Palladium-catalyzed Silylstannylation-cyclization of 1,6-diynes ; Axial Chirality in (Z, Z)-1,3-dienes PDF Download
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Author: Sandra Warren Publisher: ISBN: Category : Chirality Languages : en Pages : 886
Book Description
The silylstannane-mediated cyclization of mono-substituted 1,6-diynes is readily catalyzed by palladium in the presence of a weakly coordinating phosphine. The products of the reaction are highly functionalized carbo- or heterocycles. The reaction displays wide functional group compatibility, tolerating groups like carboxylic esters, amines and amides. The reactionis not sensitive to moisture or air, and the isolation of the products is operationally simple. The 1,2-bis-alkylidenecycloalkane products display very interesting stereochemical fearf and stannyl groups are pointing toward each other, despite the apparent steric hindrance thus generated. Second, this stereochemistry of the individual double-bonds can lead to axial chirality in the diene system. It has been shown that the 1,2-bis-alkylidenecycloalkanes are indeed axially chiral at lowtemperatures, but they display a fluxional behavior at room temperature. The coalescence temperatures have been determined for several compounds, ranging from 670ðC to + 20ðC, giving insight into the exchange process and the influence of various substituents. Also, the energies of activation of the exchange process have been determined via NMR Line Shape Analysis for a selected set of compounds. These energies appear to depend not only on the groups on the silicon and the tin, but also on the cyclopentane ring substituents. Free energies of activations in the range of 52.3 to 56.9 kJ.mol-1 were observed. The synthetic applications of these compounds have been the object of preliminary studies. Although many reactions did not yield the desired products, tin-lithium and tin-halogen exchange and epoxidation of the alkenes seem to offer some promise. Diels-Alder reactions of the destannylated products can be carried out in quantitative yield. The first example of a Stille coupling with a vinyl iodide is also described.
Author: Sandra Warren Publisher: ISBN: Category : Chirality Languages : en Pages : 886
Book Description
The silylstannane-mediated cyclization of mono-substituted 1,6-diynes is readily catalyzed by palladium in the presence of a weakly coordinating phosphine. The products of the reaction are highly functionalized carbo- or heterocycles. The reaction displays wide functional group compatibility, tolerating groups like carboxylic esters, amines and amides. The reactionis not sensitive to moisture or air, and the isolation of the products is operationally simple. The 1,2-bis-alkylidenecycloalkane products display very interesting stereochemical fearf and stannyl groups are pointing toward each other, despite the apparent steric hindrance thus generated. Second, this stereochemistry of the individual double-bonds can lead to axial chirality in the diene system. It has been shown that the 1,2-bis-alkylidenecycloalkanes are indeed axially chiral at lowtemperatures, but they display a fluxional behavior at room temperature. The coalescence temperatures have been determined for several compounds, ranging from 670ðC to + 20ðC, giving insight into the exchange process and the influence of various substituents. Also, the energies of activation of the exchange process have been determined via NMR Line Shape Analysis for a selected set of compounds. These energies appear to depend not only on the groups on the silicon and the tin, but also on the cyclopentane ring substituents. Free energies of activations in the range of 52.3 to 56.9 kJ.mol-1 were observed. The synthetic applications of these compounds have been the object of preliminary studies. Although many reactions did not yield the desired products, tin-lithium and tin-halogen exchange and epoxidation of the alkenes seem to offer some promise. Diels-Alder reactions of the destannylated products can be carried out in quantitative yield. The first example of a Stille coupling with a vinyl iodide is also described.
