Transition metal catalyzed nucleophilic allylic substitution: activation of allylic alcohols via π-allylic species

Modern organic synthesis now requires efficient atom economical synthetic methods operating under greener pathways to achieve C-C and C-heteroatom bond formation. Direct activation of allylic alcohols in the presence of transition metal catalysts leading to electrophilic π-allyl metal intermediates represents such a promising target in the field of nucleophilic allylation reactions. During the last decade, this topic of recognized importance has become an emerging area, and selected transition metals, sometimes associated with alcohol activators, have brought elegant solutions for performing allylic substitution directly from alcohols in a regio, stereo and enantioselective manner.     

SN2-Selective allylic substitution of chiral γ-aryl substituted allylic picolinates with alkynylcopper reagents

Substitution of γ-aryl secondary allylic picolinates with alkynyl copper reagents was studied. The copper reagent, prepared from TMSCCMgBr and CuBr·Me₂S in 2:1, was subjected to substitution of the picolinate derived from (E)-3-phenyl-1-methyl-2-propenyl alcohol at 0 °C for 1 h in THF to produce a mixture of α- and γ-products and the alcohol in 67:20:13, while the reagent in 3 or 4:1 ratio gave the α-product with 90-91% selectivity. On the contrary, reaction in CH₂Cl₂-THF (6-8:1) at 0 °C for 1 h furnished the α-product with 99% regioselectivity. The effect of CH₂Cl₂ was also demonstrated with eight more examples. Furthermore, 99% inversion was determined by transformation to the literature compound and by chiral HPLC.     

Chiral aminophosphine phosphinite–palladium catalysts in asymmetric allylic alkylations

Palladium complexes of chiral aminophosphine phosphinites were investigated for the asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate. The chirality of the carbon bonded to O-PPh₂ in the ligands was found to be very important to the enantioselectivity of the catalysts. Introduction of this stereogenic center has led to significantly improved enantioselectivities.     

Samarium(II)-mediated reaction of allylic phosphate esters with carbonyl compounds: A new method for “Umpolung” of allylic phosphates

Allylic phosphates allylate ketones and aldehydes in the presence of samarium(II) iodide. The coupling proceeds with the preservation of the olefin geometry, however, regio- and stereoselectivity are not high.     

Enantioselective palladium catalyzed allylic substitution with 2,2′-bipyridine ligands

A number of chiral C₁-symmetric 2,2′-bypyridines were prepared and assessed in the enantioselective palladium catalyzed allylic substitution of 1,3-diphenylprop-2-enyl acetate with dimethylmalonate. Enantioselectivity up to 89% was obtained.     

New chiral phosphine–amide ligands in palladium-catalysed asymmetric allylic alkylations

Palladium-catalysed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate 8a with a dimethyl malonate–BSA–LiOAc system has been successfully carried out in the presence of new chiral phosphine–amide, such as 5, in good yields and high enantiomeric excesses of up to 85%.     

Phosphonium salt induced stereoselective allylic rearrangement during chlorination of α-hydroxyallylphosphinates

α-Hydroxyallylphosphinates were stereoselectively converted to γ-chloro allylphosphinates when treated with Ph₃P and CCl₄, via substitution of hydroxyl and migration of double bond. The reaction was promoted by phosphonium cation that acted as Lewis acid.     

Copper-catalyzed regioselective allylic oxidation of olefins via C–H activation

A regioselective oxidation of allylic C–H bond to C–O bond catalyzed by copper (I) was developed with diacyl peroxides as oxidants. The oxidation of allylic C–H bond was accomplished with good yield and regioselectivity under mild reaction conditions. This method has a broad substrate scope including cyclic olefins, terminal and internal acyclic olefins and allyl benzene compounds. The reaction proceeds by a radical mechanism as suggested by spin trapping experiments.     

