Photochemical reaction of W(CO)6 with GeCl4 as a source of germyl and germylene compounds acting as initiators for ring-opening metathesis polymerization of norbornene

Photolysis of W(CO)₆ in a solution of n-heptane containing GeCl₄ leads to the formation of two seven-coordinate compounds with direct WGe bonds: [(CO)₄W(μ-Cl)₃W(GeCl₃)(CO)₃] (1) and [(μ-GeCl₂){W(CO)₅}₂] (2). The molecular structures of these two tungsten-germanium compounds have been established by single-crystal X-ray diffraction studies. The germyl compound 1 is a previously synthesized binuclear complex of tungsten(II) with a d⁴ electronic configuration, while in the new germylene compound 2 the tungsten atom is formally in the zero oxidation state with a d⁶ electronic configuration and a direct WW bond. In an attempt to establish whether compounds 1 and 2 could be used as precatalysts for the ring-opening metathesis polymerization (ROMP) of cyclic olefins, their reactivity towards norbornene has been studied. In chloroform-d₁ solution the ROMP reaction is accompanied by the formation of a new olefin, 2,2′-binorbornylidene (bi-(NBE)), as a result of carbene-carbene coupling. In reaction of norbornene carried out in benzene solution the ROMP reaction is accompanied by the formation of 2-phenylnorbornane (hydroarylation product) and small amounts of bi-(NBE). In general, in the presence of olefin, compound 2 undergoes rearrangement to give an olefin compound of the type [(WCO)₅(η²-olefin)].     

Atroposelective Synthesis of C−N Vinylindole Atropisomers by Palladium-Catalyzed Asymmetric Hydroarylation of 1-Alkynylindoles

Transition-metal-catalyzed hydroarylation of unsymmetrical internal alkynes remains challenging because of the difficulty in controlling regioselectivity and stereoselectivity. Moreover, the enantioselective hydroarylation of alkynes using organoboron reagents has not been reported. Herein, we report for the first time that palladium compounds can catalyze the hydroarylation of 1-alkynylindoles with organoborons for the synthesis of chiral C−N atropisomers. A series of rarely reported vinylindole atropisomers was synthesized with excellent regio-, stereo- (Z-selectivity), and enantioselectivity under mild reaction conditions. The ready availability of organoborons and alkynes and the simplicity, high stereoselectivity, and good functional group tolerance of this catalytic system make it highly attractive.