A DFT‐Based Theoretical Investigation of the Mechanism of the PtCl₂‐Mediated Cycloisomerization of Allenynes

Abstract

The mechanism of Pt(II)-catalyzed intramolecular cycloisomerization of allenyne systems has been extensively investigated by DFT calculations. Different mechanistic schemes have been proposed and discussed, including the Alder-ene reaction. The free energy results suggest that the kinetically preferred reaction pathway for precursors that are tri- and tetrasubstituted on the allene moiety should proceed by a five-step mechanism. This would involve formation of a platina(IV)cyclopentene intermediate by selective engagement of the external pi bond of the allene, which would undergo regioselective beta-H elimination from the equatorially disposed methyl group. A metal-induced H migration leads to a second octahedral Pt(IV)-chelate complex, which would yield the expected bicyclic system through an intramolecular migratory insertion step. Therefore, depending on the conformation of the initial eta(4)-reactant complex for trisubstituted patterns, two possible intermediates can be formed that would evolve through different paths. In these cases, the regio- and stereochemical outcomes predicted by the mechanistic scheme proposed agree with experimental data. Substituted precursors on the alkyne moiety follow a distinct, four-step, mechanism also involving an oxidative cyclometalation process to an octahedral Pt(IV) intermediate complex. Theoretical results reveal the kinetic preference for beta-H elimination from the allylic group rather than from the gem-dimethyl group, which should account for the observed regioselectivity.

The Role of Water in Platinum‐Catalyzed Cycloisomerization of 1,6‐Enynes: A Combined Experimental and Theoretical Gas Phase Study

“…[3] Instead, the formation of a linear (Au I ) or tetracoordinate (Au III ) alkyne complex A takes place, followed by the nucleophilic attack of the double bond in an anti fashion to give a cyclopropyl carbene D. [4] Among the possible modes of evolution of D, the rearrangement into G and then H is typi-cal. [6] However, based on our calculations, this pathway can be ruled out with PtCl 2 and 1,6-enynes because of the very high barrier to oxidative cyclization (DG°2 98 (B!E) = 41.6 kcal mol À1 ) [7] compared to the direct anti addition of the double bond to the alkyne (DG°2 98 (A!D) = 10.2 kcal mol À1 ), [8] yet the chelate that may form between PtCl 2 and the double and the triple bond of the enyne system is by far the most stable complex when compared to single coordination at the triple bond (DG 298 (B!A) = 21.3 kcal mol À1 ) or to the double bond (DG 298 (B!C) = 15.9 kcal mol À1 ). [6] However, based on our calculations, this pathway can be ruled out with PtCl 2 and 1,6-enynes because of the very high barrier to oxidative cyclization (DG°2 98 (B!E) = 41.6 kcal mol À1 ) [7] compared to the direct anti addition of the double bond to the alkyne (DG°2 98 (A!D) = 10.2 kcal mol À1 ), [8] yet the chelate that may form between PtCl 2 and the double and the triple bond of the enyne system is by far the most stable complex when compared to single coordination at the triple bond (DG 298 (B!A) = 21.3 kcal mol À1 ) or to the double bond (DG 298 (B!C) = 15.9 kcal mol À1 ).…”

“…[5] On the other hand, metallacycles are frequently invoked with platinum, because both Pt II and Pt IV can give rise to tetracoordinate species. [6] However, based on our calculations, this pathway can be ruled out with PtCl 2 and 1,6-enynes because of the very high barrier to oxidative cyclization (DG°2 98 (B!E) = 41.6 kcal mol À1 ) [7] compared to the direct anti addition of the double bond to the alkyne (DG°2 98 (A!D) = 10.2 kcal mol À1 ), [8] yet the chelate that may form between PtCl 2 and the double and the triple bond of the enyne system is by far the most stable complex when compared to single coordination at the triple bond (DG 298 (B!A) = 21.3 kcal mol À1 ) or to the double bond (DG 298 (B!C) = 15.9 kcal mol À1 ). To explain the ease by which the enyne cycloisomerization proceeds, Echavarren and coworkers.…”

Gold(I)‐ and Gold(III)‐Catalyzed Cycloisomerization of Allenynes: A Remarkable Halide Effect

“…This suggestion was later supported by the DFT calculations of Soriano and Marco-Contelles. [9] Nevertheless, some new data did not completely fit with this mechanistic rationale. For example, we noticed that the formation of hydrindiene 2 a from 1 a could also be promoted by PtCl 4 ( Table 1, entry 2) in a more rapid reaction than that with PtCl 2 ; this result would imply an unlikely Pt IV -Pt VI catalytic couple if a platinacyclopentene pathway were involved (Table 1, entry 2).…”

Theoretical Insight into PtCl2-Catalyzed Isomerization of Cyclopropenes to Allenes

“…38 This structure features octahedral coordination of the ligands with trans disposition of the chlorine ligands. According to the experiment by Lee et al, 6 for the allene skeleton with the connection of the phenylethyl group to the original C2 atom to remain unchanged, IM8 has to undergo C3 migration from the C2 atom to the C1 atom via TS12 with an energy demand of 36.3 kcal/mol, leading to IM9, a formal five-membered-ring configuration.…”