Dienyl systems

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). 

It was also of interest to generate the related symmetric primary benzylic trication. However, the ionization of the 2,4,6-tris(chloromethyl)mesitylene (112) in excess SbF5/S02ClF at -78°C gave apparently only the dienylic allylic dication Lewis acid-base complex (113). The unionized chloromethyl carbon displayed a relatively deshielded NMR absorption, 35.3, indicative of a weak Lewis acid-base interaction. The terminal methylene carbons of the dienylic system showed a 8 C of 197.7 and the terminal carbons of the allylic system displayed a 8 C of 194.8, quite similar to that of the previously described dienylic allylic dications, 107 and 108. 

The regiocontrol displayed by an alkoxy substituent at C-2 of the dienyl system has been exploited for the total synthesis of a number of relatively complex natural products. For example, the malonate ad-... 

Ab initio MO calculations have been used to examine the conformational preferences, transition structures, and products of the 2 + 2-electrocyclization reaction of the penta-1,4-dienyl system bearing various substituents on C(3) see (251) —> (252). The results... 

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. 

Coming back to the initial problem, namely the stereoselective preparation of dienyl zirconocene derivatives, it was initially thought to apply the strategy described above to the preparation of dienyl systems (transformation of het-erosubstituted conjugated dienes 90 into dienyl zirconocenes 91, as described in Scheme 32). A few examples will be shown later in the chapter. 

In this tandem allylic C-H bond activation, followed by an elimination reaction, substituted l-zircono-lZ,3 -dienes (zirconium moiety at the terminal position of the dienyl system) were easily prepared as unique isomers. With the idea of extending this methodology to the stereoselective synthesis of 3-zircono-1,3-diene (zirconium moiety at the internal position of the dienyl system), 119 was prepared and the reactivity was investigated with (1-butene)ZrCp2 21 (119 was obtained by carbocupration of the a-allyl alkoxy-allene, Scheme 35) [79]. When 119 was submitted to the tandem reaction, the diene 120 was isolated after hydrolysis as a unique ( ,Z) isomer in 75% isolated yield (Scheme 44). 

As usual, to further increase the scope of the reaction, transmetalation of dienyl zirconium complexes, such as 124-127Zr, into the corresponding dienyl organocopper derivatives was performed. Surprisingly, when 126Zr was trans-metalated to copper derivatives by addition of a catalytic amount of CuCl/2LiCl in the presence of allyl chloride for 1 h at +50 °C, a partial isomerization of the dienyl system was found (Scheme 49). 

Obtained by measuring the intensity of the absorption at 252 nm due to the cyclopenta-dienyl system of (V). In the case of NQ the IR stretching band of caAonyl was used. Polym s were twice redissolved and rqirecqpitated from acetone before anahsis. 

Because the reactions of related in -cyclohexadienyl complexes are synthetically valuable, the reactions of this ligand have been studied extensively. An outline of how this chemistry can be conducted on the Fe(CO)j fragment is shown in Equation 11.51. A variety of cyclohexadienes are readily available from Birch reduction of substituted aromatics. Coordination and abstraction of a hydride, typically by trityl cation, leads to cationic cyclohexadienyl complexes. These cyclohexadienyl complexes are reactive toward organolithium, -copper, -cadmium, and -zinc reagents, ketone enolates, nitroal-kyl anions, amines, phthalimide, and even nucleophilic aromatic compounds such as indole and trimethoxybenzene. Attack occurs exclusively from the face opposite the metal, and exclusively at a terminal position of the dienyl system. This combination of hydride abstraction and nucleophilic addition has been repeated to generate cyclohexa-diene complexes containing two cis vicinal substituents. The free cyclohexadiene is ttien released from the metal by oxidation with amine oxides. ... 

Chesnut and Bartolotti described the aromaticity of a series of substituted five-membered cyclopenta-dienyl systems using the electron localization function (ELF), which is defined for a single determinant wave function built from Hartree—Fock or Kohn—Sham orbitals as... 

In water, an increase of the facial selectivity up to 2 1 in favor of the si face (antiaddition) of the diene was observed as depicted in Figure 3. That is the contrary of what has been predicted from several models of cycloadditions. The perpendicular model in which the heteroatom is peipendicular to the dienyl system predict syn selectivity by syn periplanar approach of the dienophile (17,18). The coplanar model in which the heteroatom lies in the plane outside the dienyl system, predicts syn selectivity as well (19). Both of then failed to explain our anti selectivity without implication of the p-hydroxy group. Furthermore, the increase of the anti-selectivity in water compared with toluene could be a consequence of the hydrophobic effect which favored the dienophile approach anti to the hydrophilic face where hydroxy group are tightly bonded to water molecules. 

The 2-methoxycyclohexadienyl complexes are particularly useful in synthesis. The methoxy substituent directs nucleophiles to the remote end of the dienyl system. This directive effect is strong enough to overcome the steric hindrance of a methyl substituent at the 5-position, although a mixture of products is obtained with the 5-ethyl complex. 

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