Pyridines aldimines

Cyclotrisilanes are easily converted to a mixture of silylene and disilene. Reactions of the silylene (l-Bu)2Si and the disilene l-Bu2Si=Si(Bu-l)2 produced by photolysis of the cyclotrisilane 293 with cyclopentadiene, furan, and thiophene have been reported recently152, as were their reactions with 2,2 -bipyridyl, pyridine-aldimine and ketoimines153. 

The best method for synthesis of various pyridine aminophosphonates is a thermal addition of diethyl phosphonate to pyridine aldimines. Pyridine aldehydes react easily with aliphatic or aromatic amines in toluene solution to form corresponding aldimines (Schiff bases). The obtained imines were reacted in situ with diethyl H-phosphonate to give aminophosphonate diethyl esters [24],... 

For the reaction of the 3-pyridine aldimine 16e, the first equivalent of zinc chloride is coordinated to the more nucleophilic pyridine nitrogen and thus is inactivated. Only the second equivalent activates the imine. However, the zinc chloride coordinated to the more nucleophilic pyridine nitrogen, logically exposes more nucleophilic chlorine ligands for the interaction with the silyl dienol ether 19. Therefore, in this case the nucleophile 19 is introduced from the free back side of 16e (Formula C) and furnishes selectively the Mannich base 28 with opposite stereochemistry compared to the compounds 27. 

Some cycloaddition reactions of 4 are summarized in Scheme 1. This shows that silylene 4 undergoes reactions with nitriles [14], phosphaalkynes [15], silyl azides [16], diazabutadienes [17], 2,2 -bipyridyl and its derivatives [18, 19], a-ketoimines [19], and pyridine-2-aldimines [19]. 

Depending on the steric bulk of the nitrogen bonded substituent, the reaction of 4 with pyridine-2-aldimines can also proceed by an intermolecular insertion of the silylene into the C-H bond of the acyclic CN group [19]. 

Various phenyl-substituted ketimines and aldimines react with metallocenes 1 and 2, in a manner that depends on the substituents present [41]. In all cases, elimination of the al-kyne is observed. Complex 2b reacts with PhN=CMePh to give the r 2-complex 64, which is stabilized by an additional pyridine ligand [41a], In the reactions of 1 or 2a with the ketimine HN=CPh2, hydrogen transfer generates complexes 65. Two molecules of the aldimine PhN=CHPh are coupled by 2a to give the cyclic diamido complex 66 [41b]. 

Fig. 6.4 Reversible interconversion of amino acid and keto acid. Conjugation of the imine bond in the aldimine with the electron sink of the pyridine ring plus protonation of the pyridine nitrogen as well as the metal ion - all this results in weakening of the C-H bond of the amino acid residue. Thus, also catalyzed is a-proton exchange, racemization of a chiral center at the a-carbon atom and decarboxylation of the appropriate amino acid. ...

Auch Phenole lassen sich an Aldimine addieren (Bd.XI/1, S. 337). Ein neueres Beispiel ist die Herstellung von 2-[Anilino-( 2-pyridyl)-methvl]-phenol aus 2-(Phenylimino-methyl)-pyridin und Phenol in siedendem Benzol2. 

An interesting example is the hydroiminoacylation reaction, a good alternative to hydroacylation reactions, using aldimines as a synthetic equivalent to aldehydes (Scheme 4) [4]. The rhodium-catalyzed hydroiminoacylation of an olefin with aldimines produced a ketimine which could be further acid-hydrolyzed to give the ketone. The reaction proceeded via the formation of a stable iminoacylrhodi-um(III) hydride (this will be discussed in the mechanism section), production of which is facilitated by initial coordination of the rhodium complex to the pyridine moiety of the aldimine. This hydroiminoacylation procedure opened up the direct... 

The value of reduced LD in the quinonoid band of the enzyme complexes with L-alanine and oxindolyl-L-alanine proved to be close to that in the absorption band of the external aldimine formed with L-threonine (Table 9.2). It does not follow from this coincidence that the external aldimine and quinonoid have a similar orientation in the active site. The two intermediates, in fact, differ in their orientation this is because the directions of the transition dipole moment in the pyridine ring of the aldimine and quinonoid are very different.72 ... 

Thus, to attain the racemase reactivity it is important to learn how to block the transamination process. Our idea was to place a catalytic group that can protonate only the a-carbon of the amino acid unit but not the C4 of the pyridoxal moiety in the quinonoid intermediate (Scheme 2.5). Thus we synthesized catalyst 38, which carries a rigid pyridine side chain [39]. Catalyst 39, which lacks the double bond, was also synthesized as a less rigid control. Both catalysts catalyzed loss of optical activity from the aldimine equally well - about twice as fast as simple pyridoxal. However, 39 could catalyze the transamination reaction 2.5 times faster than 38. Therefore, 38 showed a small preference for racemization over the transamination reaction as compared with 39 by a... 

Various phenyl-substituted imines react with (26) in a maimer that depends on the substituents present but in all cases, elimination of the aUcyne is observed. The imine PhN=CMePh affords the -complex (35). On the contrary, with the ketimine HN=CPh2, complex (36) is generated by hydrogen transfer. Two molecules ofthe aldimine PhN=CHPh are coupled to give a five-membered diamido complex (37). An unusual reaction with 2-vinylpyridine takes place starting from (26) or (28). Monoazadienes (38) are formed, in which the aromaticity of the pyridine ring has been lost. 

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