Bis couple

The answer to this problem was in situ generation of the free phenylbutadiyne under standard Pd-coupling conditions [66]. Addition of a few milliliters of a concentrated KOH solution provided the bis-coupled product in 71 % yield. Desilylation and use of high dilution conditions in the oxidative coupling reaction gave 1 as the sole product in moderate yield. Compound 1 was poorly soluble in common organic solvents nevertheless, all of the spectral data (NMR, IR, UV,MS) supported the assigned structure. The minimal solubility of the product was no-doubt responsible for the low isolated yield. 

The N, C -coupling reactions of primary amines with BENAs are very sensitive to steric factors in BENAs. For example, the reactions with terminal BENAs are difficult to stop at the mono-alkylation step, whereas in internal BENAs, it is very difficult to isolate the bis-coupling product. A special procedure, based on this fact, enables one to synthesize bis-oximes (460) containing various oximinoalkyl substituents at the nitrogen atom. It should be emphasized that diastereoselectivity of /V,( -coupling reactions of amines with terminal BENA is very low. 

The required long planar shape is more readily supplied by simple disazo structures such as Cl Direct Blue 1 (4.60). Copper complexes of such disazo compounds are important and are dealt with elsewhere (section 5.5.3). A widely used method of producing A—A type disazo dyes relies on treating J acid with phosgene (COCl2) to give the bis-coupling component carbonyl J acid (4.61). 

For optimum results, it is important to monitor the reaction carefully. Extended reaction times lead to formation of bis-coupled product. Optimum results are obtained when the dibromide (Note 3) and the palladium catalyst (Note 9) are prepared as described. 

The fractions, containing the product were combined and analyzed by capillary GC, which showed them to be 87-94% pure with the major impurity (-6-11%) being the bis-coupled product. 

Each bimetallic specimen thus obtained was cut into two pieces using an electric-spark machine. The surface of the Ni-Bi couples was first ground mechanically and then polished electrolytically using an Elypovist ... 

The Ni-Bi couples were annealed in sealed glass ampoules, filled with high-purity helium (0.25 atm), at 150, 200 and 250°C for 1 to 300 h. Each couple was annealed successively a few times. After each anneal, the specimen surface was examined in the as-received conditions and after mechanical and/or electrolytic polishing. 

The compound layer thickness was determined using a conventional microhardness tester and an optical microscope. The microhardness tester was also used to put inert markers onto the surfaces of the phases involved in the interaction before each anneal of the Ni-Bi couples, except the first one when the intermetallic layer was still too thin. The microhardness indentations are known to possess advantages over other markers (inert... 

No indication of the presence of the NiBi intermetallic compound was found. The reasons for its absence from the Ni-Bi couple will be discussed in the next chapters. Here, it suffices to underline that those are of kinetic rather than of thermodynamic nature. 

The lack of the surface effect in the Ni-Bi couples was in all probability due to the presence onto the specimen surface of a very thin protective layer occurred at the end of electropolishing when the current was already switched off, while the specimen surface still continued to contact with the electrolyte. Though transparent and undetectable by EPMA, this layer consisting presumably of bismuth oxide was nonetheless sufficient to... 

This example shows how difficult the reaction-diffusion experiments are, even with most suitable binary systems. The latter means the minimal influence of (i) thermal expansion and (zz) volume effect associated with compound formation. In the Ni-Bi couple the coefficients of thermal expansion of the components are identical, " while the volume effect is within a range typical of intermetallic compounds. With respect to the influence of thermal expansion, the Ni-Bi system is preferable to the Al-Mg one in which the coefficients of thermal expansion of the components are slightly different. 

In order to visualise the NiBi layer at temperatures below the melting point of bismuth, experiments with Ni-Bi couples must evidently be carried out in the 100-1000 h time range. At such annealing times, its thickness will probably exceed a few micrometres. The NiBi layer formed should therefore be seen even under optical microscope at moderate magnifications. A serious obstacle to performing such experiments may be the rupture of Ni-Bi specimens with thick NiBi3 intermetallic layers. 

If you look back on the arylations and alkenylations that have so far been discussed in this section, you will find out that the acetylene itself was never used as the nucleophile, but always a higher alkyne. That is no coincidence, since in the presence of an amine, acetylene and Cul form the poorly soluble Cu2C2 most of which precipitates. If at all, the very small portion of this species that remains in solution would couple with the arylating and alkenylating agents on both C atoms. This would at best result in the formation of the respective bis-coupling product of acetylene. 

The coupling of an iodoalkene with a trimethylsilylacetylene and a subsequent desi-lylation result in the formation of 1,3-enynes with a terminal C=C triple bond. The parent acetylene usually does not react to give such an enyne under these reaction conditions, since Cu2C2 is formed. The solubility of this carbide is very low so that it precipitates. The very small amount of this copper species that remains in solution— if it couples at all—couples at both C atoms. Hence, the major coupling product then is the bis-coupling product of acetylene, a 3-ene-l,5-diyne, but in any case this product is formed only in small amounts because of the low solubility of Cu2C2. 

Unsymmetrical 2,5-disubstituted alkynylfurans could be prepared from 2,5-bis(butyltelluro)furan by sequential palladium-catalyzed cross-couplings. As represented in Scheme 26, the use of THE, a less effective solvent than MeOH for the symmetrical bis-coupling to alkynes, enabled the first monocoupling to occur <2003TL1387>. 

Using the Smith-Fishman technique which is based on the hydrolysis of naphthol AS-BI phosphate and on the visualization of naphthol AS-BI coupled to diazotized p-acetoxymercurianiline rather than on products of hydrolysis of /3-glycerophosphate, Sasaki and Fishman (1972) observed reaction product in the basal infolding membranes of mouse kidney cortex epithelial cells (Fig. 11). These structures would easily form vesicles during homogenization and should contribute to the microsome organelle population. This picture does make it possible to understand the biochemical findings now to be described. 

---