Bases, ambident

The cyanide ion is an ambident base. The carbon end is seen to be much softer than the nitrogen end, as would be predicted. The greater strength of the H-CN bond, compared with H-NC, means that the carbon end is much more basic than the nitrogen end. This would be true even in solution, since the ions are the same for both acids. As a result, binding to carbon will be more common than binding to nitrogen, even for hard acids. 

Ambident reactivity of A-4-thiazoline-2-thione has been discussed (101) in terms of the Hard and Soft Acids and Bases classification (199) and the Klopman-Hudson approach (200). 

The tautomeric 2-hydroxypyrroles undergo base-catalyzed reactions probably through deprotonation to the ambident conjugate base (Scheme 71). 2-Hydroxyfurans (e.g. 178) similarly exist in ketonic forms (179) and (180), the most favoured tautomer being the conjugated 2(5//)-furanone (180)... 

Other possible ambident nucleophiles include cyanii anion (CN ), methyl sulfinate anion (CH3SO2 ), ar acetone enolate (CH3COCH2 ). Identify the most electro rich atom(s) in each anion (based on charges alone), ar indicate the major product that should result from an S, reaction with methyl bromide at this atom(s). 

Treatment of an ethylidene malonic ester such as (a) with strong bases results in loss of a proton from the allylic position to produce the ambident ion fb). Alkylation of such carban-ions usually occurs at the carbon bearing the carbonyl groups, resulting in the establishment of a quaternary center and deconjugation of the double bond fc). 

Rawal s group developed an intramolecular aryl Heck cyclization method to synthesize benzofurans, indoles, and benzopyrans [83], The rate of cyclization was significantly accelerated in the presence of bases, presumably because the phenolate anion formed under the reaction conditions was much more reactive as a soft nucleophile than phenol. In the presence of a catalytic amount of Herrmann s dimeric palladacyclic catalyst (101) [84], and 3 equivalents of CS2CO3 in DMA, vinyl iodide 100 was transformed into ortho and para benzofuran 102 and 103. In the mechanism proposed by Rawal, oxidative addition of phenolate 104 to Pd(0) is followed by nucleophilic attack of the ambident phenolate anion on o-palladium intermediate 105 to afford aryl-vinyl palladium species 106 after rearomatization of the presumed cyclohexadienone intermediate. Reductive elimination of palladium followed by isomerization of the exocyclic double bond furnishes 102. 

Concerning the mechanism of the rearrangement, it is proposed to involve the addition of the nucleophilic nitrogen of the 1,3-ambident nucleophiles to C-6 (see Section III,A,2,a). This covalent adduct is in equilibrium with the open-chain diamidino compound 83 (R = OCH3, SCH3). Cycliza-tion into 84, followed by a base-catalyzed fragmentation of the nitrogen-carbon bond at N-3 and expulsion of the thiomethyl or methoxy anion, yields the 2-amino-4(5)-phenylpyrimidines (Scheme III.47). 

This procedure describes the preparation of 3-nitropropanai, 1, employing the rarely encountered 1,4-addition of ambident nitrite ion with its "softer N-atom,2 and further transformations of 1, as reported earlier.3 A similar preparation of 3-nitrobutanal from crotonaldehyde (3-butenal) is known,4 as well as analogous additions to a, 3-enones.2 The reduction of 1 to the alcohol 2, originally carried out with borane-dimethyl sulfide (BMS),3 is now more conveniently and economically done with sodium borohydride. The acetalization of 1 to yield the dimethyl acetal 3 is based on our earlier report.3... 

The reactions of the ambident ArNH ions, which do not IV-arylate to a great extent, are discussed in Section 2.2.3.1.4. Based on the negative results obtained with IV-acyl anions such as AcNH and AcNMe-,68 and the anion of phthalimide,134 it would appear that these anions do not undergo IV-arylation under SrnI conditions. 

Methanol-0-4 methyl nitrite, and dimethyl disulfide have been examined as potential chemical probes for distinguishing between alkoxides and enolates in the gas phase.171 Methanol-0-d proved to be unsuitable and methyl nitrite reacts too slowly in contrast, the reactive ambident behaviour of dimethyl disulfide results in elimination across the C—S bond on reaction with alkoxides ( hard bases ) and attack at sulfur by enolates ( soft bases ). This probe has been applied to investigation of the anionic oxy-Cope rearrangement. The dianionic oxy-Cope rearrangement is a key step in a squarate ester cascade involving stereoinduced introduction of two alkenyllithium reagents cis to each other.172... 

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