Vinylidene to alkyne rearrangement to form polyynes: synthesis and photolysis of dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-triene derivatives

Photochemical Generation of Allenylidenes from Cyclopropanated Phenanthrenes: An Experimental and Computational Study

To address the scarcity of generally applicable photochemical routes to allenylidenes in solution, phenanthrene-based sources have been investigated. Specifically, the syntheses of 1-vinylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, 1-(2-phenylvinylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, and 1-(2-methylvinylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene, photochemical precursors to propadienylidene, 3-phenylpropadienylidene, and 3-methylpropadienylidene have been carried out. Photolysis of these new precursors in olefin traps and benzene afforded the expected cyclopropane adducts of the corresponding allenylidenes. Quantum chemical calculations show that the ground state of all three carbenes is a singlet with a singlet-triplet gap of ∼29, 30, and 33 kcal/mol for propadienylidene, 3-phenylpropadienylidene, and 3-methylpropadienylidene, respectively.

Carbenes from cyclopropanated aromatics

Although a ripe old discipline by now, carbene chemistry continues to flourish as both theorists and experimentalists have shown sustained interest in this area of research. While there are numerous ways of generating carbenes, the thermal and/or photochemical decomposition of diazo compounds and diazirines remains, by far, the most commonly used method of producing these intermediates. There is no disputing the fact that these nitrogenous precursors have served carbene researchers well, but their use is not without problems. They are often sensitive and hazardous to handle and, sometimes, the desired nitrogenous precursor simply may not be available, e.g., for synthetic reasons, to study the particular carbene of interest. Furthermore, there is a legitimate concern that the photochemical generation of carbenes in solution from diazo compounds and diazirines may be contaminated by reactions in the excited states (RIES) of the precursors themselves. As an alternative, several laboratories, including ours, have used cyclopropanated aromatic systems to generate a wide range of carbenes. In each case, the cheleotropic extrusion of carbenes is accompanied by the formation of stable aromatic by-products such as phenanthrene, indane, naphthalene, and 1,4-dihydronaphthalene. The emergence of these "non-traditional" carbene sources, their versatility, and promise are reviewed in this work.

Adamantylidenecarbene: Photochemical Generation, Trapping, and Theoretical Studies

Photolysis of 1-(2-adamantylidene)-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene in benzene (or benzene-d6) at ambient temperature produces adamantylidenecarbene. The carbene undergoes dimerization to a cumulene and may also be trapped in a stereospecific fashion by cis- and trans-4-methyl-2-pentene. No products attributable to 4-homoadamantyne, resulting from ring expansion of the carbene, could be detected. Coupled cluster/density functional theory calculations place the singlet carbene ∼49 kcal/mol below the triplet and show that the former must overcome a barrier of ∼13.5 kcal/mol to rearrange into 4-homoadamantyne.

Masked Alkyne Equivalents for the Synthesis of Mechanically Interlocked Polyynes*

Polyyne polyrotaxanes, encapsulated cyclocarbon catenanes and other fascinating mechanically interlocked carbon-rich architectures should become accessible if masked alkyne equivalents (MAEs) can be developed that are large enough to prevent unthreading of a macrocycle, and that can be cleanly unmasked under mild conditions. Herein, we report the synthesis of a new bulky MAE based on t-butylbicyclo[4.3.1]decatriene. This MAE was used to synthesize a polyyne [2]rotaxane and a masked-polyyne [3]rotaxane by Cadiot-Chodkiewicz coupling. Glaser cyclo-oligomerization of the [2]rotaxane gave masked cyclocarbon catenanes. The unmasking behavior of the catenanes and rotaxanes was tested by photolysis at a range of UV wavelengths. Photochemical unmasking did not proceed cleanly enough to prepare extended encapsulated polyyne polyrotaxanes. We highlight the scope and challenges involved with this approach to interlocked carbon-rich architectures.

Carbyne: The Molecular Approach

For the last 60+ years, the synthesis and study of cumulenes and polyynes have been the focus of a small, but dedicated, group of researchers. Many of the remarkable electronic, optical, and structural properties of cumulenes and polyynes had already been identified in the earliest reports. The molecular lengths achievable by the initial syntheses were, unfortunately, somewhat limited by synthetic methods available. For the past 15 years, we have worked toward expanding on the synthesis of cumulenes and polyynes through the development of new methods and stabilization motifs. As new compounds have become available, homologous series of cumulenes and polyynes have then been examined as a function of molecular length. While we are not yet there, we would like to eventually provide a general description of the sp-carbon allotrope carbyne, and this account presents some of our efforts toward this goal.

