Transition closed

Figure 4.7 Circuit diagram for a closed transition AIT switching...

Four different orientations are possible when the enantiofaces of (E)- and (Z)-enolates and an ( )-enone combine via a closed transition state, in which the olefinic moieties of the donor and the acceptor are in a syn arrangement. It should be emphasized that, a further four enan-tiomorphous orientations of A-D are possible leading to the enantiomers 2 and 3. On the basis of extensive studies of Michael additions of the lithium enolates of esters (X = OR) and ketones (X = R) to enones (Y = R) it has been concluded ... 

Structures F and G are two model transition states for the reaction of an ( )-enolate with an ( )-enone. Although the same syn and anti diastereomeric pairs are formed as in the case of the closed transition states, the ratios will in general be different. 

Based on the same strategy, Denmark and coworkers developed a vinylogous aldol reaction using enolate activation with a catalyst derived from SiCl4 and dimeric phosphoramide 47 [24,25]. This strategy relies on the observation that not all Lewis acid - Lewis base interactions diminish the Lewis acidity [26-28]. Due to the formation of a pentacoordinated silicon cation (48), both the enolate and the substrate can be assembled in a closed transition state, giving rise to the observed high selectivities (Scheme 19) [29,30]. 

Flouk has also considered the alkylation reactions of pyrroles and indoles using the same class of catalyst. The report addresses the fact that while catalyst 12 provides high ees in the alkylation of pyrroles (Scheme 15), the same is not true of indoles and catalyst 21 is required instead (Scheme 16). A thorough examination of the accessible transition states for the reaction of iminium ion 184 with pyrrole and with indole led to the conclusion that the two reactions occur through different transition states. Pyrrole adopts a closed transition state reminiscent of that of the Diels-Alder reaction whereas indole adopts an open transition state (Fig. 19) [233]. 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

The general problem of simple diastereoselection (see Section A. 1.), as outlined below refers to the combination of two enantiofaces, which is possible in 4 different orientations (1, 3, 5 and 7). 1/7 and 3/5 are enantiomorphous and lead to enantiomeric pairs (2/8 and 4/6, respectively). In reactions with closed transition states , this means that the two olefins are in a syn orientation. Generally, this orientation is enforced by the transfer of a metal cation or a proton from D to A [ene-type reaction]8. D is a donor and A an acceptor group. 

Although the same combination of enantiofaces (e.g., Re + Re, 1/10) leads to identical stereoisomers 2 in both orientation, the stcric interaction of the substituents is different. Therefore, the ratio of the diastereomers (2 and 8) (4 and 6) crucially depends on the formation of an open or closed transition state. 

The following examples show how open and closed transition states may be invoked by the choice of the reaction type. For instance, aldol-type addition normally proceeds via a closed transition state because the metal ion is shifted from the enolate oxygen to the carbonyl oxygen in an ene-like mechanism ( Zimmerman-Traxler transition state 9). The crucial interactions in the Zimmerman-Traxler transition state 16 are those between the 1,3-diaxially oriented substituents around the chair-like structure. R2 adopts the location shown, thus R3 avoids the 1,3-interaction and assumes an equatorial position. Therefore, the diastereomeric ratio depends mainly on the ( )/(Z) configuration of the enolate. Whereas (Z)-enolates 13 afford syn-config-urated enantiomers, 17 and 18, the corresponding ( )-enolates 14 lead to anti-configurated adducts 19 and 20 10. 

Open transition states have been postulated in the aldol-type additions of ( )- and (Z)-crotyl-stannanes (21/22) to aldehydes. Irrespective of the ( ) or (Z) configuration of the stannane only. yyn-adducts 23/24 are formed. Due to the Lewis acid (LA) complexation of the carbonyl oxygen, a cyclic ( closed ) transition state cannot be adopted. Instead, an open geometry is preferred, in which the methyl and the R group move apart as far as possible to generate the enantiomorphous arrangements 25/2611. 

Sigmatropic rearrangements proceed via closed transition states in the Claisen-Ireland variation a silyl-enol ester, 27 or 28, is used, which may be selectively generated in (Z) or (E) configuration12 (Section A. 1.6.3.1.). As shown in the transition states 32 and 29, this results in the formation of 30/31 and 33/34. 

