Practical Synthesis of the Dendralene Family Reveals Alternation in Behavior

Reductive Metallation of Dendralenes and Myrcene Using Dimagnesium(I) Compounds: A Facile Route to Unsaturated Organomagnesium Compounds

The two-electron reduction of 2,3-dimethylbuta-1,3-diene (DMB) with β-diketiminate and guanidinate substituted dimagnesium(I) compounds has given complexes in which two bidentate amido-magnesium fragments are bridged through the π-system of the DMB dianion, viz. [(LMg)2 (μ-DMB)] (L=Xyl Nacnac, [HC(MeCNXyl)2 ]- , Xyl=2,6-xylyl; or Priso=[(DipN)2 CNPri 2 ]- , Dip=2,6-diisopropylphenyl). Similar double reductions of [4]dendralene (4dend) have afforded the complexes, [(LMg)2 (μ-4dend)] (L=Ar Nacnac, Ar=Xyl or mesityl (Mes); or Priso) in which the 4dend dianion is π-coordinated to the bidentate amido-magnesium fragments. Treatment of several such complexes with THF leads to Z- to E-isomerization of the dendralene fragment, and formation of purely σ-bonded Mg-C interactions in the THF coordinated products [{(Ar Nacnac)(THF)Mg}2 (μ-4dend)] (Ar=Xyl, Mes or Dip). Reaction of myrcene (Myr) with [{(Xyl Nacnac)Mg}2 ] proceeds via reductive coupling of Myr to give a previously unknown acyclic, branched C20 tetra-olefin dianion complex [{(Xyl Nacnac)(THF)Mg}2 (μ-Myr)2 ]. Preliminary reactions of [(LMg)2 (μ-DMB)] with H2 and/or CO yielded a series of products, including novel magnesium hydride compounds, products derived from couplings of CO with the reduced DMB fragment (viz. magnesium dimethylcyclohexadienediolates), and one magnesium cyclopropanetriolate complex from the magnesium(I) induced coupling of DMB with H2 and CO.

Mild Stereoselective Synthesis of Densely Substituted [3]Dendralenes via Ru-Catalyzed Intermolecular Dimerization of 1,1-Disubstituted Allenes

Described here is a mild and stereoselective protocol for the synthesis of [3]dendralenes via the intermolecular dimerization of allenes. With the proper choice of a ruthenium catalyst, a range of unactivated 1,1-disubstituted allenes, without prefunctionalization in the allylic position, reacted efficiently to provide rapid access to densely substituted [3]dendralenes. An intermolecular C-C bond and three different types of C═C double bonds (di-, tri-, and tetrasubstituted) embedded in an acyclic structure were constructed with good to high E/Z stereocontrol. This is in contrast to the known catalytic protocols that focus on allenes with prefunctionalization at the allylic position and/or monosubstituted allenes, which would proceed by a different mechanism or require less stereocontrol. The silyl-substituted dendralene products are precursors of other useful dendralene molecules. Density functional theory (DFT) studies and control experiments supported a mechanism involving oxidative cyclometalation, β-H elimination (the rate-determining step), and reductive elimination.

A Rapid Aza-Bicycle Synthesis from Dendralenes and Imines

The diene-transmissive 2-fold Diels-Alder sequence between carbon-based dienophiles and [3]dendralenes is becoming an established method for polycarbocycle synthesis. Here, we demonstrate for the first time that imines are competent participants in intermolecular formal [4 + 2] cycloadditions with dendralenes. After a second Diels-Alder process with a carbadienophile, hexahydro- and octahydro-isoquinoline structures are formed. The formal aza-Diels-Alder reaction, which requires Lewis acid promotion, proceeds in high regio- and stereoselectivity under optimized conditions. ωB97XD/Def2-TZVP//M06-2X/6-31+G(d,p) calculations reveal a stepwise ionic mechanism for the formal aza-dienophile cycloadditions and also explain an unexpected Z → E olefin isomerization of a non-reacting C═C bond in the first formal cycloaddition.

Substituents Regulate the Cyclization of Conjugated Alkynes to Accurately Construct Cyclo-(E)-[3]dendralenes

Substituent-regulated cyclization of conjugated alkynes with acid catalysis was developed in this paper, and it provides a straightforward synthesis of cyclic-(E)-[3]dendralenes. Depending on the electronic effect of the aromatic ring pairing, a variety of phosphinyl quintuplet/hexa cyclo-[3]dendralenes with diverse substitution patterns are accessible, with good efficiency and high stereoselectivity. This self-cyclization process achieves the first precise construction of a phosphinylcyclo-(E)-[3]dendralene from conjugated alkynes to aromatization.

