Palladium 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]. 

Trifluoromethyl imidates show similar reactivity.262 Imidate rearrangements are catalyzed by palladium salts.263 The mechanism is presumably similar to that for the Cope rearrangement (see p. 555). 

The Cope rearrangement of 24 gives 2,6,10-undecatrienyldimethylamine[28], Sativene (25j[29] and diquinane (26) have been synthesized by applying three different palladium-catalyzed reactions [oxidative cyclization of the 1,5-diene with Pd(OAc)2, intramolecular allylation of a /i-keto ester with allylic carbonate, and oxidation of terminal alkene to methyl ketone] using allyloctadienyl-dimethylamine (24) as a building block[30]. 

The [3,3]-sigmatropic rearrangement of a 1,5-hexanediene is known as the Cope rearrangement and usually proceeds through a chair transition state. Generally, a large substituent at C-3 (or C-A) prefers to adopt an equatorial-like confirmation.303 304 As the reaction is concerted, chirality at C-3 (or C-4) is transferred to the new chiral center at C-l (or C-6). The reaction can be catalyzed by transition metals.305 The use of a palladium catalyst allows for the reaction to be conducted at room temperature instead of extremely high temperatures (Scheme 26. lO).306-307... 

Palladium(O) complexes also catalyse the low-temperature 3-aza-Cope rearrangement of jV-allylenamines in the presence of trifluoroacetic acid as co-catalyst62 (Scheme 15). 

The palladium-catalyzed decarboxylative coupling of allyl 2-(benzo[c(jthiazol-2-yl)acetates 118 provides a facile approach to 2-(but-3-enyl)benzo[c(jthiazoles 122 <07JA4138>. The reaction is initiated by nucleophilic attack of Pd(0) on the allyl ester to give Pd-7t-allyl complex 119, which undergoes nucleophilic attack at the less substituted allylic carbon from the benzothiazole nitrogen to produce 120. Decarboxylative dearomatization leads to intermediate 121, and a subsequent aza-Cope rearrangement driven by rearomatization affords the final product 122 and accounts for the unusual regioselectivity. This appears to be the first report of a tandem allylation/aza-Cope reaction driven by decarboxylative dearomatization/ rearomatization. 

The Cope- and Claisen rearrangements are also catalyzed by square-planar transition metal complexes. Palladium complexes have been responsible for spectacular increases in the rates of Cope rearrangements by factors of up to 10 . Certain other sigmatropic rearrangements are also catalyzed by transition metal complexes. 

The Cope rearrangement takes place with hydrocarbon substrates. Nickel(O) and palladium(II) species were first demonstrated to show opposite effects in the rearrangement, the nickel(O) catalyst being responsible for ring expansion (equation 28) and the palladium(n) complex for ring contraction (equation 29). The reaction shown in equation (29) is catalyzed less effectively by H2[PtCl4]. 

Synthesis of useful 5-oxohexanals can be carried out via the following three reactions (1) 1,2-addition of dlylmagnesium bromide to a, 3-unsaturated aldehydes (61) to give 3-hydroxy-1,5-dienes (62) (2) conversion to 5-hexenals (63) by Ae oxy-Cope rearrangement (3) the palladium-catalyzed oxidation of... 

Photochemical rearrangements have also been reported, as shown in equation (53). " Transition metal catalyzed rearrangement [palladium(O)] of a dienylaziridine has been reported in one case, and a radical opening of a dienylaziridine led to pyrroline formation under the conditions of radical initiation with AIBN/PhsSnH (equation 54). For those vinylaziridines that contain additional unsaturation, the corresponding aza equivalent of a divinylcyclopropane Cope rearrangement is the usual pathwayThe subject of heterodivinylcyclopropane Cope rearrangement is covered in detail elsewhere." The... 

---