The Formation of Nitrogen-Carbon Bonds

Initially it was thought that the mechanism of both protonation and alkylation would be the same, involving electrophilic attack by a proton or a carbo-cation such as RCO. It soon became clear, however, that the mechanism of N-C forming reactions was much more complicated. Moreover, several types of reaction were discovered. 

Alkyl bromides and iodides were found to react with the trans-[M(N2)2(dppe)2] complexes to give diazenido-compounds these could proto-nate at the terminal nitrogen to give hydrazido(2—)-complexes which reverted to the diazenido parents on treatment with base [Equation (12)].  

The range of metals was also extended to include rhenium in the preparation of [ReCl2(N2COR)(PMe2Ph)3] from [ReCl(N2)(PMe2Ph)J,3 but other dinitrogen complexes proved resistant. 

In the above reactions and related ones, such as the formation of [MoCl(N2COPh)(dppe)2] from [Mo(PhCN)(N2)(dppe)2], the halide of the attacking reagent was included in the complex product. However, the reaction of [Mo(SCN)(N2)(dppe)2] with Bu I produced [Mo(SCN)(N2Bu )(dppe)2], which suggested that the mechanism of the reaction must differ in detail from those of the previously described reactions. The studies which led to a detailed understanding of the mechanism of N-C bond forming reactions are described below, but first the discovery of further types of complex will be described. 

One type was essentially found by accident. When the complexes trans-[M(N2)2(dppe)2] were treated with MeBr in thf rather than benzene (the previous solvent of choice) instead of methyldiazenido-compounds, another type of 

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Miscellaneous Nitrogen Donors. Azo compounds, R—N=N—R, which have both a and n electrons, characteristically use their a lone pairs,25 as in the examples (21-XXXV1) and (21-XXXV11). Coordinated azo groups play an important role in the complexes formed by many azo dyes and in the formation of metal—carbon bonds to phenyl groups, as in the compound (21-XXXVIII). See also Sect. 24-A-4. 

Substitution chemistry remains one of the most popular and simplest approaches to the formation of nitrogen-carbon(sp ) bonds. An example of this method involved the addition of pyrimidines to brominated esters (Scheme 3.1) [1]. The first step in this sequence was the protection of the primary amine in order to direct the reaction to the N l)-H of the pyrimidine. Once the protection was complete, addition of a strong base deprotonated the N l)-H and generated the amide that attacked the brominated ester to generate the new nitrogen-carbon(spO bond. 

Gabriel synthesis (Section 22 8) Method for the synthesis of primary alkylamines in which a key step is the formation of a carbon-nitrogen bond by alkylation of the potassium salt of phthalimide... 

This apparent characteristic enhancement in the basicity has been used quite frequently for the determination of the position of a double bond with respect to the nitrogen atom in unsaturated amines. The cases such as neostrychnine (134) and dehydroquinuclidine (139) in which the protonation at the 8-carbon atom cannot occur due to the lack of overlap between the electron pair on the nitrogen atom and the tt electrons of the double bond, since this would involve the formation of a double bond at the bridgehead— a violation of Bredt s rule—show a decrease in basicity. For instance the basicities of quinuclidine (140) and dehydroquinuclidine (139) have been shown by Grob et al. (82), to differ by 1.13 pK units in aqueous solution at 25. This decrease in basicity has been attributed to the electron-withdrawing inductive effect of the double bond. 

Route b involves the formation of one carbon-carbon bond and one carbon-sulfur bond. It belongs to the category of sulfene chemistry143. Sulfene intermediates react readily with diazoalkanes to produce, after the loss of nitrogen, thiirane dioxides. So far, this appears to be the method of choice for the preparation of thiirane dioxides of all types. 

For reviews of reactions of carbonyl compounds leading to the formation of C=N bonds, see Dayagi, S. Degani, Y. in Patai The Chemistry of the Carbon-Nitrogen Double Bond Ref. 40, p. 64 Reeves, R.L. in Patai, Ref. 2, p. 600. 

The scheme proposed by the authors81 includes the formation of a hydrogen bond between the proton at the ortho-carbon atom of the phenyl ring and one of the nitrogen atoms of cyclostannylene followed by the Sn-N bond cleavage and C-Sn bond formation. The second Sn-N bond is cleaved... 

The formation of a double bond during anodic oxidations can result from eliminations of protons, carbon dioxide or acylium cations. The electrooxi dative aromatization of dihydropyridine derivatives and heterocycles containing nitrogen atom (di-hydroquinoxalines, tetrahydrocinnolines) involves an ECE mechanism as previously... 

This reaction was intensely investigated by Mloston et al. In a classical, but not general reaction, they observed that heating aromatic or aliphatic thioketones [251, 252], thionoesters [253] and dithioesters [252, 253] with aryl-or benzylazides led to the formation of a carbon-nitrogen double bond. A [3+2] cycloaddition was assumed, with successive elimination of nitrogen and sulfur. 

As usual, the best strategy is to identify the nucleophile and the electrophile. This chapter introduced a new electrophile, the carbonyl carbon of an aldehyde or ketone. The nucleophiles are listed in Table 18.2. Hydride, water, HCN, and organometallic nucleophiles result in the addition of the nucleophile to the carbon and a hydrogen to the oxygen of the carbonyl group. Ylides and nitrogen nucleophiles result in the formation of a double bond between the carbonyl carbon and the nucleophile. And alcohols and thiols add two nucleophiles to the carbonyl carbon. 

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