3-Phenyl-2-butyl tosylate

Although there is usually a preference for anti elimination in acyclic systems, syn elimination is competitive in some cases. In acyclic systems, the extent of anti versus syn elimination can be determined by use of stereospecifically deuterated substrates or by use of diastereomeric reactants which will give different products by syn and anti elimination. The latter approach showed that elimination from 3-phenyl-2-butyl tosylate is a stereospecific anti process. ... 

Winstein provided powerful support for the phenonium ion hypothesis it 1952. He followed the rate of solvolysis of tAreo-3-phenyl-2-butyl tosylate botl titrametrically, by titrating the toluenesulfonic acid formed, and polarimetrically... 

The concept of an intermediate phenonium ion was, at first, controversial, and its chief detractor was H. C. Brown.25 Although 3-phenyl-2-butyl tosylate showed the stereochemical behavior expected if an intermediate phenonium ion were formed, it did not, in his opinion, show the rate acceleration that should attend anchimeric assistance to ionization of the tosylate.26 Brown said that the stereochemical results could be accounted for by invoking rapidly equilibrating open carbocations (15). According to his explanation, ionization of the tosylate... 

The solvolysis of 3-phenyl-2-butyl tosylate is only half as fast as that of 2-butyl tosylate. However, Winstein suggested that the inductive effect of the phenyl group should retard the rate by a factor of ten and that neighboring-group participation therefore has given a fivefold rate enhancement [see note 14(a), p. 273]. See p. 280 for the actual rate enhancement. 

Predict the products expected from (a) SN1 and (b) Sw2 substitutions of acetic acid on l-threo- and L- r)>Mro-3-phenyl-2-butyl tosylate and compare with the results actually obtained in Section 6.1, p. 275. 

Heating (2.S )-3-mctiiyl-3-phenyl-2-butyl tosylate in edianol leads to skeletal rearrangement and die formation of (3. S ) - 2 - e tho xy - 2 - me thy 1 -3-phenylbutane. What does diis information tell you about the stereoelectronic course of die skeletal rearrangement ... 

An interesting example of this type has been provided by Cram/ who studied the rates of acetolysis of optically active 3-phenyl-2-butyl tosylate in glacial acetic acid ... 

Relatively extensive studies have been performed on the solvolysis of several arenechromium tricarbonyl tosylates (6, 7). Comparison of the solvolysis data of the complexed and noncomplexed optically active threo-and ervfAro-3-phenyl-2-butyl tosylates led the authors to conclude that complexation of these tosylates inhibited phenyl and hydrogen migration, prevented racemization in the threo case, and reduced direct displacement... 

Solvolysis of 3-phenyl-2-butyl tosylate in acetic acid yields the acetate. The tosylate contains two chiral centers, and exists as two racemic modifications so. 

Figure 28.7. Acetolysis of racemic 3-phenyl-2-butyl tosylates.

Problem 28.17 (a) Drawing structures of the kind in Fig. 28.5 (p. 907), show how Cram s mechanism accounts for the conversion of optically active //ireo-3-phenyl 2-butyl tosylate into racemic acetate, (b) In contrast, optically active erythro tosylate yields optically active erythro acetate. Show that this, too, fits Cram s interpretation of the reaction. 

In the controversy that developed, the point under attack was not so much the existence of the intermediate bridged ion—although this was questioned, too— as its mode of formation. The 3-phenyl-2-butyl tosylates undergo solvolysis at much the same rate as does unsubstituted e c-butyl tosylate formolysis a little faster, acetolysis a little slower. Yet, as depicted by Cram, phenyl gives anchimeric assistance to the reaction. Why, then, is there no rate acceleration ... 

Such participation leads to a bridged carbocation with the positive charge delocalized into the aromatic ring. Evidence for this type of participation was first obtained by a study of the stereochemistry of solvolysis of 3-phenyl-2-butyl tosylates. The erythro isomer gave largely retention of configuration, a result that can be explained via a... 

The 7t systems of aromatic rings can act as neighboring groups. The reactions of the 3-phenyl-2-butyl tosylates are good examples. D. J. Cram [1919-2001, awarded the Nobel Prize in Chemistry in 1987 for work on crown ethers (p. 254)] and his co-workers showed that in acetic acid, the optically active 3-phenyl-2-butyl tosy-late shown in Figure 21.23 gives a racemic mixture of the corresponding 3-phenyl-2-butyl acetates. 

FIGURE 21.23 This optically active isomer of 3-phenyl-2-butyl tosylate gives a racemic mixmre of the corresponding acetates. 

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