Pyridine solvates

The Grubbs pyridine solvates are the fastest initiators of alkene metathesis and are valuable as synthetic intermediates to prepare other ruthenium carbene complexes. In particular, the 18-electron pyridine solvates 4a,b are very fast initiators that were developed to catalyze difficult alkene metatheses (e.g., the cross metathesis of acrylonitrile) [6]. The rates of initiation for several complexes are provided in Table 9.9. The pyridine solvate 4a has been found to initiate about 105 times faster than the parent Grubbs complex 2 and at least 100 times faster than the second-generation triphenylphosphine variant 26. When compared with the Hoveyda-Blechert complex 3a, 4a initiated about 100 times faster (c entry 3 vs. entry 5). The bromopyridine solvate 4b exceeded all of these in its initiation rate it was at least 20 times more reactive than 4a. 

Recently, Grela and coworkers [38] formed the pyridine monosolvate of a chelated complex related to the Hoveyda chelate. Exposure of the sulfoxide-based catalyst 27 to excess 3-bromopyridine resulted in the quantitative formation of complex 28 (Eq. (9.3)). In the RCM of diethyl allyl dimethallylmalonate, the initiation of complex 28 was much faster than that for the parent complex. Through various structural modifications as well as supplemental DFT calculations on the 6-coordinate complex, it was determined that 28 likely initiates by a different pathway than 27. Somewhat surprisingly, dissociation of the sulfoxide ligand from the metal center in 28 was found to precede olefin coordination and the subsequent loss of the pyridine ligand. Relative to 27, the overall initiation process was determined to be 5 kcalmol lower for 28, having an energy barrier of 11.7 kcal mol versus 17.4 kcal mol .  

Mechanistic studies have been used to attempt to explain the rapid initiation rate of the 18-electron pyridine solvates. Using the reported initiation rate of 4a [6], the upper limit of free energy of activation was determined to be 15.45kcalmol at 5°C using the Eyring equation [39], Similar to the Grubbs-Hoveyda complexes, these precatalysts have multiple potential pathways by which they can initiate (e.g., interchange or dissociative). An associative mechanism can be ruled out due to the coordinatively saturated, six-coordinate nature of these complexes. 

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Pyridine solvate of diazaphosphorin-mercaptane 182 undergoes S-benzylation to provide compound 183 (Equation 17) <2004CHE106>. 

The majority of Th(OR)4 have been isolated in the crystalline form only as pyridine solvates and Th(OBu )4 — as alcohol solvate (with 2 1 composition), the yields of complexes being rather far from quantitative [387]. This fact in combination with the observed molecular complexities (3-4) for unsolvated Th(OR)4 (R = Pf, Bu, C5H , C5H , CjH, ) — amorphous solids or viscous liquids—permits to suppose (in analogy with Zr(OR)4 and Hf(OR)4 , see Section 12.12) the existence in their samples oftri- and tetranuclear oxocom-plexes — MjO(OR)10 and M40(0R),4 — to be supposed. Crystalline Th3O(OBu )i0 has in fact been isolated on the hydrolysis of [Th(OBul)4Py2] [387] (Table 12.14). 

The homoleptic derivatives of Mo and W(VI) are rather scarcely studied. The only structurally characterized complex, W(OMe)5, possesses the molecular structure analogous to those of alkoxide halids, i.e. a dimer built up of two edge-sharing octahedra. The structure of monooxo homometallic derivatives is unknown and their individuality appears questionable. The only dioxocom-plex of molybdenum(V) isolated as pyridin solvate demonstrates the [Ti(OMe)w]-type structure (Table 12.19). 

Calix[8]arenes are also found to act as a ditopic receptor in most cases. The calixarenes assume various pinched conformations in these complexes, resembhng two calix[4]arene cone units hnked together. Two 1 1 complexes, of Ca° and Eu, have been characterized where a roughly planar circular conformation is observed, with only two of the phenolic O atoms interacting with the metal atom. It is interesting to note that the free jo-t-butylcalix[8]arene has been found to crystallize as a pyridine solvate with a planar, circular conformation in one case, and a pinched conformation in a subsequent report. 

The methods routinely used for preparing hydrazones of aldoses yield only a gel-like, orange mass from D-fructose and (2,4-dinitrophenyl)-hydrazine. i However, by use of p-dioxane containing catalytic proportions of water (3%) and hydrogen chloride (0.3%), small needles of the p-dioxane solvate are obtained after 5 minutes. Dissolution of these crystals in 1 1 pyridine-96% ethanol gives the pyridine solvate. These crystals (m.p. 173-175°) were found to belong to the monoclinie system, elongated parallel to the 5-axis. ... 

Figure 8-4. (a) Blue crystals of the protein /3-CR (grown by Prof Naomi Chayen of Imperial College London figure reproduced from Chayen (1998) with permission of the author and of lUCr Journals) and for comparison those of several unbound carotenoids (Bartalucci et al 2007) (b) chloroform solvate of ASX (c) pyridine solvate of ASX (d) unsolvated form of ASX (e) canthaxathin (f) zeaxanthin (g) /3,/3-carotene (see color plate section)... 

Both protocatechuate 3,4-dioxygenase and dihydroxyphenylacetate 2,3-dioxygenase activities were decreased by Cu(IIXsalicylate)2 but not Cu-Zn SOD, supporting the notion that smalTmolecular-weight and lipophilic SOD-mimetics were able to reach the site of superoxide generation [694]. Similarly, a Cu(I)(chloride) pyridine solvate was found to decrease the tryptophan... 

The large difference between the magnitudes of the quadrupole splittings observed in nitrobenzene and in pyridine solutions permitted the separation of two line pairs in the Mossbauer spectra of iron(III) chloride in a mixture of these two solvents and thereby the study of the formation of the pyridine solvate in the solvent mixture. 

It may be observed that the stabilities of the solvates compared with that of the DMF solvate increase as the donicity of the solvent increases. The stability of the iron(III)-pyridine solvate exceeds those of the solvate complexes formed in the... 

Scheme 9.2 Simplified initiation pathways for pyridine solvate precatalysts [39].

The Grubbs pyridine solvates have proven valuable in materials applications and as synthetic intermediates. These solvates are not particularly shelf-stable, and they easily lose one pyridine in solution. However, they are well-suited toward applications involving electron-poor alkenes (e.g., acrylonitrile), where their rapid initiation and lack of phosphine to intercept the intermediate carbenes are particularly kinetically advantageous. 

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