Alkene also olefin route

The carbene route to bridgehead olefins is a well established reaction and has been developed to a major method for the generation of bridgehead alkenes. The field has been recently reviewed by one of the main contributors to this area.1 The reversed reaction, the formation of carbenes from distorted olefins, has also been known for a long time. Distortion of the tt bond in alkenes is easily affected by photoexitation. In connection with those reactions, carbene chemistry has been observed and the field has also been reviewed some years ago.2... 

Trost reported the synthesis of 1,4-dienes with ruthenium catalysis through regioselective carbometallation of alkynes with alkenes.51 Di- and trisubstituted olefins can also be obtained with arylboronic acids through an intermolecular process under rhodium,30 52 55 nickel,56 and palladium catalysis.57 Recently, Larock has reported an efficient palladium-catalyzed route for the preparation of tetrasubstituted olefins.58,59... 

In addition to the examples outlined above, various funetionalized olefins are also known to undergo CM with high E-selectivity. Functional groups that contribute to high E-olefin formation will be discussed in Section 11.06.4. In these cases, CM provides an orthogonal route to products that are typically generated via either the selective C-H activation of alkenes or allylic oxidation. 

Reactions of transition-metal coordinated olefins with diazo compounds as a route to cyclopropane products have not yet been rigorously established. Catalysts that should be effective in this pathway are those that are more susceptible to olefin coordination than to association with a diazo compound and also those whose coordinated alkene is sufficiently electrophilic to react with diazo compounds, especially diazomethane. Pd(II), Pt(II), and Co(II) compounds appear to be capable of olefin coordination-induced cyclopropanation reactions, but further investigations will be required to unravel this mechanistic possibility. 

Hydroxylation of the olefinic bond is also a convenient route for utilization of the alkenes being considered. Treatment of alkene 26 with aqueous alkaline potassium permanganate resulted in formation of compound 45, the structure of which was established by a de-gradative procedure11. It followed from this result that the initial alkene (26) was the cis isomer. Treatment of alkene 28d with iodic acid in the presence of a catalytic amount of osmium tetraoxide, with propyl alcohol as the solvent, led918 to a mixture of two alditols (46). 

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. 

The regenerated carbonium ion can of course continue the process, a key feature being that under alkylation conditions this active species is formed from saturated alkane, not an olefin as required by polymerization. Different alkenes, such as propylene, 1-butene, or the 2-butenes may also form carbonium ions in a similar manner to the process of Eq. 18.25. However, neither /7-butane nor /7-pentane can replace an isoalkane for the hydride transfer since an /7-alkane is not capable of forming a stabilized carbonium ion. Nevertheless, this is one advantage that the alkylation process has over polymerization as a route to gasoline it is able to use both light hydrocarbon alkanes (as long as they are branched) and alkenes. Alkylation and polymerization both produce branched products, but the alkylation products are saturated (Table 18.5) whereas the polymerization products are alkenes. 

Dialkylzincs can be prepared by transmetallation of zinc sails with alkylating reagents of the main group elements. Transmetallation of Me2Zn[23] with alkylborons is a particularly useful route to dialkylzincs (eq (12)) [24]. Since alkylborons are readily accessible by well documented hydroboration reactions of alkenes, a wide variety of dialkylzincs can be synthesized by this route. Dialkylzincs can be also obtained by the reaction of Et Zn [21] with olefins in the presence of a catalytic amount of Ni(acac) (eq(13))[25], ... 

These olefination reactions can be applied with confidence to the stereoselective synthesis of alkenes. Both isomers of a wide variety of alkenes can be obtained with very high stereoselectivities when suitable reaction conditions are selected. Compared with other methods, the Julia reductive elimination has some advantages. First, sulfones are more readily available and easily purified than the corresponding phosphorus and silicon derivatives. There is a wide range of mild and high-yielding routes to synthesize sulfones.3 Furthermore, the sulfone group also confers stability and frequently crystalline properties to the substrate. 

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