Enäther als Reaktionspartner von Organometallen II Hydroxyäthyl-allene und andere Folgeprodukte aus Dihydrofuran-Derivaten

Upon treatment with organolithium compounds, 2-alkyl-4,5-dihydrofurans undergo ring opening through β-elimination leading to the corresponding 3,4-dien-1-ol. If 3-chloro-2-methyl-4,5-dihydrofuran serves as a substrate, however, no 3-chloro-3,4-dien-1-ol can be isolated though it acts as a reaction intermediate. Its formation is slow compared to subsequent replacement of halogen by the organic moiety of the alkyllithium reagent. Thus penta-3,4-dien-1-ols are formed, which may isomerize, however, under certain reaction conditions affording terminal acetylenes. These as well as their allene precursors can be converted with sodium in ammonia into pent-4-en-1-ol or, respectively, pent-3-en-1-ol derivates.     

Total asymmetric syntheses of β-hydroxy-δ-lactones via Umpolung with sulfur dioxide

Cyclic stereotriads and stereotetrads of the β-hydroxy-δ-lactone type, e.g. prelactones B and E, common in polyketides and polypropionates, are prepared via SO(2)-induced oxyallylations of enoxysilanes with (1E,3Z)-1-(1-phenylethoxy)penta-1,3-dien-3-yl carboxylates. Using (Z)- or (E)-enoxysilanes both 4,5-cis- or 4,5-trans-δ-lactones are obtained. Depending on the reduction method applied to the obtained aldol intermediates 5,6-trans or 5,6-cis-derivatives are formed. The δ-lactones can be prepared in both their enantiomeric forms depending on the (1R)- or (1S)-configuration of the starting 1-(1-phenylethoxy)penta-1,3-dienes.     

Formal alkylation of allenes through highly selective radical cyclizations of allene-enes

Something radical: the first example of alkene-to-allene radical cyclization of allene-enes is reported. The highly chemoselective intermolecular radical addition reaction of the alkene and subsequent, exclusive exo-radical addition to the allene was realized with perfluoroalkyl radicals. A subsequent TBAF-promoted dehydroiodination of the cyclization products forms cyclopentanes and regenerates an allene moiety (TBAF=tetra-n-butylammonium fluoride).     

Generation of alpha-phosphonovinyl radicals and development of a new route to highly functionalized vinylphosphonates and vinylphosphonate-incorporated carbocyclic or heterocyclic compounds via a radical trapping sequence

The first direct generation of synthetically useful alpha-phosphonovinyl radicals was achieved by treatment of alpha-phosphonovinyl halides with a tributyltin radical. The alpha-phosphonovinyl radicals 2a-d were trapped with electron-rich olefins and an electron-deficient olefin to produce alpha-functionalized vinylphosphonates 3a-f in 16-55% yields. The alpha-phosphonovinyl radicals 7e-g containing the YCH2CH=CH2 (Y = O, CH2, S) substituent at the beta-position afforded mixtures of 5-exo and 6-endo cyclization products, 5e-g and 6e-g, in good yields. The 5-exo/6-endo product ratios increase in the following order of the beta-substituent: OCH2CH=CH2 > CH2CH2CH=CH2 > SCH2CH=CH2. The effects of the beta-substituents upon the cyclization reaction were discussed. Radical cyclization of alpha-phosphonovinyl radicals bearing functional groups such as geranyloxy, geranylthio, and (2-cyclohexen-1-yl)thio groups at the beta-position afforded 5-exo, 5-exo and 6-endo, and cis-fused-5,6-ring cyclization products incorporating an alpha,beta-unsaturated phosphonate unit within the ring, respectively, in good yields. The alpha-phosphonovinyl radical 20 underwent tandem radical cyclization-radical cyclization to produce a mixture of two isomeric bicyclo[4.3.0]nonenes including a vinylphosphonate moiety in high yield.     

Synthesis,X-ray structure,and stevens rearrangement of 1,1-di(prop-2-yn-1-yl)-2,5-dihydro-1H-pyrrol-1-ium bromide

1,1-Di(prop-2-yn-1-yl)-2,5-dihydro-1H-Pyrrol-1-ium bromide was obtained with interaction of 3-pyrroline with propargyl bromide and its structure was studied by X-ray analysis. It has been shown that the Stevens rearrangement of obtaining salt leads to the formation of 1-(hexa-4,5-dien-1-yn-3-yl)-2,5-dihydro-1H-pyrrol.     

