Syn-periplanar elimination

The syn-periplanar eliminations by pyrolysis of esters, xanthates, sulfoxides and amine oxides are symmetry-allowed. With respect to the alkene portion of the transition state, the centers presumably are or... 

The pyrolysis of amine oxides is called Cope elimination and typically takes place at 120 °C (Scheme 6.21). The reaction is a syn periplanar elimination in which six electrons move in a five-membered ring according to a concerted, thermally induced mechanism to yield an alkene and a hydroxylamine. 

Why do you think the enthalpy of activation, AH for a syn-periplanar elimination is higher than that for an anti-periplanar elimination ... 

Syn elimination and the syn-anti dichotomy have also been found in open-chain systems, though to a lesser extent than in medium-ring compounds. For example, in the conversion of 3-hexyl-4-d-trimethylammonium ion to 3-hexene with potassium ec-butoxide, 67% of the reaction followed the syn-anti dichotomy. In general syn elimination in open-chain systems is only important in cases where certain types of steric effect are present. One such type is compounds in which substituents are found on both the P and the y carbons (the unprimed letter refers to the branch in which the elimination takes place). The factors that cause these results are not completely understood, but the following conformational effects have been proposed as a partial explanation. The two anti- and two syn-periplanar conformations are, for a quaternary ammonium salt ... 

With acylic molecules elimination could be envisaged as taking place from one or other of two limiting conformations—the anti-periplanar (24a) or the syn-periplanar (24b) ... 

ANTI elimination [(32) — (33)] was found to proceed only 14 times faster than SYN elimination [(31)— (33)] reflecting the fact that the energy needed to distort the ring, so that (32) can assume an approximately anti-periplanar conformation, almost outweighs the normal energetic advantage of the staggered conformation over the, syn-periplanar, eclipsed one, i.e. (31). 

The six-membered rings in these T.S.s are more flexible than the five-membered T.S.—(81) above—and need not be planar (cf cyclohexanes v. cyclopentanes). Elimination may thus proceed, in part at least, from conformations other than the syn-periplanar, with the result that the degree of SYN stereoselectivity in these eliminations may sometimes be lower than that observed in the Cope reaction. Both reactions require higher temperatures than for the Cope reaction, carboxylic esters particularly so. 

A is called anti-periplanar, and this type of elimination, in which H and X depart in opposite directions, is called anti elimination. Conformation B is syn-periplanar, and this type of elimination, with H and X leaving in the same direction, is called syn elimination. Many examples of both kinds have been discovered. In the absence of special effects (discussed below) anti elimination is usually greatly favored over syn elimination, probably because A is a staggered conformation (p. 139) and the molecule requires less energy to reach this transition state than it does to reach the eclipsed transition state B. A few of the many known examples of predominant or exclusive anti elimination follow. 

To explain this phenomenon, there are two seperate processes to consider. The first and most important one for this reason is the formation of the oxaphosphetane. The addition of the ylide to the aldehyde can, in principle, produce two isomers with either a Z or E substituted double bond. The following elimination step is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state. With unstabilized ylides the syn diastereomer of the oxaphosphetane similar to 61 is formed preferentially. This step is kinetically controlled and therefore irreversible, and predominantly the Z-alkene 62 that results reflects this. 

In a cyclopentane or norbomane, the steric factors change, for the initial choice in an elimination is between two eclipsed forms. We have already seen that, according to PLM, syn-periplanar (0°) elimination of X and Y syn is preferable to X and Z anticlinal (120°). In 106, the possibilities are illustrated for a cyclopentane (or norbornane). Note that atoms y or 8 and X or Y are skew , while jS and Z are eclipsed. For exo-syn departure (as in 93) X and Y would move anticlockwise in the direction of the vertical arrow along the incipient it orbital, while Z and W would move away from j8 and a into the horizontal position. F or anticlinal depart-ture of X and Z in the direction of the incipient n orbital, Z would suffer nonbonded syn repulsion of /3. Obviously the least favorable elimination is endo-syn, in which both Z and W would have to move toward the incipient rr orbital and encounter nonbonding repulsions from /3 and a. Without a detailed calculation, it is not clear to what extent the other terms of equation (197), e.g. I tora. and I bend have to be considered here. But, it does seem clear that the relative rates (circled atoms depart) given by LeBel et al. (1964) for 92-95, are plausible for steric reasons. If this is so, the coplanarity principle also has a steric basis, in these eliminations. 

Mechanism of E2 elimination from syn-periplanar and anti-periplanar conformations. 

The anti-periplanar confonnation is more stable because it is staggered rather than eclipsed. Therefore, anti elimination is preferred in the E2 reaction. (Syn elimination is much less common but does occur when the leaving group and the hydrogen are held syn-periplanar in an eclipsed, or nearly eclipsed, confonnation of a rigid compound.) The elimination reactions of the diastereomers of l-bromo-l,2-diphenylpropane are illustrated in Figure 9.2. 

In an E2 elimination, the new 7t bond is formed by overlap of the C-H a bond with the C-X a antibonding orbital. The two orbitals have to lie in the same plane for best overlap, and now there are two conformations that allow this. One has H and X syn-periplanar, the other anti-periplanar. The anti-periplanar conformation is more stable because it is staggered (the syn-periplanar conformation is eclipsed) but, more importantly, only in the anti-periplanar conformation are the bonds (and therefore the orbitals) truly parallel. 

An anti-periplanar arrangement of C-Br and C-H is attainable with a vinylic bromide too, provided the Br and H are trans to one another. E2 elimination from the Z isomer of a vinyl bromide gives an alkyne rather faster than elimination from the E isomer, because in the E isomer the C-H and C-Br bonds are syn-periplanar. 

The next step is the loss of bromide ion in an elimination reaction. This is the step that is difficult to believe as the intermediate we are proposing looks impossible. The orbitals are bad for the elimination too—it is a syn- rather than an anti-periplanar elimination. But it happens. 

There is another, complementary version of the Peterson reaction that uses base to promote the elimination. The starting materials are the same as for the acid-promoted Peterson reaction. When base (such as sodium hydride or potassium hydride) is added, the hydroxyl group is deprotonated, and the oxyanion attacks the silicon atom intramolecularly. Elimination takes place this time via a syn-periplanar transition state—it has to because the oxygen and the silicon are now bonded together, and it is the strength of this bond that drives the elimination forward. 

In Chapter 19 you saw that anti-periplanar transition states are usually preferred for elimination reactions because this alignment provides the best opportunity for good overlap between the orbitals involved. Syn-periplanar transition states can, however, also lead to elimination—and this particular case should remind you of the Wittig reaction (Chapter 14) with a four-membered cyclic intermediate. 

We concluded in Problem 11.61 that E2 elimination in compounds of this bicyclic structure occurs with syn periplanar geometry. In compound A, -H and -Cl can be eliminated via the syn-periplanar route. Since neither syn nor anti periplanar elimination is possible for B, elimination occurs by a slower, El route. 

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