Sara Cembellín

Gold-catalysed tuning of reactivity in allenes: 9-endo hydroarylation versus formal 5-exo hydroalkylation

The divergent gold-catalysed reactivity (C(sp(2))-H versus C(sp(3))-H) of aryloxy-tethered allenes has been uncovered.

Controlled rearrangement of lactam-tethered allenols with brominating reagents: a combined experimental and theoretical study on α- versus β-keto lactam formation.

N-Bromosuccinimide (NBS) smoothly promotes the ring expansion of lactam-tethered allenols to efficiently afford cyclic α- or β-ketoamides with good yields and high chemo-, regio-, and diastereoselectivity, through controlled C-C bond cleavage of the β- or γ-lactam nucleus. Interestingly, in contrast to the rearrangement reactions of 2-azetidinone-tethered allenols, which lead to the corresponding tetramic acid derivatives (β-keto lactam adducts) as the sole products, the reactions of 2-indolinone-tethered allenols under similar conditions give quinoline-2,3-diones (α-keto lactam adducts) as the exclusive or major products. To rationalize the experimental observations, theoretical studies have been performed.

Redox‐Neutral Manganese(I)‐Catalyzed C‐H Activation: Traceless Directing Group Enabled Regioselective Annulation

A strategy is reported in which traceless directing groups (TDGs) are used to promote the redox-neutral MnI -catalyzed regioselective synthesis of N-heterocycles. Alkyne coupling partners bearing a traceless directing group, which serves as both the chelator and internal oxidant, were used to control the regioselectivity of the annulation reactions. This operationally simple approach is highly effective with previously challenging unsymmetrical alkyne systems, including unbiased dialkyl alkynes, with perfect regioselectivity. The simple conditions and the ability to carry out synthesis on a gram scale underscore the usefulness of this method. The application of this strategy in the concise synthesis of the bioactive compound PK11209 and the pharmaceutical moxaverine is also described.

Manganese(I)‐Catalyzed C−H (2‐Indolyl)methylation: Expedient Access to Diheteroarylmethanes

An unprecedented MnI -catalyzed (2-indolyl)methylation of heteroarenes is reported. This method makes use of an aromatizing cascade strategy to install a (2-indolyl)methyl group into target molecules, thereby leading to the expedient synthesis of previously challenging and important unsymmetrical diheteroarylmethanes, in particular bis(2-indolyl)methanes. The proposed cascade process comprises the reorganization of multiple bonds with controlled regioselectivity and high atom economy and can be performed on a gram-scale. Furthermore, a metal-free C-H propargylation is observed. The diverse application of this method is also demonstrated.

Iodine recycling via 1,3-migration in iodoindoles under metal catalysis

3-Substituted (indol-2-yl)-α-allenols show divergent patterns of reactivity under metal catalysis. An unprecedented intramolecular 1,3-iodine migration is described.

Highly Selective Manganese(I)/Lewis Acid Cocatalyzed Direct C−H Propargylation using Bromoallenes

A manganese(I)/Lewis acid cocatalyzed direct C-H propargylation with high selectivity has been developed. BPh3 was discovered to not only promote the reactivity, but also enhance the selectivity. Secondary, tertiary, and even quaternary carbon centers at the propargylic position could be directly constructed. Both internal and terminal alkynes are easily accessible. The chirality was successfully transferred from an axially chiral allene to central chirality. The reactivity of the manganese catalyst in this reaction was found to be unique among transition metal catalysts.

Gold as Catalyst for the Hydroarylation and Domino Hydroarylation/N1-C4 Cleavage of β-Lactam-Tethered Allenyl Indoles.

Gold-catalyzed hydroarylation reaction of β-lactam-tethered allenyl indoles gives azeto-oxepino[4,5-b]indol-2-ones, tetrahydroazeto-azocino[3,4-b]indol-2-ones, and hexahydroazeto-azepino[3,4-b]indol-2-ones with very high levels of stereo- and regioselectivity, the 7-exo and 8-endo carbocyclization modes by attack of the indole group toward either the internal or the terminal allene carbon, respectively, being favored. Hydroarylation across the central carbon of the allene moiety has not been detected. The controlled gold-catalyzed annulations allowed the formation of fused β-lactams without harming the sensitive four-membered heterocycle. Besides, a novel gold-catalyzed domino process, namely, the allenic hydroarylation/N1-C4 β-lactam bond breakage to afford dihydro-oxepino[4,5-b]indole-4-carboxamides, has been discovered.

Allenols versus Allenones: Rhodium-Catalyzed Regiodivergent and Tunable Allene Reactivity with Triazoles

2-Pyrrolines and 6-oxo-hexa-2,4-dienals have been prepared through the divergent reactions of 1-benzenesulfonyl-4-aryl-1,2,3-triazoles with functionalized allenes. The rhodium-catalyzed reactions between allenols and 1-benzenesulfonyl-4-aryl-1,2,3-triazoles yielded 2-pyrrolines. This transformation is compatible with the presence of aliphatic, aromatic, heterocyclic, amide, and halogen functional groups. Interestingly, a reactivity switch took place when the allene-tethered alcohol substrate was replaced by its ketone counterpart. When the rhodium-catalyzed reaction of 1-benzenesulfonyl-4-phenyl-1,2,3-triazole was performed with allenones, acyclic 6-oxo-hexa-2,4-dienals were stereoselectively formed as (2Z,4E) isomers.

Metal‐Catalyzed Cyclization Reactions of 2,3,4‐Trien‐1‐ols: A Joint Experimental–Computational Study

Controlled preparation of tri- and tetrasubstituted furans, as well as carbazoles has been achieved through chemo- and regioselective metal-catalyzed cyclization reactions of cumulenic alcohols. The gold- and palladium-catalyzed cycloisomerization reactions of cumulenols, including indole-tethered 2,3,4-trien-1-ols, to trisubstituted furans was effective, due to a 5-endo-dig oxycyclization by attack of the hydroxy group onto the central cumulene double bond. In contrast, palladium-catalyzed heterocyclization/coupling reactions with 3-bromoprop-1-enes furnished tetrasubstituted furans. Also studied was the palladium-catalyzed cyclization/coupling sequence involving protected indole-tethered 2,3,4-trien-1-ols and 3-bromoprop-1-enes that exclusively generated trisubstituted carbazole derivatives. These results could be explained through a selective 6-endo-dig cumulenic hydroarylation, followed by aromatization. DFT calculations were carried out to understand this difference in reactivity.

MnI/AgI Relay Catalysis: Traceless Diazo‐Assisted C(sp2)–H/C(sp3)–H Coupling to β‐(Hetero)Aryl/Alkenyl Ketones

An unprecedented MnI /AgI -relay-catalyzed C(sp2 )-H/C(sp3 )-H coupling of (vinyl)arenes with α-diazoketones is reported, wherein the diazo group was exploited as a traceless auxiliary for control of regioselectivity. Challenging β-(hetero)aryl/alkenyl ketones were obtained through this operationally simple approach. The cascade process merges denitrogenation, carbene rearrangement, C-H activation, and hydroarylation/hydroalkenylation. The robustness of this method was demonstrated at preparative scale and applied to late-stage diversification of natural products.