Gabriel dos Passos Gomes

Synthesis of Functionalized Phenanthrenes via Regioselective Oxidative Radical Cyclization

The majority of Sn-mediated cyclizations are reductive and, thus, cannot give a fully conjugated product. This is a limitation in the application of Sn-mediated radical cascades for the preparation of fully conjugated molecules. In this work, we report an oxidatively terminated Bu3Sn-mediated cyclization of an alkyne where AIBN, the commonly used initiator, takes on a new function as an oxidative agent. Sn-mediated radical transformation of biphenyl aryl acetylenes into functionalized phenanthrenyl stannanes can be initiated via two potentially equilibrating vinyl radicals, one of which can be trapped by the fast 6-endoclosure at the biphenyl moiety in good to excellent yields. The efficient preparation of Sn-substituted phenanthrenes opens access to convenient building blocks for the construction of larger polyaromatics.

Stereoelectronic Chameleons: The Donor–Acceptor Dichotomy of Functional Groups

Stereoelectronic factors account for the apparent reversal of donor-acceptor properties of a variety of functional groups by a simple change of their orientation in space. The new reactivity patterns that arise from spatial anisotropy are associated with chameleonic behavior of common organic functionalities.

Domino Fragmentations in Traceless Directing Groups of Radical Cascades: Evidence for the Formation of Alkoxy Radicals via C–O Scission

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.

Regioselective One-Pot Synthesis of Triptycenes via Triple-Cycloadditions of Arynes to Ynolates

We developed the novel one-pot synthetic method of substituted triptycenes by the reaction of ynolates and arynes. This four-step process involves three cycloadditions and electrocyclic ring opening of the strained Dewar anthracene. Each of the three related but structurally distinct classes of nucleophiles (ynolate, enolate, and anthracenolate) reacts with o-benzyne in the same predictable manner controlled by chelation and negative hyperconjugation. The resulting functionalized C3 -symmetrical triptycenes hold promise in the design of functional materials.

Combining Traceless Directing Groups with Hybridization Control of Radical Reactivity: From Skipped Enynes to Defect-Free Hexagonal Frameworks.

This work discloses the first general solution for converting oligoalkynes into polyaromatic polycyclic systems free of pentagonal defects. The efficiency and selectivity of this cascade originate from the combination of the Bu3 Sn-mediated TDG (traceless directing group) cascade transformations of skipped alkynes where the reactivity of the key radical precursor is tempered by hybridization effects. This approach ensures that the final structure consists of only six-membered rings. Practical implementation of this strategy is readily accomplished by incorporation of a suitably-substituted alkene as a final unit in the domino transformation. This strategy opens a new avenue for the controlled preparation of polyaromatic ribbons. The resulting ester functionality can be used for an additional Friedel-Crafts ring closure which effectively anneals two extra cycles with distinct electronic features to the extended aromatic system formed by the radical cascade.

tert-Butyl Carbocation in Condensed Phases: Stabilization via Hyperconjugation, Polarization, and Hydrogen Bonding

Despite the seeming similarity of the infrared (IR) spectra between tert-butyl cations (t-Bu(+)) in gaseous and condensed phases, there are important but so far unrecognized differences. The IR spectroscopic investigation of the hydrogen (H)-bonding of t-Bu(+) with the immediate environment together with the X-ray crystallographic data shows that one CH3 group of t-Bu(+) differs from the other two. In the Ar-tagged t-Bu(+) in vacuum, this group is predominantly polarized, showing three C-H stretch vibrations at 2913, 2965, and 3036 cm(-1) whereas the other two methyls are predominantly involved in strong hyperconjugation, yielding an intense triple IR band with a maximum at 2839 cm(-1). In a condensed phase, the bulk solvent effect promoted participation of the polarized CH3 group in additional hyperconjugation, decreasing its νCH3 frequencies by approximately 120 cm(-1), whereas frequencies of the other CH3 groups decreased by only ca. 4-10 cm(-1). This observation indicates that the influence of the condensed phase on t-Bu(+) stabilization is substantial. Thus, enhancement of H-bonding between t-Bu(+) and Anion(-) strengthens hyperconjugation and promotes further cation stabilization.

