Shielded group

The importance of the o-hydroxyl moiety of the 4-benzyl-shielding group of R,R-BOX/o-HOBn-Cu(OTf)2 complex was indicated when enantioselectivities were compared between the following two reactions. Thus, the enantioselectivity observed in the reaction of O-benzylhydroxylamine with l-crotonoyl-3-phenyl-2-imi-dazolidinone catalyzed by this catalyst was 85% ee, while that observed in a similar reaction catalyzed by J ,J -BOX/Bn.Cu(OTf)2 having no hydroxyl moiety was much lower (71% ee). In these reactions, the same mode of chirality was induced (Scheme 7.46). We believe the free hydroxyl groups can weakly coordinate to the copper(II) ion to hinder the free rotation of the benzyl-shielding substituent across the C(4)-CH2 bond. This conformational lock would either make the coordination of acceptor molecules to the metallic center of catalyst easy or increase the efficiency of chiral shielding of the coordinated acceptor molecules. 

Is the stability of 8Ad due to unfavorable kinetics, i.e., the bulky adamantyl groups blocking reaction, or to unfavorable thermochemistry, i.e., loss of aromaticity of the imidazole ring as a result of reaction, or both The distinction is potentially important as understanding could assist in designing stable carbenes. To decide, compare the kinetics and thermodynamics of the insertion of 8Ad into the central CH bond in propane with reactions of 8Me, which should also be aromatic but lacks shielding groups , and 9, which is neither aromatic nor crowded. 

Figure 3. General configuration of suppressive shield groups...

The tests with Shield Group 5 are summarized in Table II... 

TABLE II. SUMMARY OF TESTS WITH SHIELD GROUP 5... 

Department of Defense Explosives Safety Board has approved five groups of suppressive shields for protection of munitions production operations in US Army Ammunition Plants. Safety approved shields encompass seven different designs which range in size from a two foot diameter steel shell (Shield Group 6) to a ten foot diameter steel cylinder (Shield Group 3). 

Wilcox, W.R., Shield Group 9 Suppressive Shield Plastic Liner and Propellant Testing, Contractor Report AR-TSD-CR-78001, NASA National Space Technology Laboratories,... 

The Carbodiimide group is a weakly shielding group on the ortho aromatic hydrogens similar to a methyl substituent. Its presence in a molecule would be dif-ficultto detect without the corresponding infrared data. 

Further improvements of the method are also possible. In particular, at the stage processing ETMC for active and inactive compounds in order to reveal the ETSA, one can introduce in the algorithm the requirement that only the steri-cally accessible active elements of the ETMC of molecules have to be compared with their activity. In the version of the ET method presented in this review, the effect of shielding groups is taken into account visually in addition to the predictions of the ETSA. 

Shielded group. A functional group with extra electron density. 

These results agree with the formation of a classical cation in either case considering the shielding group effect and the solvent backside attack which was supposed to result in an inverted configuration the predominance of exo acetate 117, however, indicates the 2-norbomyl cation to be less accessible from the endo side than from the exo side. The formation of 34% of endo isomer 116 from exo tosylate 113 testifies... 

Vinyl groups bound to silicon will act as shielding groups, because addition of HBr or Br yields a Si—Br bond by (3-elimination [85]... 

This often enables Si—Ph groups to be used as shielding groups in organometallic synthesis [60]. As more electronegative substituents are introduced onto silicon, cleavage is greatly retarded, and is eventually completely prevented. 

The organometallic syntheses of cyclic and polycyclic carbosilanes have already been summarized [60]. These syntheses are made possible partly by initial incorporation of a shielding group which will be transferred into a functional group before the last reaction stage. Especially suitable in such circumstances are Si—Ph groups, which are easily cleaved ... 

The advantages of this method are that the p-hydroxybenzoic acid itself acts as a blocking group and it is not necessary to use shielding groups which must then be eliminated, as proposed in [5, 7] ... 

However, the quantitative elimination of the shielding groups has not been conducted. The intermediate products and final target acid dichloride were not separated as pure individual substances [S, 7], and for this reason polyethers of low molecular weight were obtained [7]. On the contrary, the use of the acid dichloride permitted obtaining polyethers of high molecular weight [8, 10]. 

Acyclic structures which can form secondary structures may not be rigid a dynamic equilibrium may exist in which structures can form and break. It is possible to estimate the amount of a folded structure by comparison of the observed shift for the shielded groups with those of known solvent-shielded groups in rigid systems. Furthermore, the relative extents of solvent shielding for C=0 and N-H groups involved in the same hydrogen bond should be the same. If the observed chemical shift and 1.xi 1 then the mole fraction of molecules in the solvent-shielded... 

Amide hydrogen atoms of a polypeptide chain in a random conformation do not exchange at the same rate, but 10 -fold variations of rates have been observed (Molday et a/., 1972). For exchange of hydrogen atoms in native proteins, simple biphasic curves were obtained for several proteins (Nakanishi et al, 1972,1973 Takesada et a/., 1973). The fast and slow exchanging atoms corresponded to the fraction of exposed and shielded groups (Nakanishi and Tsuboi, 1974). 

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