Carbenium ions alkynyl

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

Lockwood, R. F., Nicholas, K. M. Transition metal-stabilized carbenium ions as synthetic intermediates. I. a-[(Alkynyl)dicobalt hexacarbonyl] carbenium ions as propargylating agents. Tetrahedron Lett. 1977, 4163-4166. 

The parameters E and N can be used to compare the reactivity of compounds and even to predict the possibility that a reaction occurs. The parameter E represents the electrophiUcity parameter, which is defined by the use of a reference electrophilic compound. The values of E for many carbenium ions have been determined [6]. Contrary to common sense, which considers the carbenium ion to be intractable, it is possible to classify several carbenium ions that differ in elec-trophihdty by more than 16 orders of magnitude. Their stability is strictly connected to the presence of aryl, alkenyl, or alkynyl groups directly connected to the carbenium ionic center. For these reasons, sometimes the phrase n activated alcohol [7] is encountered for this type of compounds. The tables compiled by Mayr (Figure 26.2) offers a more readable and precise definition for the stability and reactivity of carbenium ions. Carbenium compounds with E < 0 (benzhy-dryhum ion) are almost impossible to isolate and to store without their rapid decomposition [8]. 

---