1.2- Diimines formation

If this is the case, it would be necessary to remove all the water present in order to force the equilibrium completely toward diimine formation and the establishment of permanent cross-links. Experimental evidence for this proposition is that a full state of cure is not attained if the long heating process is carried out in the mould, i.e., in a virtually closed system. Hydrolysis is also demonstrated by the fact that much of the diamine present in a vulcanizate can be extracted by treatment with water. 

Dye formation is complex because shading is achieved by employing several developers and several couplers in the same dye bath. The process is illustrated by -phenylenediamine, which is oxidized by the peroxide to a quinone diimine. This short-Hved intermediate can react, for example, with resorcinol to yield a brownish indoaniline. Table 17 provides some insight into the many interactions that exist from just a few components. Further shading is possible by including semipermanent colorants (see Table 16), especially nitroaniline derivatives. 

The synthesis of metal-eoordinated 1-azirines and the reaetions of azirines indueed by metals have opened a new area in the ehemistry of this small ring heteroeyele. Many of the reaetions eneountered bear resemblanee to previously diseussed thermally and photo-ehemieally indueed reaetions of 1-azirines. The reaetion of a series of diiron enneaearbonyls in benzene results in eoupling and insertion to give diimine eomplexes and ureadiiron eomplexes as well as pyrroles and ketones (76CC191). A meehanism for the formation of these produets whieh involves initial 1,3-bond eleavage and generation of a nitrene-iron earbonyl eomplex as an intermediate was proposed. 

Singlet phenylnitrene, and hence /V,A -diethyl-3//-azcpin-2-amines, e. g. 102, can be generated by the thermolysis of A,-phenyl-Af,<9-bis(trimcthylsi]yl)hydroxylamine (100) in the presence of dialkylamines the reaction fails, however, with arylamines.210 Photofragmentation of the spiro oxaziridine 101 in diethylamine solution also produces the 3//-azepine 102,2,1 and an oxaziridine intermediate is probably involved in the formation, in low yield (1 %), of azepine 102 by the photolysis of A/,A( -diarylbenzoquinonc diimine A/,A/ -dioxides in benzene/die-thylamine solution.212... 

Several studies characterizing the reactions of alkenyl radicals with quinone dumines and quino-neimines were published in the late 1970s. Quinone dumines react with allylic radicals yielding both the reduced PPD and the alkylated product. In these experiments 2-methyl-2-pentene served as a model olefin (model for NR). Samples of the olefin and quinoneimines or quinone diimine were heated to 140°C. Isolation and analysis of products demonstrated that 40%-70% of the imine or diimine was reduced to the corresponding PPD, while 20%-50% was isolated as the alkylated product. This alkylation reaction (via an allylic radical) represents the pathway to the formation of rubber-bound antidegradant. ... 

The reaction of AgOTf with a flexible diimine ligand in 1 2 and 1 1 ratio leads to the mono- or dinuclear complex, respectively. They further react with AgOTf to give coordination polymers. These complexes are models for the chain-growth process required in the formation of coordination polymers (see Scheme 2).642,643 Similar complexes are obtained with the 1,4-di-t-butyldia-zabut- 1,3-diene derivative.644... 

When less bulky ancillary ligands are used /3-hydride elimination leads to the formation of Q-olefins. As a consequence iminopyridine complexes are typically much less active than the diimine catalysts and afford lower-molecular-weight PE.321-324 For example, MAO/(122) polymerizes ethylene to branched oligomers with Mn < 600, and 240 branches per 1,000 carbons.325 Complex (123), is highly active for ethylene polymerization (820gmmol 1 h bar ).326 As with the diimine systems, reduction in the steric bulk of the ligand substituents results in reduced activity and lower-molecular-weight products. 

The best combinations were found to be L5 /A4 and L5 /A5, ee-values of up to 65% being observed in the Et2Zn addition to benzaldehyde with formation of (51). Based on these results a new library composed of chiral diimines as activators was scrutinized (Scheme 13).87,89... 

