Alkenes opening

As the proceeding chapters demonstrate, Ni(0)- and Pd(0)-catalyzed [3-1-21-cycloadditions of methylenecyclopropanes with alkenes open a new, simple, and useful route to a number of substituted methylenecyclopentanes. This catalytic generation of a trimethylenemethane synthon and its addition to olefinic double bonds not only lead to five-membered rings but also introduce an exocyclic methylene group, which is a useful functionality for further structural elaboration. 

Octcure reactivity, 312 0dm, Svante, 1030 Olno VaHey, 1050-1051 Olefins, sss Alkenes Open system, 38d-390 Operator ... 

Figure 6.18 Correlation of hydroxyl radical rate constants for un-ionized forms of compounds in water versus those in air. Substituted alkanes, open squares alkenes, open circles aromatics, Hlled circles. [Reproduced with permission from W. R. Haag and C. C. D. Yao, Environ. Sci. Technol. 26, 1005 (1992). Copyright 1992, American Chemical Society.]...

The addition of nitronates to alkenes opens up a route to isoxazolidines, useful precursors for the synthesis of 2-isoxazolines. Tandem [4+2]/[3+2] cycloaddition reactions of nitroalkanes involving cyclic alkyl nitronates have been reported by Denmark et al. ... 

These studies, although they did not deliver the much desired direct characterisation of a dihydride intermediate with the coordinated alkene, opened the door to the alternative mechanism, known as dihydride mechanism. An example of this mechanism, the hydrogenation of MAC with Rh/t-Bu-BisP complex, is depicted in Scheme 7.8. As with the unsaturated mechanism, this particular case is presented in the scheme, the mechanism is discussed in a broader context. 

The six-membered ring heterocyclic analog of the isoxazolidines (a dihydro-1,2-oxazine such as 2.192) is available by a Diels-AIder reaction with nitroso derivatives. Reaction of methyl sorbate (2.190) and 1-chIoro-l-nitrosocyclohexane (2.191) gave 2.192 Catalytic hydrogenation followed by hydrolysis reduced the alkene, opened the ring and converted the ester to an acid 5-amino-2-hydroxyhexanoic acid, 2.193. 

One aim of the workers in the field of solid-phase synthesis has been the development of reagents that can be used to carry out reactions in a column and that can be regenerated in a single step. To some extent this has been achieved in the epoxidation of the alkenes, opening up the possibility of automating the process. 

On pages 193-4 we saw how the double bond in ethene is formed from a a bond and a n bond. There are four electrons in total in these two bonds. So although ethene is a non-polar molecule, there is an area of high electron density around the C=C bond. This makes the alkenes open to attack by electrophiles (see page 197). 

Open chain hydrocarbons which are undersaturated, i.e. having at least one carbon-carbon double bond are part of the olefin series, and have the ending -ene . Those with one carbon-carbon double bond are called mono-olefins or alkenes, for example ethylene CH2 = CH2. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

An efficient carboannulation proceeds by the reaction of vinylcyclopropane (135) or vinylcyclobutane with aryl halides. The multi-step reaction is explained by insertion of alkene, ring opening, diene formation, formation of the TT-allylpalladium 136 by the readdition of H—Pd—I, and its intramolecular reaction with the nucleophile to give the cyclized product 137[I08]. 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Monocyclic Aliphatic Hydrocarbons. Monocyclic aliphatic hydrocarbons (with no side chains) are named by prefixing cyclo- to the name of the corresponding open-chain hydrocarbon having the same number of carbon atoms as the ring. Radicals are formed as with the alkanes, alkenes, and alkynes. Examples ... 

The parallel ability of oxiranes to undergo ring opening to carbonyl ylides was first noted in the case of tetracyanooxirane (68T2551), but such reactions have not been widely exploited. The addition to alkenes, leading to formation of tetrahydrofurans, is stereospecific (Scheme... 

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

Nucleophilic opening of oxiranes to give ultimately 1,2-diols is usually effected without isolation of the oxirane oxiranation (epoxidation) of alkenes with unbuffered peroxy-ethanoic acid or hydrogen peroxide in methanoic acid (Section 5.05.4.2.2(/)) tends to give monoesters of 1,2-diols (e.g. 53), which can be hydrolyzed to the diols (Scheme 46). 

Other isocyanates undergo [2 + 2] cycloaddition, but only with very electron rich alkenes. Thus phenyl isocyanate gives /3-lactams with ketene acetals and tetramethoxyethylene. With enamines, unstable /3-lactams are formed if the enamine has a /3-H atom, ring opened amides are produced 2 1 adducts are also found. Photochemical addition of cis- and traH5-stilbene to phenyl isocyanate has also been reported (72CC362). 

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