Pyridine initiator

As can be deduced from Eq.(2), the liberation of the catalytically active [YCo] species is of prime importance, the neutral ligand L merely stabilizing the catalyst as the isolable complexes YCoL. The influence of the neutral ligand at cobalt on the temperature at which initial pyridine formation occurred was investigated using the test reaction [Eq.(42)]. 

Ring fluorination of pyridine and its benzo derivatives is suggested to occur through nucleophilic attack of fluoride ion on an initial pyridine fluorine complex (87TL255, 91BCJ1081, 93AHC(58)29l). In electron-deficient pyridines and their benzo derivatives, fluorination on the pyridine ring is kinetically competitive with annular and side chain fluorination. 

Molecular weights of polymers, as with the initiator pyridine, are very high, unaffected by polymerization conditions and presumably determined by the level of weak acids present. Only at low temperatures, in the presence of lithium bromide, are sufficient chains initiated for molecular weights to be uninfluenced by adventitious chain transfer agents. 

Fieure 1. Rate of perhaloacetaldehyde polymerization NMR study. Initiator, pyridine initiator concentration, 2 mol % polymerization bath temperature, —78°C. B, DCBA C, CDBA D, hrotrud. 

Initially, the meto-bromopyridine complex 13b was used since it initiated at a very rapid rate. However, it was found in subsequent studies that the slower initiating pyridine complex 13a was more stable and still provided good control. ... 

This reaction consists of the condensation of two molecular equivalents of a 1,3 diketone (or a J3-keto-ester) with one equivalent of an aldehyde and one of ammonia. Thus the interaction of ethyl acetoacetate and acetaldehyde and ammonia affords the 1,4-dihy dro-pyridine derivative (1), which when boiled with dilute nitric acid readily undergoes dehydrogenation and aromatisation" to gb e the diethyl ester of collidine (or 2,4,6-trimethyl-pyridine-3,5 dicarboxylic acid (II)). For the initial condensation the solid aldehyde-ammonia can conveniently be used in place of the separate reagents. 

Repeat the proeedure using HMO. HMO requires entry of the entire lower semimatrix, ineluding the diagonal and all zero elements. Beeause the matrix element format is II, only one symbol ean be entered for eaeh element. The numbers 0.5 and 1.2 eannot be entered in this format instead enter 1, whieh will be modified later. The initial unmodified input for pyridine is the same as that for benzene, 010010001000010100010 henee, we ean make a trial run on benzene to see if everything is working properly. 

These compounds may be obtained by the Hantszch heterocyciization method (see Chapter II, Section 11.3). A -widely used two-step preparative method (Scheme 195) involves initial reaction of a 2-amiriothiazole -with 339 in pyridine (631-638) in aqueous sodium carbonate (639) or by fusion without solvent (640). The formed 340 is then hydrolyzed in acidic (641, 642, 1593) or alkaline medium (643-646). The direct reaction of 342 (Scheme 196) -with 2-aminothiazoles is less common and takes place in... 

Soum and Fontanillet prepared a living polymer of 2-vin yl pyridine using benzyl picolyl magnesium as the initiator. The values of were measured experimentally for polymers prepared with different concentrations of initiator and different initial concentrations of monomer. The results are given below calculate the theoretical molecular weights expected if polymerization proceeds completely from 100% predissociated initiator and compare the theoretical and experimental values ... 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Homopolymer. Formaldehyde polymerises by both anionic and cationic mechanisms. Strong acids are needed to initiate cationic polymerisation. Anionic polymerisation, which can be initiated by relatively weak bases (eg, pyridine), can be represented by the following equations Initiation... 

The route from o-phthalodinitnle [91-15-6] can be represented 4 CgH4N2 + M — MPc, where M is a bivalent metal, metal haUde, metal alcoholate, or an equivalent amount of metal of valence other than two in a 4 1 molar ratio. If a solvent, eg, trichlorobenzene, benzophenol, pyridine, nitrobenzene, or quinoline, is used, the reaction takes place at approximately 180°C. Without a solvent the dry mixture must be heated to ca 300°C to initiate the exothermic reaction (50). 

Halophenols without 2,6-disubstitution do not polymerize under oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or intermpt the propagation step of the chain process. To prepare poly(phenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmaim ether synthesis. A cuprous chloride—pyridine complex in 1,4-dimethoxybenzene at 200°C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1) ... 

