Reverse Dimerization analogous

Isolated Fe -heme readily reacts with O2. Flowever, under ambient conditions the reaction of Fe -heme or synthetic Fe -porphyrins is not the reversible one analogous to reaction (1), but rather the irreversible reaction (2), resulting in formation of the so-called /u-oxo dimer ... 

The methano-dimer of a-tocopherol (28)50 was formed by the reaction of o-QM 3 as an alkylating agent toward excess y-tocopherol. It is also the reduction product of the furano-spiro dimer 29, which by analogy to spiro dimer 9 occurred as two interconvertible diastereomers,28 see Fig. 6.23. However, the interconversion rate was found to be slower than in the case of spiro dimer 9. While the reduction of furano-spiro dimer 29 to methano-dimer 28 proceeded largely quantitatively and independently of the reductant, the products of the reverse reaction, oxidation of 28 to 29, depended on oxidant and reaction conditions, so that those two compounds do not constitute a reversible redox pair in contrast to 9 and 12. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

Pteridine (222) is analogous to quinazoline in that it adds water reversibly to the nucleus 222 then forms 3,4-dihydro-4-hydroxypteridine (223). In contrast to hydrated quinazoline, 223 is reducible both 222 and 223 are reducible in the pyrazine ring to the 5,8-dihydro derivative. Whereas 222 is reversibly reduced, 223 is reduced in an irreversible reaction. The electrochemical behavior of pteridine is further complicated by the reaction of 5,8-dihydropteridine (224) with 222 to a reducible dimer.5,356... 

The preparation and properties of Fe(r-BuSXant)3 have been reported (163, 227). Unlike the analogous complexes with R = Et, n-Pr, n-Bu, and Bz, this complex does not eliminate CS2 to form the dimeric [Fe(RSXant)2(SR)] 2 complexes (163). The Fe(r-BuSXant)3 complex undergoes reversible one-electron oxidation (+0.73 V) and one-electron reduction (—0.36) (in CH2Q2 versus an Ag AgI reference electrode). 

Figure 2-47 after Cinquini et al. [75] who painstakingly documented the analogy with the pomaceous model. Only examples of the reverse coupe du roi had been known prior to their work. Thus Anet et al. [76] reported the synthesis of chiral 4-(bromomethyl)-6-(mercaptomethyl)[2.2]metacyclophane. They then showed that two homochiral molecules can be combined to form an achiral dimer as shown in and illustrated by Figure 2-48. 

An analogous reaction had already been observed for the benzothiazolium salts of the dicarbocyanine dye series, with the difference that the dimerization was reversible and an equilibrium was established with the monomers in acetonitrile solution100 (Scheme 73). 

---