Carbene complexes addition-rearrangement reactions

The addition of an aikene to a carbene complex may lead to a metathesis reaction with exchange of the carbene ligand with one of the two halves of the aikene. The reaction proceeds via formation of an alkene-carbene complex, which rearranges via a metallacyclo-butane intermediate. ) This reaction is synthetically useful when a terminal aikene is used and vacuum evaporation of the more volatile aikene product (typically 3,3-dimethyl-but-l-ene) is possible to displace the equilibrium. An excess of the aikene reagent is also used to ensure a favorable equilibrium position (see Scheme 5). The properties of the an-... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

Almost at the same time, Pirrung and Johnson " independently described the rearrangement of oxonium ylides prepared by intramolecular rhodium-catalyzed carbene addition. The reaction appears to have a broad scope and gives an easy entry into complex oxygenated polycycles (Scheme 61). When the possibility arises, high levels of diastereocontrol are exercised (Scheme 62). 

A novel route for the generation of trialkylboron-carbene complexes has been reported (34). As summarized in Scheme 4, the intial step of the reaction sequence involves the N-complexation of 1,4,5-trimethylimidazole 38 with BEts to form 39, followed by deprotonation and rearrangement of resulting ionic species to 40. The final step involves methylation with Mel to form the desired triethylboron-carbene complex, 41. Compound 41 was characterized by mass spectroscopy and H-, C-, and B NMR spectroscopy the B NMR chemical shift for 41 S -12.6) appears in the tetracoordinate boron region. The 1,4,5-trimethylimidazole 38 is also the source of an unusual boryl-borane compound, 43 (34). Treatment with BH3 thf, followed by deprotonation with "BuLi, resulted in the formation of the ionic carbene/borane complex, 42. In turn, 43 was produced via the addition of chlorobis(dimethylamino)borane to 42. Compound 43 exhibits B NMR chemical shifts at S -21.8 (BH3) and 8 27.8 (BNMe2). In related work, N-methyl-AT-borane complexes 44 and 45 have been de-... 

Other reactions of sulphonium ylides include o /9 -elimination,metal-mediated carbene-transfer to olefins, insertion into aromatic C—H bonds or other carbenoid-type processes, formation of Pd" complexes, addition to enones (forming cyclopropyl ketones or heterocycles ), reaction with isoquinoline 2-oxide, and [2,3]-sigmatropic rearrangements " [as in the case of (12) 1 or... 

Anoflier reasonable alternative would be to suppose fliat some kind of rearrangement has occurred in the reaction and that the metal could be playing an active role during the process. Considering fliat at least one of die reaction products (allyl ether 3) would come from a 1,2-hydride addition to the carbene complex, we could employ this premise as a starting point for the new mechanistic proposal. 

Synthesis of (-t-j-Dumetorine A and (-i-)-Astrophylline The first enantioselective synthesis of (-t-)-dumetorine was also achieved through an RCM-ROM-RCM domino reaction as a key transformation in only six steps and a 27% overall yield from the functionalized cyclopentene 87 [43]. Preliminary investigation with catalyst [Ru]-I (10 mol%) allowed obtaining the desired product 89 in only 7% yield. The low yield was attributed to the preferential formation of a stable cyclic carbene complex 88 (Scheme 11.24) which inhibited further reaction and intermpted the catalytic cycle. With the use ofTi(Oi-Pr)4 (30mol%) as a co-additive, the formation of the chelated intermediate was inhibited and the desired ring-rearranged product 89 was obtained in good yield (80%) with only 5 mol% of catalyst [Ru]-I. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

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