Nickel-catalyzed cycloaddition of unsaturated hydrocarbons and carbonyl compounds

A mild and general route for preparing pyrans and dienones from carbonyls and diynes is described. Ni imidazolylidene complexes were used to mediate cyclizations between diynes and aldehydes. The reaction of an enyne and an aldehyde afforded a mixture of cyclized products. In addition, a spiropyran was prepared from the cycloaddition of a diyne and cyclohexanone. [reaction: see text]     

Gold-Catalyzed Cyclisation by 1,4-Dioxidation

Amide-substituted diynes were cyclized in the presence of a cationic gold catalyst and an external nucleophile leading to 1-indenones and 1-iminoindenones. The electron-donating features of the nitrogen atom enable the formation of a reactive ketene iminium ion, which can be trapped by either diphenyl sulfoxide or anthranil as nucleophiles in a subsequent oxidation step, providing substituted inden-1-on-3-carboxamides.     

An efficient synthesis of diynes using (diacetoxyiodo)benzene

A novel and facile method for synthesis of symmetrical conjugated diynes, using (diacetoxyiodo)benzene as oxidant under palladium-catalyzed conditions is presented, in which diynes are prepared in good yields in a short period of time at room temperature.     

1,3-Diyne chemistry: synthesis and derivations

Conjugated diynes have attracted more and more attention not only for their unique rod like structures and wide existence in nature product, but also the abundant properties and derivations of them. Although oxidative dimerization of alkynes or Cadiot–Chodkiewicz reactions were the main pathway and have achieved great success in the synthesis of diynes, oxidative cross coupling, FBW rearrangement as well as diyne metathesis emerged rapidly recently. Moreover, diynes could be precursors of basic heterocycles, which represented an emerging research area. This Letter will cover the recent progresses in the synthesis and further derivations of diynes.     

Organocatalyzed arylalkylation of activated alkenes via decarboxylation of PhI(O2CR)2: efficient synthesis of oxindoles

A novel organocatalyzed arylalkylation of activated alkenes has been developed. This reaction was initiated from the decomposition of PhI(O₂CR)₂ to generate alkyl radical, followed by addition to alkenes. Then the formed radical was trapped by aromatic ring to generate the cyclized products. This method presents an efficient road to synthesis of a variety of oxindoles.     

Evidence for single electron transfer in metal-halogen exchange. The reaction of organolithium compounds with alkyl halides.

The reaction of $\text{t}$-BuLi with cyclizable 1° and 2° alkyl halide radical probes at low temperature produced stab]e cyclized and uncyclized organolithium products as well as cyclized hydrocarbons which clearly indicate the presence of radical intermediates during the course of these reactions.     

Syntheses and reactions of ferrocene-containing polymers. III. Cyclopolymerization of 1,1′-divinylferrocene

1,1′-Divinylferrocene was polymerized with BF₃OEt₂ and AIBN initiators. Polymers were separated into benzene-soluble and benzene-insoluble fractions, the latter probably being crosslinked. The polymers obtained with BF₃OEt₂ were shown by infrared and NMR spectroscopy to contain both cyclized (70–80%) and uncyclized units, whereas the radical polymer consisted of more than 96% cyclized units. The benzene-soluble fraction of the cationically obtained polymer softened at temperatures below 150°C, but the insoluble fraction decomposed at 240°C. The radical polymers were stable up to 250–280°C (dec).     

Polyunsaturated alkyl amides from Echinacea: synthesis of diynes, enynes, and dienes

The synthesis of 20 alkyl amides, including 15 naturally occurring polyunsaturated alkyl amides previously identified from Echinacea spp. (1-13 and 62) or from Achilla sp. (55) and five previously unknown geometric isomers (23, 28, 67, 73, and 80), is described. Importantly, these amides include all of the major alkyl amides present in commercially used Echinacea extracts. The syntheses demonstrate methodology used for constructing alkyl amides containing conjugated diyne and isomerically pure enyne and diene moieties and may be adapted easily for the preparation of other alkyl amides present in Echinacea spp. Terminal-conjugated diynes were prepared by a Cadiot-Chodkiewitz coupling/deprotection sequence utilizing a protected bromoacetylene, and methyl-substituted diynes were made via a base-catalyzed rearrangement of terminal-skipped diynes. Conjugated dienes were prepared conveniently and with high stereoselectivity by the reduction of enynes or diynes with Rieke zinc. With the exception of 1-2 and 11-12, the alkyl amides are synthesized here for the first time, and their NMR data are consistent with that of the reported isolated natural compounds.     