Interrupted Baeyer–Villiger Rearrangement: Building A Stereoelectronic Trap for the Criegee Intermediate

The instability of hydroxy peroxyesters, the elusive Criegee intermediates of the Baeyer-Villiger rearrangement, can be alleviated by selective deactivation of the stereoelectronic effects that promote the 1,2-alkyl shift. Stable cyclic Criegee intermediates constrained within a five-membered ring can be prepared by mild reduction of the respective hydroperoxy peroxyesters (β-hydroperoxy-β-peroxylactones) which were formed in high yields in reaction of β-ketoesters with BF3 ⋅Et2 O/H2 O2 .

Ozone-Free Synthesis of Ozonides: Assembling Bicyclic Structures from 1,5-Diketones and Hydrogen Peroxide

Reactions of 1,5-diketones with H2O2 open an ozone-free approach to ozonides. Bridged ozonides are formed readily at room temperature in the presence of strong Brønsted or Lewis acids such as H2SO4, p-TsOH, HBF4, or BF3·Et2O. The expected bridged tetraoxanes, the products of double H2O2 addition, were not detected. This procedure is readily scalable to produce gram quantities of the ozonides. Bridged ozonides are stable and can be useful as building blocks for bioconjugation and further synthetic transformations. Although less stabilized by anomeric interactions than bis-peroxides, ozonides have an intrinsic advantage of having only one weak O-O bond. The role of the synergetic framework of anomeric effects in bis-peroxides is to overcome this intrinsic disadvantage. As the computational data have shown, this is only possible when all anomeric effects in bis-peroxides are activated to their fullest degree. Consequently, the cyclization selectivity is determined by the length of the bridge between the two carbonyl groups of the diketone. The generally large thermodynamic preference for the formation of cyclic bis-peroxides disappears when 1,5-diketones are used as the bis-cyclization precursors. Stereoelectronic analysis suggests that the reason for the bis-peroxide absence is the selective deactivation of anomeric effects in a [3.2.2]tetraoxanonane skeleton by a structural distortion imposed on the tetraoxacyclohexane subunit by the three-carbon bridge.

Radical Alkyne peri‐Annulation Reactions for the Synthesis of Functionalized Phenalenes, Benzanthrenes, and Olympicene

Radical cyclization reactions at a peri position were used for the synthesis of polyaromatic compounds. Depending on the choice of reaction conditions and substrate, this flexible approach led to Bu3 Sn-substituted phenalene, benzanthrene, and olympicene derivatives. Subsequent reactions with electrophiles provided synthetic access to previously inaccessible functionalized polyaromatic compounds.

Organocatalyzed synthesis of fluorinated poly(aryl thioethers)

The preparation of high-performance fluorinated poly(aryl thioethers) has received little attention compared to the corresponding poly(aryl ethers), despite the excellent physical properties displayed by many polysulfides. Herein, we report a highly efficient route to fluorinated poly(aryl thioethers) via an organocatalyzed nucleophilic aromatic substitution of silyl-protected dithiols. This approach requires low catalyst loadings, proceeds rapidly at room temperature, and is effective for many different perfluorinated or highly activated aryl monomers. Computational investigations of the reaction mechanism reveal an unexpected, concerted SNAr mechanism, with the organocatalyst playing a critical, dual-activation role in facilitating the process. Not only does this remarkable reactivity enable rapid access to fluorinated poly(aryl thioethers), but also opens new avenues for the processing, fabrication, and functionalization of fluorinated materials with easy removal of the volatile catalyst and TMSF byproducts.Fluorinated poly(aryl thioethers), unlike their poly(aryl ethers) counterparts, have received little attention despite excellent physical properties displayed by many polysulfides. Here the authors show a highly efficient route to fluorinated poly(aryl thioethers) via an organocatalyzed nucleophilic aromatic substitution of silyl-protected dithiols.