Reprint of: Recent advances in the functionalization of allenes via radical process

This review is focused on the recent advances in the functionalization of allenes via radical process. Different radical partners including carbon radicals and heteroatom radicals are discussed in the reactions of allenes. Generally, the radical formed in situ would attack the allene at the central carbon leading to allyl radical intermediate. However, the formation of alkenyl radical intermediate from allene could be observed as well in some cases with high regioselectivity and stereoselectivity.     

Recent advances in the functionalization of allenes via radical process

This review is focused on the recent advances in the functionalization of allenes via radical process. Different radical partners including carbon radicals and heteroatom radicals are discussed in the reactions of allenes. Generally, the radical formed in situ would attack the allene at the central carbon leading to allyl radical intermediate. However, the formation of alkenyl radical intermediate from allene could be observed as well in some cases with high regioselectivity and stereoselectivity.     

5-Exo versus 6-endo cyclization of primary aminyl radicals: an experimental and theoretical investigation

The cyclization of neutral primary pent-4-enylaminyl radicals was investigated experimentally and theoretically. Unlike the corresponding secondary aminyl radicals, primary pent-4-enylaminyl radicals underwent efficient cyclization to afford the pyrrolidine and/or piperidine products in good to high yields. While the simple pent-4-enylaminyl radical gave predominately the 5-exo cyclization product, 4-chloropent-4-enylaminyl radicals led to the formation of the corresponding 6-endo cyclization products in excellent regioselectivity. Theoretical calculations revealed that the 5-exo cyclization rate of primary aminyl radicals is about 3-4 orders of magnitude higher than that of secondary aminyl radicals.     

Synthesis and characterization of oligocyclosiloxanes via the hydrosilation of vinylsilanes and vinylsiloxanes with heptamethylcyclotetrasiloxane

The preparation of oligocyclosiloxanes via hydrosilation reactions has been investigated. Hydrosilation reactions employing heptamethylcyclotetrasiloxane and vinyl-containing silanes and siloxanes yielded a variety of oligocyclosiloxanes with various numbers of cyclosiloxane rings connected through tri-, tetra-, penta-, hexa-, hepta-, or octafunctional junctions. The oligocyclosiloxanes were characterized by gas chromatography, IR spectroscopy, ²⁹Si-, ¹H-, and ¹³C-NMR spectroscopy, elemental analysis, and vapor phase osmometry. The polycyclics were redistributed with hexamethyldisiloxane and the resulting fragments analyzed by gas chromatography. All results were consistent with the formation of isomeric oligocyclosiloxanes due to a lack of absolute regioselectivity of the hydrosilation reaction. © 1993 John Wiley & Sons, Inc.     

A rapid divergent synthesis of highly substituted delta-lactones

[reaction: see text] Nucleophilic 1,2-addition of (Z)-gamma-silyloxyvinylzinc reagents to ethyl glyoxylate followed by desilylation and cyclization affords 3,6-dihydro-3-hydroxypyran-2-ones in good chemical yields. In situ formation of allylic phosphates followed by reaction with RCu(CN)Li reagents affords substituted 5,6-dihydropyran-2-ones. The parent compound, 3,6-dihydro-3-hydroxypyran-2-one, undergoes allylic phosphate formation, cuprate-mediated allylic substitution, and 1,4-conjugate addition to afford trans-4,5-disubstituted tetrahydropyran-2-ones in a one-pot process.     

Reactivity and selectivity in the cyclization of sila-5-hexen-1-yl carbon-centered radicals

