Allenyl complexes formation

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

An interesting sequence, again overall an isomerization, is the stoichiometric formation of the manganese complexes 68, which, on basic alumina, isomerize to the allenyl complexes 69 from the latter the allenes 70 can be liberated with cerium(IV) ammonium nitrate (CAN) in good yields [128] (Scheme 1.30). 

A cycloaddition process between the Rh=C bond of the allenylidene derivative 38 and the C=C bond of the terminal alkyne has been evoked in the formation of the zwitterionic 71-aUyl-allenyl complexes 81 (Scheme 28), the initially formed metaUacyclobutenes 80 evolving into 81 by formation of carbene intermediate [RhCl(P/-Pr3)2(=CHCR=C=C=CPh2)] (R = Ph, p-MeC6H4, SiMe3) and subsequent migration of one of the phosphine ligands from the metal to the carbene carbon atom [205]. 

The conversion of a dimeric rr-bound methyl propargyl ether complex [Mo2(CO)4Cp2(At-Tj, i7--CH=CCH20Me)] (35) to the cationic allenyl complex [Mo2(CO)4Cp2(m-i7% i -CH=C=CH2] (36) has been described by Curtis et al. (25). Protonation of 35 with HBF4 induced the loss of methanol and formation of the required complex. Alternatively the same complex was reported accessible via the acid-promoted elimination of methanol from [Mo2(CO)4Cp2 M-T7%Tj -MeO(H)C=C=CH2 ], a rare example of an intact T7%Tj -bound allene (Scheme 9). 

In the presence of the radical substitution promoter [Fe2(CO)4()u,-SR)i(PPh3)2], tertiary amines react with transition metal carbonyl clusters under exceptionally mild conditions. Modified allenyl and allylic clusters similar to those described earlier have been isolated from such reactions. Two distinct types of products have been isolated (i) those involving the elimination of an alkyl group and (ii) those involving C-C coupling reactions. The product formation described in Scheme 31 was preceded by amine coordination, C-H activation, C-N cleavage, carbene-amine complex formation, transamination, and C-C coupling (6Ia.h)- Such processes are of interest in the area of hydrodenitrification (6/c). 

The formation of allenylidene derivatives from ethynyl-hexanol and alkenyl-vinylidene mononuclear complexes (9), the formation of mononuclear ruthenium allenyl complexes from terminal alkynes (10), the intermediacy of ruthenium-allenylidene complexes in forming propargylic alcohols (II), and in the cyclization of propargyl alcohols (12), and the use of mononuclear ruthenium compounds in allylic alkylation catalysis (13) have also been reported. 

The reaction of [Ru2Cl(PhNpy) ] with [Li(CECPh)] produces [RU2(CECPh)(PhNpy) ], a diruthenium(II, III) compound with an axial n -acetylide ligand.Carbene (N2CR2) addition to [RU2(CO)g(u2-n -CECPh)(u-PPh2)] results in the formation of the u,-n -allenyl complexes [Ru., (CO), (u -n -R C=C-=CPh)(M-PPh ) ] (R=H,Me,Ph). The heteronuclear ruthenium-phenyl... 

Primary propargylic formates decarboxylate in the presence of Pd(acac)2 and Bu3P at room temperature to give mainly allenic products (Eq. 9.115) [91]. Initial formation of a propargylic palladium complex, which rearranges to the more stable allenylpalladium species, accounts for this transformation. Under similar conditions, a terminal allenyl formate afforded a 99 1 mixture of allene and acetylene product (Eq. 9.116) [91]. However, a mixture of enyne elimination products was formed when a secondary propargylic carbonate was treated with a palladium catalyst (Eq. 9.117). 

Treatment of the azido complexes 19 with CO leads also to the migration of the N3 ligand to the aiienyiidene unit (Scheme 2.9). Nevertheless, the initially formed azido-alkynyl compounds 20 are in this case thermally unstable, evolving slowly into the metallated acrylonitrile derivatives 23 via extrusion of N2. The mechanism of formation of 23 involves the migration of the azido moiety from Cy to the C atom of the alkynyl ligand to generate the allenyl intermediates 21, which by elimination of... 

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Zwitterionic rhodium(I) complex, Rh (CODX -CfiHsBPtn), is also found to be an efficient catalyst for the silylformylation of 1-alkynes at 40 °C and 40 atm of CO in CH2C12 (equation 125) although no reaction occurs with internal alkynes327. However, silylhydroformylation takes place when the reaction is carried out under hydroformylation conditions, i.e. in the presence of CO and H2 (CO/H2 = 1/1), to give (E)-2-silylmethyl-2-alkenals (319) in 54-92% isolated yields (equation 128). The intermediacy of 7r-allenyl-Rh species is proposed to account for the formation of 31 9327. When 4-acetoxy-l-butyne and 4-(p-tosyloxy)-l-butyne are used as the substrates, saturated silylhydroformylation products are obtained327. 

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