Dow Chemical Company purchased the rights to MacaHum s patents (14), initiated a detailed study of the process and other improved syntheses of PPS in the 1950s and early 1960s, and pubUshed the results of their investigation (9,15,16). Clearly, alternative routes to PPS were desirable and the most promising of these involved the nucleophilic self-condensation of cuprous -bromothiophenoxide, carried out at 200—250°C in the soHd state or in the presence of pyridine (16) ... 

Compound (27) may also be obtained dkecdy by oxidation of P-picoline (3) or by exhaustive oxidation of 5-ethyl-2-methylpyridine (7), followed by decarboxylation of the initially formed pyridine-2,5-dicarboxyhc acid [100-26-5] (28) (eq. 8) (30). 

Nitrogen nucleophiles used to diplace the 3 -acetoxy group include substituted pyridines, quinolines, pyrimidines, triazoles, pyrazoles, azide, and even aniline and methylaniline if the pH is controlled at 7.5. Sulfur nucleophiles include aLkylthiols, thiosulfate, thio and dithio acids, carbamates and carbonates, thioureas, thioamides, and most importandy, from a biological viewpoint, heterocycHc thiols. The yields of the displacement reactions vary widely. Two general approaches for improving 3 -acetoxy displacement have been reported. One approach involves initial, or in situ conversion of the acetoxy moiety to a more facile leaving group. The other approach utilizes Lewis or Brmnsted acid activation (87). 

The reaction of 1-aminopyridinium iodide (429) with dimethyl chlorofumarate in ethanol/K2C03 to form, ultimately, a pyrazolo[l,5-n]pyridine also occurs via a 1,5-dipolar mechanism. The initially formed 1 1 adduct (430), stabilized by delocalization of the negative charge, underwent disrotatory ring closure as shown to give (431) in which the 3... 

The greater preference of molecular sieves for combining with water molecules explains why this material can be used for drying ethanol and why molecular sieves are probably the most universally useful and efficient drying agents. Percolation of ethanol with an initial water content of 0.5% through a 144 cm long column of type 4A molecular sieves reduced the water content to lOppm. Similar results have been obtained with pyridine. 

A number of other syntheses of coniine have been effected, of which that of Diels and Alder is of special interest. The initial adduct of pyridine and methyl acetylenedicarboxylate, viz., tetraraethylquinolizine-1 2 3 4-tetracarboxylate (IX) on oxidation with dilute nitric acid is converted into methyl indolizinetricarboxylate (X). This, on hydrolysis and decarboxylation, furnishes indolizine, the octahydro-derivative (XI) of which, also known as octahydropyrrocoline, is converted by the cyanogen bromide method (as applied by Winterfeld and Holschneider to lupinane, p. 123) successively into the broraocyanoamide (XII), cyanoaraide (XIII) and dZ-coniine (XIV). A synthesis of the alkaloid, starting from indolizine (pyrrocoline) is described by Ochiai and Tsuda. ... 

The method of choice for preparing tropone (45) is to treat the initial mixture of monoadducts (43a) and (43b) with methanolic 1 TV hydrochloric acid to complete ketal hydrolysis and then carry out the pyridine rearrangement to give 3-bromo-4-methoxy-A-homo-estra-2,4,5(10)-trien-17-one (44) as described above for monoadduct 17-ketone (43b). 

Figure 4-12. Stopped-flow study of the pyridine-catalyzed hydrolysis of acetic anhydride, showing the formation and decay of the acetylpyridinium ion intermediate. Initial concentrations were 0.087 M pyridine, 2.1 x im M acetic anhydride the pH was 5.5 ionic strength, 1.0 M temperature, 25 C. Five hundred data points tabsorbance at 280 nm) were measured in I s. The smooth curve is a ht to Eq. (3-27). Source Data of D. Khossravi and S.-F. Hsu, University of Wisconsin.

Nicolaou and co-workers established the severely strained A-ring oxazole (21) in their total synthesis of antitumor agent diazonamide A through initial oxidation of the hindered alcohol of intermediate 20 with TPAP and subsequent Robinson-Gabriel cyclodehydration of the resultant ketoamide with a mixture of POCI3 and pyridine (1 2) at 70°C. ... 

Subsequent to Hantzsch s communication for the construction of pyridine derivatives, a number of other groups have reported their efforts towards the synthesis of the pyridine heterocyclic framework. Initially, the protocol was modified by Beyer and later by Knoevenagel to allow preparation of unsymmetrical 1,4-dihydropyridines by condensation of an alkylidene or arylidene P-dicarbonyl compound with a P-amino-a,P-unsaturated carbonyl compound. Following these initial reports, additional modifications were communicated and since these other methods fall under the condensation approach, they will be presented as variations, although each of them has attained the status of named reaction . 

---