A General Stereoselective Synthesis of [4]Dendralenes

The first general synthesis of branched tetraenes ([4]dendralenes) involves two or three steps from inexpensive, commodity chemicals. It involves an unprecedented variation on Suzuki-Miyaura cross-coupling, generating two new C-C bonds in a one-flask operation with control of diastereoselectivity. The broad scope of the method is established through the synthesis of more than 60 diversely substituted [4]dendralene molecules, along with substituted buta-1,3-dienes and other [n]dendralenes. [4]Dendralenes are demonstrated to be significantly more kinetically stable than their well-known [3]dendralene counterparts. The first stereoselective synthesis of these compounds is also reported, through the catalyst-controlled generation of both E- and Z-diastereomeric products from the same precursor. Novel, through-conjugated/cross-conjugated hybrid molecules are introduced. The first selective dienophile cycloadditions to substituted [4]dendralenes are reported, thus paving the way for applications in target-oriented synthesis.

Palladium-Catalyzed [2+3] Cycloaddition/Cross-Coupling Reaction: Z/E and Diastereoselective Synthesis of Dendralene-Functionalized Dihydrofurans

Herein, we describe a Pd-catalyzed [2+3] cycloaddition/cross-coupling reaction of allenyl acetates for the Z/E selective and diastereoselective synthesis of dendralene-functionalized dihydrofurans. Remarkably, mechanistic studies show the formation of an epoxide from a carbonyl bond via cycloaddition, which is practically and mechanistically significant for the construction of other bioactive heterocyclic epoxides. This research also revealed the utility and potential of allenic esters as C2 synthons and 1,2-biselectrophiles in cycloaddition reactions.

Palladium-Catalyzed Oxidative Coupling of the Allenic C-H Bond with α-Diazo Esters: Synthesis of [3]Dendralenes

A palladium-catalyzed highly regio- and stereoselective allenic C-H oxidative coupling with α-diazo esters is developed. The reaction pathway involves allylic palladium carbene as the key intermediate, which is followed by a carbene migratory insertion process. The reaction proceeds efficiently under mild conditions without external base, providing substituted [3]dendralenes bearing various functional groups.

Asymmetric β,γ′-regioselective [4 + 3] and [4 + 2] annulations of α-vinylenals via cascade iminium ion-dienamine catalysis

α-Vinylenal substrates have been designed and applied in β,γ′-regioselective asymmetric [4 + 3] and [4 + 2] annulation reactions via cascade aminocatalysis.

[n]Dendralenes as a Platform for Selective Catalysis: Ligand-Controlled Cu-Catalyzed Chemo-, Regio-, and Enantioselective Borylations

We report the development of two complementary methods for the Cu-catalyzed anti-Markovnikov borylation of one specific olefin in 2-substituted [n]dendralenes (n = 3-6). The first protocol operates with a bisphosphine ligand and occurs with high regio- and chemoselectivity for the terminal double bond, independently of the number of cross-conjugated alkenes. We show that the use of a chiral phosphanamine ligand enables the highly chemo-, regio-, and enantioselective borylation of the alkene cross-conjugated with the terminal olefin in [n]dendralenes.

Copper-catalyzed protoboration of borylated dendralenes: a regio- and stereoselective access to functionalized 1,3-dienes

A copper-catalyzed protoboration of borylated dendralenes is reported. The method employs an NHC-Cu catalyst and provides access to 1,4-addition products with excellent levels of chemo-, regio- and stereoselectivity. The resulting diene bis(boronates) are oxidized to the corresponding diene diols which are synthetically versatile building blocks.

Synthesis and Properties of 2,3-Diethynyl-1,3-Butadienes

The first general preparative access to compounds of the 2,3-diethynyl-1,3-butadiene (DEBD) class is reported. The synthesis involves a one-pot, twofold Sonogashira-type, Pd⁰ -catalyzed coupling of two terminal alkynes and a carbonate derivative of a 2-butyne-1,4-diol. The synthesis is broad in scope and members of this structural family are kinetically stable enough to be handled using standard laboratory techniques at ambient temperature. They decompose primarily through heat-promoted cyclodimerizations, which are impeded by alkyl substitution and accelerated by aryl or alkenyl substitution. An iterative sequence of these unprecedented Sonogashira-type couplings generates a new type of expanded dendralene. A suitably substituted DEBD carrying two terminal alkyne groups undergoes Glaser-Eglinton cyclo-oligomerization to produce a new class of expanded radialenes, which are chiral due to restricted rotation about their 1,3-butadiene units. The structural features giving rise to atropisomerism in these compounds are distinct from those reported previously.

A general synthesis of dendralenes

The first broad spectrum dendralene synthesis permits the widest structural and substituent variation and promotes applications in step economic synthesis.