A trio of sila-5-hexen-1-yl radicals has been prepared from the corresponding halides by reaction with tri-$\text{n}$-butyltin hydride (deuteride). The radicals possessing a dimethylsilyl function α or β to the carbon radical center demonstrated marked reduction in total (but especially $\text{exo-trig}$) cyclization compared to the all-carbon system. The γ-silyl radical behaved, contrariwise, quite comparably to the all-carbon system. The difference in cyclization found in the α-silyl radical was demonstrated to result from both a pronounced decrease in cyclization rate $\text{via}$ the expected $\text{exo-trig}$ mode and $\text{and from a significantly enhanced rate of hydrogen abstraction from TBTH}$. Both the α- and γ-silyl radicals cyclized $\text{via}$ the $\text{endo-trig}$ mode at rates close to that of the parent 5-hexen-1-yl radical itself. The cyclizations studied were demonstrated to be irreversible. The kinetic control thus shown by the preferred formation of $\text{endo}$ cyclized product from the α- and β-silyl radicals is highly unusual and represents $\text{the first report of carbon-centered 5-hexen-1-yl type radicals violating the Baldwin-Beckwith rule}$$\text{exo-trig}$ cyclization preferred by 5-hexen-1-yl radicals). Rationalization of the cyclization behavior of the α- and γ-silyl radicals involves both steric and electronic factors. The behavior of the most unusual case, the β-silyl radical, which has the lowest cyclization propensity and no $\text{exo}$ mode product, remains largely unexplained because its hydrogen abstraction rate from TBTH is unavailable as yet. Some speculative considerations involving the preferred radical conformation in this system and its relation to cyclization are given.     

Synthesis and Photoinitiated Isomerizations of 3-(4-Nitrophenyl)-and 3-(4-Aminophenyl)bicyclo[2.2.1]hepta-2,5-diene-2-carbaldehyde and -2-carboxylic acid Derivatives

3-(4-Nitrophenyl)bicyclo[2.2.1]hepta-2,5-diene-2-carbaldehyde and its derivatives at the formyl group were synthesized. By reduction of the nitro group the corresponding 3-(4-aminophenyl)-substituted compounds were obtained, as well as 4,4'-bis(bicyclo[2.2.1]hepta-2,5-dien-2-yl)azobenzenes. Irradiation of the resulting norbornadiene derivatives in the region of their absorption maxima resulted in valence isomerization with formation of the corresponding quadricyclanes. The reverse transformations are promoted by molybdenum(VI) oxide as heterogeneous catalyst. 4,4'-Bis(bicyclo[2.2.1]hepta-2,5-dien-2-yl)azobenzenes undergo reversible (on heating) photochemical EZ/ isomerization at the NÍN bond.     

One product, two pathways: initially divergent radical reactions reconverge to form a single product in high yield

The paper describes examples of net diastereotopic-group-selective radical processes having the unusual feature that a single product is formed even though the key reaction of the two diastereotopic radical precursors is nonselective. For example, reaction of (R)-N-(cyclohex-2-en-1-yl)-N-(2,6-diiodo-4-methylphenyl)acetamide with tributyltin hydride produces 1-((4aR,9aR)-6-methyl-2,3,4,4a-tetrahydro-1H-carbazol-9(9aH)-yl)ethanone with high product selectivity and in high yield. Analysis of the concentration profiles of the closed-shell intermediates at the halfway point of the reaction shows that nonselective abstraction of diastereotopic iodides by tin radicals occurs, leading to diastereomeric aryl radicals. These isomeric intermediates evolve via two nonintersecting reaction pathways, cyclization and bimolecular trapping or vice versa, into the same final product. Origins of the selectivity are suggested on the basis of conformational analysis of the products using both X-ray crystallography and density functional theory calculations.     

Rhodium complex-catalyzed cycloisomerization of allenenes: exo and endo cyclization depending on the auxiliary ligands

In the presence of a catalytic amount of a rhodium(I) complex, allenenes undergo cycloisomerization reactions resulting in the selective formation of exo-alkylidenecarbocycles and heterocycles. In the catalytic system of rhodium complexes with triaryl phosphites, cyclic 1,4- or 1,5-dienes are formed in good to excellent yields in the formal exo-cyclization mode via the metallacycle intermediate having an exo-alkylidene moiety. In this cycloisomerization, (E)- and (Z)-allenenes are transformed stereospecifically to the corresponding cyclic (E)- and (Z)-1,4-dienes, respectively. On the other hand, the reactions under carbon monoxide atmosphere exclusively afford seven-membered-ring products through an endo-mode cyclization. The unusual cyclization involves an allylic C-H activation process. The allenene bearing a silicon substituent at the olefinic terminus incorporates carbon monoxide to give the corresponding [2+2+1] cycloaddition product. This result apparently indicates that the catalysis of the rhodium complex is explained in terms of the oxidative cyclization of an allenene to furnish the key exo-alkylidene metallacycle intermediate at the first stage of the catalysis.     

