C-Coupling

The P-coupling of 1,2- and 1,4-quinonediazides with phosphines (mainly tri-phenylphosphine) was studied intensively in the 1960s by the groups of Ried and Horner (see summary by Ershov et al., 1981, p. 147). The azo derivatives formed in these reactions are more stable than those of arenediazonium salts because of the stabilization by mesomeric delocalization (6.35 a ++ 6,35 b). 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the 

The arenediazocyanides have been known since 1879. They played an important role in the Hantzsch-Bamberger debate on the (Z)/( ,)-isomerism of diazo compounds (see Sec. 7.1). When an aqueous solution of a diazonium salt is added to a solution of sodium or potassium cyanide, both in relatively high concentration, at a temperature below 0°C, a yellow to red (Z)-arenediazocyanide starts to crystallize. Hantzsch and Schulze (1895 a) found that these compounds rearrange into the (ii)-isomers, which have a bathochromically shifted visible absorption (see Sec. 7.1). Under strongly alkaline conditions a 1 2 adduct is formed, to which Stephenson and Waters (1939) assigned the structure 6.36. It was never corroborated, however, by modern instrumental analysis. 

As there is no separate section on the reactions of arenediazocyanides in this book, we will make short reference to four examples. They illustrate the wide variety of reactions undergone by these compounds. 

The azomethine imine 6.39 reacts readily to give various 1,3-dipolar cycloaddition products (Huisgen and Eckell, 1977 for the naming of cycloadditions see Huisgen 1968). 

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Aminothiazoles and their Derivatives C. Coupling with Diazonium Salts... 

Decoupling (Section 13 17) In NMR spectroscopy any process that destroys the coupling of nuclear spins between two nuclei Two types of decoupling are employed in NMR spectroscopy Broadband decoupling removes all the H- C couplings ojf resonance decoupling removes all H- C couplings except those between directly bonded atoms... 

Figure 7.16 (a) Hund s case (a) and (b) Hund s case (c) coupling of orbital and electron spin... 

How would the components of a electronic state be described in the case (c) coupling approximation ... 

Pigment Red 53, barium salt [5160-02-1] 15585 1 BONA (Ba salt) (Lake Red C) coupling of dia2oti2ed 2-amino-4-methyl-5-chloroben2enesulfonic acid with 2-naphthol, foUowed by salt formation... 

Fig. 7. Spin-drawing processes for nylon yam (a) draw-twisting process, (b) conventional spinning process, and (c) coupled process.

Glass-reinforced melamine-formaldehyde laminates are valuable because of their good heat resistance (they can be used at temperatures up to 200°C) coupled with good electrical insulation properties including resistance to tracking. 

An interesting feature of tliese compounds is tliat, upon tlierniolysis in benzene at ao C, tlie unsymmetrical C C coupling product 2-Me2NCi-,H C-CR is fornied exclusively. Hie selectivity of tliis reaction is probably directly related to tlie structural features of tliis betemleptic aggregate [a9]. 

Cationic phosphine ligands containing guanidiniumphenyl moieties were originally developed in order to make use of their pronounced solubility in water [72, 73]. They were shown to form active catalytic systems in Pd-mediated C-C coupling reactions between aryl iodides and alkynes (Castro-Stephens-Sonogashira reaction) [72, 74] and Rh-catalyzed hydroformylation of olefins in aqueous two-phase systems [75]. 

The Heck reaction and other related transformations for selective C-C couplings are receiving a great deal of attention among synthetic chemists, due to their versatility for fine chemical synthesis. However, these reactions suffer in many cases from the instability of the Pd-catalysts used, resulting in high catalyst consumption and difficult processing. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

All the RyR channels are gated by cytoplasmic Ca2+, known as Ca2+-induced Ca2+ release (CICR). CICR functions as an amplifier of small Ca2+ signals in various excitable and non-excitable tissues and well documented in E-C coupling in the heart. In addition, RyRl can also mediate depolarization-induced Ca2+ release (DICR) , which is controlled by some protein-protein interactions. DICR is the principal Ca2+ release mode in E-C coupling in the skeletal muscle. 

In E-C coupling in the heart, the RyR2 channel releases Ca2+ from the SR on depolarization of the plasma membrane or T-tubules by CICR mechanisms. The... 

An example of C.O-coupling of a-ketoalkyl radicals with reversible formation of an enol ether has also been reported for a system where C,C-coupling is very hindered (Scheme 1.10).154 However, this pathway is not observed for simpler species (Section 5.2.2.1.2). 

Cyanoisopropyl radicals (15) undergo unsymmetrical C-N coupling in preference to C-C coupling.11 1 The preferential formation of the ketenimine is a reflection of the importance of polar and steric influences.1"1 However, the ketenimine is itself thermally unstable and a source of 15, thus the predominant isolated product is often from C-C coupling. 

There appears to be general agreement that termination in AN polymerization under a variety of conditions (10-90 °C, DMSO, DMF, F O) involves mainly combination.12ft 122,164,165 It was suggested that this may involve either C-N (kcteniminc formation) or C-C coupling.166... 

If an aromatic o-diamine such as 1,2-diaminobenzene (2.24) is diazotized in dilute aqueous acid, the 2-aminobenzene-l-diazonium ion formed first (2.25) undergoes a rapid intramolecular N-azo coupling reaction to give 1,2,3-benzotri-azole (2.26). Both amino groups of 2.24 can, however, be diazotized in concentrated acid (Scheme 2-18), forming the bis-diazonium ion 2.27. 1,3- and 1,4-diamines must also be bisdiazotized in concentrated acids in order to avoid inter-molecular N- or C-coupling. 

As discussed in Sections 5.1-5.3, arenediazonium ions are Lewis acids in which the (3-nitrogen forms the center of electrophilic character. This was demonstrated by the addition of hydroxide ions and water molecules. Other nucleophiles can also be added and, in principle, these reactions display the same mechanistic characteristics as those with OH and H20. According to the nature of the atom of the nucleophile that provides the lone pair of electrons, O-, S-, Se-, N-, P-, or C-coupling can occur. With N- and C-coupling, important and large groups of compounds are formed, namely azo compounds (mainly important as azo dyes) and triazenes, respectively. These compounds will be discussed in Chapters 12 and 13, respectively. 

An analogous regioselective effect of silver ions on an addition reaction of arene-diazonium ions was found by Ignasiak et al. (1975) with cyanide salts. Potassium cyanide yields diazocyanides (Ar— N2 — C = N see Sec. 6.6), i.e., C-coupling products, but with silver cyanide - albeit in low yield (7-9%) - diazoisocyanides (Ar—N2 — N = C) are formed (a better synthesis of diazoisocyanides is described in Sec. 6.4). 

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