Silver-catalyzed rearrangements

Bishomocubanes have also been used as substrates for silver-catalyzed rearrangements (Scheme 3.26).40... 

It is worth noting that a silyl group at the propargyl position did not interfere with the silver-catalyzed rearrangement of a propargyl acetate, despite the potential for further evolution of the product (Scheme 3.34).55... 

Hiyama et al.69 showed that monoacetylated 1,4-butynediols nicely afforded acetoxy allenols on silver-catalyzed rearrangement. These compounds were ideal substrates for silver-catalyzed cyclization, so the overall sequence could directly be performed in one pot, leading to substituted 2,5-dihydrofurans in high overall yields (Table 3.5). In this process, 5-10 mol% of silver perchlorate or tetrafluoroborate was used in refluxing benzene. 

Silver(l) has a marked ability to catalyze rearrangement and degradation reactions of highly... 

This silver-catalyzed ring rearrangement could be explained by preferential interaction of the silver cation with the most substituted edge of the cyclopropane to form an argento cation. In the latter, the presence of C-Si or Ge bond with the right orientation stabilized the cation sufficiently so that it classically evolved through (3-H... 

Further mechanistic work with optically active and labeled compounds confirmed that the rate-determining step in such rearrangement was the silver coordination to the alkyne moiety and revealed that the silver-catalyzed allene epimerization was 2-40 times faster than the propargyl ester rearrangement.53... 

Sigmatropic rearrangements catalyzed by silver salts have been used in total synthesis. The synthesis of three monoterpene compounds that comprise the pheromone of male boll weevil Anthonomus grandis has been achieved using the silver-carbonate catalyzed rearrangement of an acetoxycyclohexyl acetylene as the key step (Scheme 3.35).56... 

In contrast to the preceding mechanisms proposed for [3,3]-sigmatropic shifts, the mechanism of the silver-catalyzed oxy-Cope rearrangement was proposed as a stepwise process (Scheme 3.37). As usual, the reaction would be initiated by silver coordination to the alkyne moiety. Nucleophilic attack of this complex by the double bond would then lead to a cyclic cationic vinylsilver intermediate. Fragmentation would then give the dienone. 

In more sophisticated approaches, [3,3]-sigmatropic rearrangements have been incorporated in cascade reactions in which each step could be catalyzed by silver salts. Usually, the silver promoted [3,3]-sigmatropic shift is combined with a silver-catalyzed cyclization,68 as outlined in the general scheme below for alkynyl compounds (Scheme 3.44). 

The silver-catalyzed, propargyl Claisen rearrangement has also been combined with cyclization reactions. Propargyl ethers derived from [3-ketoesters were rearranged by silver hexafluoroantimonate and the resulting allenic [3-ketoesters cyclized on treatment with base, leading to the formation of 2/7-pyrans in moderate to excellent yields (Scheme 3.48).75 The cyclization process was described as a base-catalyzed... 

Very recently, Davies and coworkers have reported a silver-catalyzed Doyle-Kirmse reaction of ally lie and propargylic sulfides with ethyl diazophenylacetate (35). A selection of examples was investigated providing the rearranged products, either homoallylic thioethers (76 —> 77, Scheme 8.14a) or allenyl thioethers (78 —> 79, Scheme 8.14b) in good yields, and with good overall scope.40... 

Oxy-tetrasubstituted furans are obtained in a silver-catalyzed isomerization sequence involving an initial [3,3]-sigmatropic rearrangement (Scheme 7). Phosphatyloxy and sulfonyloxy groups can be employed as well <2004AGE2280>. 

Methoxy-N-chloromorphinan may be prepared in 78% yield by treatment of 3-methoxymorphinan with sodium hypochlorate.(139) Upon silver ion catalyzed rearrangement, it gave 13% of 146. The alkyl migration to the electron-deficient nitrogen was proposed as being mediated by a nitrenium ion transition state (145). 

Another striking example of metal-assisted S5mimetry-forbidden valence isomerizations involves the silver ion catalyzed rearrangement of homo-cubyl systems 2) i.e., 58 - 5P). A similar rearrangement was reported for cubane itself using silver salts Interestingly, other metals [e.g., rho-... 

Rearrangements of strained a-bonds sec Silver fluoroborate, 3, 250). Paquette et at have now examined the Ag "-catalyzed rearrangement of the less strained seco-cubanc derivatives. Treatment of the endo,endo-silver perchlorate resulted in quantitative and stereospecific conversion to the endo endo-letracyclo[3.3.0.0. 0 ]octane derivative (2). In like manner, the endo,exo-isomer (3)... 

The silver-catalyzed reactions of bicyclobutanea were thoroughly studied by Paquette s group. They showed that in some cases, the reaction sequence involves first cleavage of a side bond to form an argentocarbocation, which is then followed by a skeletal rearrangement " . Two examples are shown below (equations 114 and 115). (For the last... 

Rearrangement of polycyclic systems. (4, 432-435 5, 585-587). Paquette has reviewed silver ion-catalyzed rearrangements of cyclic systems, particularly those that are useful synthetically. A few of these are formulated. One is the preparation of homotropilidene (2) from 3-norcarene (1) shown in equation I. Note that rearrangement of (3), with a methyl group at a bridgehead position. 

Wolff s discovery in 1902-1912 was accomplished by using thermal and silver-catalyzed conditions. Photochemical Wolff rearrangements were discovered only 50 years later (Horner and Spietschka, 1952). During the last 40 years, more preparative and mechanistic work was made under photolytic conditions. Nevertheless, we will discuss first mechanistic investigations of the thermal method. 

Pentacyclo[3.3.0.0. " 0. 0 ]octane (or cuneane, CU), the silver ion catalyzed rearrangement product of cubane," gives SB at roughly 1 atm pressure in a first-order reaction with log = 13.82 - 37 700/2.3/ T (Scheme 9.20)." At lower pressures COT is also formed probably from vibrationally hot SB. 

Formulate a mechanism for the following silver-ion-catalyzed rearrangements. Be sure that the mechanism accounts for the observed stereochemistry. 

Kirsch has reported the conversion of propargyl vinyl ethers to form pyrroles via a multi-step transformation involving the silver-catalyzed propargyl Claisen rearrangement to form the a-allenyl P-keto ester, condensation with a primary aryl amine. 

Georg and coworkers reported a C=C bond cycloaddition with a ketene to provide six-membered enaminones, under mild conditions (Scheme 92) (2010JA15512). From this intramolecular cycloaddition of a ketene created from diazoketone 289, employing a silver-catalyzed Wolff rearrangement, a six-membered enaminones such as 290 was obtained. 

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