Cyclic transition-state structures

OTHER VIEWS ABOUT SIGMATROPIC TRANSITION STATE 1. Formation of a cyclic transition state structure... 

From this mechanism the linear complex formed in reaction (47) may be visualized as the intermediate in the formation of the cyclic transition-state structure in step (48). Further implications of these quenching studies with respect to the mechanism of the primary bond-scission reaction (49) will be discussed in the next section. 

It will now be instructive to examine the n-butane reaction (76). In this case the reaction follows almost exclusively a single path leading to the formation of sec-butyl radicals. The percentage of the quenching done by the two methylene groups is very nearly the same as that for the tertiary C-H bond in isobutane (i.e. >90%). However, the primary yield of w-butyl radicals ( 2%) from w-butane is decidedly less than that for isobutyl radicals ( " 14%) from isobutane. This behavior can be readily interpreted on the basis of a cyclic transition-state structure, but not with an open-chain transition state. For the two reaction sequences, we may write ... 

The addition of acyclic substituted enolates 19d-e and cyclic enolates 21a-c were also examined in this study. Stannanes 21a-c afforded the anti adduct with excellent simple diastereoselectivity and up to 96% ee (Eq. 8B2.6). In contrast, the use of acyclic Z-enol stannanes 21 provided the complementary syn adducts in equally high levels of diastereoselectivity and enantioselectivity. The correlation of enolate geometry with simple induction (K-enolates yield antiadducts, whereas Z-enolates yield syn adducts) has led Yamamoto to invoke a cyclic transition-state structure 22. 

The gas-phase unimolecular elimination reactions of 2-substituted ethyl N,N-dimethylcarbamates23,24 and several heterocyclic carbamates25 have been studied using the Moller-Plesset MP2/6-31G method. On the basis of these calculations, the mechanism appears to be concerted, asynchronous, through a six-membered cyclic transition-state structure. 

The reaction was found to be first order with respect to amines and acrylamides and no base catalysis was observed. The reaction is believed to occur in a single step in which the addition of amine to Cp of acrylamide and proton transfer from amine to Ca of acrylamide take place concurrently with a four-membered cyclic transition-state structure. The Hammett (px) and Brpnsted (/3X) coefficients are rather small, suggesting an early transition state (TS). The sign and magnitude of the cross-interaction constant, pxy(= —0.26), is comparable to those found in the bond formation processes in the. S n2 and addition reactions. The normal kinetic isotope effect ( h/ d > 1.0) and relatively low A and large negative Avalues are also consistent with the mechanism proposed.192... 

Scheme 5.17).40 A three-step reduction-deprotection protocol liberated the aratz -vicinal diamine 21. A six-membered cyclic transition-state structure was proposed to account for the anti selectivity after a two-electron reduction of the nitrone. The chiral A-tert-butylsulfinyl group directs the attack of the carban-ion to the Sz-face of the C=N double bond of the imine. 

Eormation of a Cyclic Transition State Structure Erontier Orbital Approach Some Examples of Hydrogen Shifts Migrations in Cyclopropane rings Migrations of Atoms or Groups other than Hydrogen Selection Rules... 

These results clearly show that the diastereoselectivity depends on the geometry of the enol stannane, and that cyclic transition-state structures (A and B, Fig. 1) are probable models. Thus, from the (i )-enolate, the and-aldol product can be obtained via a cyclic transition state model A, and another model B connects the (Z)-enolate to the sy -product. Similar six-membered cyclic models containing a BlNAP-coordi-nated silver atom instead of tributylstannyl group are also possible alternatives when transmetalation to silver enolate is sufficiently rapid. 

The use of C unsubstituted and substituted stannyl enolates has been studied by Yamamoto in a series of elegant reports involving a novel bisphosphine Ag(I) complex 64 as a catalyst for C-C bond formation [30]. The addition of methyl ketone and acetate-derived enolates furnishes adducts in up to 96% ee. The use of E-stannyl enolates yields the 2-anti diastereomer as the major product in up to 96% ee. The use of acyclic Z-enol stannanes provided the complementary syn-substituted adducts as the major adduct in equally high diastereoselectivity and enantioselectivity. The observed correlation between enolate geometry and the simple diastereoselectivity of the product (E-enolates yield anti adducts while Z-enolates yield syn adducts) has led Yamamoto to postulate the involvement of a closed, cyclic transition-state structure. 

The other method to determine reactivity for reactions with synchronous concerted cyclic transition state structures is evaluation of the transition state ring aromaticity through bond order deviation. The results of the exo cyclopropene addition to the heterocycles and to cyclopentadiene are presented in Table 33. The higher the sum of bond order deviation from average bond order (x) is, the lower aromatic character the transition state structure has. The most reactive dienophile was cyclopentadiene, followed by furan, and then heterocycles. The most reactive heterocycle with heteroatoms in 1,3-position was 1,3-oxazole as was predicted on the basis of the FMO energy changes (Table 32). The least reactive was 1,3-diazole, as one would expect on the basis of experimental observations. It is very difficult to rely on the transition state structure bond order deviation to determine the experimental feasibility of a reaction but, because SBOD for furan and 1,3-oxadiazole were very similar, one can conclude that the cycloaddition with 1,3-oxadiazole is also experimentally feasible. 

Like Norrish type I reaction, the intermolecular 1, 5-hydrogen transfer to alkoxy radical involves a six memhered cyclic transition state structure, as demonstrated by the photolysis of series of co-phenylalkyl nitrites. 

The directing effects of homoallylic alcohols were methodically investigated by Mihelich (Equation 10) [67]. It was suggested that the stereoselectivity observed in the epoxidation of substrates such as 36 resulted from a cyclic transition state structure 37. In this assembly. A, 3 interactions are avoided and the attendant substituents are accommodated in a pseudoequa-torial manner that minimizes energetically costly gauche interactions. 

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