Carbon-nitrogen bonds double, stereochemistry

Fry and Newberg 1,2> examined the electrochemical reduction of nor-camphor oxime (109) and camphor oxime (110) to the corresponding amines. The results of this study are shown in Table 3. It is clear from a comparison of these data with those in Table 2 that the electrochemical reduction of oximes 109 and 110 takes a very different stereochemical course from reduction of the corresponding anils 103 and 104. Reduction of oximes apparently proceeds under kinetic control, affords products corresponding to protonation at carbon from the less hindered side of the carbon-nitrogen double bond, and affords the less stable epimeric amine in each case. It is not evident why the stereochemistry of reduction of anils and oximes should differ, however. 

No reaction is observed when 50 and 1,1-dimethoxyethene are allowed to stand in the dark for an equivalent period of time. Irradiation of 50 with 4 equiv. of cyclohexene in methylene chloride solvent gives a 51% isolated yield of the (2 + 2)-cyclo-adduct 52 assigned cis-anti-cis stereochemistry. These two cycloadditions were the first two well defined examples of the (2 + 2)-photocycloaddition reaction of an olefin to a carbon-nitrogen double bond. With furan the (2 + 2)-photocycloadduct 53 is formed, again regiospecifically. 

VIII. STEREOCHEMISTRY OF THE CARBON-NITROGEN DOUBLE BOND IN ENAMINES... 

An interesting variant of the Peterson reaction involves the addition of an a-silylbenzylic anion (173) to a carbon-nitrogen double bond such as an imine. In this case the loss of RN—SiMes occurs less readily than the loss of alkoxytrialkylsilane (O—SiRs). The initial addition reaction is reversible and the stereochemistry of the elimination reaction is predominantly trans (Scheme 76). Both acid- and base-catalyzed elimination reactions lead to the trans product (174). 

Stereochemistry of Carbon-Carbon and Carbon-Nitrogen Double Bonds... 

Write down the structures of the imines formed in the following reactions, and assign /Z stereochemistry to the carbon-nitrogen double bonds. It is reasonable to assume that the less sterically hindered isomer will be formed. 

All of the reactions shown are fairly standard in type. The first is a Strecker synthesis the first formed product is the a-aminonitrile, which is then hydrolyzed to an amino acid. The second reaction is a standard oxime synthesis as the ketone is symmetric, there is no stereochemistry to worry about. In (c), an imine is formed the stereochemistry about the carbon-nitrogen double bond is determined by steric hindrance. Since the SM is racemic, two diastereoisomers will be formed, hence the wavy bond to the methyl group. In the final example, a hydrazine is prepared while it s not possible to be certain of the stereochemistry about the C=N double bond, it s logical to assume that the less sterically hindered isomer will predominate ... 

The diagnostic carbon for determination of the stereochemistry of methio-dides containing a double bond in the N-substituted quinolizidine ring is C-6, which is situated a to the quaternary nitrogen and a to the double bond. This... 

The second very important conclusion is related to the nature of the covalent bond between the nitrogen and the carbon atom. This bond has a single bond character in one of the resonance structures and a double bond character in the other. Hence, the amide bond has a bond order of 1.5, i.e. it has an intermediate character between a double and a single bond. In the chapter on alkanes we have mentioned that while the rotation around a single bond is free, the rotation around a double bond is forbidden. Because the amides bond has a partial double bond character the rotation around it is hindered. This evidence is important in the study of the stereochemistry of complex molecules with amide groups, such as polyamides and polypeptides. 

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