Allylic substitution regiospecificity

The use of the o-diphenylphosphanylbenzoyl leaving group in copper-catalysed allylic substitution reactions with a Grignard reagent has been studied extensively.7 ( ) High SN2 regiospecificity (>95%) and complete stereospecificity are observed at room temperature when the solvent is a dichloromethane - diethyl ethyl mixture. 

The Lewis acid-catalyzed aUylsilylation of alkynes and alkenes with allylsilanes was introduced by Jung [511] and Yamamoto [512]. Jung and co-workers found fhat AICI3 catalyzes the aUylsilylation of simple alkenes and phenyl-substituted alkynes wifh allylsilanes (Scheme 10.191) [513]. The allylation occurs regiospecifically at the y-position of the allylsilanes. When cycloalkenes are used as substrates, trans adducts are obtained exclusively. Jung et al. initially reported [513 a] that the aUylsilylation of alkynes proceeded in a cis-addition mode they later noted, however, fhat fhe stereochemical assignment of fhe aUylsilylation products was incorrect and the products were not cis but trans adducts [511]. It has been proposed that the mechanism of fhe AlCls-catalyzed aUylsilylation involves addition of a silyl cation or its equivalent to fhe carbon-carbon multiple bond and subsequent aUylation of the resulting -silylcarbenium ion [511]. 

In 2004, Krische and colleagues demonstrated that exposure of Morita-Baylis-Hillman acetates to tertiary phosphine catalysts in the presence of 4,5-dichlorophthalimide enabled regiospecific allylic substitution through a tandem Sn2 -Sn2 mechanism. Through the use of the chiral phosphine catalyst, (i )-Cl-MeO-BIPHEP, the racemic Morita-Baylis-Hillman acetate depicted in Scheme 2.108 was converted into the corresponding enantiomerically enriched allylic amination product, thus establishing the feasibility of DKR. 

Substitution of one carbonyl by PI13P in (diene)Fe(CO)3 complexes results in a change of regiospecificity of electrophilic attack and thus provides an easier access to [(allyl)FeL,4]X salts. Similar PI13P substitution in [(dienyl)Fe(CO)3]X complexes decreased reactivity towards nucleophiles495. 

Four reviews on allylic and vinyl substitution have been published.20-23 The use of pentamethylcyclopentadienylruthenium catalysts for the. S n reactions of allyl substrates has been reviewed.20 The Sn reactions of allyl substrates in the presence of ruthenium catalysts occur primarily at the most substituted position of the allylic group. All the catalysts involve formation of an intermediate where the allyl compound becomes associated with the Ru atom in the catalyst. The regiospecificity (50-98%) depends on the structure of the allylic substrate, the nucleophile, the solvent, the temperature, and the catalyst. These catalysts have also been used for protection of allylic alcohol and amino groups. Some of the reactions are stereospecific. 

Subsequent substitutions in these systems are often regiospecific (Scheme 44).459 460 461 Unlike the reductive elimination of cis hydrides in [Co(H)3(PPh3)3], ethane is not formed from cis Me groups in (120 Scheme 44), but for L = CO insertion into the Co—Me bond occurs with the release of acetone (Scheme 45) no intermediates have been observed.459 Using the diphosphomethamide anion as an analogue of allyl an interesting bonding isomerization has been observed (121 to 122 Scheme 46).462... 

In the presence of a Lewis acid (such as Et2AlCl), allylsilanes react with electrophiles in a regiospecific manner. The intermediate (3-carbocation is stabilized by (a-Tc)-conjugation with the C-Si bond. The most important feature of this reaction is that the electrophile reacts with the terminus (y-carbon) of the allyl system, and the n-system is relocated adjacent to its original position. Even substituted allylic silanes can be acylated at the more hindered site. Because of this predictability and their high nucleophilicity, allylsilanes are valuable in many synthetic transformations. 

The first step of the mechanism involves the initial complexation of titanium tetrachloride to the carbonyl group of the electron-deficient alkene (enone) to give an alkoxy-substituted allylic carbocation. The allylic carbocation attacks the (trimethylsilyl)allene regiospecifically at C3 to generate vinyl cation I, which is stabilized by the interaction of the adjacent C-Si bond. The allylic Ji-bond is only coplanar with the C-Si bond in (trimethylsilyl)allenes, so only a C3 substitution can lead to the formation of a stabilized cation. A[1,2]-shift of the silyl group follows to afford an isomeric vinyl cation (II), which is intercepted by the titanium enolate to produce the highly substituted five-membered ring. Side products (III - V) may be formed from vinyl cation I. 

Wang, M.-X., Huang, Z.-T. Regiospecific Allylation of Benzoyl-Substituted Heterocyclic Ketene Aminals and Their Zinc Chloride-Promoted 3-Aza-Cope Rearrangement. J. Org. Chem. 1995, 60, 2807-2811. 

The pinacol rearrangements of several cyclobutanediols, mostly induced by BFs-OEta, have been examined by Conia. The reactions of symmetrically substituted materials occurred with high specificity and yield. Both isomers cis and trans) gave the same product in instances where this question was examined. As shown in equation (17), for the substituents R = H, alkyl or phenyl, only ring contraction was observed, while equally regiospecific formation of the cyclobutanone product was found for the substituents R = allyl or vinyl. 

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