1,3-Allylic shift

Reactions of allylic systems that yield products m which double bond migration has occurred are said to have proceeded with allylic rearrangement, or by way of an allylic shift... 

However, IR and NMR spectral data indicated beyond doubt that the product isolated by these authors, even on lowering the temperature of esterification to — 70°, was allyl trichloromethyl sulfoxide (8a) and not allyl trichloromethanesulfenate (7a) as claimed4,0. This observation indicates that the initially formed ester undergoes spontaneous rearrangement to sulfoxide. Similarly, the attempted preparation of a,a-dimethylallyl trichloromethanesulfenate (7b) afforded y, y-dimethylallyl trichloromethyl sulfoxide (8b), thus proving the occurrence of a simultaneous 1,3-allylic shift. 

When the migrating group is allyl, an additional concerted ([2,3] sigmatropic) pathway for rearrangement becomes available. In this an allylic shift must also occur. Nevertheless, the radical pathway is not always excluded. For example, rearrangement of ylids such as 36 (R = CHs.CO) leads to product 37 (R = CH3.CO) in which the allylic protons adjacent to the amido-nitrogen atom appear in emission (D. G. Morris, 1969). No polarization is observed in the much readier... 

Scheme 3 n-allyl shift mechanism for double bond migration... 

Addition products are often derived from the latter compound, although it is the minor component at equilibrium.102 Addition is believed to occur through a cyclic process that leads to an allylic shift. 

Alkylsilanes are not very nucleophilic because there are no high-energy electrons in the sp3-sp3 carbon-silicon bond. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond that is followed by desilylation. Attack on alkenylsilanes takes place at the a-carbon and results in overall replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y-carbon and results in loss of the silicon substituent and an allylic shift of the double bond. 

In unsymmetrical olefines, a mixture of products is obtained probably due to allylic shift in the molecule... 

Some support for the allylic shift pictured above comes from the work of Goetz and Orchin (23) on the isomerization of allyl alcohol to propionaldehyde by DCo(CO)4. These authors found that in the deuterated aldehyde all the D was on the methyl carbon and the following reaction path was suggested ... 

Allylic halides usually give both SN2 products and products of substitution with an allylic shift (SN2 products) although the mixed organocopper reagent RCu— BF3 is reported to give mainly the SN2 product.22 Allylic acetates undergo displacement with an allylic shift (SN2 mechanism).23 The allylic substitution process may involve initial... 

In many allylic oxidations, the double bond is found in a position indicating that an allylic shift occurs during the oxidation. 

The oxypalladation reaction of I takes place partly in the anti-Markovnikov direction (Reaction 9) leading to 2-hexen-l-yl acetate via Reaction 10. Subsequently an allylic shift (19) can generate l-hexen-3-yl acetate. 

There are many variants on these reactions, including the [3,4] shift in the cation 5.87 created by protonating the ketone 5.86. The allylic shift is proved by labelling the carbon marked with an asterisk using deuterium and showing that the deuterium in the product 5.88 is at the benzylic position. 

As reactions 68 and 69 above exemplify, the substitution of the allylsilane usually takes place with an allylic shift (Se2 ). This can be synthetically useful, for example in the isomerization of allyl sulphones 123 to vinyl sulphones 124 (equation 74)145. The reaction is also highly stereospecific (>90% E) (vide infra). 

The reaction of dichlorocarbene with A, A -diethyl-3-methyl-2-butenamine (100) produced A V-diethyl-4-methyl-2-pentenamide (103) as the major product.51 The formation of this material was attributed to the generation of ammonium ylide 101 followed by a [l,2]-allylic shift to give intermediate 102 which then hydrolyzed during workup to produce amide 103. No product resulting from a [2,3]-sigmatropic rearrangement of ylide 101 was detected in the crude reaction mixture. 

Another possibility, involving an allylic shift, exists for the exchange of ligand. It is shown in Reaction 8. 

First, antimony trifluoride is treated with chlorine and thus antimony dichlorotrifluoride is formed. This compound catalyzes intramolecular rearrangement of fluorine with the simultaneous allylic shift of the double bond. As in Explanation 99, the tendency is to accumulate fluorine at the same carbon. The product is 1-chloroperfluoropropene Z [756]. 

The general mechanism for allylic bromination shows that either end of the resonance-stabilized allylic radical can react with bromine to give products. In one of the products, the bromine atom appears in the same position where the hydrogen atom was abstracted. The other product results from reaction at the carbon atom that bears the radical in the second resonance form of the allylic radical. This second compound is said to be the product of an allylic shift. 

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