Allenes and computational chemistry: from bonding situations to reaction mechanisms

Abstract

The present review is focused on the application of computational/theoretical methods to the wide and rich chemistry of allenes. Special emphasis is made on the interplay and synergy between experimental and computational methodologies, rather than on recent developments in methods and algorithms. Therefore, this review covers the state-of-the-art applications of computational chemistry to understand and rationalize the bonding situation and vast reactivity of allenes. Thus, the contents of this review span from the most fundamental studies on the equilibrium structure and chirality of allenes to recent advances in the study of complex reaction mechanisms involving allene derivatives in organic and organometallic chemistry.

Chirality transfer in metal-catalysed intermolecular addition reactions involving allenes

“…The second possibility is the η1-coordination, in which the metal would coordinate exclusively to the central carbon of the allene. 46 While in the η2-coordination the allene retains its chirality, the σ-allylic cation form 69 in the η1-coordination is a planar structure which would lose the chiral information of the allene, and therefore could explain the racemization. It is worth mentioning that the η1-coordinated bent allene 68 still retains the stereochemistry of the initial allene, so reactions in which this structure participates as an intermediate could still have high degrees of chirality transfer.…”

Cationic AuIII versus AuI: Catalyst‐Controlled Divergent Reactivity of Alkyne‐Tethered Lactams

“…Subsequent 7‐ exo ‐carboauration via TS1 (Δ G ≠ =17.8 kcal mol −1 ) leads to the tetracyclic species INT2 in an exergonic transformation (Δ G R =−6.8 kcal mol −1 ). This species is then transformed into INT3 through a Au−C bond protonolysis reaction assisted by the counteranion (Cl − or more likely, SbF 6 − ) following a reaction pathway similar to that described for strongly related processes . INT3 isomerizes then into the more stable (Δ G R =−4.8 kcal mol −1 ) endocyclic intermediate INT4 , which coordinates to the previously released catalyst [Au III ] + through the β‐lactam carbonyl group in a strongly exergonic transformation (Δ G R =−15.4 kcal mol −1 ).…”

“…A conventional 8‐ endo ‐ dig carbocyclization pathway followed by proto‐deauration for the cationic Au I ‐catalyzed generation of fused indoles 3 is outlined in Scheme S1 (Supporting Information) . However, the rationalization for the formation of bridged adducts 4 is more intriguing.…”

Can substituted allenes be highly efficient leaving groups in catalytic processes? A computational investigation

“…Thus, in the distorted allene structures having a bent angle, the symmetry point group is reduced from D 2d to C 2 . Indeed, as shown in recent studies, when the allenes are bent to a sufficient degree, they can be considered as systems with carbone character—systems that can bind strongly to the metal center . If one considers an allene ligand where the CCC angle is less acute, it may lead to a weak metal–ligand bond in the complex in comparison with the highly bent carbone ligand cases, because of the greater delocalization of electron density into the CCC backbone in such systems.…”

“…[16] Indeed, as shown in recent studies, when the allenes are bent to a sufficient degree, they can be considered as systems with carbone character-systems that can bind strongly to the metal center. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] If one considers an allene ligand where the C@C@C angle is less acute, it may lead to a weak metal-ligand bond in the complex in comparison with the highly bent carbone ligand cases, because of the greater delocalization of electron density into the C@C@C backbone in such systems. Therefore, such ligands can be used as leaving groups in catalytic processes where the dissociation of a labile ligand is necessary to generate the active catalytic center.…”