Shielding steric

This is again the result of steric shielding by the trichloromethyl group, which causes the bromine atom to be abstracted from the endo face of the intermediate radical. 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

A detailed NMR study and determination of the X-ray structure of the ligand has suggested the occurrence of 7t-stacking of the 2,6-diisopropoxybenzene ring and coordinated aldehyde [5 c]. Because of this stacking, the si face of the CAB-co-ordinated a,/ -unsaturated aldehyde is sterically shielded (Fig. 1.1). 

How- does this reaction take place Although it appears superficially similar to the SN1 and S 2 nucleophilic substitution reactions of alkyl halides discussed in Chapter 11, it must be different because aryl halides are inert to both SN1 and Sj 2 conditions. S l reactions don t occur wdth aryl halides because dissociation of the halide is energetically unfavorable due to tire instability of the potential aryl cation product. S]sj2 reactions don t occur with aryl halides because the halo-substituted carbon of the aromatic ring is sterically shielded from backside approach. For a nucleophile to react with an aryl halide, it would have to approach directly through the aromatic ring and invert the stereochemistry of the aromatic ring carbon—a geometric impossibility. 

A conceptually new direct oxidative glycosylation with glycal donors, employing a reagent combination of triflic anhydride and diphenyl sulfoxide, has recently been reported by Gin [83], This new 3-glycosylation method works very well with hindered hydroxy nucleophiles, including sterically shielded carbohydrate hydroxy systems, and can be run on large scales. 

Experience in PTC with cationic catalysts showed that, in general, the most suitable compounds have symmetrical structures, are lipophilic, and have the active cationic charge centrally located or sterically shielded by substituents. For anionic catalysis sodium tetraphenylborate fulfills these conditions, but it is not stable under acidic conditions. However, certain derivatives of this compound, namely sodium tetra-kis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB, 12.162) and sodium tetrakis[3,5-bis-(l,l,l,3,3,3-hexafluoro-2-methoxy-2-propyl)phenyl]borate (HFPB) are sufficiently stable to be used as PTC catalysts for azo coupling reactions (Iwamoto et al., 1983b 1984 Nishida et al., 1984). These fluorinated tetraphenylborates were found to catalyze strongly azo coupling reactions, some of which were carried out with the corresponding diazotization in situ. 

Although the number of applications of olefin metathesis to transition metal complexes is small compared to the number of applications in organic synthesis, this field is becoming increasingly important. Spectacular examples are the double RCM reactions of copper phenanthroline complexes as a synthetic route to catenanes [113] or a recently reported approach to steric shielding of rhenium complex terminated sp-carbon chains [114]. 

It has been possible to record the IR and UV spectra of several derivatives of the carbene [75] - tetrachlorocyclopentadienylidene [80], indenylidene [81] and fluorenylidene [82] (Bell and Dunkin, 1985). These carbenes were formed by UV photolysis of the corresponding diazo precursors frozen in inert matrices and have a triplet ground state. The carbenes [80]-[82] react with CO in inert matrices at 30 K, but exhibit a lower reactivity than the carbene [75]. Furthermore, they were stabilized in a pure CO matrix at 12 K, whereas the free carbene [75] could not be detected under these conditions. The different reactivity towards CO between [75] and [80]-[82] may be associated with the different steric shielding of the carbene centres and with the different triplet-singlet gap as well. 

In this work a new approach is desribed, which can help to understand ED over heterogeneous catalysts We also hope that this approach can be used to find new modifiers for enantioselective heterogeneous catalytic reactions. The basis for this approach is the steric shielding known in organic chemistry [7,8]. A chiral template molecule can induce shielding effect (SE) in such a way that it preferentially interacts with one of the prochiral sites of the substrate. If a substrate is preferentially shielded its further reaction can take place only fi"om its unshielded site resulting in ED. 

Recently it has been evidenced that a large aromatic substituent, such as naphtyl, can provide a intramolecular steric shielding for an a-keto ester moiety [9] resulting in enantio-... 

The first approach applied for [cinchonidine (CD) - a-keto ester] complex was also unsuccessful. In the open conformation CD cannot provide the required steric shielding. In open form either the quinuclidine or the quinoline moiety of CD will interact with the substrate. It has already been demonstrated that the quinuclidine moiety has a crucial role both in the rate acceleration and the induction of ED [13]. 

Perchlorotriphenyl methyl radicals are particularly persistent . Among the factors contributing to the exceptional persistency of this kind of radicals the steric shielding of the a-(tricovalent) carbon is predominant. Only hydrogen or electron can reach the carbon radical. Thus, when perchloro radicals are formed in a DMSO-alkaline hydroxide solution an electron transfer occurs, leading to the perchlorocarbanions. It is assumed that the donor is the DMSO carbanion. 

O- versus C-a kyIation product ratios in the methylation of desoxybenzoin by dimethyl sulphate can be varied between 0.75 and 63 by the choice of catalyst. The reaction can be steered towards enol-ether formation by large, sterically shielded ammonium ions, while C-alkylation is favoured by small ammonium ions (e.g. RMejN" ) and by crown ethers (Dehmlow and Schrader, 1990). 

This stereoselectivity is the result of the conformation of the enolate and steric shielding by the silyl substituent. 

The /rau.v-2-naphthyl cyclohexyl sulfone 15 can be prepared readily in either enantiomeric form. The corresponding ester enolates can be alkylated in good yield and diastereoselectivity.98 In this case, the steric shielding is provided by the naphthyl... 

A crystal structure is available for the SnCl4 complex of 2-benzyloxy-3-pentanone.89 The steric shielding by the methyl group with respect to the C=0 is evident in this structure (Figure 2.1). NMR studies indicate that the reaction involves... 

An 8-phenylmenthol ester was employed as the chiral auxiliary to achieve enantioselectivity in the synthesis of prostaglandin precursors.83 The crucial features of the TS are the anti disposition of the Lewis acid relative to the alcohol moiety and a tt stacking with the phenyl ring that provides both stabilization and steric shielding of the a-face. 

The preparation of cyclobutanes via the catalytic conditions can be extremely efficient provided that the radical formed after epoxide opening is sterically shielded and cyclization promoted by the Thorpe-Ingold effect. It... 

Although hydrolysis as well as other nucleophilic reactions of A-acylazoles (alcoholysis, aminolysis etc.) most likely follow the addition-elimination (AE) mechanism, there are indications that more complex mechanisms must be taken into account for hydrolysis under specific structural conditions. For example, for neutral hydrolysis of imidazolides with increasing steric shielding of the carbonyl group by one, two, and three... 

In 1988 Wiberg et al. presented reactions of afree, stable iminosilane.14,27 Since that time the number of products has rapidly increased.28 Although the double bond is sterically shielded, the reactivity of iminosilanes is immense. 

With increasing steric shielding of the radical center AH i — AHgr also increases. 

A further interesting example of a steric effect was recently published106. The sterically shielded 2.2.6.6-tetramethyl piperidinium radical cation adds to cyclohexene by almost three powers of ten slower than the piperidinium radical cation itself107. ... 

In a similar fashion therefore, quaternary substituents can also be introduced to aromatic ring systems by the aromatic counterpart SRN1-procedure as investigated mainly by Bunnett115. In an extreme situation of steric shielding, however, a response... 

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