Selective allylation

Allylic carbonates are better electrophdes than allylic acetates for the pallathirm-catalyzed allylic alkylation. Reaction of Eq. 5.54 shows the selective allylic alkylation of ct-nitro ester v/ith allylic carbonates v/ithont affecting allylic acetates. ... 

Chiral phosphinous amides have been found to act as catalysts in enantio-selective allylic alkylation. Horoi has reported that the palladium-catalyzed reaction of ( )-l,3-diphenyl-2-propenyl acetate with the sodium enolate of dimethyl malonate in the presence of [PdCl(7i-allyl)]2 and the chiral ligands 45 gave 46 in 51-94% yields and up to 97% ee (Scheme 38). It is notorious that when the reaction is carried out with the chiral phosphinous amide (S)-45a, the product is also of (S) configuration, whereas by using (R)-45b the enantiomeric (R) product is obtained [165]. 

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

Luche found that tin-mediated allylations can also be performed through ultrasonic radiation, instead of using aluminum powder and hydrobromic acid to promote the reaction.82,83 The use of a saturated aqueous NH4C1/THF solution, instead of water/THF, dramatically increased the yield. When a mixture of aldehyde and ketone was subjected to the reaction, highly selective allylation of the aldehyde was achieved. 

As the nitro group is removed by radical denitration with Bu3SnH, allylic alkylation of a-nitro ketones with allyl carbonates in the presence of Pd(0) followed by denitration with Bu3SnH provides a new regio-selective allylation of ketones under neutral conditions (Eq. 5.55).79... 

TABLE 10. Equilibrium acidities of selected allylic compounds in dimethyl sulfoxide at 25°Ca... 

BM Trost. New rules of selectivity allyl alkylations catalyzed by palladium. Acc Chem Res 13, 385, 1980. 

CuCN-2LiCl. This can then be selectively allylated vith allyl bromide to provide the dibromoimidazole 36, which can now be magnesiated by treatment with a further equivalent of iPrMgBr, providing the ester-substituted imidazole 37 in 55% yield after carboxylation with ethyl cyanoformate [25]. 

Considering the excellent chemoselectivity observed in the allylic oxidation of dehydroepiandrosterone (Scheme 16), it was interesting to evaluate the selective allylic alcohol oxidation in the presence of a secondary saturated hydroxyl group using the BiCls/f-BuOOH system. This study was performed using androst-... 

These multicomponent catalyst systems have been employed in a variety of aerobic oxidation reactions [27]. For example, use of the Co(salophen) cocatalyst, 1, enables selective allylic acetoxylation of cyclic alkenes (Eq. 6). Cyclo-hexadiene undergoes diacetoxylation under mild conditions with Co(TPP), 2 (Eq. 7), and terminal alkenes are oxidized to the corresponding methyl ketones with Fe(Pc), 3, as the cocatalyst (Eq. 8). 

Fig. 5. GPC (RI) trace f two select allyl fun ionalized octa-arm PIB stars with different molecular weights a M =5.77xi0 g/mo b M =11.0xl0Vmol...

Table 2. Molecular characteristics of two select allyl functionalized octa-arm PIB stars...

The allylation and the conjugate addition (vide supra) of zinc homoenolate proceed under essentially the same conditions except that the latter requires the presence of Me,SiCl as well. Due to this subtle difference, selective allyl substitution is possible even with an enone function is present in the same molecule (Table 9). 

A catalytic method for the allylic oxidation of alkenes was first reported by Umbreit and Sharpless in 1977428, who utilized TBHP as oxidant and Se02 as catalyst for selective allylic oxidation. Yields were moderate providing allylic alcohols or ketones with 54-86% yield. The reaction did not proceed under strictly anhydrous conditions but with one equivalent of water present the oxidation proceeds smoothly at room temperature. In... 

Allylation of aldehydes. Aldehydes undergo selective allylation on reaction with allylic bromides and tin (1.2 equiv.) in H20/THF (5 1) in a sonicator.1 Similar results can be obtained with zinc powder in saturated aqueous NH4C1-THF (5 1).2 Example ... 

Regio- and stereo-selective allylation of sulfonylimines has been carried out with trifluoro(allyl)borates and allylstannanes, using palladium-pincer complexes as catalysts.47 Syn products predominate, in contrast to the corresponding reaction of aldehyde electrophiles DFT calculations have been employed to probe the mechanistic differences. 

Pinacols (61) derived from a variety of aromatic aldehydes have been employed in enantio- and diastereo-selective allylations of aliphatic aldehydes.174 Their allyl-boronate derivatives react under Lewis acid conditions (SnCU) with a variety of aldehyde types, in good yield and ee. Even better results are obtained by addition of (61) as a I irons led acid (auto)catalyst, via coordination/activation of the tin catalyst. 

A number of selective transformations (Fig. 10) have been described which include the selective allylation on alcohols in the presence of amides [47], the Lewis acid catalyzed cleavage of benzyl alcohol esters with secondary amines to afford tertiary amides [48], the synthesis of ketones from Weinreb-type amides [49], and the synthesis of tertiary amines by a Michael addition/alkylation/Hoffman elimination sequence [50],... 

Nucleophilic amination of alkenes functionalized by an allylic C-X (x = heteroatoms, halides) as outlined in Eq. (1) is a simple and direct procedure for the synthesis of allyl amines, since very efficient methods for the selective allylic functionalization of alkenes are available. 

Selected allyl boronates synthesized according to Equation (57) are shown in Scheme 18. The reaction provided a convenient access to allylboranes possessing a carbonyl group 168 and 169,310,311 which underwent in situ intramolecular allylboration. A variety of 5-5, 6-5, and 7-5 m-fused exomethylene cyclopentanols were synthesized... 

The first catalytic asymmetric allylation of imines has been reported using allyltri- -butyltin in the presence of a chiral 7r-allylpalladium complex.179 Zirconium is also demonstrated as a metal center for the design of chiral Lewis acid catalysts that are suitable for the activation of bidentate imino compounds.180 Jprgensen reports high enantio-selective allylation of ct-imino esters (Equation (52)).181... 

Under palladium catalysis, triethylborane promotes the C(3)-selective allylation of indoles and tryptophans using a wide structural variety of allyl alcohols (Equation 124) <2005JA4592>. The yields of allylation are excellent and in most cases exceed 80%. 

That mercury(ll) acetate allylic oxidatimi can be a usrful reaction in the case of complex and sensitive substrates is demonstrated by the oxidation of avomectin A2a (38). The reaction, carried out in anhydrous toluene at 100 C for 40 min, was mnaikably selective, allylic oxidation occurring exclusively at the 3,4-double bond with rearrangement to give (39) in up to 73% yield (equation 17). 

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