Oxalamidines

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

The readily accessible oxalamidine derivative PhN = C(NHBu )C(NHBu ) = NPh provides another useful entry into the coordination chemistry of oxalamidinato ligands. Scheme 195 summarizes the results of an initial study. Mono- and dinuclear complexes of Ti, Zr, and Ta have been isolated and fully characterized. Silylation of both N-H functions was achieved by subsequent treatment with 2 equivalents of n-butyllithium and MesSiCl. The preparation of a nickel(II) complex failed and gave a hydrobromide salt instead. ... 

The palladium(ll) and heterobimetallic palladium(ll)/zinc oxalamidinate complexes depicted in Scheme 197 have been prepared analogously from the free ligands and Pd(acac)2 or ZnEt2/Pd(acac)2, respectively ... 

Attachment of pyridine- and phosphine-functionalities to oxalamidine ligands has also been achieved. Scheme 199 exemplifies the use of such ligands in the synthesis of novel organopalladium oxalamidinate com-... 

Nuclear tunnelling in the aqueous iron (2+)-iron (3+) electron transfer has been investigated467 and the rate enhancement for H2O has been assessed to be 65 times the classical rate, and that for D2O 25 times the classical rate, yielding a H/D isotope effect of 2.6. The occurrence of the general base catalysis and sizable primary D KIEs indicated that the isomerization of l//-indene-l-carboxylic acid to l//-indene-3-carboxylic acid in aqueous solution takes place through an enolization-reketonization sequence468. Kinetic HH/HD/DD isotope and solvent effects have been used in a dynamic NMR study469 of the tautomerization of 15N-and 2H -labelled bicyclic oxalamidines. 

This means that at low temperature where P is large the HD reaction is ca. twice as fast as the DD reaction. Equation (6.31) has been used in connection with the Bell-Limbach tunneling model to describe the stepwise double proton transfer in porphyrins, azophenine, and oxalamidines, as will be discussed in Section 6.3. Smedarchina et al. [16] used the same equations for their quantum-mechanical treatment of the porphyrin tautomerism. 

Here and represent the single H-transfer rate constants of the formation of the individual intermediates in Fig. 6.21(a). Equations (6.31) and (6.48) had already been discussed by Limbach et al. [58] but used only after independent confirmation in the cases of azophenine and oxalamidines [21, 22[, discussed below. Equation (6.48) was visualized in Fig. 6.12 and 6.14(b). The reaction energy profile of the HH-transfer involves two transition states of equal height. Thus, the product side is reached only with probability V2 as the internal return to the initial state also exhibits the same probability. The same is true for the DD reaction, only the effective barriers are larger. However, the symmetry is destroyed in the HD reaction. The rate-limiting step is the D-transfer which involves the same barrier as the corresponding process of the DD reaction. But as there is only a single barrier of this type in contrast to the DD reaction the HD reaction is about 2 times faster than the DD reaction. 

Let us first discuss the intramolecular degenerate double proton transfers in azo-phenine [21] and in oxalamidines [22a[. By liquid state NMR of the labeled compounds the intramolecular pathways of the transfer processes were established and the rate constants fenn, k = and when possible k were measured. The Arrhenius diagrams are depicted in Fig. 6.40. In all cases, the reactions... 

Figure 6.40 (a) Arrhenius diagrams of the tautomerism ofazophenine (AP) dissolved in CD2CI2 (top, [22a]) and of the seven-mem-bered bicydic oxalamidine OA7 (bottom) dissolved in methylcyclohexane [22b, 22cj. (b) Arrhenius diagrams of the tautomerism of... 

Figure 6.42 Heavy atom reorganization during the HH-trans-fer in oxalamidines calculated using the semiempirical PM3-MNDO method. Adapted from Ref. [22d].

A study of the tautomerization of 2,2 -bis(4,5,6,7-tetrahydro-l,3-diazepine) within the context of a theoretical study of oxalamidines by semiempirical PM3-MNDO calculation <94CCA43i> has revealed syn and anti forms which differ in the conformation about the methylene bridges. The geometry of the anti form is reported to agree remarkably well with the crystal geometry. 

Both binuclear and trinuclear oxalamidinate palladium complexes were employed by Rau, Walther and coworkers [79] in copper-free Sonogashira reactions. The peculiarity of the system was seen to reside in the presence of chemical bridges that allowed electronic communication between metal centers (Scheme... 

Figure 14.7 Tautomerizations of azophenine 11 [34, 35] and tetraphenyloxalamidine 12 [36]. No tautomerization is observed for the bicydic oxalamidine 13, but a fast tautomerization...

Scherer, G, and Limbach, H.H. (1989) Observation of a stepwise double proton transfer in oxalamidine which involves matched kinetic HH/HD/DD isotope... 

Proton and heavy atom motions during 46. the stepwise proton tautomerism of various oxalamidines. A semiempirical PM3-MNDO study. Croat Chem. Acta,... 

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