Dynamic allyls

The structure and chemical properties of metal-allyl compounds (ir-allylic, dynamic and a-allylic) which can be considered as models of a living polymer chain in butadiene polymerization have been studied. The polymerization of dienes proceeds only in dynamic allylic systems through the metal-ligand ir-bond in a-isomers. 

Butenyllithium and butenylmagnesium chloride were used as "dynamic allylic compounds. The former was selected because of the ability of lithium catalysts to provide high rates of diene polymerization and to give stereoregular polymers the latter was selected for its availability and simplicity of synthesis. 

The solutions are nonconducting, and the compounds display no tendency to undergo chemical reactions (hydrolysis, polymerization, etc.). However, as shown later, heating these compounds in the presence of donor reagents causes significant changes in most of their properties (NMR spectra, chemical activity, electrical conductivity, etc.), thus making them similar in properties to the typical dynamic allylic compounds. 

Discussion. It follows from the traditional representation of a dynamic allylic system that the exchange occurs between two a-allylic isomers, the transformation of one isomer into the other is instantaneous,... 

The fact that the transition of the spectrum from a 7r-allylic to a dynamic one is accompanied by a change in the properties of these compounds show that dynamic allylic compounds of Li, Mg, and Pd have the same nature. 

Figure 9. Energy diagram of dynamic allylic (b) and 7T-allylic systems (a)...

The dynamic allylic system may be represented as resulting from the exchange of ir-, o-isomers (through excited or ionic forms) ... 

Various types of metal allylic compounds differ significantly in their physical and chemical properties. Dynamic allylic compounds exhibit the highest chemical activity. 

Dynamic allylic compounds are formed as a result of rapid exchange (intra- and intermolecular) between tautometric tr-, a-forms and an intermediate (ionic) form. 

Hydrolysis of dynamic allylic compounds proceeds through 7r-bond with 7r, [Pg.278]

Finally, the method was used to prepare allyl- and methallylmagnesium chlorides, the C-spectra of which were consistent with the expected dynamic allyl structure [10]. 

Distinguish the following allyl groups with respect to their NMR spectra cr-allyl, Tu-allyl, and dynamic allyl. 

Figure 35. Dynamic change of lifetime in an n-type silicon/polymer (poly(epichlorhydrine-co-elhylenoxide-co-allyl-glycylether plus iodide) junction during a potential sweep. The arrows show the direction of sweep (0.25 V s" ). A shoulder in the accumulation region and a peak in the depletion region of silicon are clearly seen.

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

In support of the alkylation studies, the putative minimum energy conformation of the l, 4-as and 1,4-trans diastereomers of 23 (R4 = Me, Pr1 R1 = Me, Bn, allyl R2 = Me, Bn) were calculated in a high-temperature molecular dynamic study using the Hyper-Chem 3 program and MM calculations. The higher stability of the m-diastereomers was confirmed by the calculations <1997JOC6424, 1998TA4275>. 

Calculations of alkali metal allyl derivatives involving all alkali metals (Li-Cs) indicate a preferred geometry with the metal symmetrically bound in a predominantly electrostatic manner to all three carbon atoms.143 Solution studies of allyllithium in ether indicate the compounds to be highly aggregated in THF complex dynamic behavior is observed. 

The allylic conversion processes clearly represent the most feasible ones among all the critical elementary steps along the Cg-channel, involving the [NiII(octadienediyl)L] complex. Consequently, the several forms as well as the different stereoisomers are in a dynamic, pre-established equilibrium, that can likely be assumed as always being attained. 

Among the several configurations of the crucial [Nin(octadienediyl)L] complex, all of which are in equilibrium, the p3, 1 1) species 2a and the bis(p3) species 4a are predicted to be prevalent. The odonor/71-acceptor ability of the ancillary ligand is shown to predominantly determine the position of the kinetically mobile 2a 4a equilibrium. The conversion of the terminal allylic groups via allylic isomerization and/or allylic enantioface conversion are indicated to be the most facile of all the elementary processes that involve the [NiII(octadienediyl)L] complex. Consequently, the several octadienediyl-Ni11 configurations and their stereoisomers are likely to be in a dynamic pre-established equilibrium, that can be assumed to be always present. 

Considering an olefinic functionality as a chromophore, the absolute configuration of cyclic allylic alcohols can be determined using a method that involves the conversion of the alcohol to the corresponding benzoate.60 This can also be extended to acyclic alcohols where the conformations are dynamic (see Fig. 117). Interested readers may consult the literature for details.61... 

Our calculations show that the isomerization of the silyl-alkyl complex to form a V-allyl complex affords a significant stabilization as summarized in Figure 11. TheiV toil3 isomerization of 9a to the anti silyl-allyi complex, 10a-anti, results in a 9.5 kcal/mol stabilization and isomerization to the syn isomer, lOa-syn, results in a 7.2 kcal/mol stabilization. Isomerization of 9b to the anti silyl-allyi complex, 10b-anti, results in a 6.1 kcal/mol stabilization and isomerization to the syn isomer, lOb-.vyn results in a 5.4 kcal/mol stabilization. High temperature (500 °C) molecular dynamics simulations initiated at the V complex, 9a, reveal that the rj1 to T 3 isomerization has a minimal barrier and occurs in the sub-pico time frame. The inter-conversion between the syn and anti isomers has not been examined since both isomers are stereochemically equivalent, however, we expect the barrier to be small. 

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). 

Berkessel and co-workers have demonstrated the utility of the bifunctional cyclohexane-diamine catalysts in the dynamic kinetic resolution of azalactones (Schemes 60 and 61) [111, 112]. The authors proposed that the urea/thiourea moiety of the catalyst coordinates and activates the electrophilic azlactone. The allyl alcohol nucleophilicity is increased due to the Brpnsted base interaction with the tertiary amine of the catalyst. 

SCHEME 24. (-)-Sparteine-induced deprotonation of allyl carbamate 170. Dynamic resolution by crystallization and enantioselective homoaldol reaction... 

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