The first general synthetic approach to substituted [3]- and higher dendralenes is reported. Fifty-one mono- through to penta-substituted dendralenes carrying alkyl-, cycloalkyl-, alkenyl-, alkynyl-, aryl- and heteroaryl-substitutents are accessed, and the first (E)/(Z)-stereoselective syntheses of dendralenes are reported (twenty-eight examples). The approach involves twofold Pd(0)-catalyzed Negishi couplings of 1,1-dibromoalkenes with alkenylzinc reagents, and exploits both substrate- and catalyst-controlled aspects of chemo-, regio- and stereoselectivity in the two C(sp²)–C(sp²) bond forming steps. The value of the new hydrocarbons in rapid structural complexity generation is demonstrated through their deployment in unprecedented diene- and triene-transmissive pericyclic reaction sequences.

Secondary phosphine oxides stabilized Au/Pd nanoalloys: metal components-controlled regioselective hydrogenation toward phosphinyl (Z)-[3]dendralenes

A series of gold/palladium nanoalloys stabilized by secondary phosphine oxides have been prepared and applied in selective hydrogenation for the first time.

Thieno[3,4-c]pyrrole-4,6-dione Oligothiophenes Have Two Crossed Paths for Electron Delocalization

A new series of electron-deficient oligothiophenes, thieno[3,4-c]pyrrole-4,6-dione oligothiophenes (OTPD_n ), from the monomer to hexamer, is reported. The optical and structural properties in the neutral states have been analyzed by absorption and emission spectroscopy together with vibrational Raman spectroscopy. In their reduced forms, these molecules could stabilize both anions and dianions in similar ways. For the dianions, two independent modes of electron conjugation of the charge excess were observed: the interdione path and the interthiophene path. The interference of these two paths highlighted the existence of a singlet diradical ground electronic state and the appearance of low-energy, thermally accessible triplet states. These results provide valuable insights into the device performance of TPD-based materials and for the rational design of new high-performance organic semiconductors.

Selective Palladium-Catalyzed Allenic C-H Bond Oxidation for the Synthesis of [3]Dendralenes

A highly selective palladium-catalyzed allenic C-H bond oxidation was developed, and it provides a novel and straightforward synthesis of [3]dendralene derivatives. A variety of [3]dendralenes with diverse substitution patterns are accessible with good efficiency and high stereoselectivity. The reaction tolerates a broad substrate scope containing various functional groups on the allene moiety, including ketone, aldehyde, ester, and phenyl groups. Also, a wide range of olefins with both electron-donating and electron-withdrawing aryls, acrylate, sulfone, and phosphonate groups are tolerated.

Synthesis and Diels-Alder Reactivity of Substituted [4]Dendralenes

The first synthesis of all five possible monomethylated [4]dendralenes has been achieved via two distinct synthetic strategies. The Diels-Alder chemistry of these new dendralenes (as multidienes) with an electron poor dienophile, N-methylmaleimide (NMM), has been studied. Thus, simply upon mixing the dendralene and an excess of dienophile at ambient temperature in a common solvent, sequences of cycloadditions result in the rapid generation of complex multicyclic products. Distinct product distributions are obtained with differently substituted dendralenes, demonstrating that dendralene substitution influences the pathway followed, when a matrix of mechanistic possibilities exists. Dendralene site selectivities are traced to electronic, steric and conformational effects, thereby allowing predictive tools for applications of substituted dendralenes in future synthetic endeavors.

Discovery and Computational Rationalization of Diminishing Alternation in [n]Dendralenes

The [n]dendralenes are a family of acyclic hydrocarbons which, by virtue of their ability to rapidly generate structural complexity, have attracted significant recent synthetic attention. [3]Dendralene through [8]dendralene have been previously prepared but no higher member of the family has been reported to date. Here, we describe the first chemical syntheses of the "higher" dendralenes, [9]dendralene through [12]dendralene. We also report a detailed investigation into the spectroscopic properties and chemical reactivity of the complete family of fundamental hydrocarbons, [3]dendralene to [12]dendralene. These studies reveal the first case of diminishing alternation in behavior in a series of related structures. We also report a comprehensive series of computational studies, which trace this dampening oscillatory effect in both spectroscopic measurements and chemical reactivity to conformational preferences.

Synthesis of highly functionalized [3]dendralenes and their Diels–Alder reactions displaying unexpected regioselectivity

Sequential olefinations were applied for the synthesis of acyclic tetrasubstituted [3]dendralenes, which did not cyclodimerize and displayed unusual regioselectivity in the Diels–Alder reaction.

