Alkenes interesting examples

A co-solvent that is poorly miscible with ionic liquids but highly miscible with the products can be added in the separation step (after the reaction) to facilitate the product separation. The Pd-mediated FFeck coupling of aryl halides or benzoic anhydride with alkenes, for example, can be performed in [BMIM][PFg], the products being extracted with cyclohexane. In this case, water can also be used as an extraction solvent, to remove the salt by-products formed in the reaction [18]. From a practical point of view, the addition of a co-solvent can result in cross-contamination, and it has to be separated from the products in a supplementary step (distillation). More interestingly, unreacted organic reactants themselves (if they have nonpolar character) can be recycled to the separation step and can be used as the extractant co-solvent. 

The isomerization takes place because the excited states, both 5i and T, of many alkenes have a perpendicular instead of a planar geometry (p. 311), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the So state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer." Another interesting example of this isomerization involves azo crown ethers. The crown ether (5), in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds and Rb (see p. 105). Thus, ions can be selectively put in or taken out of solution merely by turning a light source on or off." ... 

Another interesting example of dehydrative C-C coupling involves the alkylation of benzimidazole 36 with allyl alcohol 37, which is catalysed by complex 39 [15], The reaction is believed to proceed by alkene complex formation with the allyl alcohol 37 with loss of water from the NH proton of the NHC ligand and OH of the allyl alcohol to give an intermediate Ji-allyl complex. The initially formed 2-allylbenzimidazole isomerises to a mixture of the internal alkenes 38 (Scheme 11.9). 

Reaction of atomic carbon with alkenes generally involves both DBA and vinyl C—H insertion. An interesting example is the reaction of C atoms with styrene in which the major products are phenylallene (21) and indene (22). The synthesis of a number of specifically deuterated styrenes and the measurement of the deuterium isotope effects on the 21/22 ratio led to the conclusion that 21 was formed by DBA followed by ring expansion and by C—H(D) insertion into and followed by rearrangement of the resultant frawi-vinylcarbene (23). The indene was formed by C—H(D) insertion into Xb followed by cyclization of the resultant cw-vinylcarbene (24) (Eq. 18). An examination of the product ratios and their label distributions when atoms are used leads to the conclusion that the ratio of C=C addition to C—H insertion is 0.72 1 in this case. 

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

An interesting example of an indirect acylation of alkenes is the reaction of small olefins (ethylene, propylene, isobutylene) over H-ZSM-5 in the presence of CO at 23°C to give unsaturated ketones.122 In this case the acylium ion is formed in situ from the alkene and CO in the pores of the zeolite at ambient temperature ... 

Alkenes have also been prepared by retro-Diels-Alder reaction of resin-bound cyclohexenes. Figure 3.40 depicts an interesting example of this strategy, in which the retro-Diels-Alder reaction is induced by conversion of a thermally stable, resin-bound 7-oxanorbornadiene into a thermally unstable 7-oxanorbornene [795]. 

Epoxides can also be formed from the oxidation of alkenes by molecular oxygen via in situ generation of hydroperoxides by autoxidation.251,252 An interesting example is the direct stereoselective oxidation of cyclohexene by 02 to syn-l,2-epoxycyclohexan-3-ol catalyzed by CpV(CO)4 with a 65% yield and 99% stereoselectivity (equation 78).253... 

A representative and interesting example for the dependency on the product s ring-size is the PET-cyclization of alkene-silylamine derivatives as described by Pandey et al. (Sch. 33) [35a]. 1,5-Cyclization (19, n= 1) predominantly gave the ds-isomer whereas the corresponding 1,6-cyclization (19, n — 2) yielded the opposite trans-isomer, respectively. Both findings were rationalized by chair-like transition states. 

The treatment of 7V-tosyl-/>-anisidine (75) and several of its derivatives with BTIB in the presence of activated alkenes provides an interesting example of an intermolecular cyclodehydrogenation sequence (99H1785). Arylpropenes are converted to 7V-tosylindolines 76 under these conditions, while alkenyl sulfides lead to the 7V-tosylindoles 77 (Scheme 22). The intermediate formation of 7V-tosylnitrenium ions in these reactions was suggested. 

An interesting example of an intramolecular 1,3-dipolar cycloaddition of an isomunchnone with an unactivated alkene to produce a complex polycyclic compound in one step has been reported <89TL4077 89CB1081>. The isomunchnones derived from the Rh2(OAc)4 catalyzed reaction of acyclic diazoimides 92-96 were found to undergo facile cycloaddition onto the tethered it-bond to provide polycyclic adducts 97-101. A notable feature of this cycloaddition is that only one diastereomer is formed. The relative... 

An important aspect of the metal catalyzed hydroboration reaction is its ability to selectively reduce certain functionalities within a molecule. For instance, a key step in the synthesis of a tripeptide derivative containing the Phe-Arg hydroxyethy-lene dipeptide iosostere is the selective rhodium-catalyzed hydroboration of a lactone. The use of disiamylborane, 9-H-BBN, dicyclohexylborane, and (.9)-alpmeborane, however, gave only low to variable yields of the alcohol due to competitive reduction of the y-lactone to the hemiacetal (equation 8). In another example, hydroboration of the diene illustrated in equation (9) with HBcat and RhCl(PPh3)3 gave exclusive formation of the terminal alcohol derived from reaction of the less substituted alkene. Interestingly, uncatalyzed reactions failed to hydroborate this substrate selectively. ... 

In contrast to lead tetraacetate, simple addition to the double bond does not occur as a side re-action. While allylic rearrangement is common and mixtures of products are frequently obtained, the reaction often proceeds in very high yield and is simple to carry out the alkene is simply heated in an appropriate solvent with mercury(II) acetate until reaction is complete. Mercury(II) acetate has also been us for dehydrogenation, particularly in the steroid field. One interesting example incorporating simultaneous dehydrogenation and allylic oxidative rearrangement is seen in the reaction of abietic acid (37 equation 16). ... 

This substrate, along with the corresponding substrate 6, which has a cyclo-butanation rate of 1.2 x 10 , have been proposed as sensitive cation radical probes, for detecting the presence of cation radical intermediates in various reactions [74]. An interesting example is the reaction of tetracyanoethylene (TCNE) with electron rich alkenes, a reaction for which an electron transfer mechanism had been considered (Scheme 39) [75],... 

The cationic site is of the immonium type, and is relatively iinreactive, but the radical site is reactive toward electron deficient alkenes such as acrylonitrile, yielding a 50 50 mixture of the diastereoisomeric pyrrolines, after being neutralized by back electron transfer from the sensitizer anion radical. The net result is an interesting example of a net 1,3-dipolar cycloaddition which, in the Huisgen method of classification of cycloadditions, is of the [3 -l- 2] type. The same general reaction had previously been carried out by Padwa, using direct photochemical excitation, a procedure which, in contrast, was highly diastereoselective (90 10, in favor of the trans isomer) [92]. 

Further interesting examples of C-S bond formation involve the reaction of previously prepared (or in situ anodically generated) thianthrene or phenothiazine radical cations with alkenes or alkynes, to give l,2-bis(hetaryl) alkanes (or the respective alkenes) [82]. With cyclooctene a 1 1 adduct is obtained instead. Another valuable application is the smooth reaction with ketones (Scheme 26). The thian-threnium salts (40) now obtained are readily deprotonated to the corresponding ylides (41) [83]. The latter compounds are directly obtained when yff-dicarbonyls are used. 

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