Louis Fensterbank

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.

Visible-Light-Induced Photoreductive Generation of Radicals from Epoxides and Aziridines†

Epoxides are readily available and highly valuable radical precursors, as demonstrated by their omnipresence in radical transformations. Several methodologies have been reported so far for their radical ring-opening. Epoxides are particularly prone to reductive ring-opening via one-electron transfer. Lithium 4,4′-di-tert-butylbiphenylid (LDBB) has thus been widely used to generate lithiated radical anions such as 99. This latter is immediately reduced to dianion 100 which can add to various electrophiles.

N-heterocyclic carbene boryl radicals: a new class of boron-centered radical.

Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 x 10(4) M(-1) s(-1), and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a pi-radical, analogous to the benzyl radical.

Gold‐Catalyzed Cross‐Couplings: New Opportunities for CC Bond Formation

Cross‐coupling is a powerful tool for the rapid construction of highly valuable compounds. In this area of chemical transformations, new catalysts have recently emerged. Combining both a π‐Lewis acid character and interesting redox properties, gold has proven to be an expedient choice for mediating such transformations. Recent developments in the use of gold to mediate a variety of CC coupling reactions are summarized, including Suzuki and Sonogashira cross‐coupling reactions, as well as the development of tandem processes and the combination of gold with palladium to enable CC bond formation via transmetalation.

Complexes of Borane and N-Heterocyclic Carbenes : A New Class of Radical Hydrogen Atom Donor

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.

Recent advances in the use of phosphorus-centered radicals in organic chemistry

This tutorial review aims at presenting recent contributions dealing with organic chemistry of organophosphorus radicals. The first part briefly lays out the physical organic background of such intermediates. In a second part the use of organophosphorus radicals possessing a P-H bond that can undergo homolytic cleavage as alternative mediators is detailed. The third part is focused on radical additions of phosphorus-centered radicals to unsaturated compounds, an old reaction that is being rejuvenated. Lastly, radical eliminations of phosphorus-centered radical are introduced in the fourth part. Most of the latter are relatively novel reactions, and have never been reviewed previously.

Generation and Reactions of an Unsubstituted N‐Heterocyclic Carbene Boryl Anion

Lying low: A lithiated unsubstituted N-heterocyclic carbene (NHC) boryl anion can be generated by reduction, and trapped by electrophiles (see scheme; dipp=2,6-diisopropylphenyl) to provide new substituted NHC boranes. It is yet another example of a low-valent boron compound or boron-containing reactive intermediate stabilized by an NHC, thereby extending the scope of NHC borane chemistry. Copyright

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.

Silicates as Latent Alkyl Radical Precursors: Visible‐Light Photocatalytic Oxidation of Hypervalent Bis‐Catecholato Silicon Compounds

This works introduces hypervalent bis-catecholato silicon compounds as versatile sources of alkyl radicals upon visible-light photocatalysis. Using Ir[(dF(CF3)ppy)2(bpy)](PF6) (dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine, bpy = bipyridine) as catalytic photooxidant, a series of alkyl radicals, including highly reactive primary ones can be generated and engaged in various intermolecular homolytic reactions. Based on cyclic voltammetry, Stern-Volmer studies, and supported by calculations, a mechanism involving a single-electron transfer from the silicate to the photoactivated iridium complex has been proposed. This oxidative photocatalyzed process can be efficiently merged with nickel-catalyzed Csp2-Csp3 cross-coupling reactions.