867 eV and 4.755 eV) play an active role, their PE-parameters similar to PE-parameter of radical COOH. 

Fig. 3.1 Structures of (a) catechin, (b) epicatechin, (c) epicatechin gallate, (d) general structure of PAs, and (e) procyanidin dimer...

A combination of gel filtration, CPC, and semipreparative HPLC was reported for the isolation of eight dimeric proanthocyanidins of general structure 1 from the stem bark of Stryphnodendron adstringens (Leguminosae). The CPC step involved separation with the upper layer of Et0Ac-n-Pr0H-H20 (35 2 2) as mobile phase. " ... 

Analytical data on the soluble products isolated from chloroform are in excellent agreement with the composition 1 Ni+2 1 monoalkylated ligand 1 I or Br. The magnetic moment of this methylated complex was found to be 1.89 Bohr magnetons per nickel (II). The molar conductivities of the methylated and benzylated complexes in methanol at 25° C. are 75.4 and 68.4 ohm-1, respectively. These values approximate those expected for uni-univalent electrolytes in this solvent. The formulation of these alkylated compounds as dimeric electrolytes (structure VII) does not appear to be totally consistent with their physical properties. One or both halide ions may be bound to the metal ion. These results lead to the easily understood generalization that terminal sulfur atoms alkylate more readily than bridged mercaptide groups. 

Phosphines also are able to (a) add to a vacant coordination site in planar dithiolenes or split dimers such as [Fe(mnt)2]2 to form52 square pyramidal adducts of structure (36) and (b) displace certain dithiolene ligands to form dithiolato complexes of the general structure (37). These reactions also are briefly discussed in Section 16.5.3.8. 

The probable general structure of the dimers was established in elegant experiments by Fruchter and Crestfield 381) involving alkylation with iodoacetate. The two isomeric dimers referred to above behave identically in these reactions. The two active sites in the dimers behave just like that of the monomer. Histidines 12 and 119 both react, but the reactions are mutually exclusive. The proposed structure is outlined in Fig. 19. The tail of one monomer combines with the body of the other and vice versa. The His 12 and 119 pairs are now on separate molecules. When the dimers, fully inactivated by reaction with iodoacetate, are dissociated by heating at neutral pH, the following monomers would be expected native RNase (active), CM-His-12-RNase (inactive) CM-His-119-RNase (inactive), and di-CM-His-12-His-119-RNase (inactive). These were, in fact, found. About 2b% activity reappeared from the inactive dimer. Equally important the di-CM compound was found. This material... 

Figure 4.5 General structure of dimeric cinchona-PTCs.

In another theoretical study on metal-metal interactions in indium(I) and thallium(I) cyclopentadienyls (178), the authors interpret the short M—M contacts in the above dimers as weak donor-acceptor interactions, similar to the Sn—Sn bond in dimeric stannylenes (12) (163,175). However, they also attribute the general structural arrangement as being determined by crystal packing forces. ... 

The equilibrium as shown by 9.6 has been observed by NMR, and the product formed in solution has been shown to be dimeric in nature by molecular weight measurements and multinuclear NMR. The proposed structure, 9.35, has two inequivalent C02R groups. One of these is bonded to the Ti center, while the other is free. X-ray structures of several Ti-tartarate complexes have also been determined, and they all exhibit the general structural features of 9.35. In other words, there is good evidence to suggest that the solid-state structure of 9.35 is retained in solution, and this species indeed is the true precatalyst. 

Dimers of the general structure [PtCl2 (olefin)] 2 may be obtained by displacing the ethylene in [PtCl2 (C2 H4 )] 2 with an excess of the desired olefin. Styrene4 and l-dodecenes complexes have been prepared in this manner. However, [PtCl2 (C2 H4 )] 2 is relatively sensitive to moisture and air and accordingly is not conveniently stored. 

In addition to (3-carotene there are a variety of other C j pro-vitamin A carotenes that differ from (3-c.arotene in the nature of the terminal cyclic moieties. Thus, representing the right cyclic moiety as X, we can represent (3-carotene as X—(IP)2—(PI)2—X that yields two molecules of vitamin A or X—(IP)2—OH. Carotenes can have different cyclic moieties X (where X X) or no cyclic isoprene dimer moieties. Other C () pro-vitamin A carotenes that yield only one vitamin A molecule on oxidation include a-carotene, (3-cryptoxanthin, (3-c.arotene epoxide, echinenone and mutachrome (generalized structure X—(IP)2—(PI)2—X ) and y-carotene and torulene (X-(IP)2-(PI)4). 

The condensation of pyrazole (=Hpz) or C-substituted derivatives thereof with a trigonal borane (which may be employed as its adduct with a Lewis base) proceeds readily to yield N-borylated pyrazoles. However, the resultant species generally exist in the dimeric pyrazabole structure, i.e., R2B(p-pz)2BR2 =... 

Chapman and Liljas, Fig. 12. The shell-forming proteins of bluetongue virus and reovirus (a) bluetongue VPS protein (Grimes et al, 1998) (b) reovirus 11 protein (Reinisch et al., 2000). In the bluetongue VPS protein, three domains (apical, carapace, and dimerization domains) have been identified. The secondary structure elements have been colored to emphasize the general structural similarity between the two proteins. 

Generally, specific proteins can bind to each other In the body to form dimers (duplex structures), trimers, tetramers, or even larger multiples. These subunit proteins may be of identical or different structure. The different proteins in these mullimeric structures arc bound to each other by hydrogen bonds and other weak interactions. Ihese multimers often perform physiological functions that cannot be carried out by the individual separated proteins. 

Phenyllithium dissolves in hexane by addition of TMEDA. Hie phenyllithium TMEDA adduct subsequently crystallizes out of solution as the dimer (113) corresponding to general structural type (16)." With diethyl ether solvation, phenyllithium exists as a solid tetramer (114). In ether solution PhLi is known to be either dimeric or tetrameric. Monomeric phenyllithium was successfully crystallized with PMDETA as the ligand. This monomer is depicted as (115). Note the difference in the coordination number of the carbanionic center in the monomer (115), the dimer (113), and the tetramer (114), i.e. one, two and three, respectively. 

---