Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

1.1. General Reactivity of Alkyne-Gold(I) Complexes
For centuries, gold had been considered a precious, purely decorative inert metal. It was not until 1986 that Ito and Hayashi described the first application of gold(I) in homogeneous catalysis.1 More than one decade later, the first examples of gold(I) activation of alkynes were reported by Teles2 and Tanaka,3 revealing the potential of gold(I) in organic synthesis. Now, gold(I) complexes are the most effective catalysts for the electrophilic activation of alkynes under homogeneous conditions, and a broad range of versatile synthetic tools have been developed for the construction of carbon–carbon or carbon–heteroatom bonds.

Gold(I) complexes selectively activate π-bonds of alkynes in complex molecular settings,4−10 which has been attributed to relativistic effects.11−13 In general, no other electrophilic late transition metal shows the breadth of synthetic applications of homogeneous gold(I) catalysts, although in occasions less Lewis acidic Pt(II) or Ag(I) complexes can be used as an alternative,9,10,14,15 particularly in the context of the activation of alkenes.16,17 Highly electrophilic Ga(III)18−22 and In(III)23,24 salts can also be used as catalysts, although often higher catalyst loadings are required.

In general, the nucleophilic Markovnikov attack to η2-[AuL]+-activated alkynes 1 forms trans-alkenyl-gold complexes 2 as intermediates (Scheme 1).4,5a,9,10,12,25−29 This activation mode also occurs in gold-catalyzed cycloisomerizations of 1,n-enynes and in hydroarylation reactions, in which the alkene or the arene act as the nucleophile.



Scheme 1

Anti-Nucleophilic Attack to η2-[AuL]+-Activated Alkynes

/pdf/gold-i-catalyzed-activation-of-alkynes-for-the-construction-50i37g5kgj.pdf

Gold(I)-Catalyzed Activation of Alkynes for the Construction of Molecular Complexity

A.L.-P. thanks CSIC for a contract under the JAE-doctor
program. Financial support by PLE2009 project from MCIINN
and Consolider-Ingenio 2010 (proyecto MULTICAT) are also
acknowledged.

Gold-catalyzed carbon-heteroatom bond-forming reactions.

Allenes are highly valuable synthetic precursors in preparative organic chemistry because of their ability to undergo a variety of transformations. Among many different reaction modes, activation of the cumulated double bonds by treatment with a Brønsted or Lewis acid is particularly useful because it allows a nucleophilic attack, which leads to the formation of a new C-C or Cheteroatom bond in an interor intramolecular fashion. Because of their axial chirality, allenes can undergo this bond formation with chirality transfer, which renders the method attractive for stereoselective target-oriented synthesis. Because of their soft and carbophilic character, gold catalysts are particularly well suited for the selective activation of allenes in the presence of other reactive functionalities. Compared to intermolecular additions, gold-catalyzed cyclization reactions of allenes by intramolecular nucleophilic attack have received much more attention. Here, the gold catalyst can coordinate to either allenic double bond, and the regioselectivity of the subsequent nucleophilic attack depends on the structure of the substrate, in particular the length of the tether connecting allene and nucleophile (Scheme 1). Hence, four different endoor exo-cyclization products can be obtained, but the formation of fiveor sixmembered rings via σ-gold species A or D, i.e., by nucleophilic attack at a terminal allenic carbon atom, is favored in most cases, whereas products arising from nucleophilic attack at the central allenic carbon atom (via intermediates B and C) are rare. Normally, these cyclization products are chiral and can be accessed in a stereoselective manner either from chiral allenes by axis-to-center chirality transfer or from achiral allenes utilizing chiral gold catalysts. This account comprises gold-catalyzed cyclization reactions of allenes by attack of carbon or heteroatom nucleophiles. Intermolecular addition reactions to allenes, as well as gold-catalyzed transformation of nonallenic substrates in which an allene is generated in situ (e.g., by sigmatropic rearrangement) and converted without isolation, will not be covered.

Gold-catalyzed nucleophilic cyclization of functionalized allenes: a powerful access to carbo- and heterocycles.

In this tutorial review directed towards chemists interested in synthesis or catalysis, the application of gold catalysis in total synthesis is summarised and the mode of activation of the substrate by the gold catalyst is discussed.

Gold catalysis in total synthesis.

