Carbonyl compounds, conformation

Synthetically useful stereoselective reductions have been possible with cyclic carbonyl compounds of rigid conformation. Reduction of substituted cyclohexanone and cyclopentan-one rings by hydrides of moderate activity, e.g. NaBH (J.-L. Luche, 1978), leads to alcohols via hydride addition to the less hindered side of the carbonyl group. Hydrides with bulky substituents 3IQ especially useful for such regio- and stereoselective reductions, e.g. lithium hydrotri-t-butoxyaluminate (C.H. Kuo, 1968) and lithium or potassium tri-sec-butylhydro-borates or hydrotri-sec-isoamylborates (=L-, K-, LS- and KS-Selectrides ) (H.C. Brown, 1972 B C.A. Brown, 1973 S. Krishnamurthy, 1976). 

In contrast to the open-chain and dipolar models, which are based on conformations of the carbonyl compound not representing energy minima, Karabatsos proposed a different model assuming an early, reactant-like transition state in which the most stable conformation of the free carbonyl compound is preserved1314. Thus, the C-M bond eclipses the carbonyl double bond and, in order to minimize the energy of the transition state, the nucleophile approaches close to the small substituent on the stereogenic center (Figure 5). 

In the transition state, the torsional strain involving the partially formed bond between the nucleophile and the carbonyl group represents a substantial fraction of the total strain, even when the degree of bonding is low. Thus, in the case of acyclic carbonyl compounds, a staggered conformation is preferred in the transition state (Figure 6). 

Simple allyl alkali metal compounds have only a small capability for discriminating between diastereotopic faces of carbonyl compounds. Although a matter of simple diastereoselectivity, this can be concluded from the reaction of conformationally locked 4-/erf-butylcyclohexanone... 

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

Other carbonyl compounds exhibit rotation about sp -sp bonds, including amides. In M-acetyl-iV-methylaniline, the cis conformation (i ) is more stable than... 

The reactivity of diazo carbonyl compounds appears to be related to the conformational equilibria between s-cis and s-trans conformations. A concerted rearrangement is favored by the s-cis conformation.237 The /-butyl compound 19, which exists in the s-trans conformation, gives very little di-/-butylketene on photolysis.238 A similarly... 

The carbonyl compounds and carboxylic acids would be other examples of conformations involving sp -sp1 hybridisatram. 

Epoxides can be isomerized to carbonyl compounds by Lewis acids.104 105 Boron trifluoride is frequently used as the reagent. Carbocation intermediates appear to be involved, and the structure and stereochemistry of the product are determined by the factors which govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. 

The reduction of thieno[3,2-6]thiophene-2,5-diaIdehyde or dithieno-[3,2-i 2, 3 -d]thiophene-2,6-dialdchyde with potassium in tetra-hydrofuran or 1,2-dimethoxyethane yields the corresponding anion-radicals, the ESR of which give information on the conformation of similar carbonyl compounds.Cation-radicals have also been studied (see Section IV). 

There are some cases where both types of photocycloaddition take place. For example, cinnamaldehyde and crotonaldehyde yield, upon irradiation with 2-methyl-2-butene, both the oxetane and the cyclobutane products.26 In marked contrast, mesityl oxide, as similar as it would appear to be to crotonaldehyde (Table I), is stable to irradiation in the presence of both isobutylene and isopropanol.37,74 These differences in reactivity of a,/9-unsaturated carbonyl compounds have been attributed to conformational (that is, s-cis or s-trans) differences.74... 

Diastereomeric 1,3-amino alcohols 1 have been obtained by reduction of 4,5-dihydroisoxa-zoles350-353. 3C chemical shifts allow a stereochemical differentiation due to the formation of energetically preferred chelated conformations. Similar to /3-hydroxy carbonyl compounds and 1,3-diol derivatives, the chemical shifts of the backbone carbons are larger in the syn 1,3-amino alcohols than in the awn -isomers353. 

Iron-acyl enolates, such as 2, prepared by x-deprotonation of the corresponding acyl complexes with lithium amides or alkyllithiums, are nearly always generated as fs-enolates which suffer stereoselective alkylation while existing as the crmt-conformer which places the carbon monoxide oxygen anti to the enolate oxygen (see Section 1.1.1.3.4.1.). These enolates react readily with strong electrophiles, such as primary iodoalkanes, primary alkyl sulfonates, 3-bromopropenes, (bromomethyl)benzenes and 3-bromopropynes, a-halo ethers and a-halo carbonyl compounds (Houben-Weyl, Volume 13/9 a, p 413) (see Table 6 for examples). 

Organic carbonyl compounds—aldehydes, ketones, amides, and acyl halides—in which the carbonyl group is not part of a cyclic structure have interesting conformational properties that may differ widely according to the molecular system bearing these substituents. 

Electronic and conformational effects on jt-facial stereoselectivity in nucleophilic additions to carbonyl compounds have been studied by the use of RHF/3-21G and RHF/6-31G methods ". Figure 10 shows a comparison of predicted and experimental selectivities for methyl Grignard additions. Satisfactory agreement of the ratios of anti and equatorial attacks of MeMgX on the carbonyl carbon atoms was reported. 

Radical anions derived from 2,5-diformylthieno[3,2-6]thiophene (39) as well as (40) were studied in connection with the conformational analysis of heteroaromatic carbonyl compounds. Different ESR signals were given by distinct rotational isomers. Information on interconversion of the rotamers could not be obtained since the radicals were unstable at the temperatures necessary for interconversion. On the other hand, ketyl radicals derived from ketones (41) and (42) are relatively more stable at the temperatures needed for the study of the conformational mobility in these systems. The ESR spectra of the bis-thienothienyl ketyls from (41) and (42) at room temperature show that the unpaired electron is coupled to three pairs of equivalent protons. Both spectra exhibited a certain amount of asymmetry, which was enhanced by lowering the temperature. At -10°C the highfield part of the spectra split into new lines arising from two species which have similar hyperfine splitting, but different g factors. These have been identified as the rotational isomers of the radicals. The two preferred conformations are cis-trans and trans-trans. An examination... 

An empirical increment system permits prediction of charge distribution in a,/ -unsaturated carbonyl compounds, assuming additivity of electronic effects and neglecting the conformational dependence of carbon-13 chemical shifts [290]. Moreover, carbonyl and alkenyl carbon shifts of a, /3-unsaturatcd ketones may be used to differentiate between planar and twisted conjugated systems, as shown in Table 4.29 [291] and outlined for phenones in Section 3.1.3.8. 

Deprotonation of carbonyl compounds by chiral amide bases followed by trapping with silylating agents or aldehydes has become a common method for de-symmetrizing prochiral and conformationally locked 4-substituted cyclohexanones and bicyclic ketones. The literature through 1997 has been reviewed [45]. 

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