Additive-product interactions

Additive-additive interaction Additive-product interaction ... 

Scheme 8 Scenarios of additive-additive or additive-product interactions that have been evaluated as the possible cause for enantioselectivity reversal in the Soai reaction in the presence of chiral and achiral additives...

In the second option, additive-product interactions are assumed. These can lead to the reproduction of enantioselectivity reversal by considering only one additive. The driving force for the enantioselectivity reversal in this case originates from a competition between kinetic steps that give rise to the product R on one hand and to the product S on the other. Due to mutual inhibition, as expressed by the minimal or alternative kinetic model, i.e., R + S++RS (k2, /t3), a process that gives rise to the formation of R is in some way equivalent to an inhibition of the S formation and vice versa. Due to additive-product... 

In the following, we will exemplify this effect by considering an enan-tiopure pro-R catalyst (X) as the only additive. The main additive-product interaction is given by the two equilibria ... 

The effect of enantioselectivity reversal serves as an additional experimental observation that gives a possible clue for the reaction mechanism. By the proposed additive-product interactions it was predicted that even poor stereoselectivity and discriminating capability of the catalytic additive can give rise to enantioselectivity reversal. This also gives a possible kinetic explanation for the effect of miscellaneous chiral additives in the Soai reaction and their role as potent chiral initiators. 

The hypothesized binding pose places the C-5 and C-6 atoms of the benzimidazole ring proximal to Asp200, precipitating the hypothesis that the introduction of basic or polar substituents at these sites of the heterocycle may establish additional productive interactions with the acid moiety. The model also predicts that substituents at C-7 of the benzimidazole ring would be poorly tolerated while C-4 would be more accommodating of substitution but, nevertheless, restricted in size since these sites are in close proximity with the protein. These hypotheses were explored experimentally with the synthesis and evaluation of the series of compounds compiled in Table 4, which systematically survey common substituents at the C-4, C-5, C-6 and C-7 sites of the benzimidazole heterocycle. It is clear from... 

In contrast to the addition products of hydrazines, the interaction of arenediazonium ions with acid hydrazides (Scheme 6-18) yields tetrazenes (6.26), which can be isolated. Tetrazoles (6.27) are obtainable by cyclization (Dimroth and de Mont-mollin, 1910). 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

Alkyl radical addition reactions to styrene chromium tricarbonyl can be accomplished using alkyl halides (10 equiv) and (TMSlsSiH (5 equiv) in the presence of AIBN in refluxing benzene, for 18 h (Reaction 66). " These reactions are believed to proceed through intermediates in which the unpaired electron is interacting with the adjacent arene chromium tricarbonyl moiety since the analogous reaction with styrene affords only traces of addition products. 

Facial selectivities of spiro[cyclopentane-l,9 -fluorene]-2-ones 30a-30e were studied by Ohwada [96, 97]. The carbonyl tz orbital can interact with the aromatic % orbital of the fluorene in a similar manner to spiro conjugation [98-102]. The ketones 30 were reduced to alcohols by the action of sodium borohydride in methanol at -43 °C. The anti-alcohol, i.e., the syn addition product of the reducing reagent with respect to the substituent, is favored in all cases, irrespective of the substituent at C-2 or C-4 of the fluorene ring (2-nitro 30b syn anti = 68 32), 4-nitro... 

Obviously, use of such databases often fails in case of interaction between additives. As an example we mention additive/antistat interaction in PP, as observed by Dieckmann et al. [166], In this case analysis and performance data demonstrate chemical interaction between glycerol esters and acid neutralisers. This phenomenon is pronounced when the additive is a strong base, like synthetic hydrotalcite, or a metal carboxylate. Similar problems may arise after ageing of a polymer. A common request in a technical support analytical laboratory is to analyse the additives in a sample that has prematurely failed in an exposure test, when at best an unexposed control sample is available. Under some circumstances, heat or light exposure may have transformed the additive into other products. Reaction product identification then usually requires a general library of their spectroscopic or mass spectrometric profiles. For example, Bell et al. [167] have focused attention on the degradation of light stabilisers and antioxidants... 

The a-n interaction in the excited-state n electron systems is also successfully treated. The 1,2-addition will take place with cis mode as is indicated in Fig. 7.39. This was predicated in reference B6>. Experimental evidence 64>149> is the photoinduced addition of IV-chlorourethane to olefins which gives mainly cis addition product, while thermal addition produces a dominantly trans adduct. 

Violent explosions which occurred at —100 to —180°C in ammonia synthesis gas units were traced to the formation of explosive addition products of dienes and oxides of nitrogen, produced from interaction of nitrogen oxide and oxygen. Laboratory experiments showed that the addition products from 1,3-butadiene or cyclopentadiene formed rapidly at about — 150°C, and ignited or exploded on warming to —35 to — 15°C. The unconjugated propadiene, and alkenes or acetylene reacted slowly and the products did not ignite until +30 to +50°C [1], This type of derivative ( pseudo-nitrosite ) was formerly used (Wallach) to characterise terpene hydrocarbons. Further comments were made later [2],... 

Dialkylamino derivatives of elements located in the periodic table to the left or below those listed above cannot be prepared by the above method due to either the ionic character of some of the inorganic halides or the formation of stable metal halide-amine addition products. Therefore, other methods must be applied. Dialkylamino derivatives of tin7 and antimony8 are conveniently obtained by reaction of the corresponding halides with lithium dialkylamides. Others, such as the dialkylamino derivatives of aluminum,9 are made by the interaction of the hydride with dialkylamines. Dialkylamino derivatives of beryllium10 or lithium11 result from the reaction of the respective alkyl derivative with a dialkylamine. 

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441. ... 

The extreme stereoselectivity toward the synthesis of cis-1,4-hexadiene is attributed to the fact that only cisoid-coordinated 1,3-diene can undergo the addition reaction (65, 66). 1,3-Dienes whose cisoid conformations are stoically unfavorable do not react with ethylene under the dimerization conditions. For example, Hata (65) was able to show that, using an Fe-based catalyst system, l-tra/is-3-pentadiene (40) and 2-methyl-1 -trans-3-pentadiene (41) reacted readily with ethylene to form the expected 1 1 addition products, while l-c/s-3-pentadiene (42) and 4-methyl- 1,3-penta-diene (43) failed to interact with ethylene. The explanation is that the cisoid conformations of 40 and 41 are stoically favorable while those for 42 and 43 are not. 

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