Reduction olefin, stereochemistry

The potential utility of an asymmetric addition to a prochiral carbonyl can be seen by considering how one might prepare 4-octanol (to take a structurally simple example) by asymmetric synthesis. Figure 4.16 illustrates four possible retro-synthetic disconnections. Note that of these four, two present significant challenges asymmetric hydride reduction requires discrimination between the enantiotopic faces of a nearly symmetrical ketone a), and asymmetric hydroboration-oxidation requires a perplexing array of olefin stereochemistry and regiochemical issues h). In contrast, the addition of a metal alkyl to an aldehyde offers a much more realistic prospect (c) or (d). 

The stereochemistry of the dibromides does not appear to affect the usefulness for regenerating olefins with zinc or chromous ion however, significant differences in ease of reduction have been observed. Thus, the diequatorial 3j5,4a-dibromocholestane (121) is 1 % reacted under conditions which give a 91 % conversion of the diaxial 3a,4j5-isomer (122). 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Disulfonate esters of vicinal diols sometimes undergo reductive elimination on treatment with sodium iodide in acetone at elevated temperature and pressure (usually l(X)-200°). This reaction derived from sugar chemistry has been used occasionally with steroids, principally in the elimination of 2,3-dihy-droxysapogenin mesylates. The stereochemistry of the substituents and ring junction is important, as illustrated in the formation of the A -olefins (133) and (134). 

CDP840 is a selective inhibitor of the PDE-IV isoenzyme and interest in the compound arises from its potential application as an antiasthmatic agent. Chemists at Merck Co. used the asymmetric epoxidation reaction to set the stereochemistry of the carbon framework and subsequently removed the newly established C-O bonds." Epoxidation of the trisubstituted olefin 51 provided the desired epoxide in 89% ee and in 58% yield. Reduction of both C-O bonds was then accomplished to provide CDP840. 

Monoalkylthallium(III) compounds can be prepared easily and rapidly by treatment of olefins with thallium(III) salts, i.e., oxythallation (66). In marked contrast to the analogous oxymercuration reaction (66), however, where treatment of olefins with mercury(II) salts results in formation of stable organomercurials, the monoalkylthallium(III) derivatives obtained from oxythallation are in the vast majority of cases spontaneously unstable, and cannot be isolated under the reaction conditions employed. Oxythallation adducts have been isolated on a number of occasions (61, 71,104,128), but the predominant reaction pathway which has been observed in oxythallation reactions is initial formation of an alkylthallium(III) derivative and subsequent rapid decomposition of this intermediate to give products derived by oxidation of the organic substrate and simultaneous reduction of the thallium from thallium(III) to thallium(I). The ease and rapidity with which these reactions occur have stimulated interest not only in the preparation and properties of monoalkylthallium(III) derivatives, but in the mechanism and stereochemistry of oxythallation, and in the development of specific synthetic organic transformations based on oxidation of unsaturated systems by thallium(III) salts. 

Initially, two plausible mechanisms were considered, as depicted in Scheme 5.16. The first was a hydroboration route (a), where the B-H bond was added across the olefin from the same face of S-0 and upon aqueous work-up, the resulting C-B bond was replaced with a C-H bond. The tis B-H addition to the olefin led to the cis-stereochemistry of the two adjacent aryl substituents. The reduction of the sulfoxide oxygen occurs in the next step. The alternative mechanism was the borane reduction route (h), which was similar to 1,4-addition of hydride,... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

A wide variety of activated olefins (126) undergo reductive electrochemical dimerization to compounds of structure 127 (electrolytic hydrodimerization) 129 i. While the product 127 is capable of existing in either dl or meso modifications, relatively little attention has been paid to the stereochemistry of hydrodimers... 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

On treatment with benzeneselenenyl chloride two olefinic urethanes (214 and 217) underwent cyclization to afford piperidine derivatives (215 and 218, respectively) having the cis stereochemistry. Their reduction with triphenyltin hydride gave the same product (216). Removal of the blocking group from the nitrogen gave ( )-isosolenopsin A (Ic) (Scheme 6) (392). 

However, use of a less reactive reagent where [R = R =(CH2)4, (CH2)s, (CH2)20(CH2)2] led to the isolation of products 61 and 62, with a reduction in the yields of the desired cycloadducts. The product 62 arises from Michael addition of the liberated methanethiol to A-methylmaleimide. The protocol was further extended to olefinic dipolarophiles with dimethyl fiimurate, dimethyl maleate, fumaronitrile, and 2-chloroacrylonitrile leading to the corresponding adducts, although these dipolarophiles proved somewhat less reactive with reduced yields being observed. Where applicable, the alkene configuration was reflected in the relative stereochemistry of the cycloadducts (Fig. 3.5). 

The intervention of olefin intermediates has significant effects on the stereochemistry of the hydrogenation of aromatics. Similarly to the reduction of alkenes, cis isomers are the main products during the hydrogenation of substituted benzenes,... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

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