Cyclic alkenes and alkynes

Name acyclic and cyclic alkenes and alkynes, and draw structures corresponding to names. 

Alkene and alkyne r-complexes see Alkene Complexes and Alkyne Complexes are known both for Au and Au. They are prepared at low temperature from AuCl or AuCls with an excess of the alkene or alkyne in the absence of any other potential donor molecules. The products, for example, of the types (MeCH=CHMe)AuCl and MeC=CMe-(AuCl3)2, are generally of low stability, and the complexation is reversible in a vacuum or on heating. Representative examples have recently been structurally characterized.Strained cyclic alkenes and alkynes give the most stable products. Multiple coordination of monoalkenes or of dialkenes (like butadiene) is known, but information about the products is limited. Alkene coordination to neutral gold atoms has been studied by matrix-isolation techniques at very low temperature. The adduct (C2H4)Au appears to be stable only below 40 K. 

Isolated carbon-carbon double or triple bonds do not usually take part in inter-molecular Diels-Alder reactions, but a number of cyclic alkenes and alkynes with pronounced angular strain are reactive dienophiles. The driving force for these reactions is thought to be the reduction in angular strain associated with the transition state for the addition. Thus, cyclopropene reacts rapidly and stereoselectively with cyclopentadiene at 0°C to form the endo adduct 7 in 97% yield, and butadiene gives norcarene 8 in 37% yield (3.13). ... 

Several other cyclic alkenes and alkynes have been used for the reaction [167b]. In general, strained alkenes are better substrates. For example, cyclohexene (10 h, 180-190 °C, 4% yield) is less reactive than the more strained cyclopentane (6h, 200 °C, 34% yield) [167c], whereas norbomene reacts much more rapidly than either and the product 444 (Scheme 16.89) is formed in 83% yield. 

In the case of vinylfurans and vinylpyrroles there is the possibility of cycloaddition involving either the cyclic diene system or the diene system including the double bond. 2-Vinylfuran reacts in high yield with maleic anhydride in ether at room temperature to form the adduct involving the exocyclic double bond. Similarly, 2- and 3-vinylpyrroles react with 7T-electron-deficient alkenes and alkynes under relatively mild conditions to give the corresponding tetrahydro- and dihydro-indoles (Scheme 51) (80JOC4515). 

The Pauson-Khand reaction was originally developed using strained cyclic alkenes, and gives good yields with such substrates. Alkenes with sterically demanding substituents and acyclic as well as unstrained cyclic alkenes often are less suitable substrates. An exception to this is ethylene, which reacts well. Acetylene as well as simple terminal alkynes and aryl acetylenes can be used as triple-bond component. 

Alkenes and alkynes can also add to each other to give cyclic products in other ways (see 15-61 and 15-63). The first exclusive exo-dig carbocyclization was reported using HfCU as a catalyst. Alkynes also add to alkenes for form rings in the presence of a palladium catalyst or a zirconium catalyst. " Carbocyclization of an alkene unit to another alkene unit was reported using an yttrium catalyst and alkenes add to alkynes to give cyclic compounds with titanium catalysts. ... 

Some years ago we began a program to explore the scope of the palladium-catalyzed annulation of alkenes, dienes and alkynes by functionally-substituted aryl and vinylic halides or triflates as a convenient approach to a wide variety of heterocycles and carbocycles. We subsequently reported annulations involving 1,2-, 1,3- and 1,4-dienes unsaturated cyclopropanes and cyclobutanes cyclic and bicyclic alkenes and alkynes, much of which was reviewed in 1999 (Scheme l).1 In recent days our work has concentrated on the annulation of alkynes. Recent developments in this area will be reviewed and some novel palladium migration processes that have been discovered during the course of this work will be discussed. 

The current scope of the controlled monocarbotitanation of alkenes and alkynes is still very limited at least in part due to competitive side-reactions arising via ft- and a-agostic interactions, as alluded to Scheme 6. On the other hand, polymerization, also shown in Scheme 6, may be largely avoided or minimized in most cases. T o overcome some of the difficulties mentioned above, cyclic version of monocarbotitanation have been explored,25-27 as shown in Scheme 12. None of these reactions has as yet been widely used, but their further development might lead to synthetically useful methods. 

As already indicated, the carbometallation reactions of zirconacyclopropanes and zirconacyclopropenes with alkenes and alkynes are in many ways similar to the corresponding reactions of titanacycles developed more recently. At the same time, however, there are a number of significant differences, as detailed in Section 10.06.2.2. At the present time, synthetically useful carbotitanation reactions are predominantly cyclic and stoichiometric in Ti and more so than the corresponding chemistry of Zr. It seems reasonable to state that Ti and Zr are complementary to each other more often than not. The cyclic carbozirconation may be either stoichiometric or catalytic. Frequently, the difference between the two is that the stoichiometric reactions lack one or more microsteps for completing catalytic cycles. Otherwise, they often share same stoichiometric microsteps. With this general notion in mind, many stoichiometric carbozirconation reactions have indeed been developed into Zr-catalyzed reactions, as discussed later. 

In summary, the most popular hydrogen donors for the reduction of ketones, aldehydes and imines are alcohols and amines, while cyclic ethers or hydroaromatic compounds are the best choice for the reduction of alkenes and alkynes. 

Scheme 1.52. Cyclic carbozirconation of three-membered zirconacycles with alkenes and alkynes.

In addition to the concerted [2 + 2] cycloadditions of cyclic allenes reported by Elliot and colleagues, Kimura and coworkers22 reported [2 + 2] cycloadditions of several 4-ethenylidene-l,3-oxazolidin-2-ones 35 with alkenes and alkynes (equation 11). The... 

