Alkenes transformation

Another approach in surface characterization of metal catalysts by alkene transformations is the use of (-l-)-apopinene (23) as the probe molecule194,196. Due to its peculiar geometry this molecule can expose only one face of its double bond to a catalyst surface. 

Table 2. Halogenated alkenes transformed by Pd catalysts in aqueous solution...

The relative rates of the cobalt-catalyzed silyl peroxidation reactions were determined for different substitution patterns of 337. Terminal alkenes are the least reactive substrates. The reaction is sensitive to steric factors. 1,1-Disubstituted olefins convert most easily, while tri- and tetrasubstituted alkenes transform more slowly. Both electron-rich and electron-poor alkenes are substrates in the silyl peroxidation however, both types react more slowly than styrenes [379]. These reaction conditions were also applied to the silyl peroxidation of several 1,1-disub-stituted alkenes bearing alcohol or peroxy functionalities in the molecule [380]. 

For a slereospecific alkene transformation, choose the right geometry of the starting material to get the right diastereo isomer of the product. Don t try to follow any rules over this—just work through the mechanism. 

A variation of the ketone to alkene transformation is conversion of quinones into arenes, for which a number of reagent systems have been employed. Over a century ago, the reduction of polycyclic quinones to aromatic systems with phosphorous and hydroiodic acid at elevated temperatures was reported. These conditions, however, resulted in complex reaction mixtures consisting of phenols and polyhydrogenated products. More recently, Konieczny and Harvey have reported the reduction of qui-... 

In a similar way, hydride transfer reactions in alkane/alkene transformations depend in a nonlinear fashion upon the varying concentration of acid sites. Post et al. [50] showed elegantly that the rates of these bimolecular reactions depend upon the square of the concentration of the acid sites, while the rates of the monomolecular reactions (protolytic cracking [51]) were linearly dependent on the proton concentration. This suggests that similar effects can also be expected in more complex organic transformations, where less thoroughly developed structure-activity relations exist. 

We return to two compounds we made earlier by the AE reaction propranolol 7 and diltiazem 67. In both cases the synthesis is easier as we do not have to start with an allylic alcohol. The synthesis of propranolol41 uses the allyl ether 188 that gives the diol 189 with good ee in the AD reaction for a monosubstituted alkene. Transformation to the epoxide 190 shows no loss of ee. Reaction with /-Pi NIE was already known to give propranolol 7. This is a very short synthesis from easily made starting materials. 

It will not have escaped notice that adsorbed species implicated in the exchange of alkanes with deuterium are formally the same as those invoked in the hydrogenation of alkenes indeed the reiteration of the alkyl-alkene transformation (process 6.J) held responsible for multiple exchange in linear and branched alkanes, and designated the afi exchange mechanism, is on the face it of identical with the old and well-tried Horiuti-Polanyi mechanism for alkene hydrogenation. This will be discussed further in the next chapter (sections 7.1 and 7.21), but briefly it supposes the sequential addition of two hydrogen atoms to some adsorbed form of the alkene, e.g. 

Oxidative vicinal functionalization of alkenes represents an important reaction among alkene transformations as it allows the introduction of two functional groups within a single transformation. In the area of palladium catalysis, this type of transformation usually consists of a two-step procedure, during which the two heteroatoms are introduced into the carbon framework. Despite this simplicity at first sight, the actual reaction mechanism may be very complex and, in principle, several mechanistic pathways are possible. 

One of the most powerful of alkene transformations is enantioselective epoxidation. T sutomu Katsuki of Kyushu University has developed (Angew. Chem. Int. Ed. 2007, 46, 4559) a Ti catalyst that with H Oj, selectively epoxidized terminal alkenes with high ee. The same catalyst converted a Z 2-alkene such as 3 into the epoxide. This is significant, because such epoxides are opened with nucleophiles selectively at the less congested center. 

