Mechanism of the Platinum(II)-Catalyzed Hydroamination of 4-Pentenylamines

Abstract

The mechanism of the platinum(II)-catalyzed intramolecular hydroamination of benzyl 4-pentenylamines has been evaluated under stoichiometric and catalytic conditions. Reaction of a benzyl 2,2-disubstituted 4-pentenylamine with [(PPh 3 )Pt(μ-Cl)Cl] 2 forms a thermally sensitive platinum amine complex that undergoes irreversible, intramolecular ligand exchange with the pendant CC bond to form a reactive platinum π-alkene complex. The π-alkene complex undergoes rapid, outer-sphere C−N bond formation, evidenced by the anti addition of Pt and N across the complexed CC bond, to form a thermally stable zwitterionic platinamethylpyrrolidinium complex. The zwitterionic complex is rapidly and exergonically deprotonated by free amine to form a neutral, bicyclic azaplatinacyclobutane complex that likely exists as a discrete 1:1 adduct with ammonium salt in the nonpolar reaction medium and that represents the resting state of the catalytic cycle. Turnover-limiting intramolecular protodemetalation of the azaplatinacyclobutane−ammonium adduct followed by ligand exchange releases the 2-methylpyrrolidine product.

Mechanism of the Stereomutation of an Azaplatinacyclobutane Complex

“…We have reported the mechanism of the Pt­(II)-catalyzed intramolecular hydroamination of γ-alkenyl benzyl amine 1 to form pyrrolidine 2 (Scheme ). These investigations established a mechanism involving the initial formation of an N-bound 4-pentylamine complex 3 , followed by irreversible, intramolecular ligand exchange to generate the unobserved platinum π-alkene complex 4 , which is trapped via outer-sphere C–N bond formation to generate the zwitterionic platinamethylpyrrolidinium complex 5 . Complex 5 undergoes N-deprotonation followed by cyclization to form the azaplatinacyclobutane complex 6 , which retains HNR 3 Cl released in the formation of 6 , presumably as a hydrogen-bonded adduct and which undergoes turnover-limiting intramolecular protodemetallation to release pyrrolidine 2 .…”

“…The modest KIE resulting from deuteration of the Cβ atom ( k H / k D = 1.32), the modest entropy of activation (Δ S ‡ = 6 ± 6 eu), and the observed increase in the rate of stereomutation with increasing solvent polarity were consistent with a mechanism for stereomutation involving rapid and reversible amine dissociation, reversible β-hydride elimination, and rate-limiting rotation about the Cα–Cβ bond of the resulting platinum enamine complex. These results reveal additional complexity in the previously established mechanism of platinum-catalyzed hydroamination of 1 . In particular, because the energy barrier for stereomutation (Δ G 303K ‡ = 24 kcal mol –1 ) is significantly lower than the barrier for protodeauration of 6 under catalytic conditions (Δ G 393K ‡ = 27 kcal mol –1 ), stereomutation of 6 via the pathway depicted in Scheme would presumably occur prior to turnover-limiting protodeauration .…”

“…The challenges associated with late transition metal-catalyzed hydroamination of alkenes with alkyl amines stems in part from the high basicity and nucleophilicity of alkyl amines vis-a-vis carboxamide derivatives, which introduce challenges including alkene displacement by amine and slow protodemetallation of metal alkyl intermediates. Another ramificiation of the nucleophilicity of alkyl amines is the strong tendency of the β-amino alkyl species generated via inner- or outer-sphere C–N bond formation to cyclize to form azametallacyclobutane complexes. , Indeed, late transition metal azametallacyclobutane complexes have been generated via the stoichiometric reactions of amines with metal olefin complexes − and have been directly observed or invoked as intermediates in the late transition metal-catalyzed hydroamination of alkenes with aliphatic amines. − Nevertheless, the behavior of these azametallacyclobutane complexes under catalytic conditions has not been fully delineated.…”

Development of highly efficient platinum catalysts for hydroalkoxylation and hydroamination of unactivated alkenes

“…(1) Anionic systems, such as the famous Zeise’s salt (K[PtCl 3 (C 2 H 4 )]·H 2 O) and Chojnacki’s salt (K[PtBr 3 (C 2 H 4 )]). (2) Neutral systems 33 , 34 , such as [PtCl 2 (C 2 H 4 ) 2 ] 2 that have been introduced by Widenhoefer 4 , 19 , 33 , 34 . (3) Cationic systems, with diamine 35 – 38 , bisphosphine 39 , 40 , carbene 41 – 43 or pincer ligands 44 – 46 ; in particular, Gagné discovered a series of elegant cationic Pt(II) complexes, such as (PPP)Pt 2+ and (PP)PPt 2+ , which efficiently inhibit β -H elimination as a competing decomposition pathway 31 , 40 , 46 .…”

Nuclear Magnetic Resonance and Computational Study of trans-(μ2:η2,η2-1,3-Butadiene)bis(trichloroplatinate(II))

“…For example, palladium mediates many commercial applications of the use of alkenes, while platinum was involved in very early examples of C–H activation chemistry and perhaps its most important catalytic application is in hydrosilylation chemistry, where mechanistic questions still remain . Other, more contemporary areas of platinum alkene catalysis relate to subjects such as hydroamination and cyclization reactions . As such and in the context of the utility of third-row congeners to provide mechanistic pointers, continued insights into aspects of stability and reactivity remain valuable and are of direct interest to the wide research community in the area.…”