The observation of open-closed transitions and of different conductance levels can be interpreted according to such a molecular aggregation mechanism. Pores... 

Encounter-separation model of half channels may give rise to the open-closed transition. 

Conformational fluctuations within the transmembrane aggregate produce the open-closed transitions. 

Either or all of these processes may account for the open-closed transitions. The hypothetical scheme of supramolecular organization and open-closed transitions is summarized in Figure 5. 

Figure 5. Possible mechanisms of supramolecular organization of amphiphilic ion pair 1 to form the transmembrane channel and its open-closed transitions. The amphiphilic units are incorporated into bilayer lipid membrane and organized to the ion channel through aggregation and interconnection between two lipid layers. Ions can pass through the pore only when the aggregates are large enough, and interconnected between layers, and conformationally open.

The permeability selectivity ratio for P < /Pci estimated to be 5 for the channel of 5 pS. The selectivity again favors cation and the ratio resembles the value obtained by supramolecular oligoether channels. Open and closed transitions were relatively slow and the time distributions were 300 ms and 400 ms, respectively. Therefore, the ion pair, trans-2l, can be concluded to show typical characteristics of the single-ion channel in all respects. 

The idea that the stereochemical outcome of an intramolecular enolate alkylation is determined by chelation in the transition state was recently demonstrated by Denmark and Henke, who observed a marked preference for a "closed transition state (coordination of the cationic counterion to an enolate and the developing alcohol) resulting in a syn product. For example, the highest syn anti ratio (89 11) was obtained in toluene and the lowest syn.anti ratio (2 98) was obtained with a crown ether. These observations parallel the facial selectivities described herein and in ref 11 on the intramolecular SN2 reaction see (a) Denmark, S. A. Henke, B. R. J. Am. Chem. Soc. 1991, 113, 2177. (b) Denmark, S. A. Henke, B. R. J. Am. Chem. Soc. 1989, 111, 8022. 

Stereochemical retention of configuration around the saturated a-carbon can only result from the closed transition state, whereas the open transition state can lead to inversion (10, 193). Configurational inversions occur in many biological substrates, and much future work on biomethylation will likely involve the use of substituted methyl groups and the study of any optical changes that occur. Determination of the relative rates of cleavage for a series of normal and branched alkyl derivatives enables a distinction between retention or inversion in the SK2 (open) pathway (194). 

Mikami has carried out a number of investigations aimed at elucidating mechanistic aspects of this Si-atom transfer process. In particular, when the aldol addition reaction was conducted with a 1 1 mixture of enoxysilanes 60 and 62, differentiated by the nature of the 0-alkyl and 0-silyl moieties, only the adducts of intramolecular silyl-group transfer 63 and 64 are obtained (Scheme 8B2.6). This observation in addition to results obtained with substituted enol silanes have led Mikami to postulate a silatropic ene-like mechanism involving a cyclic, closed transition-state structure organized around the silyl group (Scheme 8B2.6). 

In conclusion, the distinguishing characteristic of this type of phosphoramide-based neutral Lewis base catalysis is the potential of the reaction to proceed through an associative ( closed ) transition structure. Thus pronounced diastereo-selection results and control of the absolute configuration are possible. Currently, however, it seems difficult to explain the sense of induction based on transition state models [59, 60],... 

A chelate will form with small counterions (Li) and in apolar solvents, leading to a closed transition state. 

The transition phrase consisting of is a closed transition that indicates that the elements immediate subsequent to the term define the sole elements of the group or subgroup those elements are a part of. Let s consider the very basic composition claim ... 

On the other end of the transition phrase spectrum are open transition phrases exemplified most commonly by comprising and less commonly by including, containing, or characterized by.16 Unlike the closed transition consisting of, the presence of one of these open transitions means that the listed elements of the claim subsequent to the transition are the minimum elements to satisfy the claim and that additional ingredients, even in very large amounts can be included and the limitations of the claim may still be met. An example of a very basic open claim is ... 

FIGURE 5.7 Claim with open and closed transitions. 

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