Transition structures boat-like

In the absence of constraints, chairlike transition structures are preferred to boat-like transition structures. 

The opposite diastereoselectivity is observed in the absence of Lewis bases. This clear switch of diastereoselectivity is rationalized by invoking a chair-like transition state in the hexacoordinate siliconate and a boat-like transition structure in the penta-coordinate siliconate. 

Denmark et al. have reported that trichlorosilyl enolates also undergo non-catalyzed aldol reaction with aldehydes (Scheme 10.30) [95]. The sense of diastereoselectivity depends on the geometry of the enolates - ( )-enolate 33 adds to aldehydes with high. syn selectivity, whereas low anti selectivity is observed for (Z)-enolate 35 [96], The stereochemical outcomes can be rationalized by boat-like transition structures arranged by the Lewis acidity of the silicon atom, in which the configuration around silicon is trigonal bipyramidal with aldehyde binding in the apical position. In the transition structure from 35 there are severe steric interactions caused by the enolic Z substituent, which is attributable to the low anti selectivity. 

The bond length of the partial single bond Cl - C6 is predicted to be 0.06 A longer by the BLYP/6-31G method than by the RHF/6-31G method, whereas the other geometric parameters are found to be approximately equal by the two methods. The shown boat-like transition structure allows optimal interaction of the n -electrons at the carbon termini to form the new a-bond. The activation energy calculated at the BLYP/6-3IG level is in much better agreement with the experimental value of 121.3 kJ mol than the corresponding value from the RHF/6-31G calculation. It is also close to the value calculated at the MP2/6-31G level The heat of reaction calculated by... 

Two minor processes sometimes operate competitively with that illustrated in the scheme. One of these involves 1,4-addition of the second vinyl anion to give a reactive intermediate that differs structurally from 1, but is capable of setting into motion a closely related sequence of chemical events leading to an isomeric diquinane.4 This is the route followed to produce the minor product characterized here. The other option consists of cis-t, 2-addition, an event that is followed by a dianionic oxy-Cope rearrangement via a boat-like transition state.4 When sufficient substitution is present to allow the installation of multiple stereogenic centers, the adoption of this pathway is easily distinguished from the electrocyclic alternative since a cis relationship between relevant substituents is in place, instead of the trans arrangement required by the electrocyclization cascade. 

In cyclic systems, however, conformational constraints can override the inherent preference for chairlike transition states in Cope as well as Claisen rearrangements and lead to a partial involvement if not a dominance of boat-like TS structures. In the Ireland rearrangement of lactones of type (247), for example, chair-like transition state (249) is accessible only when the diaxial bridging methylene chain becomes sufficient in length (n = 7, Scheme 44). The preference of boat-like transition state (250) over (251) is due to a serious A - -type interaction between the endocyclic oxygen atom and pseudoaxial substituent R in (251). 

Diels-Alderase antibody 39-All minimizes product inhibition while generating a more conventional Diels-Alder product (Fig. 11) (80). Bicyclo[2.2.2]octane hapten 29 was designed to mimic the proposed boat-like transition state 30 of the [4n -F 2n] cycloaddition between diene 31 and dienophile 32. Product inhibition was circumvented by the structural disparity between the product cycloadduct and the pseudo-boat form of the hapten employed for immunization. X-ray crystallographic data of hapten 29 complexed with the 39-All Fab indicated that the diene and the dienophile are bound in a reactive conformation that reduces... 

Analogues of Alternative Transition States. The chair- and boat-like transition state structures are shown diagrammatically in Figure 2. They illustrate two problems of transition state analogue design. First, the partial bonds in transition state structures give rise to bond lengths and hybridization states which cannot be reproduced in stable molecules. Second, the number and variety of chemical features in transition state structures may be incompatible with synthetic feasibility. 

In connection with the synthesis of podophyllum lignans, ester (62) was deprotonated and the resulting enolate condensed with 3,4,5-trimethoxybenzaldehyde to give a 1 1 mixture of diastereomeric aldols (equation 68). The structure of (63) was established by X-ray analysis the other diastereomer was assigned the 2,3-anti relative stereochemistry (64) on circumstantial evidence. It was suggested that the 1 1 mixture of isomeric products results from a 1 1 mixture of the ( )- and (Z)-enolate, each of which shows complete simple and diastereofacial selectivity in its reactions with 3,4,5-trimethoxybenzaldehyde. For this to be true, it is also necessary that the ( )-enolate reacts through a non-Zimmerman , boat-like transition state, whereas the (Z)-enolate reacts through the normal chair-like transition state. 

The failure to insert quaternary vinyl groups in a cyclization process is limited to 1,6-diene systems [17]. When a similar competition for selective insertion is attempted on a 1,5-diene, cyclization occurs because of the inherent entropic advantage of five-membered ring formation (Eq. 59). The close approach of the bulky organometallic to the axial alkoxy substituent in the chair-like transition structure (Fig. 12b) causes the reaction to proceed through the less hindered boat formation (Fig. 12a). 

An appreciation of the ability of 48 to attain appreciable levels of double diastereoselection when reacted with chiral (racemic) vinyl organocerium reagents had earlier been gained in this laboratory [32]. Consequently, it occasioned no surprise to observe that 49 [33] adds to this bicyclic ketone with customary endo stereoselectivity to deliver 50 and 51 in a relative ratio of 92 8. The major product, easily purified by chromatographic means, was smoothly isomerized to 52 under anionic conditions at room temperature. For structural reasons, this sigmatropic change is required to proceed via a boat-like transition state. The all-... 

Figure 5 The two-dimensional potential-energy. surface for the chair-to-chair rearrangment in Cp2Ti[C2S4]TiCp2. The horizontal and vertical axes are the left and right fold angles, respectively. The chair structures are at (—45, -45) and (45, 45), while the boat-like transition states are at (28, —28) and (-28, 28) and are 58 kJ mol" above the ground state. The fully planar structure is at (0. 0) and is 113 kJ mol" above the ground. state. Copyright 1994, American Chemical Society. Reprinted with permission of the American Chemical Society...

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