Chelation, internal

Enhanced anti selectivity is observed in reactions of lithiated 4.5-dihydrooxazoles bearing an additional substituent which facilitates the formation of rigid azaenolates by internal chelation of lithium13. Thus, reaction of 2-ethyl-4,5-dihydro-4,4-dimethyloxazole (10) with 2-methylpropanal gives a 56 44 mixture of adducts while (R)-2-ethyl-4,5-dihydro-4-(methoxymethyl)-oxazolc (12) reacts with the same aldehyde to yield a 90 10 mixture of adducts 1313. 

Provided that the reaction occurs through a chairlike TS, the E anti/Z syn relationship will hold. There are three cases that can lead to departure from this relationship. These include a nonchair TS, that can involve either an open TS or a nonchair cyclic TS. Internal chelation of the aldehyde or enolate can also cause a change in TS structure. 

The general trend is that boron enolates parallel lithium enolates in their stereoselectivity but show enhanced stereoselectivity. There also are some advantages in terms of access to both stereoisomeric enol derivatives. Another important characteristic of boron enolates is that they are not subject to internal chelation. The tetracoordinate dialkylboron in the cyclic TS is not able to accept additional ligands, so there is no tendency to form a chelated TS when the aldehyde or enolate carries a donor substituent. Table 2.2 gives some typical data for boron enolates and shows the strong correspondence between enolate configuration and product stereochemistry. 

It has been demonstrated that excellent diastereoselectivities for enolate alkylation also are obtained when alkyl substituents are positioned at C(4), C(5) or C(6) of benzamide 5. AryE and methoxy substituents at C(5) also are compatible, but a methyl group at C(3) leads to an inversion of the diastereoselectivity of enolate alkylation. The inverted sense of stereoselection is thought to be a result of a disruption of the internal chelation shown in enolate 6 by steric effects of the neighboring methyl substituent. 5... 

Hellmuth believes that the polymeric material forms. If magnesium acetate is simply the internally chelated material, then it should be quickly washed away. The combination of calcium and magnesium acetates is known as CMA. CMA is more expensive than sodium chloride or calcium chloride, but it does not damage plant life and it has a much longer effective life. The polymer seeks the cracks and crevices in and around the pavement and remains until needed again to lower the freezing point of water. Often, one application of CMA is sufficient for a winter season. 

The cyclopropane is inert to tributyltin chloride, but reacts readily with more highly Lewis acidic tributyltin trifluoromethanesulfonate (triflate) [11]. The 3-stannyI ester 13 showed no sign of internal chelation Eq. (16). 

Strongly Lewis acidic SbCl5 rapidly reacts with one equivalent of the cyclopropane 16 in chloroform to give monoalkylated antimony derivate 18 in 87% yield Eq. (20) [11]. The internal chelation is especially strong, as indicated by the carbonyl band in the IR spectrum at 1600 cm-1. In contrast to the common monoalkyltetrachloroantimony, the chelated homoenolate 18 is stable at room temperature for many hours. 

The presence of a chiral centre ft to a carbonyl also provides the possibility for chirality transfer upon nucleophilic attack. In cases where there is the possibility of forming an internal chelate, 25, the stereochemistry is explained by attack of the carbonyl opposite the bulky R2 group. However, when chelation is not possible, the group of Reetz has suggested that a transition state such as 26 is involved when the chiral carbon has a polar X group which points away from the carbonyl and the nucleophile attacks from the face opposite the bulky R group77. 

It is often helpful to construct models of the reactant and the reagent as a pointer to the stability of the different conformations resulting from intramolecular non-bonded interactions. In particular any internal chelation effects existing in the reagent or reactant, or which might develop during the course of the reaction need to be identified. 

The stereochemical outcome of the conversion of 4 into 2 The stereochemical outcome of this hydrazination can be explained if one assumes that the (Z)-lithium enolate 3 is internally chelated to the oxazolidinone carbonyl at low temperature. This would hold the enolate... 

Hereinafter, the extra- and intra- annular chirality transfer (CT) classification defined by Evans402 will be employed provided the original stereocenter is linked to the enol moiety by one or two points of anchorage, with the possibility of chelate-enforced chirality transfer due to internal chelation with heteroatoms (Figure 4)410. 

The action of N-oxides of a-dimediylaminocarboxylic acids on dihydroxyidienylborane produces die internal chelates of the oxidation products." Various pyridine and quinoline Af-oxides may be used to ox idize organylboranes, but there can be some alkylation of die heteroaiomatic lings. ... 

The reactivity of ggm-dizmcioalkenes (prepared by metal allylation of alkynyl metals (see Sections. )) with aldehydes leads to the corresponding aUene derivatives in moderate yields (Scheme 38). As already seen for gm-dizincioalkanes, an internal chelation of the 1,1-dimetalloalkene with a Lewis functional group allows differentiation of the reactivity of the two metal centers towards electrophile (see Electrophile) reagents. Scheme 39 summarizes the reactivity... 

In the development of chiral lithium amides which result in higher ee, the effect of a diverse set of substituents R and R in 38 was examined. It was shown that ee increases as the size of substituent R becomes bulkier, and also as the amount of fluorine in R increases. In THF, 38a occurs as a monomeric structure M-38a in either the presence or absence of HMPA. Fluorinated base 38b has also been shown to be monomeric in THF, consistent with structure M-38b where the fluorine atoms do not act as internal chelating ligands. In the presence of LiCl, the solution structure of labeled 38b was examined by Li and NMR in THF-ds- The Li- N coupling patterns showed that mixed dimer MD-6 was formed, as also illustrated with 38a. The absolute configuration of the products renders the OD-1 structure of transition state TS-1 most likely (Fig. 7) [58]. 

Snp2 seems unique for this annulation. After generation of the allylstannane, the Sn" serves as a Lewis acid catalyst and provides internal chelation. The fluoride counter ion is an ideal nucleophile for the activation of the allylsilane. The high stereoselectivity (25 1 to 75 1) is attributed to chelation (Eq. 2) [12]. 

Cyclohexanediols are prepared in approximately 20-60 % yields (diastereomeric ratios from 4 1 to 50 1) by a [3 + 3] annulation using a,/3-epoxy aldehydes (R = alkyl, H) as the dielectrophilic partner (Eq. 3) [13]. The high stereoselectivity and the lack of appreciable [3 -h 2] annulation are attributed to internal chelation and internal frans-diaxial epoxide opening (Eq. 4) [13]. 

---