Other Dimeric Systems

Observations analogous to those for carboxylic acid dimers have been reported for 2-pyridone-2-hydroxypyridine (2PY-2HP) [52, 53], illustrated in Fig. 29.9. This dimer, formed from two isomers, is asymmetric, but it has a symmetric double proton transfer potential since the transfer interchanges the isomers. In a collaborative effort, Pratt, Zwier, Leutwyler and their coworkers [52] measured and analyzed the high-resolution fluorescence-excitation spectrum of the origin 

In a careful study of HH, HD, and DD transfer, Sakota and Sekiya [54] showed convincingly that the transfer is concerted, contradicting earlier conclusions [55], and that HD transfer contains two components due to the fact that the coupling between the monomers is small (about 3.5 cm i) compared to the energy shift for NH— ND substitution (about 40 cm i), so that the excitation is effectively localized on one of the monomers in the mixed (i.e. dj) isotopomer, a situation similar to that encountered in dimeric benzoic acid and in 2PY-2HP. Specifically, they found that H D transfer is faster than HD transfer, where the asterisk indicates the monomer on which the excitation is localized. By analogy to the 2PY-2HP 

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The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

The hydrolysis of p-nitrophenyl acetate and bis(p-nitrophenyl phosphate) are frequently used to probe hydrolytic activity. A problem with some other dinuclear systems is that the Zn units are held together by bridging ligands which can be cleaved on reaction with the substrate.440 This is not the case in a ditopic ligand such as those designed by Lippard and co-workers based on Kemp s triacid imide with a xylyl spacer.441,442 Both zinc dimers and mixed metal dimers were formed and a structure characterized with a bridging phosphodiester (Figure 6). 

Although Pd-based catalyst systems form only the desirable trans-1,4-hexadiene, their industrial value is very doubtful because of the reported poor yield of the C6 diene and the very low activity of the dimerization reactions compared to other catalyst systems. 

Dimerization of conjugated dienes and trienes is generally accomplished at elevated temperatures or in the presence of metal catalysts. Linear dimerization of butadiene occurs readily at room temperature on nickel catalysts bearing aminophosphinite (AMP) ligands, and the reaction rate is reportedly twice that observed in other nickel systems employing either morpholine, ethanol or P-methyloxaphospholidines as modifiers62. 1,3-Pentadiene dimerizes in the presence of 1 mol% nickel catalyst to give a diastereomeric mixture of 4,5-dimethyl-l,3,6-octatriene as shown in equation 42. 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

In phenolic oxidative coupling reactions, these phenol-derived radicals do not propagate a radical chain reaction instead, they are quenched by coupling with other radicals. Thus, coupling of two of these resonance structures in various combinations gives a range of dimeric systems, as shown. The... 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

The studies on X-bridged systems (X = C1, Br, I, NR, etc.) indicate that they generally are slower and go through the same paths described for other bridged systems. There is, however, growing evidence that some of these may also go through a reaction involving the dimeric or polymeric species without prior dissociation. 

In contrast to cycloaddition reactions of phosphaalkenes, cycloaddition reactions between phosphaalkynes and other unsaturated systems are comparatively rare. Indeed, there are only a limited number of reports for monophos-phacyclobutadiene) complexes, which are obtained from the corresponding phosphaalkyne. Relatively recently, the reaction of phosphaalkynes with highly electron deficient alkynes was reported <19990M4838>. Treatment of a CF3C=CGF3-coordinated dimeric rhodium complex with phosphaalkynes in hexane at — 20°C followed by warming to room temperature afforded the red air- and moisture-stable phosphete complexes 60 in ca. 50% isolated yields. When phosphaalkynes are allowed to react with a kinetically stabilized cyclobutadiene, 2-Dewar-phosphinines, for example 93 (Equation 30), are obtained <1998S1305>. 

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