Antiperiplanar positions

Bohlmann et al. (118-121) observed that an infrared absorption band between 2700-2800 cm is characteristic of a piperidine derivative possessing at least two axial carbon-hydrogen bonds in antiperiplanar position to the free-electron pair on the nitrogen atom. The possibility of forming an enamine by dehydrogenation can be determined by this test. Compounds which do not fulfill this condition cannot usually be dehydrogenated (50, 122,123). Thus, for example, yohimbine can be dehydrogenated by mercuric acetate,whereas reserpine or pseudoyohimbine do not react (124). The quinolizidine (125) enamines (Scheme 4), l-azabicyclo(4,3,0)-nonane, l-azabicyclo(5,3,0)decane, l-azabicyclo(5,4,0)undecane, and l-azabicyclo(5,5,0)dodecane have been prepared in this manner (112,126). 

The fact that the rate law of hydrogen bromide elimination is first order with respect to the base may be interpreted by an E2 mechanism. The antiperiplanar position of the hydrogen and the bromine atoms in Ih also makes this mechanism very likely. Earlier the same mechanism was proposed for the elimination reaction of some tertiary a-halo ketones (ref. 19). Other mechanism, such as ElcB or El, seems to be very improbable considering the lack of any deuteration at C-2 or the lack of any rearrangement and the fact that the generation of a-keto cations requires acidic conditions (ref. 20). 

Li+ interacts with both alkyl chains is more stable, compared to the structure where it sits at the antiperiplanar position, by 3.0 kcalmoG. The two binding trends of the alkali metal cations to alkenes were rationalized, taking into account the polarization of the double bond by the cation, polarization of the alkyl chains and steric effects as well. Recently, Gal and coworkers reported a similar alkyl chain-Li+ attraction in alkylbenzenes, namely the scorpion effect ... 

More complex is the problem of syn and anti eliminations (cis and trans in the older nomenclature), i.e. whether the substituents on Ca and leave the parent compound from synperiplanar or from antiperiplanar positions, viz. 

In the explanation favored today, the reason for this stereoelectronic effect is as follows The electronically preferred direction of attack of a hydride donor on the 0=0 double bond of cyclohexanone is the direction in which two of the C—H bonds at the neighboring a positions are exactly opposite the trajectory of the approaching nucleophile. Only the axial C—H bonds in the a positions can be in such an antiperiplanar position while the equatorial C—H bonds cannot. Moreover, these axial C—H bonds are antiperiplanar with regard to the trajectory of the H nucleophile only if the nucleophile attacks via a transition state B, that is, axially (what was to be shown). The antiperiplanarity of the two axial C—H bonds in the a positions is reminiscent of the antiperiplanarity of the electron-withdrawing group in the a position relative to the nucleophile in the Felkin-Anh transition state (formula C in Fig. 8.8 cf. Fig. 8.11, middle row). 

Figure 4.1-12 Characteristic frequencies of C-X bonds (X = Cl, Br, I) in hydrocarbons P primary, S secondary, T tertiary C atom, the subscripts denote the atoms in trans (antiperiplanar) position to the C-X bond. For cyclohexane derivatives Sh h stands for an axial, Sqc for equatorial C-X bond (Schrader and Meier, 1975).

A study similar to that of the carboxylic esters was done for carboxylic amides by Chakrabarti and Dunitz [28]. A general conclusion for the amides from primary, secondary and tertiary C(a) amines is that the C(a)-CGff) bond of an alkyl substituent avoids the synperiplanar arrangement to the C(0)-N bond. For the (er(-alkyl substituents one of the C - C bonds is therefore always in an antiperiplanar position... 

According to the Delongchamps theory of stereoelectronic control8), the cleavage of a given bond is facilitated when at the neighbouring heteroatoms the electron pairs are located in the antiperiplanar positions7,9 . This is illustrated below, the arrow shows the bond in the antiperiplanar position ... 

Therefore, depropagation (from A) is unlikely, because the electron pair on the endocyclic N-atom cannot be located at the antiperiplanar position (the required position for the electron pair is occupied by a bond that is a part of the ring). The breaking of the C—O bond (B) and propagation (C) are both allowed stereoelec-tronically for the same reason. This is visualized by the scheme below, which shows the corresponding conformations and subsequent reaction steps ... 

The highest proportion of the antiperiplanar positions of electron pairs is in structure C. The propagation step (C) is additionally favoured by the release of ring strain during ring-opening. 

Large coupling values, of ca. 11 Hz, observed by Olah and co-workers between the HI and H2 protons in the spectra of chiral benzylic carboca-tions generated from the corresponding benzylic alcohols, have been used by the authors as an indication of the antiperiplanar position of these two hydrogen atoms. 

The H-n.m.r. spectra of (71) and (73) established the correct carbon skeleton and the relative stereochemistry of C-2, C-3, C-4 and C-5 in (71). With /=llHz and 12 Hz, the protons on these carbons must occupy antiperiplanar positions in a six-membered ring, a conclusion confirmed by C-n.m.r. data. 

The mechanism of this reaction involves hydroxide attack at a carbonyl ligand with concomitant cleavage of the lactone moiety to give an allyliron complex with a hydroxy group in a-position. The latter is eliminated after rotation to an antiperiplanar position in respect to the iron complex moiety. This results in formation of the (diene)iron complex with inversed configuration at Cl (Scheme 4-109). ... 

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