Trimerization cross

A variety of aryl ethynyl ketones undergo trimerization in the presence of a trace amount of diethylamine (Table 11-4) [53-55]. Moreover, not only homotrimerization but also cross-trimerization can be performed (Table 11-5) [55]. The cross-trimerized products always contain a mixture of four kinds of trimers when starting with two kinds of ethynyl ketones, but these products can be separated by GPC. 

T vo kinds of ethynyl aryl ketones were chosen as starting units. One was ethynyl ketone 50 and the other was m-propynoylphenyl trimethylsilylethynyl ketone 51. Cross-trimerization of these units afforded a mixture of four kinds of trimers trimer 53 from 3 mol of 50,2 1-trimer 52,1 2-trimer 54, and trimer 55 from 3 mol of 51. These trimers could be separated by GPC. The trimer 52 was subjected to the second trimerization reaction after deprotection to afford dodecaketone 49 [55 b]. 

Another cross-trimerization reaction was employed to corroborate mechanism A in which one ketoenamine and two ethynyl ketone molecules form a six-membered ring from which the diethylamino group is intercepted by another molecule of ethynyl ketone (Scheme 11-7). There are two more mechanisms, B and C that are conceivable (Schemes 11-8 and 11-9, respectively). The ketoenamine is regenerated and plays the role of the catalyst in mechanism B, and the ketoenamine is in equilibrium with the ethynyl ketone in mechanism C. 

Scheme 11-10 The cross-trimerization reactions employed for differentiating mechanisms A-C.

Based on previous results for the 1 2 cross-trimerization of alkynes [50], Ogata, Fukuzawa et al. [51] developed the first 1 1 1 cross-trimerization of alkynes employing Ni(cod)j/2PPhj (10mol%) as the catalyst. A high... 

This methodology was also applied to the 1 1 1 cross-trimerization between an internal alkyne 44 and two distinct terminal alkynes 40 and 45 [52], which is normally hampered by competition of the terminal alkynes for the oxidative addition to the metal center and for insertion into the... 

SCHEME3.29 Proposed mechanism for Ni-catalyzed 1 1 1 cross-trimerization of alkynes. 

K. Ogata, H. Murayama, J. Sugasawa, N. Suzuki, S.-I. Fukuzawa, J. Am. Chem. Soc. 2009,131, 3176-3177. Nickel-catalyzed highly regio- and stereoselective cross-trimerization between triisopropylsilylacetylene and internal alkynes leading to l,3-diene-5-ynes. 

Highly chemoselective nickel-catalyzed three-component cross-trimerization of three distinct alkynes leading to l,3-dien-5-ynes. 

K. Ogata, Y. Atsuumi, S.-I. Fukuzawa, Org. Lett. 2011, 13, 122—125. Highly chemoselective nickel-catalyzed three-component cross trimerization between two distinct terminal alkynes and an internal alkyne. 

Nickel-catalyzed three-component cross-trimerization has also been carried out in a highly chemo-, regio-, and stereoselective fashion. The 1 2 cross-trimerization involving TIPS-acetylene (eq IQ) and two internal alkynes and the 1 1 1 three-component cross-trimerization (eq 20) of three distinct alkynes in the presence of [Ni(cod)2] and a phosphine ligand afford the corresponding TIPS-substituted l,3-diene-5-ynes in good yield with a high selectivity. 

Combination techniques such as microscopy—ftir and pyrolysis—ir have helped solve some particularly difficult separations and complex identifications. Microscopy—ftir has been used to determine the composition of copolymer fibers (22) polyacrylonitrile, methyl acrylate, and a dye-receptive organic sulfonate trimer have been identified in acryHc fiber. Both normal and grazing angle modes can be used to identify components (23). Pyrolysis—ir has been used to study polymer decomposition (24) and to determine the degree of cross-linking of sulfonated divinylbenzene—styrene copolymer (25) and ethylene or propylene levels and ratios in ethylene—propylene copolymers (26). 

The tetramer exists in two-molal zirconium chloride and nitrate solutions, but it polymerizes into cross-linked chains on hydrolysis (190—191) in strong acid solutions, the hydroxyl bridges can be replaced by other anions to form trimers (192) and monomers (192—193). 

The hemagglutinin trimer molecule is 135 A long (from membrane to tip) and varies in cross-section between 15 A and 40 A. It is thus an unusually... 

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

There is little mention in the literature of the use of amide salts in substitution reactions on chlorophosphazene precursors. The anilide anion was shown to be a powerful nucleophile in substitution reactions on various trimer derivatives, but investigations of such reactions with the high polymer have not been reported.22 Where strong nucleophiles (such as amide salts) with low steric requirements are employed, the usual pentacoordinate transition state (Scheme 1), may be a viable reaction intermediate which can undergo alternative modes of decomposition, perhaps involving chain cleavage and/or cross-linking. 

As an alternative method, poly(cyclodiborazane)s were prepared by the reaction of bis(silylimine)s with chlorodialkylboranes or with methyl dialkylborinates (scheme 18).32 This reaction proceeds via the condensation between V-silylimine and boron halide, eliminating trimethylsilyl halide followed by dimerization. However, the isolated polymer became insoluble after several hours of exposure under air, which resulted from the cross-linking reactions of unreacted trimethylsilyl groups to form trimerized hydrobenzamide derivatives. 

The polymerization reaction (Figure I) is markedly influenced by the presence of trace impurities which was one of the difficulties encountered in earlier investigations. The conventional route is a melt polymerization of highly purified trimer (NPC1 ), or a mixture of trimer and a small amount of tetramer (NFCl.), sealed under vacuum in glass ampoules, at approximately 250°C. Proper selection of time and temperature is necessary to obtain II and avoid the formation of cross-linked matrix (III). 

Brandi et al. [71] using culture fluid of Acidovorax delafieldii and cyclic 3HB oligomers were in agreement with the presence of endo-hydrolase activity of poly(3HB) depolymerases. Similar results were obtained by de Koning et al. [72] who demonstrated that covalently cross-linked poly(HAMCL) was hydrolyzed completely by P. fluorescens. It is assumed that most - if not all - extracellular poly(HA) depolymerases have endo- and exo-hydrolase activity. Depending on the depolymerase the hydrolysis products are only monomers, monomers and dimers, or a mixture of oligomers (mono- to trimers). 

Figure 15.9 The results of capture ELISA on native RNase A and formalin-treated RNase A. Right panel, native RNase A (curve 1) and unfractionated formalin-treated RNase A (curve 2). Left panel, individual fractions of formalin-treated RNase A monomer (curve 3), dimmer (curve 4), trimer (curve 5), tetramer (curve 6), and a mixture of oligomers with >5 cross-linked proteins (curve 7). The ELISA plate wells were coated with monoclonal antibody against bovine pancreatic RNase A (1 pg/mL) overnight at 4°C and then blocked with bovine serum albumin. The wells were incubated for lh at 37°C in the presence of various concentrations of antigen in lOOpL of PBS. After washing, each plate well received a 1 4000 dilution of horseradish peroxidase conjugated rabbit polyclonal anti-RNase A antibody followed by incubation at ambient temperature for lh. After washing, detection was achieved using a mixture of 2,2,-azino-di-(3-ethylbenzthiazoline-6-sulphonate) and hydrogen peroxide. Absorbance was monitored at 405 nm. See Rait etal.11 for details.

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