Gilles Lemière

Generation and Trapping of Cyclopentenylidene Gold Species: Four Pathways to Polycyclic Compounds

Cyclopentenylidene gold complexes can easily be formed from vinyl allenes through a Nazarov-like mechanism. Such carbenes may transform in four different ways into polycyclic frameworks: electrophilic cyclopropanation, C-H insertion, C-C migration, or proton shift. We have studied the selectivity of these different pathways and used our findings for the expedient preparation of valuable complex molecules. An application to the total synthesis of a natural product, Delta(9(12))-capnellene, is presented. DFT computations were carried out to shed light on the mechanisms.

Gold‐ and Platinum‐Catalyzed Cycloisomerization of Enynyl Esters versus Allenenyl Esters: An Experimental and Theoretical Study

Ester-way to heaven: Unexpected formation of bicyclo[3.1.0]hexene 4 was the main focus of combined experimental and theoretical studies on the Au-catalyzed cycloisomerization of branched dienyne 1 (see scheme), which provided better understanding of the mechanistic details governing the cyclization of enynes bearing a propargylic ester group.Experimental and theoretical studies on Au- and Pt-catalyzed cycloisomerization of a branched dienyne with an acetate group at the propargylic position are presented. The peculiar architecture of the dienyne precursor, which has both a 1,6- and a 1,5-enyne skeleton, leads, in the presence of alkynophilic gold catalysts, to mixtures of bicyclic compounds 3, 4, and 5. Formation of unprecedented bicyclo[3.1.0]hexene 5 is the main focus of this study. The effect of the ancillary ligand on the gold center was examined and found to be crucial for formation of 5. Further mechanistic studies, involving cyclization of an enantioenriched dienyne precursor, (18)O-labeling experiments, and DFT calculations, allowed an unprecedented reaction pathway to be proposed. We show that bicyclo[3.1.0]hexene 5 is likely formed by a 1,3-OAc shift/allene-ene cyclization/1,2-OAc shift sequence, as calculated by DFT and supported by Au-catalyzed cyclization of isolated allenenyl acetate 7, which leads to improved selectivity in the formation of 5. Additionally, the possibility of OAc migration from allenyl acetates was supported by a trapping experiment with styrene that afforded the corresponding cyclopropane derivative. This unprecedented generation of a vinyl metal carbene from an allenyl ester supports a facile enynyl ester/allenenyl ester equilibrium. Further examination of the difference in reactivity between enynyl acetates and their corresponding [3,3]-rearranged allenenyl acetates toward Au- and Pt-catalyzed cycloisomerization is also presented.

The Role of Bent Acyclic Allene Gold Complexes in Axis-to-Center Chirality Transfers†

the two orthogonal C=C bonds are the most intuitive form. Depending on the substitution pattern of the allene, the contribution of the two carbon atoms to the coordination might not be strictly equivalent, leading to slipped structures of type I’ or I’’. This case will manifest itself for instance in the presence of electron-donating groups that will stabilize I’, or electron-withdrawing groups that favor I’’. The second category comprises species in which the metal fragment is coordinated to the central allene carbon only. Along with sallylic cations II, there are alternatives, such as zwitterionic carbenes II’ or h-coordinated bent allenes II’’. In this type of complex, the three allene carbons and the metal fragment lie in the same plane, from which the four substituents of the allene deviate significantly. Compounds of type II’’ have been recently isolated as tetraaminoallene rhodium or gold complexes and characterized by X-ray diffraction studies. Interestingly, gold is known to activate allenes toward nucleophilic attack. For instance, aand b-hydroxyallenes or -aminoallenes can be cycloisomerized in the presence of gold(I) or gold(III) catalysts into the corresponding 5and 6membered heterocycles. These transformations have also been accomplished with axis-to-center transfer of chirality, even in intermolecular versions. Thus, the question whether the nucleophile attacks a species of type I–I’’ or II–II’’ is of paramount importance in accounting for the chirality transfer. Indeed, although the stereochemical information is maintained in species I–I’’, the axial chirality of the allene seems to be lost in II or II’. On the other hand, bent allene complexes should retain the chirality of the starting material. We ran a set of computations on model chiral allene gold complexes to understand which factors govern the ground state of I–II’’, and how these species interconvert. We then used our findings to predict which structural variations of the allene could favor a successful chirality transfer. Finally, we verified our hypothesis on some gold(I)-catalyzed cycloisomerizations. We began our study with (R)-1,3-dimethyl allene. Figure 1 shows all the structures that converged as minima in the presence of AuBr3. [14] We found two diastereomeric complexes of type I, and two C2-coordinated complexes of type II and II’’. The type II complex is planar, and has C1 C2 and C2 C3 bond lengths of 1.39 8, and a C1-C2-C3 angle of 1168. The latter has shorter C1 C2 and C2 C3 bond lengths (1.36 8), and is strongly twisted (Me-C1-C3-Me 65.88) to minimize the allylic strain. This complex is the less stable form, the ground state being one of the type I species (DH298 = 3.5 kcalmol ). We investigated a few other metal fragments (Table 1). With the exception of Au, no gold(I) complex of type II’’ could be located. Using PtCl2, the C1 C2 and C2 C3 bond lengths (1.39 8), the small C1-C2-C3 angle (129.18), and the moderate Me-C1-C3-Me tilt angle (45.18) compare quite well with a distorted allylic cation. On the other hand, the allene ligand is less severely bent in AuX3 complexes (C1-C2-C3 140.18 and 136.68 with X=Br and Cl, respectively), the C=C bonds are much less perturbed (1.36 8), and the tilt angles are much more pronounced (65.88 and 59.78). However, in all cases (as for AuBr3; see Figure 1), the ground states are complexes of type I. For instance, with [Au(PMe3)] , the allene complexes 1 and 2 were more stable than the allylic cation 3 (Scheme 2). Whereas 3 was the only C2-coordinated Scheme 1. Various coordination modes of allenes. M = metal–ligand fragment.