A strictly "pair"-selective synthesis of conjugated diynes via Pd-catalyzed cross coupling of 1,3-diynylzincs: a superior alternative to the Cadiot-Chodkiewicz reaction

A strictly "pair"-selective synthesis of conjugated diynes via Pd-catalyzed cross coupling of 1,3-diynylzincs is described. This method, like the Cadiot-Chodkiewicz reaction, requires three steps for the synthesis of R(1)Ctbd1;CCtbd1;CR(2) from R(1)Ctbd1;CH, R(2)X, and HCtbd1;CH. However, the high "pair"-selectivity permitting high-yield production of the desired conjugated diynes without separation of symmetrical diynes promises to make the present protocol superior to the Cadiot-Chodkiewicz reaction in many cases.     

How do analogous alpha-chloroenamides and alpha-iodoenamides give different product distributions in 5-endo radical cyclizations?

5-Endo cyclizations of N-alkenyl carbamoylmethyl radicals provide gamma-lactam radicals, which in turn evolve to reduced or non-reduced (alkene) products depending on reagents and reaction conditions. Several groups have made surprising observations that chlorides are better radical precursors than iodides in such cyclizations. Here is described a detailed study of tin and silicon hydride-mediated radical cyclizations of N-benzyl-2-halo-N-cyclohex-1-enylacetamides. The ratios of directly reduced, cyclized/reduced, and cyclized/non-reduced products depend not only on the reaction conditions and reducing reagent but also on the precursor. Prior explanations for the precursor-dependent product ratios based on amide rotamer effects are ruled out. The precursor-dependent behavior is further dissected into two different effects: (1) the ratio of cyclized/reduced products to cyclized/non-reduced products depends on the ability of the radical precursor to react with the product gamma-lactam radical in competition with tin hydride (iodides can compete, chlorides cannot), and (2) the occurrence of large amounts of directly reduced (noncyclized) products in the case of iodides is attributed to a competing ionic chain reaction by which the precursor is reductively deiodinated with HI. This side reaction is not available to chlorides, thereby explaining why the chlorides are better precursors in such reactions. The ability of the iodides to provide cyclized products can be largely restored by adding base. The chlorides and iodides then become complementary precursors, with chlorides giving largely cyclized/reduced products and iodides giving largely cyclized/non-reduced products.     

The interaction of poly(methacrylate) radicals with diphenyldiacetylenes

The free radical polymerization of methyl methacrylate (MMA) in the presence of p,p′- disubstituted diphenylbutadiynes was studied. Both the rate and degree of polymerization are somewhat lowered by the presence of the diynes, but the propagating radicals were stabilized giving clear ESR signals of the interacted polyMMA radicals at the polymerization temperature of 70°C. The magnitude of the interaction depended on the electron density of the diynes; in the cases of diphenylbutadiyne and dimethoxycarbonyldiphenylbutadiyne, the intoraction was more enhanced showing ESR signals with smaller spectra widths and increasing the number of radicals with the polymerization time, while in the cases of electron donor-substituted diynes the interaction was weaker and the radical concentration remained constant during the polymerization. These systems are considered to be examples of the stabilization of transient radicals by the direct interaction of radicals with additives without formation or breaking of chemical bonds. No diacetylenic group was found in the polyMMA obtained. © 1994 John Wiley & Sons, Inc.     

Effective synthesis of aryl ethers and coumaranones employing the palladium-catalyzed enyne-diyne

Alkyl-, alkoxy-, and aryloxy-substituted conjugated enynes 1 in the presence of Pd(PPh(3))(4) catalyst smoothly underwent the regiospecific [4 + 2] cycloaddition reaction with conjugated alkyl- and alkoxy-substituted symmetric diynes 2 to give multisubstituted aryl ethers 3 in good to high yields. Benzannulation of enynes 1d-g with unsymmetric diyne 6, possessing alkyl and alkoxy groups at acetylenic terminii, in most cases produced an aromatic product 8 with an alkoxy group of diyne attached to the ethynyl moiety of the aromatic product. Remarkably, alkoxy-substituted diynes 2c and 6 underwent the benzannulation reaction with 1 at an unusually low temperature of 0 degrees C! One-pot consecutive benzannulation of alkyl-substituted enynes 1d,e and alkoxy-substituted enynes 1f,g with alkoxy-substituted diynes 2c and 6 followed by protonolysis with TsOH afforded coumaranones 9a-c and 10 in reasonable to high overall yields.     

A new synthesis of 14-tetradecanolide from cyclododecanone.

The ω-hydroxy ynamine (2) is cyclized to the ketene N,O-acetal (3) which gives, on hydrolysis, natural 14-tetradecanolide (1).     

