The chemistry of bisallenes

Allenes in Diels–Alder Cycloadditions

For a long time, allenes—and cumulenic systems in general—played a relatively minor role in Diels–Alder cycloadditions. This situation has changed, since allenes are more readily available and as their unique stereochemical features in [4+2]cycloadditions are more widely recognized. This review presents a comprehensive overview of allenes in Diels–Alder processes using selected examples. Allenes in dienes, dienophiles and cycloadducts are covered, inter- and intramolecular Diels–Alder cycloadditions are discussed, and stereochemical features of the addition process are described. Areas of emerging importance are also covered, including allenic components in dehydro-Diels–Alder processes, and dendralenic allenes in Diels–Alder sequences for the rapid generation of target-relevant molecular complexity. Preparatively useful methods for allenic precursor synthesis are also discussed.

1 Introduction

2 Allenic Dienes

2.1 Vinylallenes

2.2 Bisallenes

2.3 Cross-conjugated Allenes

3 Allenic Dienophiles

4 Intramolecular Diels–Alder Cycloadditions

5 Allenic Cycloadducts

6 Conclusions and Outlook

Synthesis of conjugated bisallenes by cooperative Cu/Pd-catalysed borylallenylation of 2-trifluoromethyl-1,3-enynes

We report a cooperative Cu/Pd-catalysed borylallenylation of 2-trifluoromethyl-1,3-enynes with diboron reagents and propargylic carbonates access to a series of conjugated bisallenes with excellent functional group compatibility.

Quantification of the Helicality of Helical Molecular Orbitals

The frontier molecular orbital (MO) topology of linear carbon molecules, such as polyynes, can be visually identified as helices. However, there is no clear way to quantify the helical curvature of these π-MOs, and it is thus challenging to quantify correlations between the helical curvature and molecular properties. In this paper, we develop a method that enables us to compute the helical curvature of MOs based on their nodal planes. Using this method, we define a robust way of quantifying the helical nature of MOs (helicality) by their deviation from a perfect helix. We explore several limiting cases, including polyynes, metallacumulenes, cyclic allenes, and spiroconjugated systems, where the change in helical curvature is subtle yet clearly highlighted with this method. For example, we show that strain only has a minor effect on the helicality of the frontier orbitals of cycloallenes and that the MOs of spiroconjugated systems are close to perfect helices around the spiro-carbon. Our work provides a well-defined method for assessing orbital helicality beyond visual inspection of MO isosurfaces, thus paving the way for future studies of how the helicality of π-MOs affects molecular properties.

Nucleophile dependent formation of 6- and 7-membered N-heterocycles by platinum-catalysed cyclisation of 1,5-bisallenes

An unprecedented Pt-catalysed cyclisation of N-tethered 1,5-bisallenes in the presence of oxygen nucleophiles is reported, where formation of 6- or 7-membered rings is driven by the choice of nucleophile and the mechanism dictated by the nucleophile and the electronic properties of the bisallene. The reaction in the presence of alcohols gives preferentially vinyltetrahydropyridines with an extra alkoxy group and Pt-H as the active species in the catalytic cycle, while formation of di- and tetrahydroazepines with an extra hydroxyl group is favoured when water is used as nucleophile, via nucleophilic attack/carbocyclization as the favoured pathway. The products obtained are frequently found in the core of natural products with important biological activities, so understanding this complex mechanistic behaviour and exploiting this new methodology will have a big impact in organic synthesis and organometallic chemistry.

Palladium-Catalyzed Oxidative Domino Carbocyclization-Arylation of Bisallenes

Herein we report a highly efficient and site-selective palladium-catalyzed oxidative carbocyclization–arylation reaction of bisallenes and arylboronic acids under operationally simple conditions for the selective synthesis of cyclohexadiene derivatives. The palladium source and the solvent proved to be crucial for the selectivity and the reactivity displayed. Interestingly, in the absence of the nucleophile, an oxidative carbocyclization-β-elimination pathway predominates. The reaction conditions are compatible with a wide range of functional groups, and the reaction exhibits broad substrate scope. Furthermore, key information regarding the mechanism was obtained using control experiments and kinetic studies.

Solving the puzzling competition of the thermal C(2)-C(6) vs Myers-Saito cyclization of enyne-carbodiimides

The mechanism of the thermal cyclization of enyne-carbodiimides 7a–c has been studied computationally by applying the DFT method. The results indicate that enyne-carbodiimides preferentially follow the C²–C⁶ (Schmittel) cyclization pathway in a concerted fashion although the Myers–Saito diradical formation is kinetically preferred. The experimentally verified preference of the C²–C⁶ over the Myers–Saito pathway is guided by the inability of the Myers–Saito diradical to kinetically compete in the rate-determining trapping reactions, either inter- or intramolecular, with the concerted C²–C⁶ cyclization. As demonstrated with enyne-carbodiimide 11, the Myers–Saito channel can be made the preferred pathway if the trapping reaction by hydrogen transfer is no more rate determining.

Nonstatistical dynamics in the thermal Garratt-Braverman/[1,5]-H shift of one ene-diallene: an experimental and computational study

A mechanistic study of the thermal Garratt-Braverman/[1,5]-H shift of ene-diallene 6 leading to alkenes (E)-8 and (Z)-8 is reported. The product ratio was found to be temperature-independent and does not agree with the computed energy barriers (i.e., TS2E(⧧) and TS2Z(⧧)). On the basis of experimental and DFT computational results, we propose a mechanism that is strongly controlled by nonstatistical dynamic effects.

Microwave-promoted synthesis of bicyclic azocine-β-lactams from bis(allenes)

A metal-free preparation of structurally novel bicyclic azocine-β-lactams has been developed. The first examples accounting for the preparation of eight-membered rings from bis(allenes) in the absence of metals have been achieved by the thermolysis of nonconjugated 2-azetidinone-tethered bis(allenes) on application of microwave irradiation. This selective carbocyclization reaction has been studied experimentally, and additionally, its mechanism has been investigated by a DFT study.

Cyclization reactions of bis(allenes) for the synthesis of polycarbo(hetero)cycles

This review presents the reactivity of a variety of substituted bis(allenes) under different reaction conditions, showing the high number of complex structures that can be obtained.

Polar reactions of acyclic conjugated bisallenes

The chemical behaviour of various alkyl-substituted, acyclic conjugated bisallenes in reactions involving polar intermediates and/or transition states has been investigated on a broad scale for the first time. The reactions studied include lithiation, reaction of the thus formed organolithium salts with various electrophiles (among others, allyl bromide, DMF and acetone), oxidation to cyclopentenones and epoxides, hydrohalogenation (HCl, HBr addition), halogenation (Br(2) and I(2) addition), and [2 + 2] cycloaddition with chlorosulfonyl isocyanate. The resulting adducts were fully characterized by spectroscopic and analytical methods; they constitute interesting substrates for further organic transformations.