We report the first example of a traceless directing group in a radical cascade. The chemo- and regioselectivity of the initial attack in skipped oligoalkynes is controlled by propargyl OR moiety. Radical translocations lead to the boomerang return of the radical center to the site of initial attack where it assists the elimination of the directing functionality via β-scission in the last step of the cascade. The Bu3Sn moiety continues further via facile reactions with electrophiles as well as Stille and Suzuki cross-coupling reactions. This selective radical transformation opens a new approach for the controlled transformation of skipped oligoalkynes into polycyclic ribbons of tunable dimensions.
Interaction between donor and acceptor groups incorporated in the backbone of cycloalkynes can be partially disrupted by twisting. The additional electronic energy accumulated as a result of this disruption can be harvested in the alkyne/ azide cycloaddition transition state, where an optimal conjugation pattern at a remote location is restored. The design provides electronically activated cyclodecynes that approach ''click'' reactivity of cyclooctynes. In addition, the twisted cyclodecynes are chiral and thus add axial chirality to the toolbox of properties introduced via ''click'' chemistry.
Direct evidence for the formation of alkoxy radicals is reported in radical cascades using traceless directing groups. Despite the possibility of hydrogen abstraction in the fragmenting step, followed by loss of R-OH, β-scission is preferred for the formation of alkoxy radicals. For the first time, the C-O radical was intermolecularly trapped using a silyl enol ether. Various C-X fragmenting groups were explored as possible traceless directing groups for the preparation of extended polyaromatics. Computational evidence shows that a combination of aromatization, steric and stereoelectronic effects assists the fragmentation to alkoxy radicals. Additionally, a new through-space interaction was discovered between O and Sn in the fragmentation as a specific transition state stabilizing effect.
Chenoweth and co-workers provide an atomic resolution crystal structure and computational analysis illustrating that aza-proline mimics l-proline stereochemistry in collagen.
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