Copper-catalyzed γ-selective and stereospecific allylic alkylation of ketene silyl acetals

Copper-catalyzed allylic alkylation of ketene silyl acetals proceeded with excellent γ-E-selectivity. Efficient α-to-γ chirality transfer with anti-selectivity occurred in the reaction of enantioenriched secondary allylic phosphates, affording enantioenriched β-branched γ,δ-unsaturated esters. Excellent functional group compatibility was observed.     

Ligand coupling through σ-sulfurane — complete retention of geometric configuration of allylic and vinylic groups in the reactions of allylic and vinylic sulfoxides with Grignard reagents

The reaction of p-benzenesulfonylphenyl crotyl sulfoxide with Grignard reagents is considered to proceed via formation of an incipient σ-sulfurane to afford the coupling product, p-benzenesulfonyl-crotylbenzene, in which the geometric configuration of crotyl group was completely preserved. No rearrangement was observed in the coupling reaction of p-benzenesulfonylphenyl α-methylallyl sulfoxide. Such a complete retention of geometric configuration was also found in the reactions of 2-pyridyl and p-benzenesulfonylphenyl styryl sulfoxides with Grignard reagents.     

α-Amino esters as nucleophiles in diastereoselective palladium catalysed allylic substitution reactions

Allylic acetates undergo palladium catalysed allylic substitution with amino ester nucleophiles. Using enantiomerically pure α-amino esters, the substitution products are obtained with moderate diastereoselectivity (up to 85:15: 70% d.e.). The reactions could also be performed using an enantiomerically pure ligand, which was found to exert a stronger influence over the stereochemical outcome.     

Ir-catalysed formation of C-F bonds. From allylic alcohols to α-fluoroketones

A novel iridium-catalysed tandem isomerisation/C-F bond formation from allylic alcohols and Selectfluor? to prepare α-fluorinated ketones as single constitutional isomers is reported.     

Rh(Ⅰ)-catalyzed regio-and enantioselective allylic alkylation of Meldrum's acid

A Rh(Ⅰ)-catalyzed highly regio-and enantioselective allylic alkylation of Meldrum acid with racemic allylic substrates bearing alkyl groups has been developed.The applying of chiral bisoxazopinephosphine ligand is essential for the high yields and selectivities.This method provides a rapid access to various chiral β-alkyl-γ,δ-unsaturated carboxylic acids and their derivatives.     

Palladium-catalyzed α-regioselective allylic amination of Morita-Baylis-Hillman acetates with simple aromatic amines

An efficient allylic amination of Morita-Baylis-Hillman acetates with simple aromatic amines provided good yields with excellent α-regioselectivity (up to exclusive α-product) under the catalysis of Pd(2)(dba)(3)/ferrocene-type diphosphine ligand.     

S,O-ligand-promoted palladium-catalyzed C–H olefination of arenes with allylic substrates

An efficient catalytic system for the C–H olefination of arenes with different allylic substrates is reported. The catalytic system is based on Pd(OAc)₂ and a readily accessible bidentate S,O-ligand. The methodology shows high activity with a wide range of arenes, including bulky and, electron-rich and -poor arenes. The applicability of this catalyst is demonstrated in the late-stage functionalization of the complex molecule O-methylestrone.     

Diastereoselectivity in the Carroll rearrangement of β-keto esters of tertiary allylic alcohols

Carroll rearrangement of β-keto esters derived from tertiary allylic alcohols, for example, 7, under basic conditions followed by decarboxylation of the resulting β-keto acids yielded the expected γ,δ-unsaturated methyl ketones 8 with a range of olefin geometries from 100:0 to 1:1.8 E/Z, depending on the relative steric requirements of the two groups at the allylic center.     

Solvent and additive-free selective aerobic allylic hydroxylation of β-pinene catalyzed by metalloporphyrins

Metallodeuteroporphyrins(MDPs) were employed as the catalysts for aerobic oxidation of β-pinene in absence of solvents and additives. Allylic hydroxylation products were found to be the main products from this protocol. The catalytic activity of MDPs with different metal nuclei and the influences of technological conditions on this reaction were investigated. This catalytic system has bright application prospect since only eco-friendly and readily available dioxygen were needed.