AUyl systems

Figure 15 Hiickel jr-system energy level diagram, energies, and wavefimctions for the aUyl system...

Figure 14 Basis set orbitals for the aUyl system in Hiickei caicuiations...

Starting from enantioenriched branched substrates, a strong stereochemical memory effect is observed. With sNus, the substitution reaction occurs with net retention of configuration at the reacting center. If hNus are used, the attack at the aUyl system proceeds according to an inner-sphere mechanism resulting in a net inversion (Scheme 12.88). 

Obtaining high branched-product selectivities is feasible with aryl-substituted substrates but not trivial starting from linear alkyl-substituted aUyl systems. However, a remarkable improvement of regioselectivity with such substrates was achieved by the use quinoUnecarboxylic acid as a hgand. The branched aUylation products were formed with regioselectivities of up to 89% [209]. 

We have met the stmcture of the aUyl system several times already (Problems 1.61,9.3), but Figure 12.47 again summarizes both the molecular orbital treatment and the resonance treatment of this system of three contiguous 2p orbitals. 

Similarly, the linear combination of three p-orbitals in aUyl system will give rise to three new MOs xl/j, l/2 and /3. The antibonding interactions increase the energy of the MO. Thus, the energy of is higher than that of l/] and is of higher energy than 2. The wave functions and their symmetry in relation to the mirror plane are shown in Fig. 1.2. 

Figure 1.2 Molecular orbitals formation in aUyl systems. 6...

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal aUyl system called an allylmethylene diradical ... 

Even under mild conditions, powerful nucleophiles can be generated that open the way for interesting aUyl systems [109] (see 261). 

The stabilization of the 2-propenyl (aUyl) system by resonance can also be described in terms of molecnlar orbitals. Each of the three carbons is sp hybridized and bears a p orbital perpendicular to the molecular plane (Figure 14-1). Make a model The structure is symmetric, with equal C-C bond lengths. 

There are many examples in which late transition metal-catalyzed hydroamination is initiated by activation of a C-C multiple bond followed by nucleophilic attack. Two types of initial coordination of a C-C multiple bond are conceivable the direct T -coordination of a C-C multiple bond and the more extended Jt-coordination system such as a ri -aUyl system (or a T] -arene system). 

Larock has developed a new catalyst system for the Pd-catalyzed cyclization of olefinic tosylamides. Whereas typical conditions require either stoichiometric amounts of Pd(II) salts or catalytic amounts of Pd(II) in the presence of benzoquinone as a reoxidant, the new catalyst system utilizes catalytic Pd(OAc)2 under an atmosphere of O2 in DMSO with no additional reoxidant <96JOC3584>. Although o-vinylic tosylamides 76 can be cyclized to Af-tosylindoles 77 using this catalyst system, PdCla/benzoquinone is more effective for such cyclizations. Interestingly, in the case of o-allylic tosylanilides, the cyclization can be modulated to afford either dihydroindole or dihydroquinoline products. In a related approach involving a common 7i-aUyl Pd-intermediate, 2-iodoanilines were coupled with vinylic cyclopropanes or cyclobutanes in the presence of a Pd catalyst to afford dihydroindoles <96T2743>. 

A detailed comparative study of the allylic substitution in allylacetates by amines has been carried ont for the two related mixed donor ligand systems 113 and 114 shown in Fig. 2.19. The P-C system shows dramatically lower activity than N-P, which has been atlribnted to the decreased electrophiUcity of the coordinated aUyl gronp, originating from the strongly o-donating NHC and P donors, compared to N and P donors. 

Enantioselectivity (which is hnked to the regioselectivity of the attack of the nucleophile to the coordinated allyl) in the allylic amination of 1,3-diphenyl-allyl ethyl carbonate was also very low compared to the P-N system. This was attributed to the comparable fran -influence of P and NHC functionalities, leading to poor regioselec-tion of the two aUyl termini trans to the P and NHC ligands by the nucleophile [95],... 

Aregioselective catalytic system for the allylic substitution of non-symmetric allyl carbonates by carbon and nitrogen nucleophiles based on [ Bu N][Fe(NO)(CO)3] and PPhj was developed (Scheme 2.26). The high regioselectivity was ascribed to the slow a-allyl- to Jt-aUyl-isomerisation relative to the rate of substitution. However, the use of high excess of the pro-nucleophile and DMF solvent are drawbacks on the atom efficiency and functional group tolerance of the system. 

Regarding the use of other metals for this transformation, Shirai and co-workers reported that a system constituted by palladium(II) complex [Pd(p-Cl)(r -aUyl)]2 and thioether-imidazolium chloride 19 achieved the arylation of aldehydes with boronic acids [33] and potassium trifluoroborates in good to excellent yields (Scheme 7.5) [34], More recently, Buffard and Itami showed that a NKcod) / IPr-HCl system could catalyse the reaction of arylboronate esters and inactivated aldehydes and ketones (Scheme 7.5) [35]. 

Eine Schltisselreaktion scheint die Amin-Eliminierung in AUyl-Stellung aus einem 2.3-Endiol-l-amin-System 54 oder 63 zu sein >. 

The closely related dilithiated compound 208, also with a central four-membered ring, was obtained from starting compound 207 in DME (Scheme 71). The molecule represents the first bis(aUyl)dianion system consisting of six carbon atoms involving eight jt electrons (6C-8 7r). The crystal structure of 208 was reported . ... 

According to equation 102, stereochemically homogeneous 3-carbonyl-substimted tetrahydrofurans are constructed in a brick-box system by sequential homoaldol and aldol reaction. The metallated aUyl carbamate serves as an equivalent for the chiral dianion A, which accepts two different aldehydes B and C in a highly controlled manner. ... 

Stereoselective, conjngate allylation of more remote positions nsing aUyl trimethylsilane and TiCU is possible. Further transformation by nucleophilic addition and ringclosing metathesis affords fused polycychc ring systems (Scheme 88). Stabilized anions and organocopper see Copper OrganometaUic Chemistry) reagents also participate in Michael-type additions to complexed Q, /3-unsaturated aromatic enones. 

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