Hydroxy dimers

The nature of the [Co(C03)3] ion in aqueous solution has been studied by i.r. and by optical resolution achieved by means of (-i-)- [Co(en)3]. Co(OAc)3 is conveniently synthesized by ozonation of Co(OAc)2 in glacial acetic acid. The triacetate has the empirical formula [Co2(OAc) (OH)2(HOAc)] and exists largely as the p-hydroxy-dimer in glacial acetic acid. ... 

The co-product of the addition of water to the 7r-oxo-allyl rhodium complex, 5, in THF at 25°C was suggested to be an (unobserved) hydroxy rhodium species which rapidly dimerises to give complex 7, a square planar /z-hydroxy dimer. This complex was easily identified by the high field shifts of the /z-hydroxy protons ( — 1.9 ppm) and its symmetry (a single Rh-coupled doublet is observed at 55 ppm in the 31P 1H NMR spectrum). The assignment was confirmed by independent synthesis [(S)-BINAP)Rh(/x-Cl)]2 reacted cleanly with KOH in aqueous THF to give 7, as did addition of (S) -BINAP to the conveniently prepared complex [Rh(/z-OH)(cod]2 (cod = 1,5-cyclo-octadiene). 

The final stage of this investigation was to study the reaction of the hydroxy dimer 7 with phenylboronic acid to test whether 7 is able to undergo efficient phenylation. The reaction with PhB(OH)2 was found to proceed smoothly and, when conducted in the presence of PPh3, gave the stable four-coordinate complex 4 which had already been characterised, Scheme 12.4. As 4 was already known to be effective as a pro-catalyst (see above), this process formally closed the circle to constitute a very convincing and conclusive collection of experiments. 

The reaction has one more secret to reveal before we move on. The careful work of Hayashi and co-workers led to the use of the //-hydroxy dimer, 7, as a pro-catalyst. Whilst... 

The concentration dependence of FTIR and UV spectra of 2-hydroxypyridine and its 6-chloro analog was measured in chloroform and CCI4 in order to elucidate their tautomeric equilibrium and determine the association constants (96MI1). For unsubstituted 2-hydroxypyridine, the hydroxy monomer or dimer was not detected in both solvents, and the equilibrium existed exclusively between oxo monomer and dimer. In contrast, for 6-chloro-2-pyridone both monomeric hydroxy (about 72%) and oxo (about 28%) forms were observed in chloroform. Increase in the substrate concentration led to decrease in the content of the monomeric species in favor of relevant dimers. In CC14, 6-chloro-2-hydroxypyridine was found to exist mainly in hydroxy monomer hydroxy dimer equilibrium. 

Further deprotonation, dehydration, and polymerization of monomers and dimers may yield ringlike stmctures of hydroxy—aluminum complexes (10). Coalescence of ring compounds into layers by further growth results in the formation of crystalline aluminum hydroxide at pH 6, the point of minimum aqueous solubiUty. 

Benz[e][ 1,2]azaborine, 1,2-dihydro-2-methyl-halogenation, 1, 656 Benz[6]- 1-azafulvenes dimerization, 4, 309 Benz[c]-2-azafulvenes dimerization, 4, 309 Benzazatropolone X-ray analysis, 7, 494 1 -Benzazepine, N-acetyl-6-hydroxy-bromination, 7, 528... 

Oxazolium hydroxide, anhydro-5-hydroxy-aromaticity, 6, 184 cycloaddition reactions, 6, 209 dimerization, 6, 207 1,3-dipolar cycloaddition reactions with alkynes, 6, 210 electrophilic reactions, 6, 207 mesoionic reactions, 6, 188 reactions, 6, 206-211 synthesis, 6, 225-227... 

SCF + MBPT + ZPE calculations [92THE(277)313] and MP2/6-31+G calculations [97JPC(B)9199] of the three possible dimers (oxo-oxo, oxo-hydroxy, and hydroxy-hydroxy). 

Type 125 0x0 forms are characteristic for 5-hydroxy-l,2,4-triazoles [76AHC(S1), pp. 379, 388], These forms are additionally stabilized by an electron-withdrawing substituent, R = NO2 (98MRC343). Both hydroxy and 0x0 tautomers are capable of forming stable dimers owing to the in-termolecular hydrogen bonds (126 and 127 [76AHC(S1), pp. 377,379). 

For 5-hydroxy-l,2,4-oxadiazoles, equilibria with dominant oxo forms 145b prone to type 146 dimerization were observed (Scheme 54) [76AHC(S1), p. 378 76BSB35 84CHEC-I(6)365 96CHEC-II(4)179]. 

For the mechanistic course of the reaction the diketone 5 is assumed to be an intermediate, since small amounts of 5 can sometimes be isolated as a minor product. It is likely that the sodium initially reacts with the ester 1 to give the radical anion species 3, which can dimerize to the dianion 4. By release of two alkoxides R 0 the diketone 5 is formed. Further reaction with sodium leads to the dianion 6, which yields the a-hydroxy ketone 2 upon aqueous workup ... 

M(0H)2SOi, H2O where M=Zr (8), Hf (12) also have been determined and reveal the presence of almost planar zigzag chains of metal atoms joined by double hydroxide bridges. The single exception to this trend toward formation of double hydroxy-bridged metal dimers or chains is the compound which is best described as CeOSOif,H20 (17). However, even in this structure the cerium ions form chains which are linked by bridging oxide ions. 

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