Pyridines addition reactions

Like pyridines (334), thiazoles undergo addition reactions with dimethyl acetylenedicarboxylate leading to 2 1 molar adducts, the structure of which has been a matter of controversy (335-339). 

Diels and Meyer found that the exothermic reaction obtained on dropping pyridine into dimethyl acetylenedicarboxylate in methanol gave a mixture of the indolizine (108) and a methoxymethylindolizine formulated as (109), and some dimethyl fumarate and dimethyl methoxyfumarate. Later workers - obtained only the methoxymethylindolizine in rather poor yield. The indolizine (108) has also been isolated from the products obtained when the addition reaction was carried out in ether, but in this case the course of the reaction was very susceptible to the presence of impurities in the ether, and the results indicated that ethanol was necessary as a reactant. ... 

Quinoxalines undergo facile addition reactions with nucleophilic reagents. The reaction of quinoxaline with allylmagnesium bromide gives, after hydrolysis of the initial adduct, 86% of 2,3-diallyl-l,2,3,4-tetrahydroquinoxaline. Quinoxaline is more reactive to this nucleophile than related aza-heterocyclic compounds, and the observed order of reactivity is pyridine < quinoline isoquinoline < phenan-thridine acridine < quinoxaline. ... 

A variation of an approach developed by Meyers was used to prepare nifedipine-type 1,4-dihydropyridines 35 from pyridine 34 using an oxazoline-directed aryllithium 1,4-addition reaction <96H(43)2425>. 

In this progress report we have reviewed the latest developments in the large area of cationic low-coordinated species and their coordination with Lewis donors. It is clear that these species are of a broad interest, in particular for catalysis. In some cases, e. g. the methylene phosphenium cation, the donor adducts also open new routes for synthesis. Regarding the mechanism for the diverse donor-addition reactions, the structural details are only poorly understood and need a better classification. In particular the variation of the Lewis-donor has to be established. Hitherto in most cases iV-donation is studied. It includes amines or pyridines. Obviously the effect of other donors, such as phosphines, thioethers needs to be studied as well. The siliconium cation for which these effects are better known could provide an understanding for further investigations within this field. 

Carbon-phosphorus double bonds are also formed in addition reactions of tris(trimethylsilyl)phosphine 1692 (which can be readily prepared from white phosphorus, sodium, and TCS 14 [13a,b,c]) to give oxazohum fluorides 1691 which then give the azaphospholes 1694, via 1693 [3, 14]. On addition of 1692 to 1695, the diazaphosphole 1696 [3, 15] is prepared, whereas l,3-azaphospholo[l,2a]pyridines 1698 [16] are formed from 1692 and 1697, and 1,3-thiaphospholes 1700 are formed from the dithiohum fluorides 1699 [17]. l,3-Benzodiphospholyl anions 1703 are generated by reaction of acid chlorides with the dihthium salts 1701, via 1702 [18] (Scheme 11.3). 

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

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). 

Metal cations can catalyze reactions of certain dienophiles. For example, Cu2+ strongly catalyzes addition reactions of 2-pyridyl styryl ketones, presumably through a chelate involving the carbonyl oxygen and pyridine nitrogen.31... 

Besides addition reactions, azides or hydrazoic acid can also yield tetrazoles through displacement reactions. Thus, halide displacement in imide chloride (78) yields 1,5-disubstituted tetrazoles (79), and in 2-chloro-pyridine (80), yields tetrazolopyridine (81) (Eq. 16a,b).141 143 Vinylogous... 

The five-coordinate complexes Ir(CO)(PPh3)2L, where HL = /3-diketone, A-benzoyl-A-phenyl-hydroxylamine, salicylaldehyde, 8-hydroxyquinoline, 2-hydroxybenzophenone, 2-hydroxy-8-methoxybenzophenone, were prepared from [Ir(CO)(PPh3)2Cl].632 The resulting compounds all underwent oxidative addition reactions with Br2. Reaction of [(cod)2IrCl]2 with N-substituted 3-hydroxy-2-methyl-4-pyridine gives the bichelated complex (389). 33... 

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... 

Michael addition of (benzotriazol-l-yl)acetonitrile 557 to a,[)-unsatu rated ketones followed by heterocyclization provides new means for preparation of 2,4,5-trisubstituted pyridines. The reaction is catalyzed by bases. In the presence of secondary amines, a nucleophilic attack of amine on the CN group in adduct 556 initiates the cyclization to tetrahydropyridine 558 that subsequently eliminates water and benzotriazole to give pyridine 559. Analogously, in the presence of NaOH, pyridone 560 forms, via intermediate 561 (Scheme 88) <1997JOC6210>. 

Here the alkene and pyridine will compete for the carbene at a constant concentration of pyridine the observed pseudo first order rate constant for ylide formation will increase with increasing alkene concentration. A plot of kobs vs. [alkene] will be linear with a slope of kad, which is the rate constant for the carbene/alkene addition reaction affording cyclopropane 5 (Scheme 1). 

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... 

Nitrogen-containing heterocyclic compounds, including 1,2,3,4-tetrahydroqui-noline, piperidine, pyrrolidine and indoline, are also popular hydrogen donors for the reduction of aldehydes, alkenes, and alkynes [75, 76]. With piperidine as hydrogen donor, the highly reactive 1-piperidene intermediate undergoes trimer-ization or, in the presence of amines, an addition reaction [77]. Pyridine was not observed as a reaction product. 

Bansal et al. found that the recently synthesized phosphatriazolo[l,5- ]pyridines can undergo addition reaction on the phosphorus atom when treated with sulfur or selenium in the presence of a secondary amine <1995ZNB558> (Scheme 15). Thus, reaction of 4 with these reagents yields under mild conditions the sulfur- or selenium containing addition products 51a and 51b in fair yield. 

As discussed in Section 11.15.4 on thermodynamic aspects, dinitrotetrazolo[l,5- ]pyridines 11 are electrophiles and can react with nucleophilic species in addition reactions as shown in Scheme 18 <1994IZV1278, 2003OBC2764>. In the presence of alcohols on addition of the alcoholate anion in position 5 of tetrazolo[l,5-tf]pyridine takes place. The primary addition product 12 formed in an equilibrium was characterized by its H NMR spectrum and can be isolated in the form of potassium salts 62 in good to high yields 53-96% <1994IZV1278>. 

A general summary of the chemistry of H4Os4(CO)12 is given in Scheme 10 (159, 166). Substitution reactions with phosphine, phosphites, or pyridine have been observed. An addition reaction occurs with I2 to yield the anionic species [H3Os4(CO)12I] The X-ray structure of this anion has established that the iodide is bridging between two metal centers, to yield an Os4 "butterfly configuration in which one of the metal- metal bonds of the initial Os4 tetrahedron has been broken (Os-Os = 3.817 A) (Fig. 38). Interestingly, if the two Os-1... 

Both amino groups of 3,5-diamino-4-phenylpyrazole reacted with EMME during 2 min. at reflux [83JCS(P1)11], The amino group at position 3 underwent a cyclocondensation reaction to form the bicyclic pyraz-olo[l,5-a]pyrimidine, while the amino group at position 5 participated in an addition reaction to give a (2,2-diethoxycarbonyl-l-ethoxyethyl)amino side-chain. Pyrazolo[l,5-a]pyridine (25) was obtained in 27% yield. 

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]). 

Next, in chronological order, Acheson and his group, in his series Addition reactions of heterocyclic compounds (cf. 63AHC(1)125), extended the range of substrates to pyridine derivatives and initially believed to have found numerous pyridoazepines. The contents of six experimental articles (1968-1975), however, were for the most part revised... 

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