Generation and trapping of a cage annulated vinylidenecarbene and approaches to its cycloalkyne isomer

A novel cage-annulated (bis-homocubyl) vinylidenecarbene has been generated and successfully trapped without any intermediacy of its cycloalkyne isomer. The greater kinetic and thermodynamic stability of the vinylidenecarbene vis-à-vis its cycloalkyne isomer has been predicted by DFT B3LYP/6-31G* calculations. The calculated results suggest the prospects of the cycloalkyne becoming amenable for trapping, if generated under suitable experimental conditions, owing to the substantial kinetic energy barrier associated with its possible ring contraction via 1,2-shift to the vinylidenecarbene isomer and marginal ground state energy difference. However, all of our attempts to directly generate and trap the cycloalkyne yielded unsatisfactory results. Attempted generation and trapping of a C2-symmetric bis-vinylidenecarbene from a bis-vinylidenedibromide met with unexpected failure.

The Fritsch-Buttenberg-Wiechell rearrangement: modern applications for an old reaction

The Fritsch-Buttenberg-Wiechell rearrangement of carbene/carbenoid intermediates has evolved into a valuable synthetic methodology for the preparation of polyyne structures. Various synthetic routes toward the formation of the corresponding precursors, alkynyl-substituted dibromoolefins, have been developed. Additionally, the scope of this methodology is expanded significantly by the development of functional group-tolerant one-pot procedures. The preparation of various polyynes up to the octa- and decaynes is, thus, possible on a scale that enables thorough physico-chemical characterization. Hence, series of polyynes have been investigated by, e.g., UV-vis, IR- and Raman spectroscopy, as well as X-ray crystallography. These investigations give unique insight into the structural characteristics of longer polyynes and hint to the structure of carbyne.

Development of a New Method for Consecutive Activation of Conjugated Alkynes Based on Intramolecular Silyl Migration

A new method for consecutive alpha- and beta-activation of propiolates toward electrophiles has been developed, which is mediated by suitable tertiary amines (e.g., DABCO) involving intramolecular silyl migration as a key step. Methyl 3-trimethylsilylpropiolate was reacted with aromatic aldehyde in the presence of DABCO in refluxing benzene to give a highly functionalized olefin product, in which new carbon-carbon bonds were formed at both alpha- and beta-positions of the starting propiolate. On the other hand, when using aliphatic aldehyde the reaction course was dramatically changed to afford a propargyl TMS ether as a sole product. However, we suppose that these reactions have a common reaction pathway partly, including ammonium ylide-alkylidene carbene equilibrium, and that the former products arise from the ylide form and the latter from the carbene form. These domino reactions were successfully applied for an intramolecular version by use of substrates having both formyl group and TMS-propiolate structure derived from salicylaldehyde, leading to a new formylcoumarin-forming reaction.

A new protocol for the consecutive alpha- and beta-activation of propiolates towards electrophiles, involving conjugate addition of tertiary amines and intramolecular silyl migration

Herein, we present a novel approach for the consecutive alpha- and beta-activation of conjugated alkynes and demonstrate the application of this methodology towards the C-C bond-forming reactions of propiolates. This new concept is based on the 1,4-addition of a tertiary amine to a conjugated alkyne, followed by an aldol-type addition to an aldehyde and subsequent intramolecular silyl migration. This sequential process is generally applicable for 3-trimethylsilylpropiolates. The combination of methyl 3-trimethylsilylpropiolate, 1,4-diazobicyclo[2.2.2]octane (DABCO), and aromatic aldehydes brought about domino-type C-C bond formations to afford highly functionalized olefins as the major products. On the other hand, aliphatic aldehydes, including the sterically demanding aromatic aldehyde, 2,6-dimethylbenzaldehyde, produced alkyne derivatives as the sole products from the reaction, presumably, by the reaction pathway common to the first cases. The intramolecular version of the reaction was successfully applied to the cyclization of trimethylsilylpropiolic esters derived from salicylaldehydes, leading to a new formylcoumarin synthesis. Studies of the reaction mechanisms are also described.