Allylic structure

The system provides an opportunity to test our method for finding the conical intersection and the stabilized ground-state structures that are formed by the distortion. Recall that we focus on the distinction between spin-paired structures, rather than true minima. A natural choice for anchors are the two C2v stmctures having A2 and B, symmetry shown in Figures 21 and 22 In principle, each set can serve as the anchors. The reaction converting one type-I structirre to another is phase inverting, since it transforms one allyl structure to another (Fig. 12). 

Allylic rearrangement (Section 10 2) Functional group trans formation in which double bond migration has converted one allylic structural unit to another as in... 

Lines represent [1 1 0] directions of underlying platinum. A unit cell ofthe adsorbate structure is drawn, (b) Diagram ofthe proposed (y/7 x yj7) R19.T model for the Jt-allyl structure in Figure 7.18a. Inset shows bonding structure. 

Compounds of the type Zr(7r-Cpd)2, Ti(Tr-Cpd)2, and Cr(CaH6)2, were found to be completely inactive with all monomers whereas a significant number of transition metal allyl compounds were found to have weak activity for ethylene polymerization. The latter results are summarized in Table I. Despite the fact that many transition metal allyl compounds are unstable above 0°C, in the presence of monomer, the metal allyl structure... 

A study of the reactions of butadiene, isoprene, or allene coordinated to nickel in a metallacycle, with carbonylic compounds, has been reported by Baker (example 11, Table IV). In the presence of phosphines, these metallacycles adopt a cr-allyl structure on one end and a ir-allyl structure on the other, as mentioned in Section II,A,1. The former is mainly attacked by aldehydes or electrophilic reagents in general, the latter by nucleophiles (C—H acids, see Table I, or amines, see Table IX). 

Monosaccharides carrying hindered allylic bromide or methanesulfonate groups react with Me3SnCu (from CuCN + BuLi, then Bu3SnH) with retention of the allylic structure, but less-hindered derivatives give some allylic rearrangement.7... 

The Sn-allyl bond (but not Sn-alkyl, Sn-vinyl, Sn-benzyl, or Sn-aryl) reacts with C02 in the presence of a palladium catalyst to give a tin buteneoate. A mechanism can be envisaged which involves the formation of the Sn-Pd-allyl structure, which undergoes C02 insertion to give Sn-Pd-OCO-allyl, followed by reductive elimination of the tin carboxylate (Equation (131)).345... 

To account for the exchange and isomerization of a number of poly-methylcyclopentanes, Rooney et al. (3S) postulated that intermediates corresponding to the w-allyl structures written above were not only able to abstract hydrogen from the surface as in the classical mechanism, but also could accept an atom from molecular hydrogen according to an Eley-Rideal mechanism (Fig. 26). 

Fig. 26. The alternative paths for the combination of a w-allyl structure with hydrogen according to the mechanism of Rooney, Gault, and Kemball.

The cationic polymerization of 2-vinyltetrahydrofuran 1 is marred by side reactions due to the allylic structure of the substituent and the participation of the ring (22). The oligomers obtained have therefore a complex structure. 

Tetraallyluranium [143, 144) and tetra(2-methylallyl)-uranium(IV) [145) are prepared by the reaction of the Grignard reagents with uranium tetrachloride in diethyl ether. The PMR spectra of these temperature-sensitive compounds are listed in Table 7. The AMgXa [146) pattern exhibited by tetraallyluranium is typical of the symmetrical jr-allyl structure in Fig. 14. The spectrum of tetra-... 

The proton spectrum of CH2=CHCH2Mn(CO)6 is wholly consistent with a simple allylic structure (69) and will not be discussed here. In the case of C3H6Mn(Co)4 the proton spectrum (69), shown in Fig. 6, requires a... 

The predominant 1,4-addition selectivity with lithium in nonpolar solvents is explained by the localization of Li+ to the terminal carbon. Morton et al. found197,198 that lithium is essentially cr-bonded to the terminal carbon (23). In polar solvents the negative charge is delocalized (24), generating a Jt-allyl structure that enhances the reactivity of the y-carbon, affording 1,2 addition ... 

