Metal-mediated rearrangements

A transition metal mediates rearrangement of a substituted (noncyclo) C5 species. This includes ring expansion reactions of, for example, substituted 3-vinyl- 1-cyclopropene (cf. Scheme 22) or cyclobutene, both bonded to a metal to give supracyclopentadiene Rh and Ru complexes (92- 94). The rearrangement of vinylcyclopropenes to cyclopentadienes can also occur photolytically (66). 

Various other pathways not following a general reaction pattern start from preformed three-carbon chains and lead to cyclopropane systems. These include various metal-mediated rearrangements of saturated and unsaturated carbon skeletons. 

Photochemical and transition-metal-mediated rearrangements are also known. In the former case the intermediates are high-energy diradicals, often leading to complex mixtures of... 

The synthesis of bicyclo[3.2.1]octadienes 86 has been accomplished by refluxing divinyl-cyclopropanes 85 in xylene. Subsequent research led to the development of conditions employing a rhodium-catalyzed step for the synthesis of bridged systems via the cyclopropanation of cyclopentadienes, furans and pyrroles (see section on transition-metal-mediated rearrangements). 

A metal-mediated rearrangement of 3-alkynyl flavone ethers has been found to involve 5-endo enyne cyclization to a platinum-containing spiro-oxocarbenium intermediate which may be trapped with methanol to give spirodihydrofurans or further rearranged to allenyl chromanediones or benzofuranones (Scheme 150). ... 

A recent application of the metal mediated [6 + 4] cycloaddition reaction is the synthesis of nine-membered carbocycles by a sequential [6 + 4] cycloaddition-pinacol rearrangement, as employed by Rigby and Fales310. 

Reduction of (312) has been found to afford the dimer (313) which upon heating rearranged to yield the unprecedented di(benzopentalene) complex (314). The regio- and stereo-specificity of the conversion (313) into (314) implies a metal-mediated pathway for the process (see Scheme 100). The first observable cis-bis(alkyne)cyclobutadiene rearrangement [see (315) to (316)] has been reported. 

A hitherto unknown type of rearrangement of l-(l-alkynyl)cyclopropanols (321) to cyclopent-2-en-l-ones (322) mediated by octacarbonyldicobalt" and hexacarbonyldicobalt" complexes has been described. A possible pathway for the transformation is outlined in Scheme 101. A -proton transfer accompanied by a metal-mediated Stevens rearrangement, which converts a coordinated dimethylsulfane... 

In the past few years, new approaches for the enantioselective synthesis of / -benzyl-y-butyrolactones appeared in the literature. Some of these approaches involve the asymmetric hydrogenation of 2-benzyl-2-butenediols (j [34]), the radical mediated rearrangement of chiral cyclopropanes (r [35]), the transition metal catalyzed asymmetric Bayer-Villiger oxidation of cyclobutanones n [36]), or the enzymatic resolution of racemic succinates (g [37]). 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

Transition metal mediated cross couplings of epoxides have remained relatively unexplored, with only a few examples of this potentially useful reaction reported in the literature. A recent report details the Suzuki-Miyaura cross-coupling of epoxides <07JOC3253>. The reaction of aryl epoxides with arylboronic acids under Suzuki-Miyaura coupling conditions provides the coupled product in good yields. Careful monitoring of the reaction is essential to avoid Pd-catalyzed rearrangement of the epoxides. 

Detailed studies have been made of metal-mediated Wagner-Meerwein rearrangements in dicobalt, dimolybdenum, and cobalt-molybdenum complexes of the 2-fenchyl cation.105... 

Certain Lewis acids are known to induce an epoxide-aldehyde rearrangement <01TL8129>, and this chemistry has recently been combined in tandem with metal-mediated allylations. For example, epoxides react with tetraallyltin in the presence of bismuth(III) triflate to give homoallylic alcohols 116. The reaction involves an initial 1,2-shift to form an aldehyde 115, which is then attacked by the allyl tin species <03TL6501>. A similar but operationally more straightforward protocol is available by combining allyl bromide with indium metal, followed by the addition of epoxide <03TL2911>. 

In conclusion, C-C bond formation on carbohydrates has absorbed lots of widely used methods from general synthetic organic chemistry. Typical among these methods are intramolecular alkylation and intramolecular condensation of aldehyde with enolates, phosphonates, and nitro-stabilized anions. Metal-mediated radical reactions, cycloadditions, and rearrangements have also been applied frequently. 

Julian, R. R., May, J. A., Stoltz, B. M., Beauchamp, J. L. Gas-Phase Synthesis of Charged Copper and Silver Fischer Carbenes from DIazomalonates Mechanistic and Conformational Considerations In Metal-Mediated Wolff Rearrangements. J. Am. Chem. Soc. 2003, 125, 4478 486. 

Drabowicz J,MikolajczykM (2000) Selenium at Higher Oxidation States. 208 143-176 Eder B,see WrodniggTM (2001) The Amadoriand Heyns Rearrangements Landmarks in the History of Carbohydrate Chemistry or Unrecognized Synthetic Opportunities 215 115-175 Ehses M, Romerosa A, Peruzzini M (2002) Metal-Mediated Degradation and Reaggregation of White Phosphorus. 220 107-140... 

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