PtCl₂-Catalyzed Rearrangement of Methylenecyclopropanes

Abstract

Alkylidenecyclopropanes readily convert into cyclobutene derivatives on treatment with catalytic amounts of PtCl2. The reaction is strongly accelerated when performed under an atmosphere of CO (1 atm). The resulting cyclobutenes are isolated in good to excellent yields for substrates bearing aliphatic as well as aromatic substituents R on their olefinic site. If the substituent R, however, is a very electron-rich arene, the cyclobutenes initially formed react further to give dimeric products with a previously unknown 1,2,2a,7a-tetrahydrospiro[cyclobuta[a]indene-7,1'-cyclobutane skeleton. A mechanism accounting for these experimental observations as well as for a deuterium-labeling experiment is proposed which implies reactive intermediates at the nonclassical cation/carbene interface. Furthermore it is shown that the PtCl2-catalyzed cyclobutene formation can be geared with subsequent ring-opening/ring-closing metathesis (ROM/RCM) events. Finally, a convenient "one pot" method for the preparation of the alkylidenecyclopropane substrates used in this study is presented, which is based on a modified Julia-Kocienski olefination of aldehydes with readily available 1-tert-butyl-1H-tetrazol-5-yl-cyclopropyl sulfone under Barbier conditions.

Gold Vinylidene Complexes: Intermolecular C(sp3)H Insertions and Cyclopropanations Pathways

“…The methylenecyclopropane IV then undergoes a gold-catalyzed ring-expansion cascade. The rearrangement of methylenecyclopropanes to cyclobutenes catalyzed by platinum [11] or palladium [12] were recently published. Upon coordination of the gold to the highly activated double bond of III, a cyclobutane carbene intermediate V is formed, which after a shift of the vinyl group delivers the molecular skeleton of the product.…”

Gold‐Catalyzed Intermolecular Anti‐Markovnikov Hydroamination of Alkylidenecyclopropanes

“…Regarding the anti-Markovnikov regioselectivity of goldcatalyzed ACP hydroamination, we initially considered that the gold p-ACP bond might be polarized toward a cyclopropylcarbinyl cation-like structure (structures c and d, Figure 1), as has been invoked to account for Pt II - [25] or Pd II -catalyzed [26] ACP-to-cyclobutene isomerization and for the stoichiometric conversion of ACPs into Ru and Os cyclobutylidene complexes. [27] To evaluate this hypothesis, we generated the thermally unstable gold p-ACP complex [(P1)Au(h 2 -1 a)] + SbF 6 À (6 a) as an equilibrium mixture (~25:1) of trans/cis isomers from reaction of 1 a with a 1:1 mixture of AgSbF 6 and (P1)AuCl at À80 8C to À40 8C [Eq.…”

Katalytische carbophile Aktivierung: Platin‐ und Gold‐π‐Säuren als Katalysatoren

“…[122,123] Wegen der zumeist höheren Reaktivität von Allenen ist es nicht verwunderlich, dass diese sich in Gegenwart p-saurer Katalysatoren ebenfalls effizient umsetzen lassen. Neben den üblichen Gold-und Platinverbindungen erwiesen sich auch Silbersalze als hoch effektiv.…”