Pentadienylation regioselective reaction

A (7] -allyl)ferralactam synthesis using Ee2(CO>9 has been reported by Aumann el a/. <1979CB3644>. Schobert el a/. have also studied the regioselective reactions of (4,6- 7 -pentadienyl)ferralactones 53 with primary amines to afford 3-( o-vinyl)-(4,6-7] -allyl)ferralactams 54 and 6-(-vinyl)-(4,6- -allyl)ferralactams 55 (Equation 7) <2004JOM575>. 

The regioselectivity in palladium-catalyzed alkylations has been attributed to the dynamic behavior of trihapto pentadienyl metal complexes60. For example, competing electronic and steric effects influence product formation in dienyl epoxides, but in palladium-catalyzed reactions steric factors were often found to be more important. Thus, alkylation of dienyl epoxide 76 with bulky nucleophiles such as bis(benzenesulfonyl)me-thane in the presence of (Ph3P)4Pd occurred exclusively at the terminal carbon of the dienyl system producing allyl alcohol 77 (equation 39). However, the steric factors could be overcome by electronic effects when one of the terminal vinylic protons was replaced with an electron-withdrawing group. Thus, alkylation of dienyl epoxide 78 affords homoal-lylic alcohol 79 as the major product (equation 40). 

The reaction of 2,4-pentadienyltrimethylsilane with IV-acyliminium ion generated in situ at —78°C results in the formation of pentadienyl-substituted lactams 87 in good yield (equation 59). It is noted that the reaction temperature is an important factor for controlling the e-regioselectivity. A mixture of e- and /-substitution products at the dienyl system is obtained when the reaction is carried out at 0 °C127. 

Intramolecular hydroamination of cyclohexa-2,5-dienes has afforded the corresponding bicyclic allylic amines with high selectivity (Scheme 13).80 The reaction does not proceed through a direct hydroamination of one of the diastereotopic alkenes but more likely involves a diastereoselective protonation of a pentadienyl anion, followed by addition of a lithium amide across the double bond of the resulting 1,3-diene and a highly regioselective protonation of the final allylic anion. 

The cleavage of the carbon-silicon bond of 1-silyl-l-vinylcyclopropane has been achieved by the fluoride ion. The reaction has been performed138) in only one case (Scheme 65) but has severd to generate a pentadienyl anion which was regioselectively hydroxy-alkylated on the cyclopropane ring 138) (Scheme 65). This reaction has been successfully used for the synthesis of ( ) a-vetispirene138) (Scheme 65). 

C-Substituted Allyl Anions—Pentadienyl Anions.315 Allyl anions with C-substituents pose a different problem. Both a attack and y attack are known, as illustrated by the reactions of the open-chain C-substituted anions 4.66,316 and 4.67.317 The problem is not only that the regioselectivity is irregular, but explaining it is not straightforward either. Simple predictions based on the n orbitals suggest that the... 

The regioselectivity of nucleophilic attack for the formation of G-G bonds depends on the nature of the nucleophile as well as the substituents attached to the dienyl ligand. Reactions of 1,2-disubstituted 42a and 42b and 1,4-disubstituted 43a and 43b cations with methanol, triphenylphosphine, lithium dimethylcuprate, and sodium dimethylmalonate nucleophiles have been investigated. It was found that cations 42a and 42b direct nucleophilic attack regiospecifically to the unsubstituted C5 position whereas cations 43a and 43b undergo G1 addition, although in a less predictable manner (Scheme 3). Weak nucleophiles such as methanol, allyltrimethylsilane, furan, and allylic organozirconiums are found to attack exclusively at the less substituted pentadienyl terminus and on the less stable, but more reactive, transoid form. Malonate anion attack on 1-substituted acyclic (77 -pentadienyl)-... 

Addition of amines and phosphines to acyclic (ry -pentadienyOiron cations is reversible in some cases. In a representative example (Scheme 4), kinetically controlled nucleophilic attack by phosphine occurs on the more abundant cisoid cation to generate an ,Z-diene 47. Since significant steric interactions exist between the phosphine nucleophile and substituents present on the pentadienyl ligand, nucleophilic attack in this case is reversible. At higher temperatures and longer reaction times, the thermodynamically more stable , -diene 48 is formed via nucleopilic attack on the less abundant transoid form of the (ry -pentadienyOiron cation. Amine nucleophiles add regioselectively to the unsubstituted terminus of the (ry -pentadienyOiron cation. 

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