Preparation and synthetic value of π-bond-rich branched hydrocarbons

The two simplest branched acyclic structures comprising only conjugated C═C units, namely, [3]dendralene (3-methylene-1,4-pentadiene) and [4]dendralene (3,4-dimethylene-1,5-hexadiene), were first reported in 1955 and 1962, respectively. No higher members of the series were described in the literature until 2000. This Account describes the modern phase of dendralene chemistry, driven to a large extent by research performed within the author's group. The first synthesis of the parent dendralene family allowed access to the hydrocarbons in batches of up to 5 mg. The synthetic approach took into account the prevailing dogma of the time, specifically that these compounds would be very reactive species and hence difficult to handle in the laboratory. As such, a route involving the cheleotropic elimination of SO2 from stable, and generally insoluble, 3-sulfolene-masked precursors was devised. Our second-generation approach was of significantly higher value in preparative terms, allowing the syntheses of the first six members of the unsubstituted [n]dendralenes (i.e., n = 3-8) directly, on scales of hundreds of milligrams to decagrams, using commercially available precursors and standard laboratory equipment and methods. This work demonstrated that the assumed high reactivity and instability this family of compounds was erroneous and ultimately led to the development of syntheses of structurally related cross-conjugated systems including substituted dendralenes, tetravinylethylene, 1,1-divinylallene, and furan-containing analogues of the dendralenes. Cross-coupling reactions feature strongly in the syntheses of these compounds, and methods involving single- to multifold Stille, Kumada, and Negishi couplings are mainstays of this work. The even parity [n]dendralenes were shown to exhibit enhanced stability over the odd parity congeners, a result that can be attributed to conformational effects. π-Bond-rich branched hydrocarbons are demonstrated to have significant value in the rapid generation of structural complexity. Pericyclic processes are particularly useful in this regard, with the dendralenes and their relatives serving as multidienes, participating in diene-transmissive cycloaddition sequences, sometimes in combination with electrocyclizations, to generate fused and bridged multicyclic systems containing many new covalent bonds. The outcomes of exploratory investigations into pericyclic sequences involving dendralenes are presented, along with methods developed to control chemoselectivity, regioselectivity, and stereoselectivity. Distinct from their use in diene-transmissive sequences, the dendralenes also serve as multialkenes, for the direct synthesis of polyols and oligo-cyclopropanes. Finally, the deployment of π-bond-rich branched hydrocarbons in the shortest total synthesis of a pseudopterosin natural product is summarized, as a prelude to future prospects in the areas of hydrocarbon chemistry and target synthesis.

Ruthenium-catalyzed synthesis of 2,3-cyclo[3]dendralenes and complex polycycles from propargyl alcohols

Ruthenium-catalyzed cascade transformations for the synthesis of 2,3-cyclo[3]dendralenes and multicomponent processes based thereon to generate complex polycycles are presented. The combination of allylation-cyclization sequences with diene-transmissive Diels-Alder reactions allows the rapid and selective construction of natural-product-like motifs from easily accessible starting materials in a one-pot process and provides a new method to access potential drug candidates.

Attempts to prepare an all-carbon indigoid system

First attempts are described to prepare a precursor for an all-carbon analog of indigo, the tetracyclic triene 4. Starting from indan-2-one (9) the α-methylene ketone 13 was prepared. Upon subjecting this compound to a McMurry coupling reaction, it dimerized to the bis-indene derivative 17, rather than providing the tetramethyl derivative of 4, the hydrocarbon 14. In a second approach, indan-1-one (18) was dimerized to the conjugated enedione 21 through the bis-1-indene dimer 19. All attempts to methylenate 21 failed, however. When 19 was treated with the Tebbe reagent, the dimer 23 was produced, presumably through a Cope reaction of the intermediately generated isomer 22. The bis-indene derivative 23 can be alkylated with 1,2-dibromoethane to produce a 1:1 mixture of the spiro compounds 24 and 25. Although 9 could be reductively dimerized to 30, the conversion of this olefin to 14 failed.

Regiospecific inverse electron demand Diels–Alder reactions of 7-methylcoumarin-4-azadienes

Condensation of 7-methylcoumarin-4-carbaldehyde with different anilines affords 7-methylcoumarin-4-azadienes which on Diels–Alder reaction with dihydropyran gives coumarinyl substituted pyranoquinolines.

Computational and synthetic studies with tetravinylethylenes

Computational and experimental studies offer fresh insights into the neglected tetravinylethylene class of compounds. Both the structures and the outcomes of exploratory reactions of the parent hydrocarbon are predicted and explained in detail through high-level composite ab initio MO G4(MP2) computational studies.

Tetravinylethylene

The first four-fold cross coupling reaction involving alkenic partners leads to the title hydrocarbon on multi-gram scale in one step from commercially available precursors. In stark contrast to its close structural relatives, tetravinylethylene is a remarkably robust, bench-stable compound. The π-bond rich hydrocarbon is shown to undergo one-pot sequences of pericyclic reactions leading to the formation of complex systems with four new rings, seven C-C bonds and ten stereocenters with a very high level of stereoselectivity. Insights into the reactivity of this and related systems is provided using the accurate composite ab initio MO G4(MP2) method.