Lewis Acid Catalyzed Rearrangement of Vinylcyclopropenes for the Construction of Naphthalene and Indene Skeletons

Synthesis of a versatile 1H-indene-3-carboxylate scaffold enabled by visible-light promoted Wolff rearrangement of 1-diazonaphthalen-2(1H)-ones

Herein, we have developed a sequential visible-light-promoted Wolff rearrangement of 1-diazonaphthalen-2(1H)-ones, followed by capturing the in situ generated ketene intermediates with various alcohols, producing diverse 1H-indene-3-carboxylates in moderate to good yields under mild reaction conditions. The broad substrate scope, high functional group tolerance, and robust conditions make the resulting derivative a versatile platform for the synthesis of plenty of bioactive molecules.

One Pot Synthesis of Spirocycles and Cyclopropa[b]pyrans by Alkenylation-Rearrangement of Cyclopropenes

A one pot alkenylation followed by a stereoselective Alder-ene cycloisomerization of cyclopropenes give (aza)spiro[2.4]heptanes and spiro[2.5]octanes in high yields. Total trans diastereoselectivity is achieved for spiro[2.4]heptanes if the cyclopropene is monosubstituted in C3. When an α,β-unsaturatedcarbonyl-containing bromide is used, an alternative cyclization takes place giving cyclopenta[c]cyclopropa[b]pyrans.

NHC-Ni(II)-catalyzed cyclopropene-isocyanide [5 + 1] benzannulation

Isocyanides are common compounds in fine and bulk chemical syntheses. However, the direct addition of isocyanide to simple unactivated cyclopropene via transition metal catalysis is challenging. Most of the current approaches focus on 1,1-insertion of isocyanide to M-R or nucleophilc insertion. That is often complicated by the competitive homo-oligomerization reactivity occurring at room temperature, such as isocyanide 1,1-insertion by Ni(II). Here we show a (N-heterocyclic carbene)Ni(II) catalyst that enables cyclopropene-isocyanide [5 + 1] benzannulation. As shown in the broad substrate scope and a [trans-(N-heterocyclic carbene)Ni(isocyanide)Br₂] crystal structure, the desired cross-reactivity is cooperatively controlled by the high reactivity of the cyclopropene, the sterically bulky N-heterocyclic carbene, and the strong coordination ability of the isocyanide. This direct addition strategy offers aromatic amine derivatives and complements the Dötz benzannulation and Semmelhack/Wulff 1,4-hydroquinone synthesis. Several sterically bulky, fused, and multi-substituted anilines and unsymmetric functionalized spiro-ring structures are prepared from those easily accessible starting materials expediently.

The direct addition of isocyanides to cyclopropenes is challenging. Here, the authors report a catalytic cyclopropene-isocyanide [5 + 1] benzannulation catalyzed by an (N-heterocyclic carbene)Ni(II) complex; this method enables the preparation of fused and multi-substituted anilines and unsymmetrically functionalized spiro-ring structures.

Straightforward Synthesis of Highly Functionalized Indanes and Tetralines through Ene-Cyclopropene Rearrangement Mediated by Ruthenium

Ene-cyclopropenes give functionalized indanes and tetralines in the presence of ruthenium dimeric catalysts. This reaction involves the cyclopropene opening by the metal catalysts with a different regioselectivity respective to gold chlorides and produces totally different products than when using semisandwich ruthenium complexes. Here, the process leads to a bridged 7-oxanorbornene-type intermediate that is converted into a functionalized aromatic ring through deoxygenative aromatization. Alternative reaction pathways occur with substrates with no possible aromatization.

Synthetic and mechanistic aspects of sulfonyl migrations

Sulfonyl migrations, frequently described as ‘unusual’ or ‘unexpected’, from the last 20 years, including 1,2-, 1,3-, 1,4-, 1,5-, 1,6- and 1,7-sulfonyl shifts, through either radical or polar processes, either inter- or intramolecularly are reviewed.

Copper-Catalyzed Cycloisomerization of Cyclopropenylimine for Synthesis of Pyrroles

Copper-catalyzed cycloisomerization of 3-iminocyclopropenes for synthesis of pyrroles has been developed. The reaction allows regioselective construction of pyrroles with various substitution patterns, including fully substituted pyrroles. The method was successfully applied to synthesis of steroidal pyrroles as well as a N-fused pyrrole.

Cu(I)-Catalyzed Intramolecular Tandem Cyclization of N-Indole-Tethered Cyclopropenes: Synthesis of Functionalized Hydrogenated Diazabenzo[ a]cyclopenta[ cd]azulene Derivatives

A Cu(I)-catalyzed [3 + 2] intramolecular cycloaddition reaction of N-indole-tethered cyclopropenes is presented in this paper. This reaction starts from the formation of π-allyl cationic intermediate or its resonance-stabilized metal carbenoid intermediate upon activation of cyclopropene with Cu(I) catalyst and a Friedel-Crafts-type cyclization to give functionalized hydrogenated diazabenzo[ a]cyclopenta[ cd]azulenes in good to excellent yields along with moderate to good dr values. The asymmetric variant of this cycloaddition reaction can be realized, giving the desired products with moderate ee values.

