Achiral alkenes

In this example addition to the double bond of an alkene converted an achiral mol ecule to a chiral one The general term for a structural feature the alteration of which introduces a chirality center m a molecule is prochiral A chirality center is introduced when the double bond of propene reacts with a peroxy acid The double bond is a prochi ral structural unit and we speak of the top and bottom faces of the double bond as prochiral faces Because attack at one prochiral face gives the enantiomer of the com pound formed by attack at the other face we classify the relationship between the two faces as enantiotopic... 

Notice further that, consistent with the principle developed in Section 7.9, optically inactive starting materials (achiral alkenes and bromine) yield optically inactive products (a racemic mixture or a rneso structure) in these reactions. 

With this reaction, two new asymmetric centers can be generated in one step from an achiral precursor in moderate to good enantiomeric purity by using a chiral catalyst for oxidation. The Sharpless dihydroxylation has been developed from the earlier y -dihydroxylation of alkenes with osmium tetroxide, which usually led to a racemic mixture. 

The reaction discussed in the previous section involves addition to an achiral alkene and forms an optically inactive, racemic mixture of the two enantiomeric products. What would happen, though, if we were to carry out the reaction on a single enantiomer of a chiral reactant For example, what stereochemical result would be obtained from addition of H2O to a chiral alkene, such as... 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

Cycloadditions of cyclic (a) and acyclic (b) nitrones to achiral (535a) and chiral a-diphenylphosphinyl alkenes (535b,c) have been reported (752). In each case, addition to allyldiphenylphosphine oxide (535a) gave a single isoxazolidine... 

Although there are many reports on the enantioselective catalytic double bond isomerization of functionalized achiral alkenes, that of alkenes bearing an isolated double bond have had limited success. The use of a chiral bis(indenyl)titanium catalyst 5 containing a chiral bridging group realized the highly enantioselective isomerizations of unfunctionalized achiral alkenes with up to 80% ee (Equation (27)).90... 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

Chain-end controlled isospecificity and syndiospecificity for 1-alkene polymerizations at low temperatures with achiral metallocenes have also been reported.2,163 81131135 The polymerization with these catalysts is highly regio-specific in favor of primary monomer insertion. 

The first application of a heterocyclic carbenoid achiral ligand for hydrogenation of alkenes was reported in 2001 by Nolan and coworkers. Both ruthenium [36] and iridium [37] complexes proved to be active catalysts. Turnover frequency (TOF) values of up to 24000 b 1 (at 373 K) were measured for a ruthenium catalyst in the hydrogenation of 1-hexene. 

Another series of achiral iridium catalysts containing phosphine and heterocyclic carbenes have also been tested in the hydrogenation of unfunctionalized alkenes [38]. These showed similar activity to the Crabtree catalyst, with one analogue giving improved conversion in the hydrogenation of 11. 

Marks and coworkers developed a series of cyclopentadienyl-lanthanide complexes. In the initial investigations on achiral catalysts 36a and 36b (Fig. 29.21), TOFs greater than 100000 IT1 were observed in the hydrogenation of 1,2-disub-stituted unfunctionalized alkenes [48]. 

Table 38.1 Aqueous-organic two-phase achiral hydrogenation of alkenes. 

Hydrosilylation can be applied to alkenes, alkynes, and aldehydes or ketones. A wide range of metal compounds can be used as a catalyst. The most common and active ones for alkenes and alkynes are undoubtedly based on platinum. Hydrosilylation of C-0 double bonds gives silyl ethers, which are subsequently hydrolysed to their alcohols. The reaction is of interest in its enantioselective version in organic synthesis for making chiral alcohols, as the achiral hydrogenation of aldehydes or ketones does not justify the use of expensive silanes as a reagent. 

The inter- or intramolecular cyclopropanation of achiral alkenes with enantiome-rically pure diazoacetic esters [1016,1363,1364] or amides [1365,1366] does not usually proceed with high diastereoselectivity. A chiral auxiliary which occasionally gives good results is pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) [1016,1367,1368]. 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

Sulfonylhydroxylamines and hydroxylamine O-sulfonic acid have found wide apph-cation in synthesis of amines from achiral or chiral organoboranes and boronate esters and the hydroboration-amination methodology is successfully used for direct amination of alkenes. 0-Sulfonyloximes were also found to be good reagents for synthesis of amines from organomagnesium, -copper and -zinc reagents. 

The hydrovinylation reaction, the codimerization of ethene and styrene (Scheme 2), provides easy access to chiral building blocks from inexpensive hydrocarbon feedstocks, which can be used further for the preparation of fine chemicals. Key problems in this reaction include the selectivity of the reaction and the stability of the catalyst. The main side reactions are oligomerization and isomerization of the product to internal achiral alkenes. The latter reaction can be suppressed by... 

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