Carbonylative double-bond migration

The double bond migration in steroid hydrocarbons catalyzed by acids or noble metals (see, for example, ref. 185) will not be discussed here. A general review of nonsteroid olefin isomerization has recently been published. Iron carbonyl has been used to isomerize steroidal dienes. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

Bicyclo[3.2.0]heptane-l,5-diols are oxidized by Jead(IV) acetate35,36 or sodium periodate35 to give cycloheptane-1,4-diones. With lead(IV) acetate in acetic acid at 100 C substrates with a hydroxy group in -position to one of the newly formed carbonyl groups eliminate water to afford a,/ -unsaturated cycloheptanediones,36 and / ,y-double bonds migrate to the apposition.35... 

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... 

The Alder-ene reaction is an atom-economic reaction which forms a new carbon carbon-bond from two double bond systems (alkenes, carbonyl groups, etc.) with double bond migration [5]. This reaction follows the Woodward-Hoffmann rules if the reaction is performed under thermal conditions. However, when transition metal catalysts are involved, thermally forbidden Alder-ene reactions can also be realized (Scheme 9.1). Examples of such processes are the formal [4 + 4]-Alder-ene reaction catalyzed by low-valent iron catalysts. 

For the Heck reaction as discussed in Section III.2.1 the final position of the olefi-nic double bond of the products must not necessarily be the same as in the starting materials (for example Schemes 8, 9, and 10 of Section III.2.1) [1], The selectivity is often driven by stereochemical requirements, because the /1-hydrogen elimination step which forms the double bond proceeds exclusively in a syn manner (if a trans /3-hydrogen is eliminated, one should suspect major deviations from the general mechanism of the Heck reaction, for example electrophilic substitution instead of carbopalladation). An impressive example of a double bond migration is depicted in Scheme 1 - instead of olefins the coupling reaction of iodobenzene 1 with the olefmic alcohol 2 results in the isomeric aldehydes 3 and 4 as final products [2], Reactions of this type have emerged as valuable tools for the synthesis of carbonyl compounds and also as crucial steps in domino processes. 

When an a,/3-unsaturated carbonyl compound having a functional group at an appropriate position in a tether is treated vith RuH(Cl)(CO)(PPh3)3 the double bond migrates from the a,/3-position of the carbonyl group to the position conjugated vith the functional group (Eq. 12.16) [21]. 

The preparation of thioacetals involves treatment of the carbonyl substrate with a dithiol in the presence of an acid catalyst, usually TsOH or Bp3 OEt2. Since thioacetals are quite stable toward hydrolysis, there is no special need to remove the H2O formed during the reaction. Also, since it is more difficult to equilibrate thioacetals than acetals via protonation, double bond migration in thioacetalization of enones is usually not observed. 

Fischer esterification would make the ester, but in the process, the acidic conditions would risk migrating the double bond into conjugation with the carbonyl group. The diazomethane reaction is run under neutral conditions where double bond migration will not occur. 

The reactions of ailylboranes, -silanes and -stannanes with carbonyl compounds and imines always take place with double-bond migration, and the structural stability of the reagent is a common problem that is encountered ( 2.7). The reactions of ailylboranes and -boronates occur without catalysis, while those of allylsilanes and -stannanes usually require the presence of a Lewis add [253], The mechanism of the reactions of allylboron derivatives is concerted, and the addition occurs via a six-membered cyclic transition state. A slightly distorted chair transition state model in which the oxygen of the carbonyl group is coordinated to the boron atom is usually invoked (Figure 6.42). Various steric and polar interactions dictate whether the Si or Re face of the prochiral aldehyde is attacked (models C j... 

The reactions of allenyl and propargyl derivatives of boron, silicon or tin with carbonyl compounds or acetals take place with double-bond migration (Se reactions). Therefore, allenyl derivatives will lead to homopropargyl alcohols and propargyl analogs to allenyl alcohols. 

A summary of some typical processes for which borosilicates are of interest was provided by Holderich in 1986 (56). Advantages in their use were reported for double bond migrations in carbonyl containing compounds (aldehyde - ketone rearrangement), for the dehydration of aldehydes, for aldol... 

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