Cyclopentadiene complexes allyl

Carboxylate anions are better nucleophiles for allylation. The monoepoxide of cyclopentadiene 343 is attacked by AcOH regio- and stereoselectively via tt-aliylpalladium complex formation to give the m-3,5-disubstituted cyclopen-tene 344[212]. The attacks of both the Pd and the acetoxy anion proceed by inversion (overall retention) to give the cis product. 

In the presence of water the tropylium ion (50) has been found to undergo polar cycloaddition of the 4 + 2 type. Ito and Itoh found that cyclopentadiene reacted with the tropylium ion to afford a 90% yield of a complex mixture of alcohols, all of which could be derived from the allylic cation (51). 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

The extensive organometallic chemistry of chromium, i.e. the hexacarbonyl and its derivatives, organochromium compounds without carbonyl ligands, cyanide and isocyanide complexes, alkene, allyl, diene, cyclopentadiene and arene derivatives, and complexes of a-donor carbon ligands, has been recorded in Chapters 26.1 and 26.2 of Volume 3 of Comprehensive Organometallic Chemistry .1 In the present section, chromium complexes... 

Reactions of the HNiL3CN complex with 1,3-cyclopentadiene, 1,3-cyclo-hexadiene, and 1,3-cyclooctadiene gave intermediates with decreasing stabilities in that order the 1,3-cyclooctadiene intermediate was not spectroscopically observable. The cyclohexadiene adduct was shown to be the cyclohexadienyl complex 12 by its proton spectra, with resonances of H , Hb, and —(CH2)3— at 14.53, 6.06, and 8.47, respectively these values are close to the chemical shifts found earlier (51) for 13 14.52,5.86, and 8.48. The reaction of DNi[P(OMe)3]X with cyclopentadiene gives 13-d, with addition of D and Ni to the same side of the ring (52). Backvall and Andell (55) have shown, using Ni[P(OPh)3]4 and deuterium cyanide (DCN), that addition of D and CN to cyclohexadiene is stereospecifically cis, as expected for jt-allyl intermediate 12. 

Iron carbonyls also mediate the cycloaddition reaction of allyl equivalents and dienes. In the presence of nonacarbonyldiiron a,a -dihaloketones and 1,3-dienes provide cycloheptenes (Scheme 1.5) [14,15]. Two initial dehalogenation steps afford a reactive oxoallyliron complex which undergoes a thermally allowed concerted [4 + 31-cycloaddition with 1,3-dienes. The 1,3-diene system can be incorporated in cyclic or heterocyclic systems (furans, cyclopentadienes and, less frequently, pyrroles). Noyori and coworkers applied this strategy to natural product synthesis, e.g. a-thujaplicin and P-thujaplicin [14, 16]. 

A Lewis acid-base complex, TiCl4-SbPh3, mediates the allylation of 2-(benzyloxy)propanal with allyltributyl-stannane, where a homoallyl alcohol is obtained with a high yy/z-selectivity (Equation (5)).40 The same complex is also effective for Diels-Alder reaction of an acrylate of (A)-ethyl lactate with cyclopentadiene, where an endo-adduct is formed predominantly with high diastereoselectivity and without polymerization of cyclopentadiene (Equation (6)). 

Cases in which allyl radicals display sufficient reactivity to participate successfully in radical chain reactions include the addition of bromotrichloromethane to butadiene the reaction of cyclopentadiene with tosyl cyanide, the addition of thiols , stannanes " and hydrogen halides . All these reactions follow the simple two-step radical chain mechanism depicted in Scheme 1, and the low reactivity of the intermediate allyl radicals can be compensated by using the trapping agent in excess or even as the solvent. In chain reactions with three or more chain-carrying radicals, this compensation is not possible anymore, because the concentration of the reaction partners has to be chosen such that the selectivity requirements for all intermediate radicals are satisfied. Complex radical chain reactions with polyenes as one of the reactants are therefore not known. 

Cyclic allyl residues enable the synthesis of cycloalkenes having a 1-hydroxyalkyl side chain. Cyclic Ti complexes, like (118) and (119) are readily obtained (Scheme 12) by treatment of (n -C5H5)2TiCl (117) with i-butylmagnesium chloride and cyclohexadiene or cyclopentadiene respectively, in TOF at -40 C for 10 min. Such complexes are quite unstable and can be prepared only in situ. Propanal... 

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