Aryl O-carbamates

Scheme 30. Nickel-catalyzed cross-coupling of O-aryl carbamates and triflates with Grignards electronic and steric effects.

Scheme 31. Nickel-catalyzed O-aryl carbamate Crignard cross-coupling regiospecific access to 1,2,3- and 2,3-substituted naphthalenes.

Metallation of 0-aryl carbamates. O-Aryl carbamates are metallated by iW-BuLi-TMEDA at the o /to-position. Metallation followed hy reaction with an electrophile offers an attractive route to o-substituted carbamates and phenols. 

Sibi, M. P., Snieckus, V. The directed ortho lithiation of O-aryl carbamates. An anionic equivalent of the Fries rearrangement. J. Org. Chem. 1983,48, 1935-1937. 

Direct ortho lithiation of O-aryl carbamates and 0 to C carbamoyl migration to give salicylamides. 

Anionic Fries Rearrangement. Lithiated O-aryl carbamates constitute one of the most powerful ortho directors in aromatic metallation chemistry. If allowed to warm in heu of quench-... 

To minimize racemization, the use of nonpolar solvents, a minimum of base, low reaction temperatures, and carbamate protective groups (R = O-alkyl or O-aryl) is effective. (A carbamate, R = O-r-Bu, has been reported to form an oxazolone that appears not to racemize during base-catalyzed coupling.) ... 

To minimize racemization, the use of nonpolar solvents, a minimum of base, low-reaction temperatures, and carbamate protective groups (R = O-alkyl or O-aryl) are effective. 

Af,Af-Diisopropylcarbamates generally offer little advantage over their A,A-diethyl analogues, and their removal is even more difficult. O-Aryl A-cumyl carbamates do, however, offer some advantages, and can be hydrolysed to phenols under acid conditions °. 

The procedure described is a good general method3 for obtaining a thiophenol from the respective phenol. It employs three steps conversion of a phenol to the O-aryl dialkylthiocarba-mate by treatment with dialkylthiocarbonyl chloride pyrolysis of the O-aryl dialkylthiocarbamate to the S-aryl dialkylthio-carbamate and hydrolysis of the latter to the aryl mercaptan. 

Rasmussen et al. [205] described the use of resin-bound triaryl bismuth diacetates as starting material for O, N and C arylations [206]. In their approach one of the non-polymer-bound aryl groups is transferred onto imides, amides, carbamates, imidazoles, ahphatic amines, anilines, and phenols to yield the corresponding N- and O-arylated compounds. In the case of p-naphthol the a-C-arylated naph-thol was obtained in 49-68% yield. The authors used 1.5 equiv. of P-naphthol, and... 

Formal replacement of an aromatic hydroxyl group by a thiol group is shown in the preparation of thiophenols by way of O-aryl iV,A-dialkylthio-carbamates, which undergo rearrangement to. the S-esters, whence hydrolysis gives the thiophenols.373 The same reaction occurs in the heterocyclic series. Good yields are obtained in all the steps. 

Ono, K., Fugami, K., Tanaka, S. and Tamaru, Y. (1994) Palladium catalyzed arylation of IV-alkyl O-allylic carbamates synthesis of cinnamyl alcohols via Heck arylation. Tetrahedron Lett., 35, 4133-6. 

Carbamates are also effective nucleophiles for the gold(I)intramolecular hydroamination of alkynes [12]. For example, treatment of O-propargylic carbamate 10 with a catalytic 1 1 mixture of (PPh3)AuCl and AgOTf in dichloromethane at room temperature led to isolation of 2,5-dihydroisoxazole 11 in 88% yield (Eq. (11.8)). The transformation was effective for alkyl-, alkenyl- and aryl-substituted internal alkynes and terminal alkynes and for N-Boc, Cbz, and Ts derivatives. A similar... 

By using the fluoride-induced generation of arynes from o-(silyl)aryl triflates, Larock and Liu have developed the aryne-mediated A-arylation of amines, sulfonamides, and carbamates as well as the O-arylation of phenols and carboxylic acids. Remarkably, a wide variety of functional... 

A thermally induced O r SeAr migration can be used to prepare aryl selenols in three steps from the corresponding phenols. (9-Aryl selenocarbamates rearrange to 5c-aryl carbamates via a four-membered transition state (Scheme 167). ... 

---