1.5- Dienes, Cope rearrangement

The 5-oxohexanal 27 is prepared by the following three-step procedure (1) 1,2-addition of allylmagnesium bromide to an a, / -unsaturated aldehyde to give the 3-hydroxy-1,5-diene 25, (2) oxy-Cope rearrangement of 25 to give 26, and (3) palladium catalyzed oxidation to afford 27. The method was applied to the synthesis of A -2-octalone (28), which is difficult to prepare by the Robinson annulation[25]. 

The mechanism of the PdCh-catalyzed Cope rearrangement has been studied by use of the partially deuterated 1.5-diene 53[46], The coordination of Pd(II) activates the alkene, and cyclization (carbopalladation) takes place to... 

The most important sigmatropic rearrangements from the synthetic point of view are the [3,3] processes involving carbon-carbon bonds. The thermal rearrangement of 1,5-dienes by [3,3] sigmatropy is called the Cope rearrangement. The reaction establishes equilibrium between the two 1,5-dienes and proceeds in the thermodynamically favored direction. The conversion of 24 to 25 provides an example ... 

The Cope rearrangement of hexa-l,5-diene does not allow for differentiation of starting material and product this is called a degenerate Cope rearrangement. Another example is the automerization of bicyclo[5,l,0]octa-2,5-diene 7 ... 

Occasionally, addition products of 4//-l,2,4-triazole-3,5-diones or diazenedicarboxylic esters and oxepins have been obtained whose formation can be rationalized by an addition to the 2,4-diene system in the oxepin, e.g. formation of 10.190191 In these cases, the primary adduct usually cannot be isolated, because it undergoes a hetero-Cope rearrangement to a tricyclic or bicyclic structure in which the oxepin oxygen has become part of a carbonyl function.190 191,227... 

J 7i-Cycloadducts at the C4 — C5 azepine positions are also formed with 1,2,3,4-tetra-chloro-5,5-dimethoxycyclopentadiene260 261 and with hexachlorocyclopentadiene.261 The reactivity of ethyl l//-azepine-l-carboxylate towards cyclopentadienones has been studied in terms of frontier molecular orbital theory which predicts that dimethyl 2-oxo-4,5-diphenyl-cyclopenta-1 (5),3-diene-1,3-dicarboxylate (20) should be more reactive towards the 1/f-azepine than other more common cyclopcntadienone derivatives.262 In fact, in refluxing benzene, the cyclopentadienone and ethyl 1/f-azepine-l-carboxylate (1) form a mixture of the [4 + 2] n-endo,anti-adduct 22, produced by Cope rearrangement of the initially formed [2 + 4] 7T-adduct 21, and the c.w-adduct 23, a rare example of a [6 + 4] rc-cycloadduct. At room temperature, only the [6 + 4] adduct 23 and a small amount of the [2 + 4] adduct 21 are obtained, the latter rearranging to the [4 + 2] adduct 22 on warming.262 Other [4 + 2] 7r-adducts with cyclopentadienones have been prepared similarly.263... 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

When 1,5-dienes are heated, they isomerize, in a [3,3] sigmatropic rearrangement known as the Cope rearrangement (not to be confused with the Cope elimination reaction, 17-8)When the diene is symmetrical about the 3,4 bond, we have the unusual situation where a reaction gives a product identical with the starting material ... 

Therefore, a Cope rearrangement can be detected only when the diene is not symmetrical about this bond. Any 1,5-diene gives the rearrangement for example,... 

Like [2 + 2] cycloadditions (p. 1082), Cope rearrangements of simple 1,5-dienes can be catalyzed by certain transition metal compounds. For example, the addition of PdCl2(PhCN)2 causes the reaction to take place at room temperature,This can be quite useful synthetically, because of the high temperatures required in the uncatalyzed process. 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

Scheme 10 Diaza-Cope rearrangement of chiral 2,5-diaza-1,5-dienes and homolytic C - C bond cleavage of dimetal piperazides cause loss of stereochemical pimity...

When there is a hydroxy substituent at C(3) of the diene system, the Cope rearrangement product is an enol that is subsequently converted to the corresponding... 

Among the best known examples, involving a carbon moiety, is the shift from one carbon atom to another observed in the Cope rearrangement of 1,5-dienes (49— 50 not to be confused with the... 

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