Olefin migration shifts

For substrates lacking olefinic migrating groups, [1,2]-shifts occur instead to give oxabicydic products, Eq. 46. 

For carbenes 25 and 26 1,2-shifts of bonds a and b will lead to different bridgehead olefins, 17 and 27 for 26, and 1-norbomene 16 and bicy-clo[3.1.1]hept-l-ene (30) for 25, whereas carbene 24a will only give alkene 15. In accordance with experimental observations,19 calculations show that the more strained bond will migrate preferentially. 

The complex, XLYI, may add to another molecule of isobutylene to yield a higher polymer complex or eliminate aluminum chloride to yield the dimer in the latter case intramolecular migration (a 1,5-shift 1) of hydrogen must be postulated in order to form an olefin. On the other hand, cyclization may readily occur (particularly after a 1,2-shift of a proton from a methyl group) with the resultant formation of a naphthene. 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable olefin is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of doublebond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the olefin to a borane (5-12). rearrangement of the borane (8-11), oxidation and hydrolysis of the newly formed borane to the alcohol (2-28), and dehydration of the alcohol (7-1) ... 

Carboxylic esters are produced by the addition of carboxylic acids to olefins, a reaction that is usually acid-catalyzed (by proton or Lewis acids182) and similar in mechanism to 5-4. Since Markovnikov s rule is followed, hard-to-get esters of tertiary alcohols can be prepared from olefins of the form R2C=CHR.183 When a carboxylic acid that contains a double bond in the chain is treated with a strong acid, the addition occurs internally and the product is a y- and/or a 8-lactone, regardless of the original position of the double bond in the chain, since strong acids catalyze double bond shifts (2-2).184 The double bond always migrates to a position favorable for the reaction, whether this has to be toward or away from the carboxyl group. Carboxylic esters have also been prepared by the acyloxymercuration-demercuration of olefins (similar to the procedures mentioned in 5-2 and 5-4).185... 

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... 

Aromatic hydrocarbons are subject to cytochrome P-450-catalyzed hydroxylation in a process that is similar to olefin epoxidation. As discussed in Section IV. G, halogen migration observed during the hydroxylation of 4-ClPhe and similar substrates, led to the discovery of a general mechanism of oxidation that invokes arene oxide intermediates and the NIH shift. Arene oxides and their oxepin tautomers have not been isolated as products of metabolism of benzenoid compounds, but their presence has been inferred by the isolation of phenols, dihydrodiols and dihydrophenolic GSH conjugates derived therefrom262. 

In agreement with Balandin s theory it was assumed by Eidus (79,80) that methylene radicals adsorbed on two adjacent centers of the catalyst are dimerized to ethylene which remains adsorbed on a doublet subsequently a new methylene group is added to one of the carbon atoms with a hydrogen atom migrating to the carbon atoms of the ethylene which is then desorbed further growth with formation of 1-olefins proceeds similarly a shift of the double bond inside of the molecule may occur. The view of Craxford and co-workers that all polymerizing methylene radicals remain adsorbed to the surface was contradicted by Eidus as inconsistent with experimental evidence (84b). 

Zelinskil and Levina studied the shift of double bonds when olefins were passed over oxide catalysts (446). Levina also established that in the presence of chromic oxide on alumina at 250° the triple bond in a 1-alkyne is shifted, giving a product one half of which consists of the corresponding 2-alkene and one-half of a 1,3-diene (206). She also reported migration of double bonds from side chains into the ring of naphthenes carrying unsaturated side chains. 

Treatment of the des-A-A -unsaturated compound (406) with hydrogen fluoride gave the 9a-fluoro-product (407) and the backbone-rearranged ketone (409). Migration of a deuterium atom from the 17a- to the 13a-position, and failure to incorporate any deuterium other than at C(ll) when the reagent was deuterium fluoride, established a mechanism of the type illustrated (408), presumably with rapid sequential hydride and methyl shifts, not involving any olefinic... 

However we rationalized that using silicon as a migrating group could result in a unique stabilization of the energy surface of olefin isomerization. This rationalization was based on silicon s well known ability to stabilize both a-carbanions and p-carbocations. Thus a hypothetical "dual-stabilized" zwitterion would be produced by a 90° twist of a vinyl silane, and a following 1,2-shift of Silicon would produce a singlet carbene possessed of the same hyperconjugative stabilization as in a P-silyl cation (Eq. 5). 

Since the more comfortable suprafacial path is forbidden in the [1,3] process, simple olefins do not undergo thermal double-bond migration through 1,3-hydrogen shifts. This process, however, can conceivably be rendered allowed through interaction with a tranmtion metal center. 

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