Electrophilic reactions ethynyl electrophiles

Benzotriazole-related methodology publications appeared in 2006. Reaction of 1-formylbenzotriazole with triphenylphosphine/carbon tetrachloride afforded l-(2,2-dichlorovinyl)benzotriazole, where lithiation followed addition of electrophiles gave a variety of functionalized M-(ethynyl)benzotriazoles <06T3794>. Novel mono- and symmetrical di-/V-hydroxy- and IV-aminoguanidines were readily prepared from the reaction of diverse hydroxylamines or hydrazines with reagent classes di(benzotriazol-l-yl)methanimine, (bis-benzotriazol-1 -y 1-methy lene)amines, benzotriazole-1 -carboxamidines, benzotriazole-1 -... 

Unlike in the case of the preparation of indoles and benzofuranes, the synthesis of benzothiophenes from o-ethynyl-thiophenols is not known. A close analogy was reported by Larock, where phenylacetylene was coupled with 2-iodothioanisole. Ring closure of the formed o-ethynyl-sulfide was initiated by the addition of different electrophiles. The reaction led to the formation of the benzothiophene core bearing the electrophile in the... 

The stepwise reduction of the ethynyl complex Fe(C=CH)(dppe)(fj-C5H5) to the neopentyl derivative has been achieved by two sequences of methylation and hydride additions, illustrating the propensity for nucleophilic addition to C , and electrophilic addition to Cfi (25). The initial conversion to [Fe(=C=CMe2)(dppe)(i/-C5H5)]+ (5) has been described above (Section III,A,2) further reactions of this complex with Na[HB(OMe)3], Me30+, and NaBH4 afford the neopentyl complex ... 

The mechanism of this new ethynylation involves an addition-elimination sequence, probably promoted by the coordinatively unsaturated center (electrophilic assistance) and by mechanoactivation (grinding up the reactants with AI2O3) <2004TL6513, 2006RJ01348>. The intermediates of this reaction, 2-(l-bromo-2-acylethenyl)pyrroles 700, are isolated and converted upon AI2O3 to 2-ethynylpyrroles 699 (Scheme 136) <2006RJ01348>. 

Ethynyl carbinols (propargylic alcohols) such as 134 (Scheme 2.58) represent another important group of oxidation level 3 compounds. Their preparation involves nucleophilic addition of acetylides to the carbonyl group, a reaction that is nearly universal in its scope. Elimination of water from 134 followed by hydration of the triple bond is used as a convenient protocol for the preparation of various conjugated enones 135. Easily prepared O-acylated derivatives are extremely useful electrophiles in reactions with organocuprates, which proceed with propargyl-allenyl rearrangements to furnish allene derivatives 136. 

The vinyl derivative was converted to ethynyl-CTM by reaction with bromine followed by HBr elimination. The double bond of vinyl-CTM turned out to be much less active than that in the ferrocene analog as regards electrophilic addition. Also, the stabilization of an a-carbonium center, the phenomenon characteristic of ferrocenyl cation, was not found in the manganese compound (379). 

Alkynes and functionally substituted alkynes may be synthesized either from starting materials that do not already contain the triple bond by elimination reactions, or the ethynyl and alkynyl groups may be introduced onto substrates by nucleophilic or electrophilic substitution reactions. 

Silyl-substituted carbenium ions have attracted considerable experimental interest because they are believed to be intermediates in electrophilic additions to vinyl, ethynyl and aryl silanes, in solvolytic reactions and in cationic cyclization reactions1 -4,322. Despite this wide interest, knowledge concerning the effect of silyl substitution on the stabilities of carbenium ions was rather qualitative and only recently more quantitative data became available. Theoretical studies centered mainly around a- and / -silyl-substituted carbenium ions, but y-silyl effects have been also studied. 

The deprotonation of N—H bonds in diverse oxazoUdin-2-ones with KHMDS as the base followed by tbe treatment of tbe crade reaction mixtures with trimethylsilylethynyl iodonium triflate electrophiles afforded trimethylsilyl-terminated IV-ethynyl oxa-zolidinones in 50-60% yields (eq 74). Desilylation could be realized on the purified products or the crude reaction mixtures, and the alk)myl oxazolidinones were elaborated into novel stannyl enamines in the subsequent steps. In contrast, protocols employing -BuLi in toluene, or CS2CO3 in DMF gave yields lower than 20%. The procedure could be successfully applied to chiral oxazolidinones, since substitution at the C4 position of the oxazolidinones did not have a detrimental effect on reactivity. 

Asymmetric 2-tosylvinylation of Af-substituted proline esters (207) using ethynyl tolyl sulfone as an electrophile has been shown to produce (208) at <95% ee in the absence of any added base. The reaction is believed to proceed via the formation of A-2-tosylvinyl ammonium ylides. ... 

Transition Metal-catalyzed Coupling Reactions. TIPS-acetylene has been widely used for transition metal-catalyzed coupling reactions with a variety of aryl-X and vinyl-X (X = 1, Br, Cl, and OTf) compounds as useful electrophiles in the presence of proper cocatalysts or additives, leading to the corresponding ethynyl-functionalized derivatives. 

Formally, the ethynylation can be rationalized both as electrophilic substitution of hydrogen in the pyrrole ring and nucleophilic addition of the electron-rich pyrrole moiety to the electron-deficient triple bond. In both cases, the one-electron transfer from pyrrole to acetylenic fragment is plausible. This is confirmed by the fact that in the ESR spectrum of the reaction mixture, a singlet (g=2.0023, A//= 1.8 mT) is observed [522]. 

However, the reaction with aldehydes and ketones affords, along with allenic alcohols, their N-ethynyl isomers, the content of which in the products mixture can reach 60% (in the case of the reaction with paraform) (Scheme 2.224). The mixture composition is defined to a large extent by the reaction conditions and the electrophile nature. 

The asymmetric a-2-tosylethenylation of (5)-2-(pyrrolidin-l-yl)propanoic acid esters using ethynyl tolyl sulfone as an electrophile produces a good yield with high enantioselectivity (up to 96% ee) without the addition of any bases The reaction would proceed via the formation of a nonracemic ammonium enolate without an external source of chirality. 

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