Acceptors hydrogen binding

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

Since they have both a vacant low-energy orbital and a lone pair, silylenes might behave either as electron pair donors or acceptors. There is scant evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as Lewis acids are now well-established these complexes can also be described as silaylides, R2>Si —B+262. Trinquier has calculated that even SiLL should form a weak complex with 112S i , in which a silane hydrogen binds to the p-orbital of the silylene263. 

The importance of hydrogen bond formation in the interaction of 35a with anions is evidenced by the fact that the fully protonated receptor, which lacks a hydrogen bond acceptor site, binds H2PO less efficiently than the pentaprotonated analog (log K., of the H2PO complex of pentaprotonated 35a amounts to 2.97 and for the hexaprotonated state to 2.12). In contrast, affinity for SO)2. an anion which remains unprotonated even at low pH and therefore does not experience repulsive interactions inside the cavity of 35a, increases from 3.74 for the pentaprotonated receptor to 5.03 for the hexaprotonated one. 

This is an unusual structure for an acceptor-hydrogen complex in other semiconductors hydrogen assumes the bond-centre position when binding to acceptors [7], As a direct consequence the vibrational frequency of the complex is not representative of a Ga-H bond, but rather of an N-H bond. The calculated vibrational frequency (in the harmonic approximation) is 3360 cm 1. Anharmonic effects may lower this frequency by as much as 170 cm 1 [8]. 

Fersht et al., in a very well-known study of the binding of tyrosyl-tRNA synthetase to its substrate [39], found that deletion of a strong hydrogen bond donor or acceptor from the enzyme reduced the free energy of binding by only 2.1-6.3 kJ mol , whereas removal of the partner of a very strong donor or acceptor weakened binding by an additional 12 kJ mol . Data on mutants of ribonuclease Tl by Shirley et al. 

Structural refinement of these host molecules was later pursued by the same group on a double track. In the non-covalent direction, studies were finalized to the design of calix[5]arene pentols fitted, at the wider rim, with a molecular hook capable of facilitating Cgo and C o capsule formation via additional self-assembling interactions between the two sub-units. To this end, ureido 2a [11] and 2,2 -bipyridyl (bpy) derivative 2b [12] were endowed with a self-complementary hydrogen-bond donor/acceptor group and a metal-acceptor (Ag binding site, respectively. 

CO2 must be reduced to carbohydrate by means of the hydrogen set free by photolysis. This reduction takes place only after the CO2 has been bound to an organic acceptor. The binding and the reduction of the CO2 are known as the secondary processes of photosynthesis. 

A vinylogous Michael addition of 3-alkylidene oxindoles (441), normally regarded as good Michel acceptors, to nitroalkenes, has been attained in the presence of the cinchona-derived thiourea catalyst (372). This rare example of organocatalytic umpol-ung, presumably proceeding via the hydrogen-binding stabilized enol (442), afforded the adducts (443) in a >99 1 y.a ratio with 10 1 to >20 1 Z/E) selectivity and 97 to >99%... 

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