Ethynyl cation

Phenyl 2-(trimethylsilyl)ethynyl sulfone (118) can act as a vinyl cation synthon (equations 93 and 94)78 79. Thus, the reaction of enolates with 118 and subsequent desulfonylation of the adduct gives a-vinyl ketone, such as 119 and 120. 

The (alkenyl-ethynyl)gold(i) complexes shown in Scheme 13 were prepared with mono- and ditertiary phosphine ligands and used as substrates for the coordination of coinage metal cations. The products are strongly luminescent.60... 

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

TABLE 1. 13C-NMR Chemical Shifts of Tris(ethynyl)methyl Cations (49a-d) and Their Precursor Alcohols 50a-d°... 

An interesting chain elongation of 4-oxo-l-tributylstannylbutyl carbamate 83 by ethynyl A,A-diisopropyl carbamate (82) to the dicarbamate 84 and the subsequent intramolecular carbolithiation to form the alkenyllithium 86 has been reported very recently (equation 18). 86 was trapped by electrophiles to yield—as expected—both epimers 87 and 88 in essentially equal amounts. From an X-ray analysis of a derivative, the shown absolute configuration and the ( )-stereochemistiy at the double bond was concluded. Thus, the attack at the triple bond here took place in the anti fashion, which is quite unusual. We suggest that a lithium cation in 85, captured by the second carbamoyl group, catalyses the anti addition. ... 

In the preceding cyclization, one concern is the behavior of intermediate 24 if its benzyl cation is replaced with a tertiary cation to avoid forming a tertiary carbon, the 1,2-aryl migration of this intermediate would be unlikely to occur. When this cyclization was extended to 2,2-dimethyl(o-ethynyl)styrene 26, the 2-alkenyl-lH-indene product 28 [16] was obtained in 76% yield, as depicted in Scheme 6.14. In this transformation, the alkenyl double bond of the 3,5-dien-l-ynes is cleaved and a... 

Ethanolysis of the a-ethynyl-substituted vinyl triflates 93-95 proceeds via the intermediacy of the cumulenic dicoordinated cations 96-98 (equation 17) and suggests the following stability order for the carbocations 98 > 96 > 9745. The same relative reactivity order was found for the triflates 99-10145. 

Bachrach, S. M. Structure, deprotonation energy, and cation affinity of an ethynyl-expanded cubane, 7. Phys. Chem. A 2003,107,4957-4961. 

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). 

The solvolysis product of alkyne-substituted 1-ethynyl cyclopropyl tosylates (6) appeared to be strongly dependent upon the nature of the substituent R of the triple bond. So, stabilization of the intermediate cyclopropyl cation (7), by delocalization of the positive charge over the three carbons of the mesomeric propargyl-allenyl system, entails a powerful electron-releasing substituent R effectively, when R = p-anisyl, the ring-opening of the cyclopropyl cation was totally suppressed (equation 5). ... 

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. 

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