Primary centers

With two competing allylic species, a secondary center -CH2- is brominated preferentially over a primary center -CH3. 

The generally observed endo preference has been justified by secondary orbital interactions, [17e, 42,43] by inductive or charge-transfer interactions [44] and by the geometrical overlap relationship of the n orbitals at the primary centers [45]. 

The route has also been applied to TBS-substituted propargylic mesylates (Eq. 9.39) [45]. Interestingly, the isomeric propargylic silanes are not formed despite the more attractive steric environment for a direct SN2 displacement at the primary center. 

The observation of exalted secondary isotope effects, i.e., those that are substantially beyond the semiclassical limits of unity and the equilibrium isotope effect. These observations require coupling between the motion at the primary center and motion at the secondary center in the transition-state reaction coordinate, and in addition that tunneling is occurring along the reaction coordinate. 

Usually, the different types of color center are formed after a certain initial concentration of F centers has been prodnced. These primary centers are typically created by two main experimental methods (i) additive coloration and (ii) irradiation. 

Because of their importance as basic primary centers, we will now discuss the optical bands associated with the F centers in alkali halide crystals. The simplest approximation is to consider the F center - that is, an electron trapped in a vacancy (see Figure 6.12) - as an electron confined inside a rigid cubic box of dimension 2a, where a is the anion-cation distance (the Cr -Na+ distance in NaCl). Solving for the energy levels of such an electron is a common problem in quantum mechanics. The energy levels are given by... 

Pretreatment temperature (CC) Primary center g tensor (27AI) (G) Oxygen adduct ... 

The move toward catalytic reactions is reflected in the increase in the number of chapters in this book on the topic compared to the first edition. The trend has been observed by noted chemists in the previous decade. Professor Seebach, for example, in 1990 stated the primary center of attention for all synthetic methods will continue to shift toward catalytic and enantioselective variants indeed, it will not be long before such modifications will be available for every standard reaction. 6 Professor Trost in 1995 was a little more specific with catalysis by transition metal complexes has a major role to play in addressing the issue of atom economy—both from the point of view of improving existing processes, and, most importantly, from discovering new ones. 7 However, the concept can be extended to biological and organic catalysts and to those based on transition metals. 

This reaction will show competition between SN1 and SN2 mechanisms due to the fact that this center is less hindered than a tertiary center but more hindered than a primary center. An SN1 mechanism will be favored using highly polar, aprotic solvents to stabilize the forming carbocation. An SN2 mechanism will be favored when nonpolar solvents are used. 

This reaction will proceed through an SN2 mechanism. In general, primary centers are not sterically encumbered enough to inhibit Sn2 reactions. Additionally, recall that primary carbocations are much less stable than tertiary carbocations, making an SnI mechanism highly unlikely for this transformation. 

When reactions are performed on tributylstannyl ethers and dibutylstan-nylene acetals of hexopyranoside derivatives that have more oxygen atoms, including the primary one, unprotected, markedly different results are obtained, as shown in Figs. 24 and 25. Dibutylstannylene acetals favor reaction on the position in the 1,2-diol unit that is adjacent to an axial substituent, whereas tributyltin ethers prefer to react at the primary centers. However, this pattern was not observed for reactions of the tributylstannyl ether of l,2-(l-methoxyethylidene)-j3-D-mannopyranose, as shown in Fig. 26.122 Methylation of this compound through the tributylstannyl ether in toluene in the presence of added tetrabutylammonium bromide also gave substitution on 0-3 predominantly.123 These results may arise from distortion of the chair conformation by the fused isopropylidene acetal. 

However, the anion of dimethyl methylmalonate preferentially attaeks the terminal primary center to give 3. 

It is also interesting to note that the distance between the substrate and the zeolite framework in the TS was much larger for the isobutane reaction, where the attack was on a tertiary center, than for the linear alkanes, where the attack was on primary centers. A similar situation was found in the case of the exchange reactions [44]. Although the nature of the TS was the same for exchanging at primary, secondary and tertiary centers, in the last case the distance from the substrate (isobutane) to the zeolite framework is larger than for the other alkanes. This can be attributed to the steric hindrance of the methyl groups as the isobutane approaches the acid site. Thus, it is... 

SCHEME 6.11 Halogenation reactions via displacement at primary centers. 

While hydride-based reagents such as lithium aluminum hydride can selectively reduce at primary centers over secondary centers, reagents such as Raney nickel show little or no selectivity. As shown in Scheme 6.71, a bis-chloride was converted to the corresponding 4,6-dideoxy sugar using this reagent [110]. 

Finally, when the selective reduction of secondary centers is desired over primary centers, free radical chemistry provides the answer. Unlike ionic mechanisms, the formation of free radicals occurs more readily at tertiary centers, with secondary radicals being more stable than primary radicals. As shown in Scheme 6.72, tributyltin hydride sequentially removed the secondary chloride with complete reduction of both chlorides over extended reaction times [111]. 

Complementary to catalytic hydrogenations, the use of diborane provides convenient routes towards the elimination of olefins. However, this reaction has the added advantage of introducing new functionality at the site of olefin reduction. Because of regiochemical considerations, this reaction is particularly useful when applied to exocyclic olefins. Scheme 6.76 shows that a sugar derivative was treated with diborane with an oxidative workup to yield the illustrated product bearing a new hydroxyl group exclusively at the primary center [117]. 

Substituted iT-allylnickel complexes which could give products derived from reaction either at a primary or a secondary/tertiary terminus almost invariably react only at the primary center (equations 62 and 63). 2... 

Migration to a primary center has also been effected, as shown by the conversion of (45) to (46 equation 25). Conditions for these reactions are quite mild, consisting of solvolysis in tetrahydro-furan/LiC104, with CaC03 present to neutralize the p-toluenesulfonic acid that is formed. 

LiBr solubilized by Bu"3PO (or HMPA) was found to catalyze the formation of 2-oxazolidones from organic isocyanates and terminal alkene epoxides. The epoxide substituent appears at the 5-position of the product as shown in equation (122). This outcome is in keeping with rapid trapping by the isocyanate of the halohydrin salt formed by attack of bromide at the primary center. This interpretation requires that oxazolidone formation be faster than epoxide rearrangement data are not available to confirm this point. 

Figure 14 The most relevant elementary steps observed at the (R,/ -enantiomer of a chiral, C2-symmetric, isospecific zirconium center with a primary growing chain end (top) and a secondary growing chain end (bottom). The (S,S)-enantiomer produces the opposite stereochemistry of each single event, but overall the same polymer chains and the same insertion mistakes. In practice, in the case of C2-symmetric metallocenes, the racemic mixture (R,R+S,S) is always used. P = growing polypropylene chain [C] = concentration of active primary centers pC] = concentration of active secondary centers.

This MErKoFer ontology is partially shown in Fig. 7.14. In the area of descriptions, the most important aspects can be found profile and material definitions, and the aforementioned categories for phase changes and other tasks, process states, and errors. In the product area, the plant and its elements are modeled as producing products, in addition to profile and material batches as produced products. The process area contains the production process itself as primary center for state and material changes, and the transport orders of the material batches which are read from the company s ERP system (SAP R/3) that is part of the storage area. 

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