Aryloxy carbene

Although mostly alkoxy carbene complexes have been benzannulated, other types of carbene complexes are equally well suited. These include aryloxy carbene complexes as well as acyla-mino and thioalkylidene complexes, and even complexes with no heteroatoms, such as diaryl carbene complexes, are suitable (see below). Besides the commonly used methoxy and ethoxy carbene complexes, alkoxy carbene complexes with a longer alkyl chain have also been successfully reacted. The benzannulation of aryloxy carbene complexes has recently been studied to probe electronic effects [28a]. Aryloxy alkylidene complexes of type 21 have been used to prepare diaryl ethers 22, which constitute a common substructure in many important types of natural products [28b]. The benzannulation methodology provides an access to phenyl naphthyl ethers in yields of 60-93 % under mild conditions (Scheme 11). 

Scheme 11. Benzannulation of aryloxy carbene complexes affording diaryl ethers.

Hemilability of the carbene moiety can be avoided, if a second anchor group is introduced. Examples include two pendant aryloxy [31] or amido groups [32,33] (see Figure 2.5). 

Diazirines (isomeric with diazoaUcanes) give carbenes/ but aryhnethyl radicals have also been generated from diazirines. In a different study, thermolysis of diaryloxydiazirines gave the anticipated carbene products, but photolysis gave both carbenes and aryloxy radicals by a-scission. ... 

The synthesis of the 3-oxo-3,4-dihydro-2H-l-benzpyran ring system 367 in high yields was effected by the decomposition of l-diazo-3-aryloxy-2-propanones 368 in the presence of catalytic amounts of Cu(hfacac)2 (84S268 86T4319). This reaction proceeds as a very selective intermolecular carbene C—H insertion. 

Alkoxy(or -aryloxy)halocarbenes are ambiphilic (for selectivity indices, see ref 9), they react at a high rate with electron-rich and electron-poor alkenes, however, they react at a slow rate, with alkenes of intermediate reactivity. Steric hindrance decreases the rate addition of fluoro- and chloro(phenoxy)carbene to 2,3-dimethylbut-2-ene. ° For discussion of structure-activity relationship of carbenes, see ref 9, and Houben-Weyl, Vol. El9b, pp 14-34. 

Table 37. l-Alkoxy(or -Aryloxy)-l-halocyclopropanes from Carbenes, Generated from Diazirines, and Alkenes... 

Highly functionalized diaryl ethers are accessible from reaction of aryloxy-substi-tuted Fischer carbene complexes with alkynes. ... 

A number of diaryloxydiazirines (4 X = Y = H, MeO, Me, Cl X = H, Y = MeO, Cl X = MeO, Y = Cl) (Scheme 2) have been synthesized and their thermolyses and photolyses investigated in various media.Although the thermolysis of (4) afforded products only from the expected carbenes (6), photolysis gave both carbenes (6) and aryloxy radicals by a-scission. The fragmentation of diaryloxycarbenes (6) to aryloxy radicals appears to be unprecedented and, moreover, thermodynamically improbable, as indicated by computations. The authors suggest, therefore, that the a-scission takes place from the excited state of the diazirine, which they depict as the ring-opened diradical (5). 

In a reaction that parallels the one described by Geoffroy and co-workers for the formation of [CpW(f/ = CPhCPh = CHR)(CO)2][BF4], protonation of (aryloxy) carbynes generates an agostic carbene complex which undergoes insertion of... 

Carbene addition to olefinic linkages to form cyclopropanes also shows stereoselectivity (32). The cis compounds are the principal or exclusive adducts when X = alkyl, aryl, halo, alkylthio, or alkylseleno group, while the trans adducts are formed preferentially when X = trimethylsilyl, alkoxy-carbonyl, or aryloxy. It is clear that carbenes containing a soft substituent X prefer the reaction path leading to products in which X and alkyl R are closer, and the opposite is true when X is hard. 

Yagyu, T., Oya, S., Maeda, M. and Jitsukawa, K. (2006) Syntheses and characterization of palladium(II) complexes with tridentate A-heterocyclic carbene ligands containing aryloxy groups and their application to Heck reaction. Chem. Lett., 35, 154-5. 

---