Hydrogen bond scheme

The extremely bulky TsiSiPh(OH)2 (see Scheme 5 for its preparation) has an even more restricted hydrogen-bonding scheme comprising simple dimers only, in which further hydrogen bonding to build up larger units is hindered by the steric effect of the Ph and Tsi groups (84). 

Figure 10. Hydrogen bonding scheme between 7,9-dimethylguanine (7,9-dmgua) and 1-methycytosine (mcyt) in fra/is-[Pt(MeNH2)2(7,9-dmgua-N1)2(CI04)2-2mcyt with two hydrogen bonds, N(2)-N(3a ) 2.881 (8) A and N(3)—N(4a ) 3.194(8) A. (Reproduced with permission from ref. 48).

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

Type A enamine catalysts include simple amino acids, such as proline 6, and most of their derivatives (such as the tetrazole 44 and various sulfonamides, e.g. 45). They are typically used for aldol, Mannich, a-amination and a-oxygenation reactions - these are all reactions where the electrophile can readily be activated by hydrogen bonding (Scheme 12) [8, 9, 12, 46],... 

Hydrogen would be the simplest center element. Indeed, chiral Brpnsted acids have emerged as a new class of organocatalysis over the last few years [3-13]. The field of asymmetric Brpnsted acid catalysis can be divided into general acid catalysis and specific acid catalysis. A general acid activates its substrate (1) via hydrogen bonding (Scheme 2, a), whereas the substrate (1) of a specific acid is activated via protonation (Scheme 2, b). 

The hydrogen bonding scheme is identical in both crystals as seen in Fig. 11 and Fig. 12. The molecules pack so as to provide what maybe aptly described as a hydrogen-bonded comb, with a GA 36 backbone and caffeine 35 teeth. In fact, the major difference between the two polymorphs on the secondary level of supramolecular architecture resides in the torsion of the GA 36 aliphatic chain. 

Figure 4-12 Proposed hydrogen-bonding scheme for the "native" conformation of hyaluronan. See Morris et al.lw...

The published hydrogen bonding scheme (62) has been slightly modified in the latest adjustment of the model to the 2-A map, M30, based on 6000 reflections. Many side chain bonds to the backbone have been assigned, and a number of solvent molecules have been tentatively located. These are indicated schematically in Fig. 6a. The number of... 

Fig. 6. (a) Backbone hydrogen bonding scheme as currently delimited. Disulfide bridges are indicated in heavy dashed lines and poor or less certain bonds in light dashed lines. W indicates solvent molecule. Side chain to side chain bonds are not generally indicated. The scheme of bonding currently assigned for 3 -CMP is also... 

The tolerance at position 3 is -)-0.8 A (CH). The tolerance at position 4 is very considerable and in position 5 essentially unlimited. Ward et al. (446) sharpened the focus on this situation in describing the rationale for activity against poly F. Meadows et al. (465) have presented the hydrogen bonding scheme to 2, 3, and 4 deduced from the RNase-S X-ray structure. Allewell (120) presented much of the evidence in her thesis. Richards et al. (459a) discussed the situation in a conference report. 

Fourier analysis of Model 2 is still in progress. However, from the above experiments it is clear that the intra-molecular hydrogen bond scheme in Klebsiella K8 polysaccharide does not have hydrogen bonds strengthening every glycosidic linkage. 

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