Phenols oxypalladation

Phenolic oxygen participates in facile oxypalladation. The intramolecular reaction of 2-hydroxychalcone (105) produces the flavone 106[127]. The ben-zofuran 107 is formed from 2-allyIphenol by exo cyclization with Pd(OAc)2, but benzopyran 108 is obtained by endo cyclization with PdChf S], Normal cyclization takes place to form the furan 109 from 2-(l-phenylethenyl)phe-nol[129]. Benzofuran formation by this method has been utilized in the synthesis of aklavinione (110)[130]. 

The isoflavone 406 is prepared by the indirect a-phenylation of a ketone by reaction of phenylmercury(II) chloride with the enol acetate 405, prepared from 4-chromanone[371]. A simple synthesis of pterocarpin (409) has been achieved based on the oxypalladation of the oriho-mercurated phenol derivative 408 with the cyclic alkene 407[372,373]. 

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

Early mechanistic studies have indicated that the oxypalladation step in the Wacker process proceeds through an <37z/z-pathway,399 although recent deuterium-labeling experiments have shown the viability of a yy/z-mechanism involving insertion of a metal-coordinated oxygen into the alkene.400,401 For example, with excess chloride ion present, the Wacker-type cyclization of a deuterated phenol system occurred in a primarily //-pathway, whereas the oxypalladation step favored a yy/z-mode in the absence of excess chloride ion (Scheme 16). Thus, either mechanism may be operative under a given set of experimental conditions. 

An interesting variant involves the use of an allylic alcohol as the alkene component. In this process, re-oxidation of the catalyst is unnecessary since the cyclization occurs with /Uoxygen elimination of the incipient cr-Pd species to effect an SN2 type of ring closure. Both five- and six-membered oxacycles have been prepared in this fashion using enol, hemiacetal, and aliphatic alcohol nucleophiles.439,440 With a chiral allylic alcohol substrate, the initial 7r-complexation may be directed by the hydroxyl group,441 as demonstrated by the diastereoselective cyclization used in the synthesis of (—)-laulimalide (Equation (120)).442 Note that the oxypalladation takes place with syn-selectivity, in analogy with the cyclization of phenol nucleophiles (1vide supra). 

The phenolic oxygen on 2-allyl-4-bromophenol (7) readily underwent oxypalladation using a catalytic amount of PdCl2 and three equivalents of Cu(OAc)2, to give the corresponding benzofuran 8. This process, akin to the Wacker oxidation, was catalytic in terms of palladium, and Cu(OAc)2 served as oxidant [17]. Benzofuran 10, a key intermediate in Kishi s total synthesis of aklavinone [18], was synthesized via the oxidative cyclization of phenol 9 using stoichiometric amounts of a Pd(II) salt. 

A phenolic oxygen participates in facile oxypalladation. 2-Allylphenol (55) undergoes clean cyclization to 2H-l-benzopyran (56) in DMSO under air with a catalytic amount of Pd(OAc)2 without a reoxidant. 2-Methylbenzofuran (57) is obtained when PdCl2 is used [50]. But different chemoselectivity with the Pd(II) salts was also reported [51]. Catalytic asymmetric cyclization of the tetrasubstituted 2-allylic phenol 58 using the binaphthyl-type chiral ligand 60, called (.S, .S )-ip-borax, afforded the furan 59 with 96% ee. Use of Pd(CF3 002)2 as a catalyst is essential in the presence of benzoquinone [52]. Formation of the benzofuran 62 from 61 has been utilized in the synthesis of aklavinione [53]. The intramolecular reaction of 2-hydroxychalcone (63) produces the flavone 64 [54]. 

Consider the first oxidative cyclization shown to produce A. It was proposed originally that the cyclization proceeded through if-allyl-Pd intermediate B. Treatment of C, however, which was predicted to form B under the same reaction conditions, led to A in less than 5% yield, even after 21 hours of reaction time. The researchers involved were forced to come up with another mechanism. Propose an alternate catalytic pathway that involves complexation of the phenolic oxygen and side chain C=C bond to Pd and then oxypalladation of the C=C bond.213... 

A phenolic oxygen participates in facile oxypalladation [16], Different chemos-electivity is observed depending on the catalytic species. The 2H-l-benzopyran 66 was formed cleanly from 2-allylphenol (65) by endo cyclization with Pd2(dba)3 or... 

B. INTRAMOLECULAR OXYPALLADATION OF ALKENES CONTAINING PHENOLIC GROUP... 

---