Cope rearrangement transfer

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

Keywords Aza-Claisen rearrangement 3-Aza-Cope rearrangement Chirality transfer Asymmetric induction Charge acceleration... 

Characteristics and energetics of the photo-induced electron-transfer degenerate Cope rearrangement of 2,5-diarylhexa-l,5-dienes have been reported in detail. Oxygentrapping experiments have established the two-step nature of the rearrangement of... 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

There appears to be much interest in the mechanism of various pericyclic transfer-mations, particularly of the Cope rearrangement. A pair of interacting allyl radicals, an aromatic species, or a 1,4-cyclohexanediyl diradical are the possible intermediates and transition states for the rearrangement represented here as resonance hybrids in the transformation of 1,5-hexadiene (Scheme 4.16). Two high-order theoretical studies indicate that the Cope rearrangement is concerted and proceeds via an aromatic chair transition state (33).362,364... 

The fluorescence of DCA is also quenched efficiently by 2,5-diphenyl-l,5-hexadiene with a nearly diffusion-limited rate constant in MeCN (1.1 x 10lodm3 mol-1 s ), since the photoinduced electron transfer from the diene ( ° = 1.70 V vs. SCE) to DCA (E ed = 1.91 V vs. SCE) [170] is exergonic [184], The photoinduced electron transfer induces Cope rearrangement of the diene via the cyclohexane-1,4-radical cation intermediate. In... 

The allyl-transfer reaction based on 2-oxonia Cope rearrangement allows highly stereocontrolled chirality transfer. Triflic acid has been shown to induce the rearrangement of the 251 allyl sterols into 22-homoallylic sterols with high stereoselectivity without side reactions86 [Eq. (5.316)]. The protocol, however, is not effective for syn substrates (for example, 251, R = H, R = COOEt). 

Cope rearrangement (10, 31). The Pd(II)-catalyzed Cope rearrangement of chiral 1,5-dienes occurs with virtually complete 1,4-transfer of chirality.1 Example ... 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

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... 

An interesting example of the transfer of center chirality to helicity is the work by Ogawa et al., based on an asymmetric aromatic oxy-Cope rearrangement to provide nonracemic [5]helicenes (Fig. 15.8) [75]. The starting material with center chirality, bicyclo[2,2,2]ketone (-)-21 (>98% ee), was obtained by enzymatic resolution. In the annelation step, the phenanthrene derivative was subjected to aromatic oxy-Cope rearrangement, to afford a pentacyclic product in 47 % yield. The corresponding [5]helicene 22 was obtained in 7 % overall yield (> 98 % ee) after six steps. 

A sub case in this category is the benzylation of enolates through the Srn 1 process. In this way, both a-nitro- and a-chloro-4-nitrocumene are alkylated by the enolates of 2-nitropropane, diethylmalonate, or diethyl 2-butylmalonate. A particular case of benzylic C-C fragmentation is the electron transfer photosensitized Cope rearrangement of 2,5-diphenyl-1,5-hexadienes [218,219]. 

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

TCNE takes place (Scheme 57). Tomioka reported the (3 + 2) photocycloaddition between 1,1,2-triarylcyclopropanes and vinyl ethers in the presence of p-DCB [162]. Mizuno and Otsuji reported the (4 -l- 2) photocycloaddition between 1,2-diarylcyclopropanes and DCA [23]. The 1,4-radical cation produced as an intermediate of the Cope rearrangement of 1,5-dienes via photoinduced electron transfer can be trapped by molecular dioxygen, giving bicyclic dioxanes (Scheme 58) [163]. This photooxygenation takes place in a stereospecific manner. 

The transposition of oxygen in allylic esters (R6 = alkyl, phenyl) or carbamates [R6 = N(CH3)2] has the same overall bonding changes as the thermal [3,3] sigmatropic Claisen or Cope rearrangements. Chirality transfer from C-l to C-3 is accompanied by 1,3 oxygen transposition (carbonyl O to allylic O ). 

The observation of high chirality transfer in examples 2-7 in Table 3 is in agreement with the proposed transition state A (see Section 7.6.1.1.), thereby allowing the prediction of the stereochemical outcome of the aza-Cope rearrangements. 

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