Felkin−Anh Stereoselectivity in Cycloadditions of Acetylketene:  Evidence for a Concerted, Pseudopericyclic Pathway

Abstract

The cycloadditions of acetylketene with alpha-chiral aldehydes and ketones are shown to be diastereoselective, forming a tertiary or quaternary chiral center at an acetal or ketal carbon with good stereocontrol. X-ray crystallography of a minor product (5b) shows that the major products (e.g., 4b) are those predicted by the Felkin-Anh model. Transition states are reported at the MP2/6-31G level for the addition of ethanal to formylketene and at the B3LYP/6-31G level for the addition of 2-phenylpropanal. The ground-state conformations of the reactants and products are used to rationalize the relative energies and geometries of the transition states without the need to invoke the Cieplak hypothesis. However, chiral substituents on the alpha-oxoketene show no diastereoselectivity. These experimental and computational results are only consistent with the nearly planar, pseudopericyclic transition state previously proposed.

A Density Functional Theory Study on the Electrocyclization of 1,2,4,6‐Heptatetraene Analogues: Converting a Pericyclic to a Pseudopericyclic Reaction

“…The dihedral angles of these H atoms are very close to planarity: 169.28 and À12.58 (see Table 1). An almost total lack of rotation of these H atoms is observed when we display the animation of the imaginary frequency for TS 6 . In TS 5 the situation is very If we take TS 6 and carry out an optimization (search for a TS) with enforcement of a planar geometry for the six-membered ring (all the atoms of the molecule were constrained to a plane except for the two H atoms of C7), we obtain a structure which is not true TS, since an additional imaginary frequency appears (corresponding to deformation of the enforced plane, as expected).…”

“…Pseudopericyclic reactions are known to exhibit low activation energies. [2][3][4][5][6][7][8][9][10] This conclusion should be interpreted in a relative rather than absolute manner as, in fact, pseudopericyclic reactions have low activation energies with respect to analogous pericyclic reactions. Thus, the pseudopericyclic Boulton-Katritzky rearrangement of (5R)-4-nitrosobenz[c]isoxazole and its anion have activation energies higher than 30 kcal mol À1 .…”

“…According to Birney et al pseudopericyclic reactions have planar transition states. [2][3][4][5][6][7][8][9][10] As can be seen in Figure 3 (which includes the normal vibrational frequency corresponding to each imaginary frequency) and Table 1, there are substantial differences in the planarity of the transition structures. Thus, TS 1 , TS 2 , and TS 3 have geometries strongly distorted from planarity, whereas the remaining transition structures, especially TS 5 , show more planar geometries.…”

“…Hence, pseudopericyclic reactions cannot be orbital symmetry forbidden. Recently, Birney et al [2][3][4][5][6][7][8][9][10] and others [11][12][13][14][15][16][17][18][19][20] showed that a number of organic syntheses involve this type of process. Although Lemals definition is seemingly quite clear, there is some ambiguity as the orbital description is not unique; thus, any unit transformation of canonical molecular orbitals can be used to reproduce molecular properties.…”

100 Years of the Wolff Rearrangement

“…[270] Thermal decomposition of 337 furnished 338 as the major (90%) ketoketene which was trapped by benzaldehyde to give 339 with almost no diastereoselectivity (ഠ 1.5:1). [271] …”