Allenes allylic alkylation

Allenes 23 can also serve as precursors for allyl complexes which undergo intramolecular allylic alkylation to give vinylcyclopropanes 25. This conversion, however, competes with cyclopentene formation. Several other examples of this type have been reported. ... 

Allylic alkylation catalyzed by Pd(dba)2 and ( PrO)3P has been used for incorporation of nucleobases (pyrimidines and purines) into carbocyclic nucleoside analogs. In certain cases, unstabilized nucleophiles have been found to participate in allylic alkylation reactions. For example, an aUenic double bond is sufficiently nucleophilic to react with the r-aUyl complex generated upon heating Pd(dba)2 and 1 in toluene (eq 15). Formation of the trans-fused product (2) was interpreted to arise via the double inversion pathway commonly observed in conventional Pd-catalyzed allylic alkylation reactions. Interestingly, changing to a coordinating solvent (CH3CN) resulted in allene insertion into the r-allyl complex to form the isomeric c/s-fused product (3). 

Cyclopropanated allylboration reagents have been used to homoallylate and homocrotylate aldehydes. A zinc-mediated a-prenylation of aldehydes and ketones has been reported conventional conditions have yielded mainly the y-adduct to date. " A racemic allene has been hydroborated in an enantioconvergent manner the enantioenriched crotylborane then reacts with aldehydes in high ee to give ( j-stannyl-homoallylic alcohols. A copper-catalysed y-selective and stereospecific allylic alkylation of ketene silyl acetals has been reported. ... 

Allene derivatives are reactive toward Pd-catalyzed allylic alkylation reactions. The addition of bis(benzenesulfonyl)-methane to a functionalized allene in the presence of dimer (1) and dmppp [(l,3-bis(di(2-methoxyphenyl)phosphino)propane)] ligand gave the substituted alkene as the exclusive -isomer in 64% yield (eq 34). ... 

Cycloheptatriene reacts with DAD to afford exclusively ene products, as do the enols of some triazene derivatives. Allenes with alkyl substituents react with DAD to yield ene products in most cases (eq 7). Ene reactions with DAD have also been found to be a useful way of cleaving allyl ethers. ... 

An allylic pertluonnated carbamon can be generated from tetrakis(trifluo-romethyl)allene and cesium fluoride and can be trapped by reactive alkylating agents 247] (equation 51)... 

Sulphones (coni.) allenic - see Allenic sulphones allyl - see Allyl sulphones aryl unsaturated alkyl - see Aryl unsaturated alkyl sulphones aryl vinyl - see Aryl vinyl sulphones as radiolytic products 907 bicyclic - see Bicyclic sulphones bis(/ -hydroxyalkyl) - see Bis... 

Brown and Suzuki have shown that treatment of trialkylboranes with ethenyl-(Scheme 42, Eq. 42a) and ethynyloxiranes (Scheme 42, Eq. 42b) in the presence of a catalytic amount of oxygen, affords the corresponding allylic or allenic alcohols. The mechanism may involve the addition of alkyl radicals to the unsaturated system leading to l-(oxiranyl)alkyl and l-(oxiranyl)alkenyl radicals followed by rapid fragmentation to give alkoxyl radicals that finally complete the chain process by reacting with the trialkylborane [104-106]. 

Although both boronates and alanates react with allylic bromides, aldehydes and C02 to afford allenic products in satisfactory yield, the alanates are more efficient in additions to ketones (Table 9.14). Boronate reagents do not require a B-C alkyl migration for their preparation. Thus the starting acetylene possesses the structural elements of the product. Additionally, the issue of dummy ligands is irrelevant. 

In 1998, Saigo et al. [150] discovered the Rh(I)-catalyzed allenic version of the vinylcyclopropane rearrangement. With a subsituent on the cyclopropane ring two different isomers can be formed depending on which of the different allylic C-C bonds in the three-membered ring is broken. Depending on R2, different regioselec-tivites were obtained with 250. Alkyl-substituted substrates lead to 251 and aryl-substituted substrates to 252 (Scheme 15.79) [150]. 

Hydroxycoumarins are alkylated under extremely mild basic liquiddiquid phase-transfer catalytic conditions to produce the ethers and 4-alkylated derivatives 141 ]. The major product tends to be the ether (50-60%) but the yield of the C-alkyl-ated product is significant with allyl bromide and with propargyl bromide, where rearrangement of the allenic derivative occurs. 

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