Alkynes as Synthetic Equivalents of Ketones and Aldehydes: A Hidden Entry into Carbonyl Chemistry

Abstract

The high energy packed in alkyne functional group makes alkyne reactions highly thermodynamically favorable and generally irreversible. Furthermore, the presence of two orthogonal π-bonds that can be manipulated separately enables flexible synthetic cascades stemming from alkynes. Behind these “obvious” traits, there are other more subtle, often concealed aspects of this functional group’s appeal. This review is focused on yet another interesting but underappreciated alkyne feature: the fact that the CC alkyne unit has the same oxidation state as the -CH2C(O)- unit of a typical carbonyl compound. Thus, “classic carbonyl chemistry” can be accessed through alkynes, and new transformations can be engineered by unmasking the hidden carbonyl nature of alkynes. The goal of this review is to illustrate the advantages of using alkynes as an entry point to carbonyl reactions while highlighting reports from the literature where, sometimes without full appreciation, the concept of using alkynes as a hidden entry into carbonyl chemistry has been applied.

1,5,7‐Triazabicyclo[4.4.0]dec‐5‐ene Enhances Activity of Peroxide Intermediates in Phosphine‐Free α‐Hydroxylation of Ketones

“…Being slightly endothermic (3.9 kcal mol −1 ), the α‐proton transfer for the substrate was reversible. Here the stabilization of enol by TBD was quite important to the whole reaction, because it led to a much lower thermodynamic penalty from ketone to enol, which has been found to be about 10 kcal mol −1 and was one of the key kinetic obstacles for this type of transformation [24] . After another endothermic process (1.4 kcal mol −1 ), there formed an enol intermediate with TBD combined at the O‐H group.…”

1,5,7‐Triazabicyclo[4.4.0]dec‐5‐ene Enhances Activity of Peroxide Intermediates in Phosphine‐Free α‐Hydroxylation of Ketones

“…Being slightly endothermic (3.9 kcal mol À1 ), the a-proton transfer for the substrate was reversible.H ere the stabilization of enol by TBD was quite important to the whole reaction, because it led to am uch lower thermodynamic penalty from ketone to enol, which has been found to be about 10 kcal mol À1 and was one of the key kinetic obstacles for this type of transformation. [24] After another endothermic process (1.4 kcal mol À1 ), there formed an enol intermediate with TBD combined at the O-H group. Theattack of triplet dioxygen on the enol intermediate should go through as pin transition process.T hus,w ep erformed aMECP (minimum energy crossing point) calculation to find the singlet-triplet crossing point, and obtained as tructure with lower energy than that of the triplet transition state.…”

Twofold π-Extension of Polyarenes via Double and Triple Radical Alkyne peri-Annulations: Radical Cascades Converging on the Same Aromatic Core

“…The high energy content and carbon-richness of alkynes makes them a privileged functional group within this reaction toolset. [32][33][34][35][36][37][38][39][40][41][42][43][44][45] Yet, most of the available examples of alkynes being used in the preparation of these carbon-rich molecules concentrate on extension at the armchair edge of the polycyclic framework, while alkyne annulations at the zigzag edge remain remarkably scarce (Scheme 1). 46,47 Scheme 1.…”