Steric constrains

Traditionally, the production of LABs has been practiced commercially using either Lewis acid catalysts, or liquid hydrofluoric acid (HF).2 The HF catalysis typically gives 2-phenylalkane selectivities of only 17-18%. More recently, UOP/CEPSA have announced the DetalR process for LAB production that is reported to employ a solid acid catalyst.3 Within the same time frame, a number of papers and patents have been published describing LAB synthesis using a range of solid acid (sterically constrained) catalysts, including acidic clays,4 sulfated oxides,5 plus a variety of acidic zeolite structures.6"9 Many of these solid acids provide improved 2-phenylalkane selectivities. 

Various works has pointed out the role of the nanostructure of the catalysts in their design.18-26 There is a general agreement that the nanostructure of the oxide particles is a key to control the reactivity and selectivity. Several papers have discussed the features and properties of nanostructured catalysts and oxides,27-41 but often the concept of nanostructure is not clearly defined. A heterogeneous catalyst should be optimized on a multiscale level, e.g. from the molecular level to the nano, micro- and meso-scale level.42 Therefore, not only the active site itself (molecular level) is relevant, but also the environment around the active site which orients or assist the coordination of the reactants, may induce sterical constrains on the transition state, and affect the short-range transport effects (nano-scale level).42 The catalytic surface process is in series with the transport of the reactants and the back-diffusion of the products which should be concerted with the catalytic transformation. Heat... 

It is important to realize that polymer configuration and conformation are related. Thus, there is a great tendency for isotactic polymers (configuration) to form helical structures (conformation) in an effort to minimize steric constrains brought about because of the isotactic geometry. 

Zinc may function to promote the nucleophilicity of a bound solvent molecule in both small-molecule and protein systems. The p/Ca of metal-free H2O is 15.7, and the p/Ca of hexaaquo-zinc, Zn (OH2)6. is about 10 (Woolley, 1975) (Table III). In a novel small-molecule complex the coordination of H2O to a four-coordinate zinc ion reduces the to about 7 (Groves and Olson, 1985) (Fig. 2). This example is particularly noteworthy since it has a zinc-bound solvent molecule sterically constrained to attack a nearby amide carbonyl group as such, it provides a model for the carboxypeptidase A mechanism (see Section IV,B). To be sure, the zinc ligands play an important role in modulating the chemical function of the metal ion in biological systems and their mimics. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

However, by considering models of the anti configured ylide (Fig. 3.18), it was concluded that the inclusion of a three-carbon tether forces the reactive centers to be too sterically constrained to suffer intramolecular cycloaddition with an alkyne dipolarophile. Conversely, the syn ylide is able to achieve the correct approach for such a process, despite the steric interaction with the phenyl ring. Extension of the interim chain by one methylene unit using 6-heptynal, introduced a greater degree of flexibility into the system, allowing for the formation of the expected diaster-eoisomers (Scheme 3.101). 

The mechanism involving simple nitrogen-coordinated complexes also accounts for reactivities of certain sterically constrained systems. For instance, 3-(diethyamino)cyclohexene undergoes facile isomerization by the action of the BINAP-Rh catalyst (Scheme 18). The atomic arrangement of the substrate is ideal for the mechanism to involve a three-centered transition state for the C—H oxidative addition to produce the cyclometalated intermediate. The high reactivity of this cyclic substrate does not permit any other mechanisms that start from Rh-allylamine chelate complexes in which both the nitrogen and olefinic bond interact with the metallic center. On the other hand, fro/tt-3-(diethylamino)-4-isopropyl-l-methylcyclohexene is inert to the catalysis, because substantial I strain develops during the transition state of the C—H oxidative addition to Rh. 

However, the more sterically constrained bisphenol 8 is oxidized to a product that apparently is an equilibrium mixture of a fully bonded singlet 9 and a biradical triplet species 10.88... 

Use of benzene as a photosensitizer with cyclic olefins produces double bond shifts and addition of alcohols.253-254 The direction of addition suggests an ionic process. The reaction does not occur with acyclic, exocyclic, or sterically constrained olefins. A likely explanation253 is that olefin triplets formed by energy transfer decay in part to highly reactive trans-cycloa 1 kenes which are readily protonated. 

One method of synthesis of 1,2-benzisoxazoles involves cyclization of o-halo- or o-nitro-benzoyl oximes via intramolecular SNAr.167 168 A recently reported variant, which gives access to sterically constrained 3-phenyl-1,2-benzisoxazoles, employs a reversed sequence of steps, e.g. nucleophilic... 

Because of steric constrains, the activation of primary and secondary alcohols is quicker than the activation of tertiary alcohols. Therefore, normally, it is possible to oxidize primary and secondary alcohols, with no interference from elimination reactions of tertiary alcohols present in the same molecule.220... 

An extensive review of recent advances in the area of asymmetric Diels-Alder reactions has been published.205 Sterically constrained tricyclic 2-oxazolidmones serve as excellent chiral auxiliaries for asymmetric Diels-Alder reactions.206 The Diels-Alder reactions of (—)-(a,. 7, )-colchicine (109) with hetero- and carbo-dienophiles show... 

TheA tr is determined by the increase in degrees of freedom for spatial conformation changes in the liquid state as compared to the crystalline state. In crystal lattices, spatial conformation change resulting from the rotation about a Lexible bond is not permitted. Such a rotation, however, is possible in the liquid state. For each Lexible bond, there are three possible conformations due to steric constrain. Thus, the probability of each conformation resulting from one single bond is one-third, and the probability of the fully stretched conformation is... 

Calculations at the B3LYP and MP2 levels of theory with the 6-31+G basis set were used to model the S 2 reactions of the intermediates formed when alkyllithiums were reacted with several 1,1-dibromo- and 1,1-dichloro-alkenes.118 The calculations showed, correctly, that the most sterically constrained halogen was attacked in the first step of the reaction and that the intramolecular substitution reaction in the second step of the reaction occurred by an SN2 mechanism. 

As well as the above 2,3-dimercaptoalkanes a few other vicinal dithiols have also been sythesized. The interaction of methylmercury(II) with chelating agents such as BAL involves linear two-coordinate complexes and in only a few cases has chelation involved a bidentate ligand. In a search for evidence of thiol-containing bidentate chelation, Alcock et al.311 synthesized three sterically constrained dithiols - cyclohexane-1,2-dithiol, toluene-3,4-dithiol and bicyclo[2.2. l]heptane-2,3-dithiol - and demonstrated by spectroscopy and crystallography their formation of chelates with methylmercury(II) of the form (40). 

In practice, and despite much study of Cr(III) systems, there is only a little definitive information about any of these factors. In this report we discuss some current studies of chromium complexes with sterically constrained ligands. These studies are providing information mostly about the fourth factor listed above. 

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