As a complement to Pd(0)-catalyzed cyclizations, seven Pd(II)-catalyzed cyclization strategies are reported. α,ω-Diynes are selectively hydroborated to bis(boronate esters), which cyclize under Pd(II)-catalysis producing a diverse array of small, medium, and macrocyclic polyenes with controlled E,E, Z,Z, or E,Z stereochemistry. Various functional groups are tolerated including aryl bromides, and applications are illustrated.

A new triarylaminium radical cation promoted coupling of catharanthine with vindoline is disclosed, enlisting tris(4-bromophenyl)aminium hexachlororantimonate (BAHA, 1.1 equiv) in aqueous 0.05 N HCl/trifluoroethanol (1−10:1) at room temperature (25 °C), that provides anhydrovinblastine in superb yield (85%) with complete control of the newly formed quaternary C16′ stereochemistry. A definition of the scope of aromatic substrates that participate with catharanthine in the BAHA-mediated diastereoselective coupling reaction and simplified indole substrates other than catharanthine that participate in the reaction are disclosed that identify the key structural features required for participation in the reaction, providing a generalized indole functionalization reaction that bears little structural relationship to catharanthine or vindoline.

Syntheses of strained cyclic dienes were accomplished via palladium(II)-catalyzed oxidative cyclizations of terminal bis(vinylboronate esters). The reactions generate strained (E,E)-1,3-dienes that undergo spontaneous 4π-electrocyclizations to form bicyclic cyclobutenes. Formation of the cyclobutenes is driven by the strain in the medium-ring (E,E)-1,3-diene intermediate. Thermal ring openings of the cyclobutenes give (Z,Z)-1,3-diene products, again for thermodynamic reasons. DFT calculations verified the thermodynamic versus kinetic control of the reactions, and kinetic studies are in excellent agreement with the calculated energy changes. An extension of the tandem coupling/4π-electrocyclization pathway was demonstrated by a palladium(II)-catalyzed oxidative homocoupling/8π-electrocyclization cascade.

b S Supporting Information W hile radical polymerization techniques are well suited for polar monomers, they are not used to polymerize simple R-olefins as the allylic hydrogens on the latter are subject to chain transfer in the presence of a reactive propagating radical. 1 Thus, with the exception of ethylene produced commercially at very high pressures and temperatures (and a few reports of propylene under similar intense conditions 2,3 ), only short oligomers of simple alkenes such as propylene or 1-hexene can be obtained using conventional radical polymerization techniques. Against this backdrop, our group recently reported that a variety of simple alkenes could, in fact, be polymerized under radical conditions at ambient temperatures and pressures in the presence of a lithium cation in the form of LiCB 11 (CH 3 ) 12 . 4 By all indications, including the use of radical and ionic traps, the polymerization proceeds as a radical process. We believe that Li þ coordination to the alkene in weakly ligating organic media alters the relative rates of propagation and degradative chain transfer in favor of propagation (Scheme 1).Following these initial reports, we began investigating this system more fully with isobutylene, whose six allylic hydrogens render it a most unlikely candidate for radical polymerization. 5 This system proved quite unique, as it appears that both a radical process and a cationic one can be induced simultaneously. Under nonoxidative conditions, the radical process produced a polyisobutylene (b-PIB) with a highly branched architecture, unlike anything previously reported or prepared commercially. Under oxidative conditions, the radical process still occurred and produced b-PIB, while a concurrent cationic one generated perfectly linear l-PIB that differed from ordinary PIB only in that a carborate anion was covalently attached at the chain end. 6 Under these conditions, LiCB 11 (CH 3 ) 12 acted not only as a catalyst but also as a reagent and could not be recovered from the reaction mixture quantitatively or at all. A concurrent study of LiCB 11 (CH 3 ) 12 -catalyzed radical polymerization of 1-hexene and 1-octene has so far only revealed the formation of low molecular weight oligomers. 7 The contrast to the polymerization of propene described below is striking, and other reaction conditions are still being examined.Here, we report on our investigation of the nature and architecture of polypropylene (PP) that is generated under nonoxidizing conditions in the weakly ligating solvent 1,2-dichloroethane (DCE), using azo-tert-butane (ATB) initiator. Figure 1 illustrates the 13 C NMR spectrum of a sample of b-PP generated at moderate pressures (15 atm) and temperatures (80°C) in DCE solvent with ATB initiation under LiCB 11 (CH 3 ) 12 catalysis (for precise experimental conditions see Supporting Information). Polymer yield in this reaction was around 40%. The b-PP formed is very highly branched, as can be seen by the plethora of resonances observed in the spectrum between 10 and 45 ppm, which is unli...

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentenones through the efficient [2 + 2 + 1] cycloaddition of dicobalt alkyne complexes with alkenes. While intermolecular and intramolecular variants are widely known, transannular versions of this reaction are unknown and the basis of this study. Macrocyclic enyne and dienyne complexes were readily synthesized by palladium(II)-catalyzed oxidative macrocyclizations of bis(vinyl boronate esters) or ring-closing metathesis reactions followed by complexation with dicobalt octacarbonyl. Several reaction modalities of these macrocyclic complexes were uncovered. In addition to the first successful transannular Pauson-Khand reactions, other intermolecular and transannular cycloaddition reactions included intermolecular Pauson-Khand reactions, transannular [4 + 2] cycloaddition reactions, intermolecular [2 + 2 + 2] cycloaddition reactions, and intermolecular [2 + 2 + 1 + 1] cycloaddition reactions. The structural and reaction requirements for each process are presented.

No abstract