Gold(i)-catalysed high-yielding synthesis of indenes by direct Csp3-H bond activation

A catalytic, practical and high-yielding procedure for the synthesis of indenes by direct Csp3-H activation under gold(i) catalysis was developed. The scope of the protocol was determined by synthesizing some electron-neutral, electron-poor as well as electron-rich derivatives including the dibenzofurane and carbazole heterocycles. The mechanism of this reaction was elucidated by theoretical calculations using a ONIOM(M08-HX/mixed-basis:PM6) hybrid scheme. Thereby we found a pericyclic transformation involving a [1,5]-H shift generating a gold(i)-carbene that evolves to the indene derivative. In comparison with several reports, our protocol presents a direct activation of the Csp3-H bond.

The Lewis superacid Al[N(C6F5)2]3 and its higher homolog Ga[N(C6F5)2]3 - structural features, theoretical investigation and reactions of a metal amide with higher fluoride ion affinity than SbF5

Herein we present the synthesis of the two Lewis acids Al[N(C6F5)2]3 (ALTA) and Ga[N(C6F5)2]3 (GATA) via salt elimination reactions. The metal complexes were characterized by NMR-spectroscopic methods and X-ray diffraction analysis revealing the stabilization of the highly Lewis acidic metal centers by secondary metal-fluorine contacts. The Lewis acidic properties of Al[N(C6F5)2]3 and Ga[N(C6F5)2]3 are demonstrated by reactions with Lewis bases resulting in the formation of metallates accompanied by crucial structural changes. The two metallates [Cs(Tol)3]+[FAl(N(C6F5)2)3]- and [AsPh4]+[ClGa(N(C6F5)2)3]- contain interesting weakly coordinating anions. The reaction of Al[N(C6F5)2]3 with trityl fluoride yielded [CPh3]+[FAl(N(C6F5)2)3]- which could find application in the activation of metallocene polymerization catalysts. The qualitative Lewis acidity of Al[N(C6F5)2]3 and Ga[N(C6F5)2]3 was investigated by means of competition experiments for chloride ions in solution. DFT calculations yielded fluoride ion affinities in the gas phase (FIA) of 555 kJ mol-1 for Al[N(C6F5)2]3 and 472 kJ mol-1 for Ga[N(C6F5)2]3. Thus, Al[N(C6F5)2]3 can be considered a Lewis superacid with a fluoride affinity higher than SbF5 (493 kJ mol-1) whereas the FIA of the corresponding gallium complex is slightly below the threshold to Lewis superacidity.

Intramolecular cyclizations of cyclopropenes with indole

Novel intramolecular cycloisomerizations of nitrogen-tethered cyclopropenes with indole in the presence of Brønsted acids have been presented.

Regioselective carboannulation of electron-deficient allenes with dialkyl (2-formylphenyl)malonates leading to multisubstituted naphthalenes

A regioselective carboannulation of allenoates (or allenylphosphonates) with dialkyl 2-(2-formylphenyl)malonates leads to multi-substituted naphthalenes via Michael addition, cyclisation, dealkoxycarboxylation and tautomerisation.

Synthesis of 2-phenylnaphthalenes from styryl-2-methoxybenzenes

A new simple and efficient method for the synthesis of 2-phenylnaphthalenes from electron-rich 1-styryl-2-methoxybenzenes has been described. The reaction proceeds via TFA catalyzed C-C bond cleavage followed by intermolecular [4+2]-Diels-Alder cycloaddition of an in situ formed styrenyl trifluoroacetate intermediate. The quantum chemical calculations identified the transition state for the cycloaddition reaction and helped in tracing the reaction mechanism. The method has been efficiently utilized for synthesis of the phenanthrene skeleton and a naphthalene-based potent and selective ER-β agonist.

Ethers as hydrogen sources in BF3·OEt2 promoted reduction of diphenylmethyl alcohols, ethers and esters to hydrocarbons

A novel ether/BF₃ reductive system is effective for the reduction of diphenylmethanols and their ether and ester derivatives into hydrocarbons.

CuO/I2-mediated intramolecular annulation for the synthesis of 2-aroyl-3-hydroxy-4-iodonaphthalenes

A novel method for the direct synthesis of 2-aroyl-3-hydroxy-4-iodonaphthalenes has been developed via CuO/I₂-mediated domino reactions.

Gold-catalyzed diastereoselective cycloisomerization of alkylidene-cyclopropane-bearing 1,6-diynes

An unprecedented gold-catalyzed diastereoselective cycloisomerization of 1,6-diynes bearing an alkylidene cyclopropane moiety has been developed. This methodology enables rapid access to a variety of 1,2-trimethylenenorbornanes, which are important building blocks in the preparations of abiotic and sesquiterpene core structures.

Superacid BF3–H2O promoted benzylation of arenes with benzyl alcohols and acetates initiated by trace water

An efficient in situ prepared superacid BF₃–H₂O promoted benzylation of arenes using benzyl alcohols and acetates achieves various diarylalkanes.

Recent developments of cyclopropene chemistry

This critical review discusses recent developments in the field of cyclopropene chemistry. Although several excellent reviews that mainly focused on the thermolysis and pyrolysis as well as metal-mediated reactions of cyclopropenes have been published, significant new developments have also been achieved in recent years. This brand new review provides an overview of the progress from 2007 to 2011 on the syntheses and transformations of cyclopropenes as well as their related mechanistic studies (238 references).

When cyclopropenes meet gold catalysts

Cyclopropenes as substrates entered the field of gold catalysis in 2008 and have proven to be valuable partners in a variety of gold-catalyzed reactions. The different contributions in this growing research area are summarized in this review.