Anti mechanisms

Zavada et ai (1973) have related the trans/cis olefin ratio in the product to competing syn- and anti-mechanisms for elimination. The transition state for syn-elimination [ 176] is stabilized by co-ordinative interactions with the cation. 

For example, see Coke Mourning J. Am. Chem. Soc. 1968, 90, 5561. where the experiment was performed on cyclooctyhrimclhylammonium hydroxide, and rrans-cyclooctenc was formed by a 100% syn mechanism, and cis-cyclooctcnc by a 51% syn and 49% anti mechanism. 

The methodology suffers from competition between syn- and anti-mechanisms in acyclic cases. Indeed, the pathway may depend on the nature of nucleophile and substrate.78139 In addition, there is no clearcut differentiation between hard and soft nucleophiles in the model described. 

If the nucleophile is water—as it might be in the work-up of the original epoxidation in acid solution—the product is a diol, which eliminates by the anti mechanism in acid solution to give initially an enol and then, under the same conditions, a carbonyl compound. All these steps are often carried... 

Yamamoto T, Saito O, Shono K, Aoe T, Chiba T (2003) Anti-mechanical aUodynic effect of intrathecal and intracerebroventricular injection of orexin-A in the rat neuropathic pain model. Neurosci Lett 347 183-186... 

Although few investigations have been made to determine the stereochemical course of the reaction of Jt-allylmthenium complexes with nucleophiles, Harman and coworkers recently reported that the reaction with soft nucleophiles exclusively proceeded via an anti mechanism [58]. The observations described here, together with information in the literature, suggest that the ruthenium-catalyzed allylic substitution reaction proceeds via a double inversion (i.e., a net retention) mechanism. 

Such reactions generally occur through an anti mechanism, although exceptions do exist. 

Upon treatment with BFj OEt2, stannane 265 is isomerized via an inter-molecular pathway, resulting in allylic transposition and stereochemical inversion of configuration to yield 266 (Scheme 5.2.57, bottom), and this process provides an efficient route to non-racemic y-(alkoxy)allylic stannanes. The reaction of stannane 267 is advanced with unsaturated aldehydes and achieves facial selectivity by the antiS mechanism giving mainly the syn product 268, containing an E-alkenyl ether. The anti-SE arrangement shown in 270 minimizes non-bonded interactions leading to the major product. Similarly, the chiral stannane 271 adds to aliphatic aldehydes to produce the E-iyn-alcohol 272 (Scheme 5.2.58). ... 

Now we are in a position to determine if hydroxypalladation is an equilibrium process as is required by the anti mechanism shown at the top of Scheme 3. The hydroxypalladation intermediate, 6, can eliminate hydroxyl from either end of the molecule and, if hydroxypalladation is an equilibrium process, oxidation of either 3a or 3b should give a 1 1 mixture of the two after the reaction has proceeded to a small extent. The deuterated allyl alcohol, 3a, was oxidized under Wacker conditions and the reaction stopped after one half-life and the unreacted allyl alcohol was analyzed for isomerization. The reaction scheme is shown in Scheme 13. After one half-life the isomerization prodnct 3b was... 

The stereochemistry of the related intramolecular amidopalladation that converts the cw-tosylamido alkenes (98) into the corresponding pyrrolidines (99) in the presence of the chiral ligand (100) has been shown to be controlled by the Pd(II) salt employed as the catalysts. Thus, with (CF3C02)2Pd, the reaction proceeds as an anft -attack of the electrophilic Pd + and the NHTs group (101) with 96% ee, whereas the less efficient syn-addition (102) is preferred by (AcO)2Pd (with mere 20% ee). The syn/anti mechanism of the ring closure was established by deuterium labelling. i... 

Calcium complex soap greases, obtained by the reaction of lime and a mixture of fatty acids and acetic acid. These greases offer good high temperature and anti-wear/extreme pressure properties related to the presence, in the soap, of calcium acetate that acts as solid lubricant they have good mechanical stability. 

The next two temis (Lorentzians) arise from the mechanical part of the density fluctuations, the pressure fluctuations at constant entropy. These are the adiabatic sound modes (l/y)exp[-FA t ]cos[co(A) t ] with (D(k) = ck, and lead to the two spectral lines (Lorentzians) which are shifted in frequency by -ck (Stokes line) and +ck (anti-Stokes line). These are known as the Brillouin-Mandehtarn, doublet. The half-width at... 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

Fn some cases, r-allylpalladium complex formation by retention syn attack) has been observed. The reaction of the cyclic allyiic chloride 33 with Pd(0) affords the 7r-allylpalladium chlorides 34 and 35 by retention or inversion depending on the solvents and Pd species. For example, retention is observed in benzene, THF, or dichloromethane with Pd2(dba)3. However, the complex formation proceeds by inversion in these solvents with Pd(Ph3P)4, whereas in MeCN and DMSO it is always inversion[33]. The syn attack in this case may be due to coordination of Pd to chlorine in 33, because Pd is halophilic. The definite syn attack in complex formation has been observed using stereoche-mically biased substrates. The reaction of the cxoallylic diphenylphosphino-acetate 36 with phenylzinc proceeds smoothly to give 37. The reaction can be explained by complex formation by a syn mechanism[31]. However, these syn attacks are exceptional, and normally anti attack dominates. 

Section 5 16 The preceding equation shows the proton H and the halogen X m the anti coplanar relationship that is required for elimination by the E2 mechanism... 

These observations must be taken into account when considering the mechanism of halogen addition They force the conclusion that a simple one step bond switching process of the following type cannot be correct A process of this type requires syn addi tion It IS not consistent with the anti addition that we actually see... 

Enediynes hold substantial promise as anti cancer drugs because of their potency and selectivity Not only do they inhibit cell growth they have a greater tendency to kill cancer cells than they do normal cells The mechanism by which enediynes act involves novel chemistry unique to the C C—C=C—C C unit which leads to a species that cleaves DNA and halts tumor growth... 

The mechanism for Stokes and anti-Stokes vibrational Raman transitions is analogous to that for rotational transitions, illustrated in Figure 5.16. As shown in Figure 6.3, intense monochromatic radiation may take the molecule from the u = 0 state to a virtual state Vq. Then it may return to u = 0 in a Rayleigh scattering process or to u = 1 in a Stokes Raman transition. Alternatively, it may go from the v = state to the virtual state Fj and return to V = (Rayleigh) or to u = 0 (Raman anti-Stokes). Flowever, in many molecules at normal... 

A typical example of a nonpolymeric chain-propagating radical reaction is the anti-Markovnikov addition of hydrogen sulfide to a terminal olefin. The mechanism involves alternating abstraction and addition reactions in the propagating steps ... 

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

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