Gold(I)-Catalyzed Cyclization of β-Allenylhydrazones: An Efficient Synthesis of Multisubstituted N-Aminopyrroles

The gold(I)-catalyzed cycloisomerization of β-allenylhydrazones provides an efficient access to multisubstituted N-aminopyrroles, which are obtained in good to excellent yields. This new intramolecular cyclization method can be applied either to alkyl- or aryl-substituted allenes. The reaction proceeds under mild conditions with short reaction times through a selective intramolecular 1,2-alkyl or -aryl migration extending the general scope of the reaction.

Bi(OTf)3-Catalyzed Cycloisomerization of Aryl-Allenes

Intramolecular hydroarylation of allenes was achieved under very mild conditions using bismuth(III) triflate as the catalyst. Efficient functionalization of activated and nonactivated aromatic nuclei led to C-C bond formation through a formal Ar-H activation. A tandem bis-hydroarylation of the allene moiety was also developed giving access to various interesting polycyclic structures.

Catalytic Activation of Olefins by Metal Triflates and Triflimides: Application to Fragrance Chemistry

Fragrance compounds constitute a wide family of relatively volatile compounds presenting interesting odour properties. Several processes catalysed by metal triflates and triflimides have been recently developed with a view to applications to fragrance chemistry. These reactions, involving the activation of non-activated olefins, mainly involve inter- and intramolecular carbon-carbon, carbon-oxygen and carbon-sulfur bond formation.

Catalytic Rearrangement of 2-Alkoxy Diallyl Alcohols: Access to Polysubstituted Cyclopentenones

A catalytic rearrangement of diallyl alcohols comprising a cyclic enol ether has been developed using very mild conditions. Bismuth(III) triflate was found to be a very active catalyst for the ring rearrangement of a range of tertiary allylic alcohols to efficiently afford polysubstituted cyclopentenones with a high degree of diastereoselectivity.

Bismuth(III) triflate-catalysed tandem cyclisations towards complex polycyclic ethers

Herein is described a double cyclisation of α-hydroxy enol ethers with a tethered olefinic double bond, readily obtained from simple ketone precursors. Interesting bridged polycyclic compounds with an additional oxaspirocycle are formed under mild conditions using a low loading of recyclable bismuth(III) triflate as the catalyst. Some experimental evidence accounts for concerted (or pseudo-concerted) polycyclisation.

Tuning the Reactivity of Functionalized Diallylic Alcohols: Brønsted versus Lewis Acid Catalysis

The chemodivergent reactivity of bifunctional, enol thioether-containing diallylic alcohols in acidic medium is disclosed, highlighting the difference between strong Lewis acid and mild Brønsted acid catalysis. In the presence of bismuth(III) triflate, allylic alcohol activation affords diversely substituted cyclopentenones in a Nazarov-type electrocyclization, whereas activation of the thioenol ether by p-toluenesulfonic acid provides an entry to α-sulfenylated β,γ-unsaturated ketones. Both methods represent a facile access to the corresponding products under mild conditions, using inexpensive and non-toxic catalytic systems.