Intermolecular allylation

Enamines derived from ketones are allylated[79]. The intramolecular asymmetric allylation (chirality transfer) of cyclohexanone via its 5-proline ally ester enamine 120 proceeds to give o-allylcyclohexanone (121) with 98% ee[80,8l]. Low ee was observed in intermolecular allylation. Similarly, the asymmetric allylation of imines and hydrazones of aldehydes and ketones has been carried out[82]. 

Intermolecular allylation of aldehydes with 1 -trialkylsilyl-1,3-dienes 22 in the presence of a stoichiometric amount of triethylsilane and a catalytic amount of Ni(cod)2 and PPI13 shows novel regio- and stereoselectivity (Scheme 6) [20-22], When a toluene solution of a 1-silyl-1,3-diene and an aldehyde is refluxed in the presence of trialkylsilane under the catalysis of Ni(cod)2 and PPh3, ( )-allylsilane (E)-23 is obtained exclusively. On the other hand, when the reaction is carried out in THF upon heating at 50 °C as... 

Diastereoselective syntheses of dihydrobenzo[f>]furans have been accomplished by a rhodium-catalyzed regioselective and enantiospecific intermolecular allylic etherification of o-iodophenols as a key step, providing the corresponding aryl ally ether 122, which leads to a dihydrobenzo[b]furan by treatment of the intermediate aryl iodide with tris(trimethylsilyl)silane and triethylborane at room temperature in the presence of air <00JA5012>. 

Transition metal-catalyzed allylic substitution with phenols and alcohols represents a fundamentally important cross-coupling reaction for the construction of allylic ethers, which are ubiquitous in a variety of biologically important molecules [44, 45]. While phenols have proven efficient nucleophiles for a variety of intermolecular allylic etherification reactions, alcohols have proven much more challenging nucleophiles, primarily due to their hard, more basic character. This is exemphfied with secondary and tertiary alcohols, and has undoubtedly limited the synthetic utihty of this transformation. 

In Equation 16, the first step is the activation of a cyclohexene molecule by electron impact, and the second step is the reaction of the activated cyclohexene molecule with a normal cyclohexene molecule resulting in intermolecular allylic hydrogen transfer. In Equation 17 the first step is ionization of a cyclohexene molecule by electron impact, and... 

The annelation may proceed through the intermolecular allylation at the a-position of the a,a -dimethoxylated starting material 39 followed by the intramolecular addition of the cation developed at the a -position to the allylic double bond. The chlorine contained in the first product may come from the Lewis acid and is easily removable by catalytic hydrogenation. 

Silyl-4-alkenals undergo cyclization which gives 3-cyclopentenols. When the intermolecular allylation of enones which favors the conjugate addition pathway is applied to l,8-bis(trimethylsilyl)-2,6-octadiene the four stereocenters of the 2 1 adduct emerge in a defined fashion. ... 

The results obtained with model systems 5 and 8 demonstrate a clear preference for the synclinal transition structure, but several questions regarding the ability of this model to predict the stereochemical outcome of the intermolecular allyl-silane-aldehyde eondensation are still at issue. Model system 10 was designed to remove any steric bias that may be present in model 5 (Scheme 10-7) [17e]. Cyclization of 10 was induced by various Lewis acids and the results are shown... 

Scheme 12.8 Palladium catalyzed intermolecular allylic amination.

Of particular note, enantioselective intermolecular allylic amination is also pos sible using this protocol, and no competing aziridination is observed. Cyclic and linear alkenes were selectively aminated with high selectivities (27-31). 

Scheme 4 DMSO promoted intermolecular allylic acetoxylation of terminal alkenes...

Scheme 11 Heterobimetallic system for intermolecular allylic oxidative amination...

Scheme 12 Pd-catalyzed intra- and intermolecular allylic alkylation...

Fig. 30 Highly chemoselective methods for intra- and intermolecular allylic C-H oxidation...

Palladium Catalysts Palladium catalysts are effective and powerful for C—H bond functionalization. Carbene precursors and directing groups are commonly used strategies. Generally, sp3 C—H bond activation is more difficult than sp2 C—H bond activation due to instability of potential alkylpalladium intermediates. By choosing specific substrates, such as these with allylic C—H bonds, palladium catalytic systems have been successful. Both intramolecular and intermolecular allylic alkylation have been developed (Scheme 11.3) [18]. This methodology has presented another alternative way to achieve the traditional Tsuji-Trost reactions. 

Intermolecular allyl, propargyl, and allenyl ligand transfer... 

The ophcally active Pd complex with a chiral allenyl ligand undergoes epimer-izahon in the presence of a catalytic amount of Pd(0) complex. This reaction does not involve the isomerization to the propargyl complex, but takes place via a dinuclear intermediate as depicted in Scheme 5.39. The -allenyl ligand in the dinuclear palladium intermediate may racemize via a vinyl-vinyidene intermediate. This type of reaction is prohahly involved in a kinetic resolution of racemic propargyl alcohols promoted hy chiral transihon metal complex [203]. The intermolecular allyl ligand transfer from Pd to Ee complexes occurs under... 

Based on early mechanistic experiments, we propose that aminoester 139 and palladium(II)-n-allyl conplex 140 establish an unfavorable equilibrium with palladium(0) and ammonium salt 141 (Scheme 1 S.3ST As soon as this unstable ammonium intermediate is formed, it undergoes a rapid deprotonation to generate ammonium ylide 142, which is transformed into the observed [2,3]-rearrangement product 143 through an exo transition state. An unfavorable equilibrium for the palladium-catalyzed ammonium salt formation, in conjunction with the facile conversion of ammonium salts into the [2,3]-rearrangement products, could explain the difficulty in observing any ammonium intermediates. This mechanistic proposal also accounts for why catalytic intermolecular allylic amination with tertiary amines has never been reported before. 

With TBHP as external oxidant, CuBr and C0CI2 co-catalyzed allylic C(sp )-H alkylation of methylenic C(sp )-H bond was first reported in 2006 [105]. Two years later, Pd(II)-catalyzed intra- or intermolecular allylic C(sp )-H alkylation with active... 

Scheme 2.22 Pd-catalyzed oxidative intermolecular allylic C(sp )-H alkylation with active C-H nucleophiles [106, 107]...

Scheme 28 Chemoselective intermolecular allylic C(sp )-H amination with sulfonimidamides.

Enantioselective, intermolecular allylic amination, catalysed by a chiral phosphoramide (302), has also been described. A variety of symmetrical or unsymmetrical allylic alcohols (300) were smoothly aminated to afford the desired products (301) in moderate to high yields with good enantioselectivities (Scheme 81). ... 

Figure 39.2 Products of asymmetric intermolecular allylation of symmetric cyclic ketones using catalyst 8.

PhI(OTf)2 is an effective oxidant for the direct formation of bicyclic diazenium salts from a variety of linear hydrazone precursors. This oxidative cyclization is postulated to occur by the iodine(III)-mediated formation of an l-aza-2-azoniaallene salt intermediate. A direct intermolecular allylic amination has been achieved with up to 99% yields using metal-free conditions. The reaction employs a hypervalent iodine(lll) reagent as an oxidant and bistosylimide as a nitrogen source. Mechanistic studies including isotope labelling and Hammett correlation indicated that depending... 

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