Pyridines additions

The procedure is experimentally simple, and the workup involves only the destruction of the traces of hydrogen peroxide with manganese dioxide and evaporation of the hexamethyldisiloxane. Pyridine additives serve to buffer the highly acidic rhenium species and to shut down the detrimental acid-catalyzed epoxideopening pathways. The scope of this transformation is best appreciated through the examples presented in Table 12.2 [28],... 

Both 2-hydroxy- and 3-hydroxypyridine are hydroxylated to 2,5-dihydroxypyridine by strains of Achromobacter sp. (Houghton and Cain 1972). These metabolites are probably, however, formed by different reactions whereas 3-hydroxypyridine behaves as a true pyridine, addition of H2O across the Cg Nj bond would produce the 2,5-dihydroxy compound 2-hydroxypyridine is a cyclic amide and hydroxylation apparently occurs at the diagonal position. The degradation of 4-hydroxypyridine is also initiated by hydroxylation and is followed by dioxygenation before ring fission (Figure 10.12) (Watson et al. 1974). 

Cyclization of 776 with ortho-esters gave (83JOC1628) [l,2,4]triazolo-[4,3-fc]pyrimido[5,4-< ][ 1,2,4]triazines 777, whereas reaction with sodium nitrite afforded the corresponding tetrazolopyrimidotriazine 778.3-Azido-pyrimido[4,5-e][l, 2,4]triazine 779 exists in a cyclic form as tetrazolo derivative 780, as shown by X-ray analysis (86KGS114). In solution the position of the 779 780 equilibrium depended on temperature and solvent. Higher temperatures favored 779. Azido compound 779 predominated in water, and tetrazolo compound 780 predominated in pyridine. Addition of sodium azide to the aqueous solution shifted the equilibrium toward 780. The... 

For products (507), the choice of reaction conditions is governed by the nature of the C,C double bond. For terminal enamines (507a,b), trialkylsilyl triflates with a pyridine additive is the reagent of choice (535) (Table 3.35, entries 1,2). By... 

The intermolecular functionalization of benzylic and activated methylene r/>3-C-H bonds represents another burgeoning field in C-H activation chemistry. A recent sp3-benzylic C-H functionalization was disclosed involving an Ir(m)acac analog. Such a reaction is retarded by pyridine addition, indicating that pyridine loss may initiate the C-H functionalization step. An Ir(v) intermediate has been postulated (Equation (30)).12... 

FIGURE 8.9 Effect of NO concentrations leaving the reaction zones of an ethylene-air flame ( = 1.68, T = 2030K) with various pydrine additions. Curve A, no pyridine addition curves B and C, 0.1-0.5 N by weight of fuel and curve D, NO addition to the fuel-air mixture (from Haynes etal. [14]). 

Diazines are generally resistant to electrophilic attack on carbon, and, as for pyridine, addition on nitrogen is observed. Alkyl halides give mono-quaternary salts di-quatemary salts are not formed under normal conditions. Of conrse, if the diazine ring carries a snbstitnent that makes the starting... 

A number of thermodynamic studies of Ni[R-dtp)2 adducts with amines have been described H2,i43,is6,is8) Daktemieks and Graddon ) have measiued the enthalpies of addition of pyridine, 4-picoline, 2,2 -dipyridyl and 2,9-dime-thyl-l,10-phenanthroline to Ni[ethyl-dtp]2 and pyridine to Nilpropyl-dtpJj. The sums of the stepwise enthalpies for pyridine addition to the ethyl and... 

A comparison of the stereochemistry of the 2-vinyl pyridine addition to [2a] with that of methylation addition is particularly instructive. Table 2 shows the stereochemistry of formation of [7a] as a function of counterion size and coordination. 

The size and coordination of the cation as well as the position of the nitrogen atom in both ultimate and penultimate pyridine groups are key parameters in the stereoselectivity of methylation and 2-vinyl pyridine addition. 

Definite compounds of niobium and iodine are unknown. The preparation of an iodide from the pentabromide has been reported2 but no details are supplied. A pyridine addition compound of the pentiodide, NbIB.(C5H5N.HI)6, has been described,2 but its existence lacks confirmation.4... 

Fig. 8. Effect of pyridine addition on integrated absorbance of 3650 cm 1 OH band of HY (41). (Reprinted with permission of the American Chemical Society.)...

Quinolinic anhydride gave the products of pyridine addition and insertion, quinoline and pyridylquinoline. Naphthyridine may have been present at a concentration too low to detect. The major insertion product, dipyridyl, was also formed by pyrolysis of pyridine itself. To demonstrate clearly the insertion products of 2,3-pyridyne, quinolinic anhydride was also pyrolyzed in benzene to give the products shown in Table 10. 

The most convenient preparative scale precursor of 1-alkoxycyclopropyllithium reagents was found to be 1-bromo-l-ethoxycyclopropane 139, prepared in good yields by reaction of 1-ethoxy-1-trimethylsiloxycyclopropane 27) with phosphorus tribromide in the absence of pyridine. Addition of 139 to r-butyllithium (2 equivalents) in ether at —78 °C resulted in immediate and exothermic halogen metal exchange to form the expected 1-ethoxycyclopropyllithium 140, Eq. (43) 74). 

The phenylllthlo intermediate has been isolated (20 ) and treated with styrene and ethylene oxides, and W-benzylldene aniline to provide various 2-phenyl-5-substituted pyridines. Addition of perhalomethanes affords bls(6-phenyl-3-pyrldyl)nethane and 6,6 -dlphenyl-3,3 -dlpyrldyl ( ). The yields of 3-substituted pyridines range, in the LAH reaction, from 89% for 3-plcoline to 63 for 3-benzylpyrldlne. Reaction of the LAH Intermediate with bromine formed 3-bromopyridlne in 1 1 yield (17). 

In order to study the factors determining the regioselectivity of sodium borohydride reduction of a, -unsaturated ketones, reactions with 3-methylcyclohexenone, carvone and cholestenone were carried out in 2-propanol, diglyme, triglyme or pyridine. Mixtures of 1,2- and 1,4-reduction products were obtained in the alcoholic and ether solvents, whereas pure 1,4-reduction was observed in pyridine. Addition of triethylamine to NaBH4 in diglyme led to formation of triethylamine borine, EtsN BHs. Similarly, with pyridine, pyridine borine could be isolated, leading to exclusive 1,4-reductions. 

Perhaps more instructive than studies dealing solely with the OH region of the IR spectrum are those studies in which a probe molecule is used either to quantify the Br0nsted sites, or to qualify the type of site as to Brpnsted or Lewis. To this end numerous studies have been conducted. Pyridine is typically the adsorbent of choice as it is well known to exhibit characteristic IR bands at approximately 1450 and 1540 cm corresponding to Lewis and Brpnsted sites, respectively. Additionally, one can measure the disappearance of the acidic OH stretch bands with pyridine addition to identify the bands corresponding to acidic hydroxyl groups. 

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