Substrate Generality

To sum up, primary and secondary substrates generally react by the Sn2 mechanism and tertiary by the SnI mechanism. However, tertiary substrates seldom undergo nucleophilic substitution at all. Elimination is always a possible side reaction of nucleophilic substitutions (wherever a P hydrogen is present), and with tertiary substrates it usually predominates. With a few exceptions, nucleophilic substitutions at a tertiary carbon have little or no preparative value. However, tertiary substrates that can react by the SET mechanism (e.g., /i-N02C6H4CMe2Cl) give very good yields of substitution products when treated with a variety of nucleophiles. ... 

However, some substrates, generally rigid bicyclic molecules, (e.g., barrelene, which is converted to semibullvalene) give the di-7t-methane rearrangement only from triplet states. 

Studies of UPD are important for a number of reasons, most importantly, because they are the formation of the first atomic layer in an electrodeposit. In the present text, they are important because they illuminate the structures of electro-deposited atomic layers, the reactants in EC-ALE. However, such studies must be kept in context, given that the structures of the first UPD layers on a substrate generally have little to do with the structures of subsequently formed compound mono-layers. It has been found that the structures of compound monolayers are determined, for the most part, by the structure of the compound that is forming, perturbed by the lattice mismatch between the deposit and the substrate. The structure of the first atomic layer on the substrate does not appear to be a significant factor in determining... 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

Bisphosphinamidites which are supported by an axially chiral framework are another important class of ligands. Although reported as early as 1980 [50], no reports on the use of binaphthyl-based bisphosphinamidite in asymmetric catalysis were published during the decade thereafter. As described above, the selectivity and substrate generality in these early attempts were very limited in scope. In 1998, we unveiled that by partially hydrogenating BINAM to H8-BINAM and... 

The first term represents the temperature dependence of the film density, being directly related to the thermal expansion coefficient of the film, and also indirectly related to that of the substrate. Generally, the thermal expansion coefficient of the organic substance is much larger than that of the inorganic substance like glass. In the case of the alternate LB film of fatty acid/alkylamine or fatty acid/alkylaniline, the pyroelectric coefficient p decreased as the thermal coefficient of the substrate increased [18]. 

No large conformational changes occur in the enzyme during catalysis, but many small movements take place. The structural basis for the catalytic power of ribonuclease thus resides in several different features tight, specihc binding of a strained conformation of the substrate, general acid-base catalysis by His-12 and His-119, and preferential stabilization of the transition state by ionic interactions with Lys-41. 

Aromatic compounds undergo carbonisation during sonication [44]. The reaction can occur either at the bubble interface or inside the cavity, according to the hydrophUicity of the substrate. Generally it would appear that apolar, hydrophobic compounds, e. g. benzene and halocarbons are pyrolysed inside the bubble [45,46]. 

In the first step, deprotonation of the methylene-containing substrate generally takes place at the interface of two phases (liquid-liquid or solid-liquid). 

We found this to be the case in each of the reaction s we have examined thus far, and it presents an important, rather unique opportunity to learn something of fundamental importance with respect to catalyst substrate generality. Why, within a series of closely related scaffolds, do some exhibit good, others poor, substrate generality Admittedly, two substrates is only a modest step toward substrate generality, but hopefully it illustrates another unique feature of the approach. It is also noteworthy that, while the data are not shown here, we see similar trends for substrate conversion and product yield as a function of SAL scaffold. This must relate to turnover frequency and/or catalyst stability and, hopefully, it will prove possible to gain new insights into these issues of fundamental importance in catalysis in the course of future studies. 

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