Modulating the Frontier Orbitals of an Aluminylene for Facile Dearomatization of Inert Arenes

Lewis bases are well known to stabilize electron-deficient species. We demonstrate herein that the redox property of a monocoordinated aluminylene 1 featuring only four valence electrons for the shell of Al can be boosted by a Lewis base. The coordination of 1 with an N-heterocyclic carbene (NHC) effectively shrinks the HOMO−LUMO gap, thereby enhancing the reactivity of the ensuing acyclic mono-NHC-stabilized aluminylene 2, which is isoelectronic with singlet carbenes. Moreover, such base coordination completely reverses the predominant chemical reactivity (i.e. electrophilicity/nucleophilicity) of aluminylenes. In marked contrast to 1, 2 readily undergoes a [4+1] cycloaddition reaction with naphthalene and biphenylene at room temperature. Remarkably, the enhanced ambiphilic nature of Al in 2 also enables facile cleavage of aromatic C−C bonds of inert arenes in both intra- and intermolecular fashion affording 3 and 5. The formation of 5 represents the first example of the cleavage of aromatic C(3)−C(4) bond in biphenylene by a single atom center.

Modulating the Frontier Orbitals of an Aluminylene for Facile Dearomatization of Inert Arenes

Lewis bases are well known to stabilize electron-deficient species. We demonstrate herein that the redox property of a monocoordinated aluminylene 1 featuring only four valence electrons for the shell of Al can be boosted by a Lewis base. The coordination of 1 with an N-heterocyclic carbene (NHC) effectively shrinks the HOMO−LUMO gap, thereby enhancing the reactivity of the ensuing acyclic mono-NHC-stabilized aluminylene 2, which is isoelectronic with singlet carbenes. Moreover, such base coordination completely reverses the predominant chemical reactivity (i.e. electrophilicity/nucleophilicity) of aluminylenes. In marked contrast to 1, 2 readily undergoes a [4+1] cycloaddition reaction with naphthalene and biphenylene at room temperature. Remarkably, the enhanced ambiphilic nature of Al in 2 also enables facile cleavage of aromatic C−C bonds of inert arenes in both intra- and intermolecular fashion affording 3 and 5. The formation of 5 represents the first example of the cleavage of aromatic C(3)−C(4) bond in biphenylene by a single atom center.

Toward Thermodynamically Stable Triplet Carbenes

<p>In sharp contrast to the widely studied and applied stable singlet carbenes, only several kinetically persistent triplet carbenes have been studied, and thermodynamically stable triplet carbenes are much less developed. With the Gibbs free energy of C-H bond insertion into methane as a probe, DFT calculations were employed to examine a variety of candidate molecules for stable triplet carbenes. Guided by these calculations, some molecules with significant stability against C-H insertion were designed by fine tuning of geometry and electronic structures. These compounds might be potential candidates for experimental development of stable triplet carbenes.</p>

Toward Thermodynamically Stable Triplet Carbenes

<p>In sharp contrast to the widely studied and applied stable singlet carbenes, only several kinetically persistent triplet carbenes have been studied, and thermodynamically stable triplet carbenes are much less developed. With the Gibbs free energy of C-H bond insertion into methane as a probe, DFT calculations were employed to examine a variety of candidate molecules for stable triplet carbenes. Guided by these calculations, some molecules with significant stability against C-H insertion were designed by fine tuning of geometry and electronic structures. These compounds might be potential candidates for experimental development of stable triplet carbenes.</p>

Photochemical Doyle-Kirmse Reaction – a Metal-Free Route to Allenes

This report describes metal-free, blue light-induced [2,3]-sigmatropic rearrangement of sulfonium ylides generated from donor/acceptor diazoalkanes and propargyl sulfides. The reaction furnishes highly functionalized allenes from a broad range of starting materials in decent yields. Mechanistic experiments supported by the literature data suggest singlet carbenes as intermediates in this reaction.

Photochemical Doyle-Kirmse Reaction – a Metal-Free Route to Allenes

This report describes metal-free, blue light-induced [2,3]-sigmatropic rearrangement of sulfonium ylides generated from donor/acceptor diazoalkanes and propargyl sulfides. The reaction furnishes highly functionalized allenes from a broad range of starting materials in decent yields. Mechanistic experiments supported by the literature data suggest singlet carbenes as intermediates in this reaction.

Ring Expansion of Alkylidenecarbenes Derived from Lactams, Lactones, and Thiolactones into Strained Heterocyclic Alkynes: A Theoretical Study

Strained cycloalkynes are of considerable interest to theoreticians and experimentalists, and possess much synthetic value as well. Herein, a series of cyclic alkylidenecarbenes—formally obtained by replacing the carbonyl oxygen of four-, five-, and six-membered lactams, lactones, and thiolactones with a divalent carbon—were modeled at the CCSD(T)/cc-pVTZ//B3LYP/6-311+G** and CCSD(T)/cc-pVTZ//CCSD/6-311+G** levels of theory. The singlet carbenes were found to be more stable than the triplets. The strained heterocyclic alkynes formed by ring expansion of these singlet carbenes were also modeled. Interestingly, the C≡C bonds in the five-membered heterocycles, obtained from the rearrangement of β-lactam- and β-lactone-derived alkylidenecarbenes, displayed lengths intermediate between formal double and triple bonds. Furthermore, 2-(1-azacyclobutylidene)carbene was found to be nearly isoenergetic with its ring-expanded isomer, and 1-oxacyclopent-2-yne was notably higher in energy than its precursor carbene. In all other cases, the cycloalkynes were lower in energy than the corresponding carbenes. The transition states for ring-expansion were always lower for the 1,2-carbon shifts than for 1,2-nitrogen or oxygen shifts, but higher than for the 1,2-sulfur shifts. These predictions should be verifiable using carbenes bearing appropriate isotopic labels. Computed vibrational spectra for the carbenes, and their ring-expanded isomers, are presented and could be of value to matrix isolation experiments.

Photoelimination of nitrogen from adamantane and pentacycloundecane (PCU) diazirines: a spectroscopic study and supramolecular control

Nitrogen photoelimination delivers carbenes from diazirines. Protonation of singlet carbenes was detected based on the PCU skeleton rearrangement. Complexation of diazirines changes the relative singlet/triplet ratio of carbene products.