Diimines, synthesis

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. 

An interesting extrapolation of this synthesis deals with the preparation of the bispyridinium salt 62 from 1,2-phthalic dicarboxaldehyde and its subsequent reaction with primary amines (92BSB509).Tlie expected diimines 63 readily cyclize so that 2-aryl-l-arylimino-2,3-dihydro-l//-isoindoles 64 can be isolated in excellent yields (90-95%). Contrary to the reactions performed by employing the dialdehyde and amines directly, the syntheses involving the azinium salts do not produce those typical dark-colored complex mixtures of products (77JOC4217 85JHC449) (Scheme 20). 

Scheme 39 Synthesis of Ci-symmetric 1,2-diamines from a chiral glyoxal diimine...

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

Synthesis of active Ni(II) a-diimine catalysts for ethylene polymerization study on substituent and cocatalyst effects... 

The methodologies described above lead to NHC precnrsors rather limited in terms of substitution at the four- and five-positions as their access is restricted to the accessibility of the appropriate diimine. As snch snbstitntions are of great interest in particular for the design of asymmetric catalysts, rontes to the synthesis of the NHC precursors have more recently been developed. Some of these approaches are described in Scheme 1.4. 

The synthesis and characterization of ci.v-[Ir(diimine)(hdpa)C12]C1, diimine = bpy, phen, hdpa = 2,2 -dipyridylamine, is reported.135 Emission studies suggest that it is the Ir(diimine)... 

The synthesis of [Ircp Cl(bpy-cd)]Cl, where bpy-cd is a /3-cyclo-dextrin attached at the 6 position to a bpy ligand, is detailed.138 The complexes [Ircp (diimine)X]+, X = C1, H, diimine = bpy, phen, are active catalysts for the light-driven water-gas-shift reaction.139 The hydride complexes luminesce at 77 K and room temperature, whereas the chloride complexes do not.140 The three-legged piano-stool arrangement of the ligands in [Ircp (bpy)Cl]+ and [Ircp (4,4 -COOFl-bpy)Cl]+ is confirmed by X-ray crystallography.141,142 Further mechanistic studies on the catalytic cycle shown in reaction Scheme 11 indicate that Cl- is substituted by CO and the rate-determining step involves loss of C02 and H+ to leave the Ir1 species, which readily binds Fl+ to yield the lrIH hydride species.143... 

The most widely applied precursors for the synthesis of monocyclic NHPs are a-diimines which can be converted to the target heterocycles either in a two-step reaction sequence involving two-electron reduction of the diimine to an enediamide, enediamine, or a-aminoamine and subsequent condensation with PC13 [18-20] or a dichlorophosphine RPC12 [21], or via direct base-promoted reaction with PC13 [20, 22], The latter reaction involves addition of a P-Cl bond to each imine moiety followed by base-promoted elimination of hydrogen chloride leading to 2,4-dichloro-... 

The two-electron reduction of a-diimines to prepare the required starting materials for a subsequent condensation is usually achieved by reaction with lithium but other alkaline (Na) or alkaline earth (Mg) metals should be useful as well. The synthesis of the heterocycles 10 is either accomplished by direct metathesis of the formed metal enediamide with PC132 [19] or, alternatively, by quenching the diamide with a suitable acid to produce an enediamine or a-aminoimine, respectively, and subsequent base-induced condensation with PC13 or RPC12 [18, 20] (Scheme 3). 1,3-l )i-/e/t-butyl-2-chloro-1,3,2-diazaphospholene was also prepared from the reaction... 

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... 

As mentioned already, Curtis reported the first (Curtis, 1960) of a number of pioneering template reactions for macrocyclic systems which were published in the period 1960 to 1965. In the Curtis synthesis, a yellow crystalline product was observed to result from the reaction of [Ni(l,2-diaminoethane)3]2+ and dry acetone. This product was initially thought to be a bis-ligand complex of the diimine species (58). However, the stability of the product in the presence of boiling acid or alkali was... 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

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 is believed to entail a kinetically favored triple helicate intermediate. Self-assembly of chiral dinuclear binaphthol-linked iron(III) porphyrin complexes into extended polynuclear species takes place through the intermediacy of fi-oxo dimers. Predetermined //-oxo-di-iron-dimers may be used in this type of synthesis. 

The synthesis of hexadentate tris-diimine ligands, e.g., (75), has resulted in Fe " eomplexes whieh are very inert to substitution, especially when the ligand is tripodal, e.g., (76), even more so when it is eneapsulating. Such extremely inert complexes are partieularly valuable for studies of eleetron transfer or of solubilities and solvation. Fe complexes of several encapsulating tris-diimine ligands, for example the cage ligands (77), (78) and (79), show the expeeted extreme substitution inertness." ... 

Gibson described the synthesis of four-coordinate cationic aluminium alkyls 1 which were reported to be well-defined aluminium polymerization catalysts [12]. However, the polydispersities of the products obtained were high (2.9-6.3), showing that there is not a single well-defined active species. The experiments were carried out in metal autoclaves, and Fe and Co complexes of pyridine-diimine ligands are extremely active in ethene polymerization [34], so a transition-metal impurity does not seem an unreasonable explanation. 

Synthesis, photophysical, and electrochemical properties of dinuclear Cd(II) diimine complexes with bridging chalcogenolate ligands were described [91]. 

Also, Yam and Cheung [62] have carried out the synthesis of a series of novel polynuclear mercury(II) diimine complexes with bridging chalcogenate ligands and studied their luminescence and electrochemical properties. 

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