Rhodium-Catalyzed Hydroformylation of Ketal-Masked β-Isophorone: Computational Explanation for the Unexpected Reaction Evolution of the Tertiary Rh-Alkyl via an Exocyclic β-Elimination Derivative

Abstract

Ketal-masked β-isophorone (7,9,9-trimethyl-1,4-dioxaspiro[4.5]dec-7-ene) is an interesting case study of Rh-catalyzed hydroformylations not only for the competition between secondary and tertiary rhodium alkyls but also for the unexpected isomerization of the tertiary Rh-alkyl to the exocyclic olefin which undergoes hydroformylation, yielding the acetaldehyde derivative (2) of 7,9,9-trimethyl-1,4-dioxaspiro[4.5]decane. Although experimental results at 100 °C pointed to reaction reversibility, the reason for this kind of behavior was however obscure. A thorough density functional theory (DFT) computational investigation of the various transition states (TS) and intermediates along the reaction pathways making use of B3P86 hybrid functionals and the 6-31G* basis set, coupled to effective core potentials for Rh in the LanL2DZ valence basis set, has been carried out to shed some light on the reaction mechanism. The TS barrier heights, based on alkyl-Rh TS free energies, computed under the hypothesis of nonreversibility were in favor of a normal hydroformylation reaction (III:II = 70:30). While the endocyclic olefins produced skew or twisted arrangements of the six-membered ring similarly to the CO insertion TS that can be even higher than the alkyl-Rh ones, grid-point calculations during the potential energy surface (PES) scan produced the much more stable chair conformation for the exocyclic olefin complex. The relevant TS were found to be very favorable as well, thus explaining the preference for the exocyclic arrangement of the tertiary intermediate, for which the reaction is therefore entirely reversible and invariably proceeds to the acetaldehyde derivative (2). Conversely for the secondary isomers, the reaction is only partially reversible, thus enriching the tertiary fraction and producing the secondary aldehyde (3) in a very limited amount.

Usingparahydrogen induced polarization to study steps in the hydroformylation reaction

“…Mechanistic studies underpin this process and normally involve a combination 1-3 of chemical, [4][5][6][7] analytical, [8][9][10][11][12] and theoretical work. [13][14][15][16][17][18][19] In situ NMR methods are gaining in importance 1, [20][21][22][23][24] in this regard despite the techniques inherent low sensitivity. This change is a result of improved hardware, 25 novel in situ high pressure methods 20,21,26 and in situ photochemical approaches, 27,28 with hyperpolarization methods in the solid and solution phases adding a further level of refinement.…”

Selective production of linear α-olefins via catalytic deoxygenation of fatty acids and derivatives

“…49,60,[62][63] Pd has shown higher activity, but Rh on the other hand has not required a large excess of ligand to ensure activity and selectivity and is thus attractive from an atom-economic point of view. 49,57,60 Rh has also been used in dehydroformylation and other similar decarbonylation reactions, 49,[142][143][144][145] and it is therefore surprising that few have further explored this metal for use in deoxygenation of fatty acids. 75 As mentioned before, this development may have been stifled because no computational investigation of Rh as catalyst for decarbonylative dehydration, in comparison to Pd and Ni, has been present.…”