A new five-membered ring forming process based on palladium(0)-catalyzed arylative cyclization of allenyl enones

A palladium(0)/monophosphine catalyst promotes a novel arylative cyclization reaction of C1-, C2-, and C3-tethered allenyl enones with arylboronic acids to produce five-membered ring containing products. The regioselectivity of the process, associated with aryl group introduction into the allene moiety, depends on the length of the tether. This finding suggests that the cyclization reaction does not proceed through a carbopalladation pathway but rather via a route involving palladacycle-forming or "anti-Wacker"-type oxidative addition to the Pd(0) catalyst.     

Mechanism and rate constants for the decomposition of 1-pentenyl radicals

This paper is concerned with the mechanisms and rate constants for the decomposition of 1-penten-3-yl, 1-penten-4-yl, and 1-penten-5-yl radicals. They are formed from radical attack on 1-pentene, which is an important decomposition product of normal alkyl radicals with more than 6 carbon atoms in combustion systems. This work is based on related data in the literature. These involve rate constants for the reverse radical addition process under high-pressure conditions, chemical activation experiments, and more recent direct studies. The high-pressure rate constants are based on detailed balance. The energy transfer effects and the pressure dependences of the rate constants are determined through the solution of the master equation and are projected to cover combustion conditions. The low barriers to these reactions make it necessary to treat these thermal reactions as open systems, as in chemical activation studies. The multiple reaction channels make the nature of the pressure effects different from those usually described in standard texts. The order of stability is 1-penten-3-yl approximately 1-penten-4-yl > 1-penten-5-yl and straddles those for the n-alkyl radicals. A key feature in these reactions is the effects traceable to allylic resonance. However, the 50 kJ/mol allylic resonance energy is not fully manifested. The important unsaturated products are 1,3-butadiene, the pentadienes, allyl radicals, and vinyl radicals. The results are compared with the recommendations in the literature, and significant differences are noted. Extensions to larger radicals with similar structures are discussed.     

Some New Radical Reactions of Xanthates

Xanthates are convenient precursors of a variety of radicals that can be captured in an inter- or intramolecular fashion. Overall, the process involves rupture of the sulfide C-S bond and addition of the elements of the xanthate to the olefinic trap. The xanthate in the product can be used in another radical sequence or ionically modified in a great variety of ways. S-Propargyl xanthates exhibit a special but interesting behaviour which involves a sigmatropic rearrangement to the corresponding allene and ring closure to a novel betaine.     

Photochemical and radical initiator-induced reaction of allylic indium compounds

Intramolecular cyclization of allylic radicals generated from allylindium compounds both by photolysis or by the reaction of radical initiators was examined. The photolysis of allylic indium compounds, prepared from 8-bromo- or 8-iodooct-1,6-dienes and powdered indium metal, led to the formation of the 5-exo-trig products. Benzoyl peroxide as a radical initiator was also effective for the cyclization. In contrast, the radical initiators with oxidizing nature, such as tert-butyl hypochlorite, induced iodocyclization producing iodomethylcyclopentane via an oxidation of the iodide on the indium atom.     

Synthesis of <Emphasis Type="Italic">para</Emphasis>-substituted 1,4,5,6,7,7-hexachlorobicyclo-[2.2.1]hepta-2,5-dien-2-ylmethyl benzoates

[4 + 2]-Cycloaddition of hexachlorocyclopentadiene to para-substituted prop-2-yn-1-yl benzoates gave the corresponding 1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hepta-2,5-dien-2-ylmethyl benzoates. The structure of the adducts was confirmed by independent synthesis, esterification of para-substituted benzoic acids with 1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hepta-2,5-dien-2-ylmethanol.     

Divergent regioselectivity in the base-promoted reactions of cyclic eight-membered alpha-ketols with activated halides

[reaction: see text] Deprotonation of 2-hydroxycyclooctanone followed by exposure to an allylic or benzylic halide proceeds very selectively to give the product of C-alkylation. The effect of Delta(5,6)-unsaturation is to promote instead the formation of the O-alkylated derivative. This crossover in kinetic preference is attributed to an inability of the olefinic system to attain a conformation conducive to proton abstraction at C-2.