Author: Sandeep D. Apte Publisher: ISBN: Category : Chirality Languages : en Pages : 127
Book Description
Abstract: The formation of silylstannyl dienes from 1,6-diynes was first reported by our former research group members. These dienes are fascinating molecules as they exhibit helical chirality. However, monocyclic dienes are known to undergo fast helical isomerization. For our main goal of making a unidirectional helical polyolefin, a diene in which this helical isomerization is completely frozen is necessary. A bicylic motif was designed to obtain such a system by slowing down this isomerization. The chemistry of these silylstannyl dienes was unexplored. We probed the Stille coupling reactions of these compounds with various electrophiles. A new protocol for the Stille coupling which used 2 mol% Pd(MeCN)2Cl2, 4 mol% AsPh3 and 8 mol% CuI in DMF at room temperature was found to be effective for several difficult substrates. Using these conditions, iodobenzene, bromotrimethylsilyl acetylene, benzyl bromide and 1,4-diiodobenzene were efficiently coupled. Synthesis of a polyolefin was attempted by coupling of a silylstannyl diene with its iodide derivative under the same conditions. However, instead of the expected tetraene, a rearranged product was observed which presumably was formed via a 1,7 -hydrogen shift. The Palladium-catalyzed addition of silylstannane reagents to alkynes is known to be a synthetically useful reaction. The tolerance of a sensitive functionality like epoxide iii had not been reported before. We examined the addition of Bu3SnSiMe3 to epoxyalkynes and found that the desired addition product was formed in excellent yields. Such products can be subjected to the well-known Ti(III)-mediated epoxide opening-radical cyclization. The reaction presumably occurs via radical formation followed by an addition-elimination sequence to give the expected product. This reaction affords five-membered carbo- and heterocyclic compounds. The presence of the vinyl-TMS group at the terminal provides a potentially useful alcohol. Similarly epoxyalkynes undergo Pd-catalyzed distannation followed by Ti(III) mediated reaction to give cyclic alcohols with vinylstannane, which is more reactive than the corresponding vinyl vinylsilane. The application of this addition-elimination reaction was expected for the formation of four-membered carbocyclic compounds. However, the radical formed by epoxide opening abstracts the vinyl hydrogen via a 6-membered transition state and an internal alkyne was obtained.
Author: Jong Uk Rhee Publisher: ISBN: Category : Catalysis Languages : en Pages :
Book Description
Abstract: Part One: The Barton-McCombie reaction has been widely acknowledged for the efficient deoxygenation of 2° alcohols through radical process. For the chain process, the radical source initially generated by the catalytic amount of AIBN attacks the thiocarbnyl sulfur to produce a radical intermediate that is cleaved at the beta-position to give alkyl radicals. Although the sp3 carbon centered radicals have been used commonly as precursors of 6-exo-trig ring closure, the radical intermediate in the thiourethane-mediated deoxygenation of alcohol has never been considered as a precursor of radical cyclization. However, the radical intermediate can participate in an exo-hex-5-enyl or exo-hept-6-enyl type intramolecular cyclization when a suitable radical acceptor is appropriately placed. Moreover, structurally rare examples of N-furanosides and N-pyranosides have been synthesized efficiently from carbohydrate-derived imidazoyl and triazoyl thioates by utilizing the new Barton's radical-mediated methodology. Depending on the radical acceptors, glycosides with either C2-carbon or C2-amino substituents are formed. The C2 stereogenic center of the N-pyranosides formed via exo-hept-6-enyl type radical cyclization can also be controlled by the stereochemistry of the radical acceptor. While (E)-olefin acceptors make allo/altro mixture, (Z)-olefin acceptors give stereoselectively altro-isomers. Part Two: Palladium catalyzed tandem bifunctionalization/cyclization of enynes, diynes, bis(allenes), alleneynes, allene aldehydes, and allene ketones with R3Sn-SiR'3 is a useful tactic for the synthesis of synthetically interesting heterocyclic and/or carbocyclic compounds. Although the reaction has a broad tolerance to functional groups, operationally simple procedure, and high catalytic turnover and regioselcetivity, the applications of the silylstannylated carbocyclization to further transformation are rare. The palladium catalyzed silylstannylation/cyclization of 1, 6-diynes gives two atropisomeric isomers with axial chirality. The silylstannyl dienes themselves do not take part in Diels-Alder reactions, but the removal of Sn has been achieved efficiently under acidic condition and the resulting silyldienes have restored this reactivity. In this study a number of fused carbocyclic or heterocyclic compounds were made by the Diels-Alder reaction and the regioselectivity was controlled by steric effects. This regioselective Diels-Alder route has been used for the synthesis of an intermediate for a Papulacandin D core synthesis. Asymmetric version of silylstannylation/carbocyclization of alleneynes has been screened with a various chiral phosphine ligands.
