Gold -catalyzed carbamate

Michael additions to conjugated carbonyls can be catalyzed by gold species. Among them, arene additions are the most studied area but other nucleophiles can attack the gold-coordinated enones as well. In fact, the intermolecular aza-Michael additions of carbamates to enones was reported in 2002 with both Au(I) and Au(III) salts, and in 2007 an intramolecular aUcoxide and amide conjugate addition has been developed and applied to the synthesis of (+)-andrachcinidine (equation 132). In the latter case, the enones are formed as intermediates in a previous gold-catalyzed step that is the hydration of an alkyne and methanol loss. Then the cyclization takes place to give piperidines. 

Examples of gold-catalyzed carbonylation of amines and olefins exist related to the activation of carbonyl. The first case involves the formation of carbamates from anilines and CO in the presence of alcohols or the production of acetamides from aliphatic amines. In these examples, Au(I) (usually [AuCl(PPh3)]) catalysts are preferred. For the carbonylation of olefins, gold(I) carbonyls are prepared in situ in sulfiu ic acid media to afford carboxyhc acids. ... 

The exo-selective gold-catalyzed cyclization of alkenyl carbamates, amides, or ammonium salts affords the corresponding pyrrolidine or piperidine derivatives. Here, cationic gold(I) catalysts bearing sterically hindered, electron-rich phosphine ligands such as P(r-Bu)2(o-biphenyl) were much more reactive than PhsPAuOTf... 

Representative procedure for the gold-catalyzed intramolecular hydroamination of alkenyl carbamates. Benzyl 2-methyl-4,4-diphenylpyrrolidine-I-carboxylat ... 

Gold-catalyzed 8-endo-dig cyclization, (see Refs. [118, 133] in Chap. 1) hydrogenation of the alkene and formation of the methyl carbamate to give indoloazocine III-40. 

After successful application of the silver catalyst shown in olefin aziridination (Section 6.1.1), He and coworkers showed that intramolecular amidation was possible with both hydrocarbon-tethered carbamates and sulfamate esters.24 They found that only the Bu3tpy silver complex could catalyze efficient intramolecular amidation, while other pyridine ligands gave either dramatically lower yields or complicated product mixtures. In an interesting control study, both copper and gold were also tested in this reaction. Both the copper and gold Bu tpy complexes can mediate olefin aziridination, but only silver can catalyze intramolecular C-H amidation, indicating that the silver catalyst forms a more reactive metal nitrene intermediate. 

Various transition metals catalyze the aza-Michael addition of carbamates to enones. Among the most efficient are gold(I) and gold(III) salts (Scheme 4-66). The catalyst of the stoichiometry AuCl3-2H20 used by the authors is probably tetrachloroauric acid hydrate (HAuCU X H2O). In the corresponding reaction of... 

Widenhoefer and coworkers have reported that the gold(I) phosphine complex [P(t-Bu)2(o-biphenyl)]AuCl is a highly active and selective precatalyst for the intramolecular exo-hydroamination of N-y- and 8-allenyl carbamates [37]. As an example, treatment of the N-6-allenyl carbamate 48 with a catalytic 1 1 mixture of [P(t-Bu)20-biphenyl]AuCl and AgOTf (5mol%) in dioxane at room temperature for 22 h led to isolation of piperidine 49 in 92% yield as a 7.0 1 mixture of cis trans diasteromers (Eq. (11.26)). Gold(I)-catalyzed hydroamination of N-y- and 8-allenyl carbamates tolerated substitution at both the internal and terminal allenyl carbon atoms and the transformation displayed modest selectivity for the transfer of chirality... 

Yamamoto has reported the intramolecular e%o-hydroamination of N-allenyl sulfonamides and carbamates catalyzed by simple, unligated gold(I) and gold(III) salts. Noteworthy was that cyclization of N-allenyl sulfonamide derivatives that possessed an axially chiral allenyl moiety occurred with highly selective transfer of chirality to the newly formed tetrahedral stereogenic carbon atom [38]. For example, treatment of enantiomerically enriched y-allenyl tosylamide 52 (96% ee) with a catalytic amount of AuCl in THF at room temperature led to isolation of ( )-2-(l-heptenyl)pyrroldine ( )-53 in 99% yield with 94% ee (Eq. (11.28)). 

In contrast to the hydroamination of alkenes with sulfonamides, the potential of an acid-catalyzed reaction pathway in the hydroamination of alkenes with carboxamide derivatives appears less likely. Hartwig found that the intramolecular hydroamination of alkenes with N-arylcarboxamides was only realized in the presence of stoichiometric amounts of triflic add [50]. In contrast, He reported that triflic add catalyzes the intramolecular hydroamination of an N-4-methyl-4-pentenyl carbamate in toluene at 85 °C [55]. However, in the corresponding gold(I)-catalyzed transformation, the intramolecular hydroamination of an N-4-methyl-4-pentenyl carbamate was markedly slower than was the intramolecular hydroamination of an N-4-pentenyl carbamate [52], which is inconsistent with the antidpated behavior of an acid-catalyzed pathway. Furthermore, control experiments firmly ruled out the presence of an acid-catalyzed reaction pathway in the gold(I)-catalyzed intramolecular hydroamination of alkenes with carboxamide derivates and ureas [53, 54]. 

In the hydroamination of unsaturated carbon-carbon bonds, gold catalysts play an important role. Intermolecular hydroamination of alkenes [177], 1,3-dienes [204], terminal and internal alkynes [205], and allenes [206] are known to proceed smoothly in the presence of PhsP AufI) or AuCls catalyst. In addition, amino olefins also efficiently undergo intramolecular hydroamination using similar gold catalysts. He and coworkers have developed the catalytic cycloaddition of tosylated amino olefins [207], A representative example is shown in Scheme 18.35. When N-tosylated y-amino olefin (97) is exposed to a mixture of PhsP AuCl and AgOTf (5 mol% each) in toluene at 85 °C, pyrrolidine (98) is obtained in 96% yield. The gold(I)-catalyzed intramolecular hydroamination is applicable to N-alkenyl carbamates [208], N-alkenyl carboxamides [209], and N-alkenyl ureas [210], The use of microwave irradiation results in completing the hydroamination in a much shorter time than that required under thermal reaction conditions [211], The... 

Zhang Z, Bender CF, Widenhoefer RA. Gold(I)-catalyzed enantioselective hydroamination of A-aUenyl carbamates. Org. Lett. 2007 9(15) 2887-2889. 

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