Mechanistic Studies of the Hydroamination of Norbornene with Electrophilic Platinum Complexes: The Role of Proton Transfer

Abstract

Hydroaminations of norbornene with arylsulfonamides and weakly basic anilines were achieved using electrophilic Pt(II) bis(triflate) complexes of the type L2Pt(OTf)2 (L2 = (t)Bu2bpy, (t)BuC6H4N== C(CH3)C(CH3)==NC6H4(t)Bu, (C6H5)2PCH2CH2P(C6H5)2, (C6F5)2PCH2CH2P(C6F5)2, S-BINAP). Pseudo-first-order kinetics reveal little to no dependence of the reaction rate on the ancillary ligand. Mechanistic studies do not favor an olefin coordination mechanism but are instead consistent with a mechanism involving sulfonamide coordination and generation of an acidic proton that is transferred to the norbornene. It is postulated that the resulting norbornyl cation is then attacked by free sulfonamide, and loss of proton from this adduct completes the hydroamination. The platinum-sulfonamide complex readily undergoes deprotonation to give a mu-amido platinum-bridged dimer that was isolated from the reaction solution. These studies also involve use of Me3SiPh and Me3SnPh as non-nucleophilic proton traps. Cleavage of the Ph-E bonds was used to detect the acidic, catalytically active species.

Stereoselectivity in the Rhodium‐Catalysed Reductions of Non‐Conjugated Dienes

“…The second double bond is also hydrogenated cleanly and stereoselectively to the cycloalkane 11, but interpretation of the results proved to be challenging, since the NMR spectra, both for 1 H and 2 H addition experiments, were more complex than expected. Chemical shift separation in the proton spectrum is small, and it required analysis at 700 MHz assisted by 13 C J-correlation in order to assign all the signals with confidence.…”

“…For reduction of the cyclopentene double bond, at least six isotopomers can be identified by analysis of C4 of the 13 C NMR spectrum. The presence of 3,5-dideuterated species (6%) as well as small quantities of both 2-and 3-monodeuterated species requires H-isomerisation in an intermediate, as well as H/D exchange with the reagent.…”

“…The endo-protons at C5 and C9 remain intact, and are clearly visible in the Hydrogenation of the homologous dienone 15 is even slower than that of dienone 13, ultimately giving the saturated ketone 16. [45] Again, full assignment of the 1 H and 13 C NMR spectra of the final product was carried out, with the necessary aid of COSY, NOESY, HMBC and HSQC spectra. This sequence located and identified the six methylene carbons and associated proton pairs, including the anomalous carbonyl-induced deshielding of endo-H10, observed at 1.58 ppm in CD 3 OD.…”

“…Related reactions with NBE were exo-selective. [7] Hydroamidation, [8] catalytic or stoichiometric hydroboration, [9] asymmetric hydrosilylation, [10] hydroamination with iridium or rhodium, [11] gold, [12] or platinum complexes, [13] C À H addition of terminal alkynes, [14] and catalysed alkyne cycloaddition, [15] and catalytic annulation, [16] of NBE are all exo-selective. Several of these reactions give exo-specific addition with norbornadiene as well.…”

Regio‐ and Enantioselective Hydroamination of Dienes by Gold(I)/Menthol Cooperative Catalysis

“…Moreover, Tilley et al proposed that the catalyst in the related platinumcatalyzed reaction is a platinum sulfonamide complex derived from coordination of the Lewis-acidic metal to the relatively acidic sulfonamide nucleophile. [8] On the basis of these reports, we envisioned that simple alcohols might serve an analogous role in Lewis acid activated Brønsted acid catalyzed [9,10] processes with gold(I) complexes playing the role of the Lewis acid. Herein, we report the application of this hypothesis to the development of an enantioselective hydroamination of 1,3-dienes.…”