Studies of the polymerization of diallyl compounds. XLI. Discussion of substantially identical gel points among three isomeric diallyl phthalates

In the bulk polymerization of three isomeric diallyl phthalates, diallyl phthalate (DAP), diallyl isophthalate, and diallyl terephthalate (DAT), no difference in the actual gel point was substantially observed. This interesting gelation behavior is discussed in terms of the correlation between gelation and the difference in cyclization modes, and the difference in reactivity between the uncyclized and cyclized radicals for crosslinking; the nonconsecutive addition in DAT polymerization led to a delayed gelation and the cyclized radical in DAP polymerization showed an enhanced reactivity for crosslinking.     

Acid-catalyzed cyclization of vinylsilanes bearing an amino group. Stereoselective synthesis of pyrrolidines

[reaction: see text] In the presence of an acid catalyst, vinylsilanes 1 bearing an amino group protected by an electron-withdrawing group were smoothly cyclized to 2-(silylmethyl)pyrrolidines 2. This cyclization was utilized for the stereoselective synthesis of 2,n-disubstituted pyrrolidines (n = 3-5). The cyclized products could be converted to the corresponding alcohols by oxidative cleavage of the carbon-silicon bond with TBAF and H2O2.     

Self-terminating, oxidative radical cyclizations: a novel reaction of acyloxyl radicals.

Acyloxyl radicals RC(O)O* (with R = alkyl, aryl) could be trapped through addition to cyclic and open-chain alkynes, where they were found to act as a donor of oxygen atoms. Mechanistically, this radical oxygenation proceeded through a transannular or intramolecular, respectively, radical cyclization cascade, which was finally terminated by release of an acyl radical RC*(O). The reaction led to stereoselective formation of cyclized products, which contained a carbonyl group at the former site of the alkyne triple bond.     

Cyclopolymerization. V. Monomer design for the synthesis of highly cyclized polymer from unsymmetrical unconjugated dienes: Synthesis and polymerization of N-methyl-N-allylmethacrylamide

N-Methyl-N-allylmethacrylamide (MAMA) was synthesized and polymerized with radical initiators, and the structure of poly-MAMA was studied, in order to establish the concept that difunctional monomers having monofunctional counterparts which do not polymerize are likely to give rise to highly cyclized polymer, if the cyclized polymer is in a lower free-energy level than the monomer. The cyclopolymerizability of MAMA was considerably higher than that of N-allylmethacrylamide, which was previously reported, and its monofunctional counterpart can be polymerized. The extent of cyclization of poly-MAMA was about 93%, even in the polymer obtained by bulk polymerization. The repeating units of poly-MAMA consist mainly of five-membered rings; six-membered rings and pendent methacryl groups were detected in addition as minor repeating components. The monofunctional counterparts of MAMA, i.e., N-methyl-N-n-propylmethacrylamide (MPMA) and N-methyl-N-allylisobutyramide (MAIA), were also synthesized. Neither MPMA nor MAIA showed any tendency toward polymerization under the same experimental conditions as used for MAMA. The results thus obtained all support the concept mentioned above.     

Synthesis of fluoroacylated 4,5-dihydrofurans and fluoroalkylated tetrahydrofurans by the radical cyclization using manganese(III) acetate. Part II

Manganese(III) acetate-based radical cyclizations of various fluorinated 1,3-dicarbonyl compounds with alkenes produced 3-fluoroacylated 4,5-dihydrofurans and 2-acetyloxy-2-fluoroalkylated tetrahydrofurans in good yields. Mechanism was proposed for the formation of all compounds. The radical cyclization of fluorinated 1,3-dicarbonyls showed to form different cyclized products depending on the structure of alkenes and enol forms of 1,3-dicarbonyls.     

Studies of the Polymerization of Diallyl Compounds. XL. Correlation between Addition Modes and Evolution of Carbon Dioxide in the Polymerization of Diailyl Oxalate

The occurrence of head-to-head (HH) addition in the radical polymerization of diallyl oxalate (DAO) was examined under various polymerization conditions. The content of HH linkage in poly (DAO) was reduced in comparison with allyl acetate and diallyl succinate; this may be ascribed to the high polarity of DAO inducing a polar effect on the intermolecular propagation of the growing polymer radical, resulting in reduced HH addition. The correlation between addition modes and evolution of carbon dioxide characteristic of DAO polymerization at elevated temperatures is mechanistically discussed in detail, with special focus on the solvent effect and the reduced dismutation of the cyclized radical compared to the uncyclized one.     

A facile photoinduced iodoperfluoroalkylation of dienes, diynes, and enynes with perfluoroalkyl iodides via selective radical cyclization

Perfluoroalkyl iodides serve as excellent mediators for the radical cyclization of dienes, diynes, and enynes upon irradiation with near-UV light, providing the corresponding iodoperfluoroalkylated cyclic compounds in moderate to good yields.