The Role of Gold Acetylides as a Selectivity Trigger and the Importance of gem-Diaurated Species in the Gold-Catalyzed Hydroarylating-Aromatization of Arene-Diynes

In the field of homogeneous transition-metal chemistry, gold plays a major role in the discovery of new reactions. Most of the transformations are based on the electrophilic activation of a multiple bond. The decrease of electron density upon p coordination of a carbophilic gold center allows nucleophilic attack. Now, in independent and parallel work the group of Zhang and our team have discovered a different reactivity which was so far unprecedented in the field of gold chemistry. Initial activation of an alkyne by s coordination to gold increases the nucleophilicity of the b-carbon atom of this alkyne. This first alkyne combines with a second alkyne, which is activated by p coordination, and highly reactive gold vinylidene intermediates are formed by this dual s/p activation. The Zhang group s publication on the synthesis of benzofulvenes now prompts us to publish our additional findings on that specific reaction. Since alkyl-substituted alkynes are readily available, a new fascinating reaction pathway was opened for the gold vinylidene intermediates. The first two elementary steps of this reaction are identical to the previous reactions, which seem to be general for this new sector of gold catalysis. Herein we report on the use of tertiary alkyl groups, the isolation of gem-diaurated species which prove to be ideal precatalysts for this type of transformation, the dynamics of the equilibrium involving these diaurated species, and the catalyst transfer in the context of a detailed mechanistic discussion. We investigated several diyne systems 1 with a terminal alkynyl group and a tert-alkyl-substituted alkynyl group as potential substrates for the gold(I)-catalyzed intermolecular arene addition. A new reaction was observed, the clean formation of a benzofulvene derivative, a class of compounds that usually is not easily available. No incorporation of the solvent benzene was observed. To optimize the reaction, we performed reactions of substrate 1 a (Table 1) with different gold catalysts and different counterions (see the Supporting Information); the well-established catalyst [(IPr)AuCl], in combination with AgNTf2 gave the best results. To evaluate the substrate scope, a small library of substrates was transformed to the corresponding products under the optimized conditions (Table 1). One advantage of substrates 1 is their easy two-step synthesis (Sonogashira reaction, Seyferth–Gilbert homologation) from commercially available 2-bromobenzaldehydes. With our test substrate 1a,

Simple gold-catalyzed synthesis of benzofulvenes--gem-diaurated species as "instant dual-activation" precatalysts.

A series of easily accessible arene-1,2-diynes, bearing one aryl substituent on one of the alkynyl groups, is readily converted to dibenzopentalenes in good yields by gold(I) catalysts The participation of gold acetylides could be proven by the direct conversion to the corresponding gem-diaurated dibenzopentalenes with a gold catalyst From an experiment with a gold acetylide complex and stoichiometric amounts of the gold “catalyst” the corresponding gem-diaurated complex of a dibenzopentalene could be obtained and characterized by X-ray crystal structure analysis Labelling studies with deuterated alkynes show the expected deuteration of the two remaining positions of the pentalene core All this provides evidence for a dual activation mode of the reaction and gold(I) vinylidene complexes as intermediates of the catalytic cycle

Gold-Catalyzed Synthesis of Dibenzopentalenes – Evidence for Gold Vinylidenes

The substrate scope, the mechanistic aspects of the gold-catalyzed oxazole synthesis, and substrates with different aliphatic, aromatic, and functional groups in the side chain were investigated. Even molecules with several propargyl amide groups could easily be converted, delivering di- and trioxazoles with interesting optical properties. Furthermore, the scope of the gold(I)-catalyzed alkylidene synthesis was investigated. Further functionalizations of these isolable intermediates of the oxazole synthesis were developed and chelate ligands can be obtained. The use of Barluenga's reagent offers a new and mild access to the synthetically valuable iodoalkylideneoxazoles from propargylic amides, this reagent being superior to other sources of halogens.

Matthias Rudolph

Papers

Gold catalysis in total synthesis.

The Role of Gold Acetylides as a Selectivity Trigger and the Importance of gem-Diaurated Species in the Gold-Catalyzed Hydroarylating-Aromatization of Arene-Diynes

Simple gold-catalyzed synthesis of benzofulvenes--gem-diaurated species as "instant dual-activation" precatalysts.

Gold-Catalyzed Synthesis of Dibenzopentalenes – Evidence for Gold Vinylidenes

Cyclization of Propargylic Amides: Mild Access to Oxazole Derivatives