Scientists classify hydrocarbons as either aliphatic or aromatic. An aliphatic hydrocarbon contains carbon atoms that are bonded in one or more chains and rings. The carbon atoms have single, double, or triple bonds. Aliphatic hydrocarbons include straight chain and cyclic alkanes, alkenes, and alkynes. An aromatic hydrocarbon is a hydrocarbon based on the aromatic benzene group. You will encouter this group later in the section. Benzene is the simplest aromatic compound. Its bonding arrangement results in special molecular stability. 

Summary of octane number aromatics, alkenes, and alkynes > cyclic alkanes and branched alkanes > straight-chain alkanes. 

The electrophile-induced cyclization of heteroatom nucleophiles onto an adjacent alkene function is a common strategy in heterocycle synthesis (319,320) and has been extended to electrophile-assisted nitrone generation (Scheme 1.62). The formation of a cyclic cationic species 296 from the reaction of an electrophile (E ), such as a halogen, with an alkene is well known and can be used to N-alkylate an oxime and so generate a nitrone (297). Thus, electrophile-promoted oxime-alkene reactions can occur at room temperature rather than under thermolysis as is common with 1,3-APT reactions. The induction of the addition of oximes to alkenes has been performed in an intramolecular sense with A-bromosuccinimide (NBS) (321-323), A-iodosuccinimide (NIS) (321), h (321,322), and ICl (321) for subsequent cycloaddition reactions of the cyclic nitrones with alkenes and alkynes. 

Alkenes, Cyclic Alkenes, and Dienes Alkenes, Cyclic Alkenes, and Dienes Ethylene Propylene Alkyl Halides Alkynes Aluminum Americium Amides Amides Carbofuran Dimethyl Acetamide Amines Antimony... 

The alkanes, alkenes, and alkynes discussed thus far are acyclic. This means that the hydrocarbon chains are linear and do not form rings that close on themselves. Alkanes, alkenes, and alkynes have the ability to form closed rings. At least three carbon atoms are needed to form a ring so the simplest cyclic alkane is a form of propane called cyclopropane. Several simple cyclic alkanes are shown in Figure 15.3. 

Alkane, alkenes, and alkynes can fold themselves to form cyclic compounds. Those represented above are cycloalkanes. 

Metathesis of enynes, which have alkene and alkyne moieties in the molecule, is also a very interesting reaction. In this reaction, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds, and the cleaved alkylidene part migrates onto the alkyne carbon to produce a cyclic compound having a diene... 

Carbon dioxide reacts either with alkene and alkyne complexes of nickel(0) or with free alkenes and alkynes in the presence of nickel(O) complexes, giving five-membered cyclic nickel(II) complexes (equations 90 and 91).278... 

The rhodium(II) complex [Rh2(OAc)4] reduces terminal and cyclic alkenes, activated alkenes and alkynes.148 Various polar solvents could be used, but DMF was preferred. Following a kinetic investigation of the hydrogenation of 1-decene, the mechanism shown in equations (32)-(35) was proposed. 

Cycloadditions are useful for the preparation of cyclic ompounds. Several thermal and photoactivated cycloadditions, typically [4+2] (Diels-Alder reaction), are known. They proceed with functionalized electronically activated dienes and monenes. However, various cycloaddition reactions of alkenes and alkynes without their electronical activation, either mediated or catalysed by transition metal complexes under milder conditions, are known, offering a useful synthetic route to various cyclic compounds in one step. Transition metal complexes are regarded as templates and the reactions proceed with or without forming metallacycles [49]. 

C(2)-C(3) fused polycyclic cephalosporins have received considerable attention as new candidates for /3-lactam antibiotics. An access to tricyclic cephalosporins based on metal-promoted alkenylation of 3-trifloxy-A3-cephem and subsequent Diels-Alder reaction has been published <1996TL5967>. Alternatively, the reaction of a cephalosporin triflate with silyl enol ethers and silylketene acetals has been described to afford tri- and tetracyclic cephalosporins <1996TL7549>. A related process is the formation of fused polycyclic cephalosporins 27 and 28 bearing a wide range of functionalities from the reaction of cephalosporin triflates 26 with unsaturated compounds (alkenes and alkynes) and a base (Scheme 5) <1997JOC4998>. These studies have suggested that the reaction proceeds via the intermediacy of a six-membered cyclic allene which undergoes concerted nZs + K2a cycloaddition with alkenes and acetylenes. 

Hydrocarbons are also divided into classes called aliphatic and aromatic. Aromatic hydrocarbons are related to benzene and are always cyclic. Aliphatic hydrocarbons may be open-chain or cyclic. Aliphatic cyclic hydrocarbons are called alicyclic. Aliphatic hydrocarbons are one of three types alkanes, alkenes, and alkynes. 

Numerous structures containing the thiocarbonyl ylide dipole are conceivable. Incorporation of the thiocarbonyl ylide dipole into a bicyclic heterocyclic system is possible by the conversion of the cyclic thione (203) into the ring-fused mesoionic system (204). The thiocarbonyl ylide dipole (205) undergoes cycloaddition with both alkenic and alkynic electron-poor dipolarophiles in refluxing benzene or xylene so that, after extrusion of hydrogen sulfide or sulfur, respectively, from the initial 1 1 cycloadducts (206) and (207), a ring-fused pyridinone is formed. The method has been used for the annelation of pyridinones to the imidazole, 1,2,4-triazole, thiazole and 1,3,4-thiadiazole systems... 

Pentafluorosulfanyl acetylene is a useful starting reagent for the synthesis of a variety of SF5 derivatives of saturated ethers, vinyl ethers, pyrazoles, cyclic alkenes, and alkyl-substituted phenyl-sulfur pentafluorides (134). It is also used for the preparation of a number of F5S-containing alkenes and alkynes (138, 139). 

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