Hydrolysis coordinated imines

The reaction of (—)-151 with imines such as 2-methyl-l-pyrrolidine illustrated in Scheme 33 affords complex 222. An X-ray structure of 222 shows that the coordinated imine is oriented perpendicular to the 1,2-azaborolyl ring. This orientation which contrasts with that assumed for 219 must be due to the greater steric bulk of the imine. Reaction of 222 with allylmagnesium bromide gives 223 with excellent stereoselectivity. Hydrolysis affords the free amine 224. The reactions illustrated in Schemes 32 and 33 demonstrate that 1,2-azaborolyl iron complexes can efficiently transfer chirality to B-bound organic substrates. The development of catalytic versions of these stoichiometric reactions would be a highly desirable extension of this work. 

Coordinated imine functions within macrocycles are generally resistant to hydrolysis. They can sometimes be reduced to secondary amines, and conversely secondary amine functions can... 

The carbon atom of an imine is susceptible to attack by nucleophiles the judicious choice of electron-rich or electron-poor metal centers allows the reactivity of coordinated imines to be modified. Numerous examples of the hydrolysis and preparation of imines have been reported, but unless they provide any mechanistic insight they have not been considered here. 

The other way of reducing nitriles to aldehydes involves using a metal hydride reducing agent to add 1 mol of hydrogen and hydrolysis, in situ, of the resulting imine (which is undoubtedly coordinated to the metal). This has been carried out with LiAlH4, LiAlH(OEt)3, LiAlH(NR2)3, and DIBAL-H. The metal hydride method is useful for aliphatic and aromatic nitriles. 

The reaction of Ni11 perchlorate with (740a) gave the complex (741), in which the imine bond of the proligand had undergone hydrolysis. In the resulting dimeric Ni2 structure with a Ni Ni distance of 3.088 A, both metal ions are in octahedral coordination environment. When (740b) was used,... 

Chiral amines, ArCH(R)NH2, can be prepared by addition of a dialkylzinc to A-(diphenylphosphinoyl)imines, ArCH=N—P(=0)Ph2, using a suitable auxiliary, followed by acid hydrolysis to cleave the phosphorus moiety. A series of 2-azanorbornylmethanols (65) give ee% up to 92%, and they also induce some enantioselectivity in additions to benzaldehyde. A highly organized transition state with two zincs is proposed one coordinates the nitrogens of substrate and catalyst and the other coordinates the oxygens. 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

Coordinated a-amino amides can be formed by the nucleophilic addition of amines to coordinated a-amino esters (see Chapter 7.4). This reaction forms the basis of attempts to use suitable metal coordination to promote peptide synthesis. Again, studies have been carried out using coordination of several metals and an interesting early example is amide formation on an amino acid imine complex of magnesium (equation 75).355 However, cobalt(III) complexes, because of their high kinetic stability, have received most serious investigation. These studies have been closely associated with those previously described for the hydrolysis of esters, amides and peptides. Whereas hydrolysis is observed when reactions are carried out in water, reactions in dimethyl-formamide or dimethyl sulfoxide result in peptide bond formation. These comparative results are illustrated in Scheme 91.356-358 The key intermediate (126) has also been reacted with dipeptide... 

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