3- pentanone conformation

Ketones also favor eclipsed conformations. The preference is for the rotamer in which the alkyl group, rather than a hydrogen, is eclipsed with the carbonyl group because this conformation allows the two alkyl groups to be anti rather than gauche. Electron diffraction studies of 3-pentanone indicate the conformation shown to be the most stable rotamer, in accord with this generalization. ... 

The first element of stereocontrol in aldol addition reactions of ketone enolates is the enolate structure. Most enolates can exist as two stereoisomers. In Section 1.1.2, we discussed the factors that influence enolate composition. The enolate formed from 2,2-dimethyl-3-pentanone under kinetically controlled conditions is the Z-isomer.5 When it reacts with benzaldehyde only the syn aldol is formed.4 The product stereochemistry is correctly predicted if the TS has a conformation with the phenyl substituent in an equatorial position. 

The is-boron enolates of some ketones can be preferentially obtained with the use of dialkylboron chlorides.17 The data in Table 2.3 pertaining to 3-pentanone and 2-methyl-3-pentanone illustrate this method. Use of boron triflates with a more hindered amine favors the Z-enolate. The contrasting stereoselectivity of the boron triflates and chlorides has been discussed in terms of reactant conformation and the stereoelectronic requirement for perpendicular alignment of the hydrogen being removed with the carbonyl group.18 The... 

The single crystal X-ray structure of fra s-methyliodo difluoro[3,3 -(trimethylenedinitrolo)bis(2-pentanone oximato)]borate rhodium(III) (62) shows that the Rh sits in the plane established by the four N atoms, with the I and the Me in the axial positions (2.813 A and 2.090 A, respectively). The two six-membered rings resemble a chair conformation, with the BF2 bridge bent toward the axial methyl group, and the propylene bent toward the iodine atom.896... 

Bcnzyne does not act as a dienophile with butadiene, presumably because the diene exists in the transoid arrangement. However, benzyne and butadiene generated from 3-sulfolene react to give 1,4-dihydronaphthalene in low yield (9%). The reaction is carried out by decomposing benzenediazonium 2-carboxylate and 3-sulfolene at 100° in pentanone-2.5 The reaction presumably occurs because the butadiene is generated in the cisoid conformation. 

Moreover, electron diffraction studies of 3-pentanone indicate the methyl-eclipsed conformation shown below to be the most stable rotamer. ... 

The reactions of bare Fe" " ions and related species in the gas phase continue to attract much interest. The remote functionalisation of 1,6-hexanediol by Fe occurs by C-H activation at C(3) and C(4).26 Functionalisation of 3-methyl-2-pentanone at C(4) is diastereoselective, probably because of the conformation of a chair-like intermediate. Reactions of Fe with anisoles and phenols have also been studied.28 Interaction of Fe with silanes gives both silene and silylene species, though the two are not interconvertible. The reactions of Fe(alkene)+ complexes with pentane were found to differ dramatically from those of bare Fe" , and C-H and C-C activation were also observed in reactions of Fe(C2H4) with oxygen. 0,31 interaction of Fe(benzyne)+ with alkyl halides led to C-X or C-C addition followed by p-elimination and loss of HX.32 The gas phase reaction of Fe(NH2)Me" with C2H4 is best explained by insertion into the Fe-C bond followed by P-elimination and loss of propene. The reaction of FeMe with 1-octyne also leads to C-C bond forming processes. 

A similar bias is seen in carbonyl compounds. Acetaldehyde shows a 1.0 kcal / mol preference for the eclipsed form over the staggered (Eq. 2.26), with the latter again being a transition state for rotation. In propanal the conformation having the methyl group eclipsed with the carbonyl (Eq. 2.27) is preferred by 1 kcal/mol. Similarly, for 2-butanone and even 3-pentanone, the conformations with the methyls eclipsed with the carbonyl are preferred. It is generally accepted that electronic interactions are controlling these preferences, but the effects are subtle. 

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