Allylic organomercurials

Considering that /Tarninornercury(II) tetrafluoroborates are polar enough to undergo nucleophilic attack by the lone electron pair of an amine, ether or alcohol in the case of the 1,3-cyclooctadiene, 179, it has been assumed that the first formed 1,4-adduct can give the reaction product by displacement of mercury by amine with direct participation of the nucleophile in an assisted breakage of the anti C—Hg bond (path a) or by spontaneous reduction of mercury in the intermediate allylic organomercurial (path b) (equation 157). 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

Formation of the intermediate organomercury peroxide 56 was rationalised in terms of an allylic mercuration providing an unsaturated hydroperoxide 55 that can cyclise by the favoured 54) 5-exo mode (equation 42 X = 02CCF3). However, this was not the main reaction pathway and the yields (2.7% and 0.6% of 2,m-10-dibromo-8,9-dioxabicyclo[5.2.1]decanes 54 were an order of magnitude lower than those of the 2,6-dibromides 53 obtained from 1,4-cyclooctadiene. 

Our solution to this synthetic problem was the development of an iterative technique for preparing hydroxypropyl ethers from allyl ethers via oxymercuration-reduction. Figure 3 illustrates the process for the preparation of a series of three chain-extended hydroxypropyl derivatives of 2,6-dimethoxyphenol. Conversion of phenol 1 to the allyl ether 2 under phase-transfer conditions (6) was followed by oxymercuration (7) to give the intermediate organomercurial 3, which was reduced without isolation to give hydroxypropyl ether 4 in 64% overall yield. Ether 4. was then allylated to provide 5, which upon oxymercuration-reduction afforded hydroxypropyl derivative 6. One further iteration of the allylation-oxymercuration-reduction sequence yielded the hydroxypropyl compound 7. 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

Quite recently, new methods for stereocontrol in cyclizations of /V-acylami nomethyl ethers have been developed. N,0-acetals of type (19 Scheme 13) were prepared from the corresponding secondary allyl alcohol and the diastereomers were separated by chromatography and/or crystallization. Cyclization with mercury(II) salts and reduction of the organomercury intermediate proceeded with high stereocontrol exerted by the amidal stereogenic center, not the stereogenic center on the original alcohol.237 ... 

The observed difference for allyl halogenides in the reaction scheme indicated by isolation of organomercurials, involves a difference in the subsequent steps, not in the uptake of the first electron. 

Vinylcyclopropanes and organomercury or thallium compounds are starting materials for a recent novel preparation of Ti-allyl palladium complexes which can finally be converted to heterocycles ... 

Whereas organomercury compounds do not add to aldehydes and ketones, cadmium organometallics show a useful reactivity. As in the case of organozinc derivatives, allylic cadmium compounds... 

Vinylic allylic, allenic and other alkenyl groups are cleaved from Sn by HgXj and such reactions are useful syntheses of organomercurials, e.g. ... 

With the more reactive organic halides, side reactions occur however the allyl adducts RInXa (diox), where R = allyl or benzyl and X = Cl, Br, or I, are obtained by shaking InX with a mixture of RX and dioxane. The aryl halides do not react with InX. To prepare arylindium halides from InX, it is necessary to use organomercurials ... 

Wacker-type oxidative reactions of olefins with nucleophiles, reactions of zr-allyl-palladium complexes with nucleophiles, reactions based on chelation, and trans-metallation of organomercury compounds. 

The synthesis began with Yb(OTf)3-catalyzed oxymercuration [88] of ho-moallyUc alcohol 204 [89], prepared by asymmetric allylation with Brown s reagent, to generate organomercury chloride 205. Rh(I)-catalyzed formy-lation [90] of 205 in the presence of DABCO (0.5 equiv.) furnished aldehyde 206. Stereoselective crotylation of 206 via Brown s reagent, followed by regioselective Rh(I)-catalyzed hydroformylation provided lactol 203. Dia-... 

Organomercury compounds found application in organic synthesis from an early date. They are generally stable to attack by atmospheric oxygen although secondary and tertiary alkyls, allyls and benzyls do need some protection. They also resist attack by water and alcohols under neutral conditions and show little or no reactivity towards carbon electrophiles such as carbonyl compounds. 

Metalation-demetalation sequences have been invoked to account for palladium(u) chloride-catalysed allylic and enol ester exchange, and organomercury(ii) and bis(organo)thallium(iii) ion catalysis of the hydrolysis of isopropenyl acetate. 

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