The introduction of cyano groups in the 3-position of diketimiminate ligands greatly increased the crystallinity of their complexes (Figure 49, 95), due to the formation of one-dimensional coordination polymers. A new f3-oxo-<5-diimine ligand, in turn, allowed the synthesis of a bimetallic organozinc complex 96, by the same methods that had been used for the syntheses of the mononuclear species 92-94.154... 

The reactions of methanol-coordinated triruthenium complex 2 with diimine ligands such as 2,2 -bipyridine and 1,10-phenanthroline at ambient temperature induced isolation of a series of oxo-centered triruthenium derivatives [Ru30(0Ac)s p.-T 1(C),T 2(N,N)-bipyridine (py)2]+ (bipyridine = dbbpy 31 dmbpy 32 bpy 33 Br2bpy 34 phen 35) containing an ort/zo-metallated bipyridine [8]. Formation of triruthenium derivatives 31-35 is involved in substitution of the coordinated methanol as well as one of the six bridging acetates in the precursor complex 2 by an orf/jo-metallated bipyridine in a p.-r 1(C),r 2(N,N) bonding fashion. 

Another compound of nitrogen and hydrogen is diimine, N2H2, which has the structure HN=NH. The compound decomposes to give N2 and H2, but it is believed to be a transient species in some reactions. The reactions that lead to formation of diimine can be summarized as follows ... 

The location of the induced unsaturation in the macrocyclic system is metal-ion dependent. This is illustrated by the examples given in Figure 8.2. In the Fe(n) complex, the imine functions form as conjugated pairs (Dabrowiak, Lovecchio, Goedken Busch, 1972 Goedken Busch, 1972) - such a-diimine species have long been known to have a special affinity for Fe(ii). In contrast, Ni(n) promotes formation of a product in which the respective imine functions are in electronically isolated positions (Curtis, 1968 1974). 

The branched polymers produced by the Ni(II) and Pd(II) a-diimine catalysts shown in Fig. 3 set them apart from the common early transition metal systems. The Pd catalysts, for example, are able to afford hyperbranched polymer from a feedstock of pure ethylene, a monomer which, on its own, offers no predisposition toward branch formation. Polymer branches result from metal migration along the chain due to the facile nature of late metals to perform [3-hydride elimination and reinsertion reactions. This process is similar to the early mechanism proposed by Fink briefly mentioned above [18], and is discussed in more detail below. The chain walking mechanism obviously has dramatic effects on the microstructure, or topology, of the polymer. Since P-hydride elimination is less favored in the Ni(II) catalysts compared to the Pd(II) catalysts, the former system affords polymer with a low to moderate density of short-chain branches, mostly methyl groups. 

The formation of diimine systems by Schiff -base-type condensation of suitable aldehydes and primary amines has been widely applied. Those reported are mostly strong field systems and their relevance to the spin crossover field is generally in systems of the kind [Fe(diimine)2(NCS)2]. The effect of the incorporation of substituents likely to hinder coordination has been studied. Robinson and Busch noted a fundamental difference at room temperature in the electronic properties of the [Fe N6]2+ derivatives of 2-pyridi-nalmethylhydrazone and 2-pyridinal-dimethylhydrazone, those of the former being low spin and those of the latter high spin [49]. The temperature-dependence of the magnetism of the latter complex was not reported but may well be of interest. However, spin crossover [Fe(diimine)3]2+ systems have been characterised for systems where the incorporation of appropriate substituents has reduced the ligand field. 

Most helicates have linear axes, though a few helicates with circular axes are known - indeed the chiral (D4) molecular squares formed from Zn2+ and 2,5 -bis(2,2 -bipyridin- 6 -yl)pyrazine, 22, may be regarded as circular helicates (450). The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2/-diimine systems with their substitutionally-inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates of this type is believed to entail a kinetically favored triple helicate intermediate (484). Self-assembly of chiral-twisted iron(III)-porphyrin dimers into extended polynuclear species takes place through the intermediacy... 