The mechanism of the toluene and xylene oxidation bears a close resemblance to the oxidation of propene. Abstraction of a H-atom from the reactive methyl group and formation of a complex between the resulting radical and the catalyst is the first and probably the rate-determining step for both. However, the effect of the mesomeric stabilization of this radical complex is different. While a symmetrical allyl structure is formed from propene, an asymmetrical situation occurs for toluene and xylene, which is illustrated below for the case of toluene, viz. 

Despite the success in the 1,3-diphenylallyl system, use of many of these ligands in the alkylation of 1,3-dialkylallyl system as Equation 8E.4 has produced mixed results, as summarized in Table 8E.5. With the phosphinooxazoline-type ligands, good selectivities (>90% ee) are still obtained from the reactions of substrates possessing bulky allylic substituents such as isopropyl groups (entries 8-10), but smaller substrates such as 1,3-dimethylallyl derivatives give only a modest level of enantioselectivities (entries 1 -7). The disparity between these results appears to be sterically derived as the enhanced preference of syn versus anti orientation in the 7t-allyl structure by the bulky phenyl or isopropyl groups may not be present with the smaller substrates. 

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

Reactions carried out with M2(DBA)3 (M — Pd, Pt DBA = dibenzylidene-acetone) and 3,5-di-t-butyl-l,2-benzquinone gave as major products the M(DBSQ)2 complexes [229]. In the case of palladium, an additional product Pd2[Pd(DBSQ)2]2 was detected, whose molecular structure consists of two planar cw-Pd(DBSQ)2 units bridged by two Pd atoms. The Pd atoms are sandwiched between semiquinone rings of adjacent Pd(DBSQ)2 units with three Pd — C lengths and an allyl structure 961 for the semiquinone rings ... 

The corresponding Heisenberg Hamiltonian derives from the connectivity of the allyl structure and is shown in the scheme below the structure. This matrix... 

High resolution MAS techniques of 13C, DEPT, correlated spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear chemical shift correlation (HETCOR) were used to examine conventional CBS and efficient TMTD vulcanisation of polybutadiene [37]. In conventional CBS vulcanisation, the major vulcanisate 13C NMR peak occurred at 44.9 ppm and was assigned to a trans allylic structure (-C=C-C-Sx with X=3 or 4). The efficient TMTD vulcanisation yielded as main product a 13C NMR peak at 54.0 ppm and was assigned to a cis allylic vulcanisate (-C=C-C-Sx x=l). While cyclic sulfur by-products were observed in both vulcanisation systems, the CBS formulations gave rise to a higher percentage postulated to be formed via a episulfide intermediate. 

Vulcanisate structures of BR crosslinked with cyclic disulfides was studied by NMR.36 Using high resolution MAS techniques of DEPT, COSY, TOCSY, and HETCOR, the resulting spectra showed that crosslinking gave an addition product to the double bond and not the allylic structure found in typical sulfur vulcanisations. 

A conjugated diene can coordinate to a transition metal by only one double bond, as an s-trans-r 2 ligand, or with the two double bonds, as an s-cis-tf or as an s-trans-rf ligand [188]. A coordinated transoid monomer (as an s-trans-rj2 or an s-trans-rf ligand) is inserted into the metal-carbon bond, acquiring the syn-/ 3-allylic structure of the growing chain end. On the other hand, when a cisoid monomer coordinates to a metal (as an s-cis-rj4 ligand), an anti-t]2-allylic structure is formed. 

Similarly, 7r-allylic structure solutions show no tendency toward hydrolysis unlike dynamic structures which hydrolyze readily. 

As stated above, for compounds having a 7r-allylic structure, reactions (including polymerization) occur after the compounds are transformed into the dynamic form. Polymerization in the presence of such compounds can be represented by Reaction 15. 

It follows from the reversibility of the transition in Reaction 15 (allylic structure to dynamic), which naturally exists during polymeriza-... 

In aerobic cells, polyunsaturated fatty acids of membrane phospholipids easily undergo such oxidative chain reactions [111,112]. This is because the double bonds of the polyunsaturated structure are repeatedly connected to each other by c/s-methylene units. Such bis-allylic structures enable electron delocalization on five carbon atoms, making the initial hydrogen abstraction on... 

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