Ammonia with methyllithium

Finally like methyllithium (ref. 121) ammonium fluoride (ref. 122), tris-(dialkylamino)sulfonium salts (ref. 123) or alkali alkoxides (ref. 124), alkali amides in liquid ammonia are able to cleave the silicium-oxygen bond of silyl enol ethers (refs. 125, 126) leading to enolates. The sodium enolate obtained (Fig. 27) by treatment of a silyl enol ether with NaNH2 can be equilibrated in the medium, leading to two alkylated products, nevertheless no polyalkylated species is detected. With the use of LiNH2 only the expected reaction product is prepared but the use of KNH2 leads to a mixture of C-mono and dialkylated and O-alkylted products (ref. 125). 

Methyllithium has proved to be a useful alternative to ammonia as a basic reagent for deacylating A-alkoxycarbonyl- and iV-benzoyl-substituted 1,2-diazepines. This methodology has found application with azetidinodiazepines (98) in which the NR group is cleaved selectively to give (99) without destruction of the -lactam moiety (Equation (18)) <90H(30)55l>. 

The current experiment involves the preparation of the sterically hindered amine A -ferf-butyl-3,5-dimethylaniline. Other preparations of this amine involve addition of methyllithium to lV-3,5-dimethylphenylacetone imine and the reaction of l-bromo-2,4-dimethylbenzene with terf-butylamine either via aryne formation or by palladium-catalyzed alkylation. The current method, the reaction of tert-butylamine with the 2,4,6-trimethylpyrylium cation, involves inexpensive starting materials and proceeds in high yield. The molybdenum(III) complex of the deprotonated form of this amine, Mo[N(f-Bu)(3,5-C6H3Me2)]3, splits the N=N triple bond in N2 to afford molybdenum(VI) nitrido products. This latter reaction is the key step in the recently discovered catalytic process to convert N2 to ammonia under ambient conditions. 

The simplest reaction characteristic of compounds that are regarded as salts of carbanions is that of proton transfer (see Proton Transfers Involving Anions and Dianions) and a number of these have been studied. A typical example is the reaction of methane with lithium amide. This reaction is a model for many known hydrogen isotope exchange reactions of hydrocarbons with lithium and cesium cyclohexylamide whose kinetics have been measured in cyclohexylamine. The model reaction does involve a preliminary weakly bound complex that isomerizes via a transition structure to methyllithium coordinated to ammonia (Figure 3). 

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