Acyclic systems induction

Several trends have emerged in the extensive carbon-13 NMR spectroscopy data that have been accumulated for sulfones and sulfoxides. Based on many studies of cyclic systems—particularly five- and six-membered ring sulfur compounds—these trends were shown to generally apply equally to both the cyclic and acyclic systems . Thus (a) oxidation of a sulfide to a sulfone results in a 20-25 ppm downfield chemical shift for sp -hybridized a-carbon atoms and 4-9 ppm upfield shift for / -carbons , and (b) there is very little difference between the chemical shifts of a-carbon atoms of sulfones and sulfoxides despite the difference in the inductive effects of these two functional groups . A difference is observed, however, in the H chemical shift of related cyclic sulfoxides and sulfones . 

Donald J. Cram, Fathy Ahmed Abd Elhafez Studies in Stereochemistry. X. The Rule of Steric Control of Asymmetric Induction in the Syntheses of Acyclic Systems, J. Am. Chem. Soc. 74, 5828-5835 (1952)... 

Nucleophilic additions to cyclic carbonyl compounds differ greatly from those of acyclic systems. In acyclic systems, only the configuration at an adjacent (1,2-asymmetric induction) or nea y center (remote asymmetric induction) is usually considered in predicting the outcome of nucleophilic attack. In cyclic systems, the conformation of the entire molecule (which is in part determined by the individual substituents) must be considered when predicting the mode of nucleophilic attack. Furthermore, a number of other factors such as torsional and electronic effects also play a role in the stereochemical course of additions to cyclic substrates. The relative importance of all of these effects (as well as others) has been the subject of considerable debate in the literature, and has not as yet been adequately resolved. ... 

Inaba, T., Fujita, M., Ogura, K. Thermodynamically controlled 1,3-asymmetric induction in an acyclic system equilibration of a-amino nitriles derived from a-alkylbenzylamines and aldehydes. J. Org. Chem. 1991, 56, 1274-1279. 

For any type of asymmetric inductions in acyclic systems to be successful, the following conditions need to be met ... 

In acyclic systems, Mukaiyama and co-workers found that addition of chiral magnesium cuprate R(A )CuLi derived from 456 gave good asymmetric induction. 20 Lithium cuprates derived from 458,... 

Isobe M, Kitamura M, Goto T (1979) Complete asymmetric induction in heteroconjugate addition on an acyclic system. A new synthetic approach directed toward ansamacrolide maytansine. Tetrahedron Lett 36 3465-3468... 

An interpretation of the magnitude of p is difficult since many factors are involved. One sees, for example, the effect of inductive electron withdrawal on the p values of (31) and (32) by their comparison with the saturated isomers (33) and (38)." Hyperconjugation is also said to be a factor " Compounds stabilized hyperconjugatively [such as (36)] have low values of p, whereas compounds with little hyperconjugative stabilization [such as (23)] have high values of p. Yet it is difficult to rationalize from our present knowledge of inductive and hyperconjugative effects why the bi-cyclic and cyclic derivatives (33) and (35), which have nearly equal p values, should have a p value so unlike the structurally similar acyclic system (37)." ... 

Induction of asymmetry into the /J-lactam-forming process was inefficient with acyclic imines having chiral groups on the nitrogen [19] but efficient with rigid, cyclic chiral imines (Table 3). One of these was used as a chiral template to produce highly functionalized quaternary systems (Eq. 5) [34]. 

Another catalyst system which shows a good performance in catalytic activity as well as asymmetric induction is formed in situ, starting from titanium(IV) alk-oxides and the Schiff bases of acyclic dipeptides 55a/b. 

Ab initio emulsion polymerisations of styrene were also conducted at 90°C, using the stable acyclic phosphonylated nitroxide radical SGI (NIO) as a mediator together with a water-soluble redox initiator (Lansalot et al, 2000). A long induction period was observed, assigned to the formation of water-soluble alkoxyamines before nucleation. In this system, molar mass of the polymer increased with conversion following the theoretical line, but the MMD was rather broad (PDI between 2.0 and 2.5). Rather small particles were obtained (average diameter was 120 nm) with a broad particle size distribution. It was also found that a few per cent of coagulum formed usually. 

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