Author: Thomas P. Fehlner Publisher: Springer Science & Business Media ISBN: 1489924590 Category : Science Languages : en Pages : 409
Book Description
There is a certain fascination associated with words. The manipulation of strings of symbols according to mutually accepted rules allows a language to express history as well as to formulate challenges for the future. But language changes as old words are used in a new context and new words are created to describe changing situations. How many words has the computer revolution alone added to languages? "Inorganometallic" is a word you probably have never encountered before. It is one created from old words to express a new presence. A strange sounding word, it is also a term fraught with internal contradiction caused by the accepted meanings of its constituent parts. "In organic" is the name of a discipline of chemistry while "metallic" refers to a set of elements constituting a subsection of that discipline. Why then this Carrollian approach to entitling a set of serious academic papers? Organic, the acknowledged doyenne of chemistry, is distinguished from her brother, inorganic, by the prefix "in," i. e. , he gets everything not organic. Organometallic refers to compounds with carbon-metal bonds. It is simple! Inorganometallic is everything else, i. e. , compounds with noncarbon-metal element bonds. But why a new term? Is not inorganic sufficient? By virtue of training, limited time, resources, co-workers, and so on, chemists tend to work on a specific element class, on a particular compound type, or in a particular phase. Thus, one finds element-oriented chemists (e. g.
Author: Karl Kirchner Publisher: Springer Science & Business Media ISBN: 3709162742 Category : Science Languages : en Pages : 165
Book Description
From the beginning of chemistry as an exact (natural) science - almost 200 years ago - there was a more or less distinct differentiation between its various branches such as organic, inorganic, physical, analytical, or biochemistry. With the increasing insight into the connections and governing laws it soon became obvious, however, that such a clear separation could be regarded as more or less obsolete; within almost any field of chemical research one has to deal with most of the branches mentioned. Especially organic and inorganic chemistry are significant examples for this statement, overlapping considerably within the important field of organome tallic chemistry. This regime of chemistry started its advance with the discovery of dimethylzinc 150 years ago, had a highlight with the introduction of Grignard reagents around 1900, developed further with the start of lithium organyls in 1925 and literally exploded after the discovery of the first transition metal cyclopenta dienyl complex ferrocene half a century ago. The chronological sequence of the important steps, i. e. 1850 (Zn) - 1900 (Mg) - 1925 (Li) - 1950 (Fe), seems rather remarkable. The increasing group of metallocenes is not only of high theoretical and, due to the potential chirality of its members, stereochemical interest, but offers also a wide variety of extremely useful catalysts, especially for stereoselective reactions. The Austrian Chemical Society took this development into account by organizing the Twelfth International Conference on Organometallic Chemistry held in Vienna in 1985.
Author: Charlie Harding Publisher: Royal Society of Chemistry ISBN: 9780854046904 Category : Education Languages : en Pages : 318
Book Description
This book covers the chemistry of the non-metallic elements (the halogens, boron, carbon, nitrogen, oxygen, silicon, phosphorus and sulfur) and uses their role in agriculture (for example, nitrogen and sulfur), industry (for example, sulfuric acid), and everyday life (for example, the chlorination of drinking water) to illustrate this chemistry. Their role in organic chemistry and biochemistry is also emphasized. Two interactive CD-ROMs accompany the book, incorporating electronic questions that facilitate revision/consolidation. This book is part of The Molecular World series which aims to provide a broad foundation in chemistry.
Author: Sandra Warren
Author: Sandra Warren
Author: Sandeep D. Apte
Author: Jong Uk Rhee
Author: 
Author: Thomas P. Fehlner
Author: Karl Kirchner
Author: Charlie Harding