Ortho allylation

You have already seen that a carbon-heteroatom bond is easy to make, since we used such bonds as natural places for disconnections (frames 234 ft). It is good strategy therefore to make a carbon-heteroatom bond and then to transform it into a carbon-earbon bond. The Claisen rearrangement is one way to do this an ortho allyl phenol (B) made from an allyl ether (A) ... 

The rearrangement of an O-allyl ether (LXXV) to form the isomeric ortho allyl phenol (LXXVI)... 

The facile electrocyclic ring closure of ortho-quinone methides provides a convenient strategy for the synthesis of 277-chromenes. Oxidation of ortho-allylic phenols using DDQ or potassium dichromate is a popular method for the formation of 277-chromenes, via ortho-quinone methide intermediates, and is discussed in detail in the preceding volumes (Scheme 6) <1984CHEC, 1996CHEC-II>. 

The palladium(n) catalyzed cyclization of ortho-allylic phenols can lead to a mixture of benzopyran and benzofuran products. The palladium(ll) catalyzed cyclization of ortho-allylic phenols, which bear a variety of substituents on both the aryl ring and the allyl group, in the presence of KHCO3 in DMSO-water (9 1) affords 2//-chromenes exclusively (Equation 15) <1998SL522>. Lower yields of 2//-chromenes are obtained when electron-withdrawing groups are... 

Iodine-mediated removal of both prenyl protecting groups from aryl ether 506 leads to the unexpected formation of 3-(3-iodo-2,2-dimethylchroman-6-yl)propan-l-ol. The reaction proceeds via formation of the ortho-allylic phenol intermediate 507 by electrophilic removal of a prenyl species, followed by an iodine promoted electrophilic cycliza-tion (Scheme 111) <2001SL1989, 2002T5689>. 

The original sigmatropic rearrange-J ment occurred when an aryl allyl ether was heated without solvent and ail ortho-allyl phenol resulted. 

Proton NMR studies show that this ji-benzyl compound (and methyl-substituted derivatives) is fluxional with the Pd equivalent populating both ortho-allyl positions. However, a 3,4-dimethyl analog did show a temperature-dependent NMR spectrum indicating a preferred conformation at low temperature . 

A similar carbonyl addition with benzaldehyde derivatives having an ortho-allylic ether led to a benzopyranone when treated with potassium hexamethyldisilazide. ° These reactions are not successful when the alkene contains electron-withdrawing groups, such as halo or carbonyl groups. A free-radical initiator is required, usually peroxides or UV light. The mechanism is illustrated for aldehydes but is similar for the other compounds ... 

Claisen rearrangement The thermal rearrangement of allylarylethers to ortho-allyl-phenols. If both ortho positions are occupied, then the allyl group migrates to the para position. 

Trinquier, G., Paillous, N., Lattes, A., Malrieu, J. P., Ground and Excited state Conformations of an ortho Allyl Aniline and Their Photochemical Implications a PCILO Study, New J. Chem. 1977, 1,403 411. 

Contrary to the above results, it has been reported that Pd(dba)2 was effective for the cyclization of ortho-allylic phenols. In the presence of Pd(dba)2, the 6-membered 2-W-l-benzopyran was the sole product while Pd(OAc)2 predominately gave mixture of 5-, and 6-membered adducts. In addition, the choice of base was crucial for the formation of the desired benzop)iran. It was observed that carbonate bases gave exclusively the 6-membered... 

The original sigmatropic rearrangement occurred when an aryl allyl ether was heated without solvent and an ortho-allyl phenol resulted. This is the Claisen rearrangement. The first step in this reaction is a pericyclic reaction of a type that you will learn to call a [3,3]-sigmatropic rearrangement. 

Other experiments have corroborated these results. It is appropriate to reiterate the results of Hayashi s study on the stereochemistry of the cychzation of the ortho-allyl phenol that was presented in Chapter 9. As shown in Scheme 16.32, the reaction with t7-ans-3-deuterio cyclohexenylphenol forms deuterated products, and the formation of these products demonstrates that the C-0 bond formation occurs by migratory insertion. As noted in Chapter 9, migratory insertion to form a cis ring juncture places the palladium trans to deuterium, and this disposition of the deuterium and the palladium leads to P-hydrogen elimination instead of p-deuterium elimination. 

The nitrogen analog of the simple Claisen rearrangement with N-allylaniUne and related derivatives is called the amino-Claisen rearrangement (or aza-Claisen rearrangement) [55]. Under thermal conditions, more drastic conditions (310-340 °C) compared with that for the oxygen substrates is required to obtain the ortho allyl aniline 59 [56]. N-Allyl aniline derivatives are, however, not stable at such... 

The )5-methyl substituent on the allyl moiety in the substrate of the aromatic Claisen rearrangement accelerates the formation of the coumaran derivative, often observed as a byproduct arising from the initially formed ortho allyl phenol. 

This sequential cyclization is strongly dependent on the nature of the solvent used [67]. As shown in the reaction of methallyl phenyl ether 17, the formation of coumaran 43 was accelerated in phenolic solvent. On the other hand, N,N-dimethyl-aniline dramatically depressed the isomerization of double bond and cyclizahon and it was the best solvent to obtain the ortho allyl phenol derivahve 83. 

Claisen rearrangement of Merrifield resin derivatized with O-allyl phenolic ethers into ortho-allylic phenols under microwave irradiation was performed by Kumar et al. [ 135]. The reactions were carried out in a household microwave oven, in which samples were irradiated in an open Erlenmeyer flask for 4-6 min (600 W) to afford products in 84-92% yield. In comparison, under thermal conditions, similar yields were obtained after 10-16 h at 140 °C (Fig. 39). 

Fig.39.Claisen rearrangement of Merrifield resin derivatized with O-allyl phenolic ethers into ortho-allylic phenols under microwave conditions. Reprinted from (2000) Synlett 1129 [135] with permission...

Arylsulfinyl groups have been reported to effect regiospecific, nucleophilic ortho-allylation of pyrroles and pyrazoles by the intermediacy of allylsulfonium salts that... 

Potassium phenolates react with allylic halides in the presence of catalytic amounts of zinc chloride to afford ortho- allylated phenols. The less hindered allylic phenol is always formed, via allylic rearrangement where necessary (Scheme 22), and the reaction fails only with highly electron-deficient phenolates or hindered allylic halides. 

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