Cyclic models

Draw the energy level spectra for the two cyclic models fill iti ati appropi iate number of electrons for the negative ion for each model. Suggest a reason why one reaetion goes and the other does not. 

Cram s open-chain model 229 Cram s rule 229, 233 Cram chelate model 229 Cram cyclic model 229 Cram-Felkin-Anh model 191,207, 236 f 246 cubane 12,318 cyanoacetic acid 636 f. cyanohydrin, protected 145, 150 f. cyclic carbonate protection 541 f., 657, 659 f., 666, 670 cyclization -,6-endo 734 -, 5-exo 733 f. 

In cases where the heteroatom substituent is the medium (M) group, the cyclic and the open-chain model predict the same stereochemistry. In cases where the heteroatom substituent is small (S), the two models predict opposite stereochemical results. This leads to an order of stereospecificity, with the stereospecificity highest when both models predict the correct stereochemistry, with substantially lower specificity when the cyclic model only applies, and with the lowest degree of stereospecificity when only the open-chain model predicts the correct stereochemical result. 

The stereoselectivity of an addition reaction is considerably lower when the reactions are conducted in polar solvents, complexing additives such as /V./V,A. A, -tetramethylethylenedi-arnine arc used, or when the stereogenic center carries a methoxy group instead of a hydroxy group. This behavior is explained as competition between the cyclic model and a dipolar model, proposed for carbonyl compounds bearing a polar substituent such as chlorine with a highly... 

In every case the dipolar and the cyclic model predict the opposite stereochemistry. Reaction conditions which allow both models to compete lower the predicted stereoselectivity from that model. 

It must be noted that the cyclic model fails to account for the role of the additional alkyllithium and diethylaluminum that are required in precise amounts to achieve high selectivity. A simple model that recognizes a possible role for the additional reagents suggests the intermediacy of an extended, noncyclic transition state G with the aldchydic oxygen coordinated to some undefined Lewis acidic species derived from the additional components of the reaction mixture26,44. Aggregates composed of enolate, alkyllithium and dialkylaluminum species are also possible. 

As outlined in Section D.2.3.5., the stereochemical outcome of the addition of nucleophilic reagents to chiral aldehydes or ketones is rationalized most plausibly by the Cram-Felkin-Anh model. On the other hand, the corresponding reactions of oxygen- or nitrogen-heterosub-stituted aldehydes or ketones may be interpreted either by the same transition state hypothesis or, alternatively, by Cram s cyclic model. 

If a chiral aldehyde, e.g., methyl (27 ,4S)-4-formyl-2-methylpentanoate (syn-1) is attacked by an achiral enolate (see Section 1.3.4.3.1.), the induced stereoselectivity is directed by the aldehyde ( inherent aldehyde selectivity ). Predictions of the stereochemical outcome are possible (at least for 1,2- and 1,3-induction) based on the Cram—Felkin Anh model or Cram s cyclic model (see Sections 1.3.4.3.1. and 1.3.4.3.2.). If, however, the enantiomerically pure aldehyde 1 is allowed to react with both enantiomers of the boron enolate l-rerr-butyldimethylsilyloxy-2-dibutylboranyloxy-1-cyclohexyl-2-butene (2), it must be expected that the diastereofacial selec-tivitics of the aldehyde and enolate will be consonant in one of the combinations ( matched pair 29), but will be dissonant in the other combination ( mismatched pair 29). This would lead to different ratios of the adducts 3a/3b and 4a/4b. 

The stereochemical course of this reaction can be rationalized by Cram s cyclic model of asymmetric induction in which lithium is coordinated between the imine nitrogen and the 2-alkoxy group. 

These results may be explained either by Cram s cyclic model in the case of lithium alkyls or by Cornforth s dipolar model if copper-boron trifluoride reagents are used. Boron trifluoride causes double complexation of both nitrogen and oxygen atoms which results in the formation of an adduct with rigid antiperiplanar conformation due to electrostatic repulsion (see 4 and 5)9. 

Fig. 5 Cyclic model enediynes used by Nicolaou to establish the critical distance model.

Cyclic imines do not have the problem of syn/anti isomerism and therefore, in principle, higher enantioselectivities can be expected (Fig. 34.8). Several cyclic model substrates 6 were hydrogenated using the Ti-ebthi catalyst, with ee-val-ues up to 99% (Table 34.5 entry 5.1), whereas enantioselectivities for acyclic imines were <90% [20, 21]. Unfortunately, these very selective catalysts operate at low SCRs and exhibit TOFs <3 h-1. In this respect, iridium-diphosphine catalysts, in the presence of various additives, seem more promising because they show higher activities. With several different ligands such as josiphos, bicp, bi-... 

Anderson, P. c Tushman, M. L. 1990. Technological discontinuities and dominant designs a cyclical model of technological change. Administrative Science Quarterly, 35 604-633... 

By the formation of complex tacticities. To identity the simplest of these stmctures the cyclic model reported earlier has been found quite useful (41). By analogy with c/ ro-inositol, 80, it was predicted that a polymer constituted of a succession of six homosubstituted tertiary atoms of the type. . . , R, S, S", S", S, R,. . . , 81, would be chiral. Until now this stmcture has neither been realized, nor have calculations been made to ascertain if it would be effectively optically active or simply crypto-chiral. 

The cyclic model is unsuitable for local structures but should be applied only to long chains in such a way that translational symmetry is not lost. 

This Hamiltonian commutes with rj2 but does not commute with S2. Therefore, the eigenfunctions of the Hamiltonian (102) can be described by quantum numbers T] and r]z. For the cyclic model the states with three different values of 77 have zero energy [17] [as it was for the model (98)]. They include one state with rj — 0 (101), all states with rj = N/2 ... 

Figure 22 (a) Model calculation for the stimulated photon echo signal of the cyclic model peptide (cydo-Mamb-Abu-Arg-Gly-Asp) based on its known structures. The same coupling constants were employed as in the model simulations of the 2D-IR spectrum in Fig. 16. The parameters for homogeneous broadening (T2 = 0.7 ps), vibrational relaxation (Tj = 1.2 ps), and inhomogeneous broadening (diagonal disorder 20 cm-1) were also the same. The pulse duration of the laser pulses was set to 120 fs. (b) The same calculation as in (a), but with -shaped laser pulses and neglecting the inhomogeneous broadening. A sharp coherence spike now occurs at T = r = 0, which is not seen experimentally.

In order to look more closely at the conditions under which sulfur radical cations in Met can be stabilized by lone pairs of electrons from neighboring sulfur atoms, two other cyclic model dipeptides, c-(D-Met-L-Met) and c-(Gly-L-Met), were studied with combined optical and conductometric detection following radiolytic oxidation. Little or no intramolecular stabilization by the unoxidized sulfur in the neighboring Met was observed in c-(D-Met-L-Met) (Fig. 1, right) in contrast to the previously observed intramolecular sulfur stabilization of the sulfur radical cation in the isomer c-(L-Met-L-Met) (Fig. 1, left). This oxidation pattern observed in c-(D-Met-L-Met) was confirmed using the oxidation of c-(Gly-L-Met) which has no chance for intramolecular stabilization of sulfur radical cation. 

---