Ligand‐Based Control of Single‐Site vs. Multi‐Site Reactivity by a Trichromium Cluster

The trichromium cluster (^tbsL)Cr₃(thf) ([^tbsL]^6?=[1,3,5‐C₆H₉(NC₆H₄‐o‐NSi^tBuMe₂)₃]^6?) exhibits steric‐ and solvation‐controlled reactivity with organic azides to form three distinct products: reaction of (^tbsL)Cr₃(thf) with benzyl azide forms a symmetrized bridging imido complex (^tbsL)Cr₃(μ³‐NBn); reaction with mesityl azide in benzene affords a terminally bound imido complex (^tbsL)Cr₃(μ¹‐NMes); whereas the reaction with mesityl azide in THF leads to terminal N‐atom excision from the azide to yield the nitride complex (^tbsL)Cr₃(μ³‐N). The reactivity of this complex demonstrates the ability of the cluster‐templating ligand to produce a well‐defined polynuclear transition metal cluster that can access distinct single‐site and cooperative reactivity controlled by either substrate steric demands or reaction media.     

EBC‐219: A New Diterpene Skeleton,Crotinsulidane, from the Australian Rainforest Containing a Bridgehead Double Bond

EBC‐219 ( 4 ), isolated from Croton insularis (Baill), was established by spectroscopic and DFT methods as the first member of a new diterpene skeletal class, uniquely defined by the presence of a bicyclo[10.2.1] bridgehead olefin. The proposed biogenetic pathway to 4 from the co‐isolated natural products EBC‐131 ( 1 ), EBC‐180 ( 2 ) and EBC‐181 ( 3 ) is highly likely. EBC‐180 ( 2 ) and EBC‐181 ( 3 ) showed moderate to strong cytotoxic activity against various cancer cell lines.     

Synthesis of Alkynylmethylidene‐benzoxasiloles through a Rhodium‐Catalyzed Cycloisomerization Involving 1,2‐Silicon and 1,3‐Carbon Migration

It is shown that a cationic rhodium(I)/biphep complex catalyzes the cycloisomerization of 2‐(alkynylsilylethynyl)phenols, leading to alkynylmethylidene‐benzoxasiloles through concomitant silicon and carbon migration. This unprecedented cycloisomerization presumably proceeds via the formation of rhodium vinylidenes through 1,2‐silicon migration, followed by 1,3‐carbon (alkyne) migration via the formation of hypervalent silicon centers.     

Insertion of CO2 Mediated by a (Xantphos)NiI–Alkyl Species

The incorporation of CO₂ into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of Ni^II to Ni^I by either Zn or Mn, followed by CO₂ insertion into Ni^I‐alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu‐Xantphos)Ni^IIBr₂ by Zn affords Ni^I species. (tBu‐Xantphos)Ni^I‐Me and (tBu‐Xantphos)Ni^I‐Et complexes undergo fast insertion of CO₂ at 22 °C. The substantially faster rate, relative to that of Ni^II complexes, serves as the long‐sought‐after experimental support for the proposed mechanisms of Ni‐catalyzed carboxylation reactions.