In a study of the methane complex [(diimine)Pt(CH3)(CH4)]+ (diimine = HN=C(H)-C(H)=NH), relevant to the diimine system experimentally investigated by Tilset et al. (28), theoretical calculations indicate preference for the oxidative addition pathway (30). When one water molecule was included in these calculations, the preference for oxidative addition increased due to the stabilization of Pt(IV) by coordinated water (30). The same preference for oxidative addition was previously calculated for the ethylenediamine (en) system [(en)Pt(CH3)(CH4)]+ (151). This model is relevant for the experimentally investigated tmeda system [(tmeda)Pt(CH3)(solv)]+ discussed above (Scheme 7, B) (27,152). For the bis-formate complex Pt(02CH)2, a a-bond metathesis was assumed and the energies of intermediates and transition states were calculated... 

One very fast and reliable method for the reduction of double bonds is that of transfer hydrogenation with diimine (Scheme 20.30). Under the influence of traces of copper ion and oxygen from air, hydrazine is rapidly transformed into diimine. This compound is able to hydrogenate double bonds with great success under the formation of nitrogen [120],... 

In recent studies on hydrogenation catalyzed by soluble iron-diimine complexes, Chirik and coworkers noted that the major deactivation pathway of these complexes occurs via formation of tj6-arene complexes [54]. 

The preparation of macrocyclic diimines and endocyclic enamines are represented by the procedures for the formation of 1,10-DIAZA-CYCLOOCTADECANE and N-METHYL-2-PHENYL-A2-TETRA-HYDROPYRIDINE. Other procedures representative of alkylation reactions and aromaticity (TRI-i-BUTYLCYCLOPROPENYL FLUOROBORATE) round out a volume of tested experimental procedures of general value. 

SCHEME 8. Formation of JV-(4 -ethoxyphenyl)-JV -(glutathion-S-yl)-4-benzoquinone diimine and subsequent reactions... 

The bicyclic quinone diimine 10a exhibits the same product pattern in the reaction with excess GSH as the monocyclic quinonimine 9a. Reduction results in slow formation of... 

It is however important to note that those complexes which have conjugated diimine groups in the macrocycle also display a reversible Fe(II)/Fe(I) reduction. This is for example the case of [Fe([14]l,3,8,10-tetraeneN4)]2 +. The formation of an unusually stable low spin Fe(I) complex (d7-t2geg, one unpaired electron) is revealed both by EPR spectroscopy and by its magnetic moment ( eff = 2.3 Pb) "... 

When the reaction is carried out with a racemic mixture of complexes, the product is a racemic mixture of the isotactic polymers. It was of interest to see what would happen if, after formation of a chiral block with one enantiomer of the bisoxazoline ligand, an equivalent of the other enantiomer was added. It was found that an excess of ligand changes the tacticity completely and the second block was syndiotactic In these diimine palladium complexes exchange of ligand is relatively fast and it can often be observed on the NMRtime scale as a broadening in the H NMR spectra. The process may well be associative. 

Products isolated from the thermal fragmentation of A-arylbenzamide oximes and A-arylbenzamide O-phenylsulfonyl oximes have been accounted for by invoking a free-radical mechanism which is initiated by the preferential homolysis of the N-O bond." Time-resolved IR spectroscopy has revealed that photolysis of A, A -diphenyl-l,5-dihydroxy-9,10-anthraquinone diimine affords acridine-condensed aromatic products via excited-state intramolecular proton transfer." The absolute and relative rates of thermal rearrangements of substituted benzyl isocyanides have been measured,and it has been found that the relative rates are independent of temperature and exhibit excellent Hammett correlations. Thionitrosoarene (25), thought to be generated by desulfurization of the stable A-thiosulfinylaniline (24), has been established" " as an intermediate in the formation of 3,3a-dihydro-2,l-benzisothiazole (26) from o-alkylthionitrosoarene (24). 

The frequent occurrence of thiocyanate in spin cross-over complexes is documented, in Section 5.4.1.4 above and in connection with diimine and with triazole ligands in Sections 5.4.3.5.9 and 5.4.3.6.3 below, while complex formation with iron(III) appears in Section 5.4.5.1.4. 

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