A list of some new catalytic alkene transformations that one might like to be able to achieve using molecular oxygen would certainly include the reactions shown in Fig. 6. The direct air-epoxidation or "glycol-ation" of alkenes, their selective allylic oxidation, and their oxidative carbonylation over active and selective catalysts would all be of immense synthetic and industrial utility. 

The reactions of the second class are carried out by the reaction of oxidized forms[l] of alkenes and aromatic compounds (typically their halides) with Pd(0) complexes, and the reactions proceed catalytically. The oxidative addition of alkenyl and aryl halides to Pd(0) generates Pd(II)—C a-hondi (27 and 28), which undergo several further transformations. 

The reaction of alkenyl mercurials with alkenes forms 7r-allylpalladium intermediates by the rearrangement of Pd via the elimination of H—Pd—Cl and its reverse readdition. Further transformations such as trapping with nucleophiles or elimination form conjugated dienes[379]. The 7r-allylpalladium intermediate 418 formed from 3-butenoic acid reacts intramolecularly with carboxylic acid to yield the 7-vinyl-7-laCtone 4I9[380], The /i,7-titisaturated amide 421 is obtained by the reaction of 4-vinyl-2-azetidinone (420) with an organomercur-ial. Similarly homoallylic alcohols are obtained from vinylic oxetanes[381]. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

Another interesting transformation is the intramolecular metathesis reaction of 1,6-enynes. Depending on the substrates and catalytic species, very different products are formed by the intramolecular enyne metathesis reaction of l,6-enynes[41]. The cyclic 1,3-diene 71 is formed from a linear 1,6-enyne. The bridged tricyclic compound 73 with a bridgehead alkene can be prepared by the enyne metathesis of the cyclic enyne 72. The first step of... 

This concludes discussion of our second functional group transformation mvolv mg alcohols the first was the conversion of alcohols to alkyl halides (Chapter 4) and the second the conversion of alcohols to alkenes In the remaining sections of the chap ter the conversion of alkyl halides to alkenes by dehydrohalogenation is described... 

Acid catalyzed hydration converts alkenes to alcohols with regioselectivity according to Markovnikov s rule Frequently however one needs an alcohol having a structure that corresponds to hydration of an alkene with a regioselectivity opposite to that of Markovnikov s rule The conversion of 1 decene to 1 decanol is an example of such a transformation... 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

Long-chain primary alcohols, eg, triacontanol, can be prepared by the hydroboration, isomerization, and oxidation of the corresponding internal alkenes (437). The less thermodynamically stable stereoisomer can be transformed into the more stable one by heating, eg, i j -into /ra/ j -myrtanjiborane (204). 

AUylic organoboranes react via cyclic transition states not only with aldehydes and ketones, but also with alkynes, aHenes, and electron-rich or strained alkenes. Bicyclic stmctures, which can be further transformed into boraadamantanes, are obtained from triaHyl- or tricrotylborane and alkynes (323,438,439). 

Reactive halogen compounds, alkyl haUdes, and activated alkenes give quaternary pyridinium salts, such as (12). Oxidation with peracids gives pyridine Akoxides, such as pyridine AJ-oxide itself [694-59-7] (13), which are useful for further synthetic transformations (11). 

Two reactions of the non-aromatic 4,4-disubstituted pyrazolones are worthy of mention. Carpino discovered that 4,4-dihalogenopyrazolones (365) and 4-substituted 4-halogenopyrazolones (366) when treated with bases yield a, 8-alkynic and -alkenic acids, respectively (66JOC2867). The reaction proceeds through an oxopyrazolenine (2,3-diazacyc-lopentadienone (367) (B-74M140408). A modification of the experimental procedure transforms (365) into bimanes (368) (82JOC214), which are formed from (367 R = X),... 

The fundamental subject of this section is the transformation of A -pyrazolines into cyclopropanes (Buchner-Curtius and Kishner cyclopropane syntheses). The cyclopropane is often accompanied by alkenes (67HC(22)l). When applied to A -pyrazolines the reaction occurs via the A isomers (Scheme 37). 

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