Kinetics and Mechanism of the Gold‐Catalyzed Hydroamination of 1,1‐Dimethylallene with N‐Methylaniline

Abstract

The mechanism of the intermolecular hydroamination of 3-methylbuta-1,2-diene (1) with N-methylaniline (2) catalyzed by (IPr)AuOTf has been studied by employing a combination of kinetic analysis, deuterium labelling studies, and in situ spectral analysis of catalytically active mixtures. The results of these and additional experiments are consistent with a mechanism for hydroamination involving reversible, endergonic displacement of N-methylaniline from [(IPr) Au(NHMePh)] + (4) by allene to form the cationic gold π-C1,C2-allene complex [(IPr)Au(η 2 -H 2 C=C=CMe 2 )] + (I), which is in rapid, endergonic equilibrium with the regioisomeric π-C2,C3-allene complex [(IPr)Au(η 2 -Me 2 C=C=CH 2 )] + (I'). Rapid and reversible outer-sphere addition of 2 to the terminal allene carbon atom of I' to form gold vinyl complex (IPr) Au[C(=CH 2 )CMe 2 NMePh] (II) is superimposed on the slower addition of 2 to the terminal allene carbon atom of I to form gold vinyl complex (IPr)Au[C(=CMe 2 )CH 2 NMePh] (III). Selective protodeauration of III releases N-methyl-N-(3-methylbut-2-en-1-yl)aniline (3 a) with regeneration of 4. At high conversion, gold vinyl complex II is competitively trapped by an (IPr)Au + fragment to form the cationic bis(gold) vinyl complex {[(IPr) Au] 2 [C(=CH 2 )CMe 2 NMePh]} + (6).

Kinetics and Mechanisms of the Gold-Catalyzed Hydroamination of Axially Chiral 1-Aryl-1,2-butadienes with Aniline

“…Most germane to the present study, we have previously studied the kinetics and mechanisms of the gold(I)-catalyzed racemization 9 of axially chiral (R)-1-aryl-1,2-butadienes [(R)-1] and the gold-(I)-catalyzed hydroamination of 3-methyl-1,2-butadiene with N-methyl aniline. 10 The former studies were in accord with a mechanism involving the initial formation of the enantiomeri-cally enriched gold−π-allene complex (R)-I, followed by unimolecular stereomutation of (R)-I via the achiral η 1 -allylic cation II and allene displacement to form the racemic allene rac-1 (Scheme 1). 9 The latter studies supported a mechanism involving the associative, endergonic displacement of N-methyl aniline from [(IPr)Au(NHMePh)] + [IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene] with allene to form the gold−π-allene complex {(IPr)Au[η 2 -H 2 C=C=CMe 2 ]} + (Scheme 2).…”

“…Interestingly, superimposed on the primary reaction manifold was the rapid and reversible, and ultimately nonproductive, outersphere addition of N-methyl aniline to the more substituted allene carbon atom of the gold−π-allene complex (Scheme 2). 10 In contrast to a general understanding of the mechanisms of both allene racemization and allene hydrofunctionalization catalyzed by gold(I) complexes, largely absent is an understanding of the kinetic phenomena that determine the extent of allene racemization, product formation, and chirality transfer in the gold-catalyzed hydrofunctionalization of enantiomerically enriched, axially chiral allenes. Although Toste has reported the mechanistic investigation of the gold-catalyzed hydroamination of a chiral 1,3-disubstituted allene with methyl carbazate, 13 this study did not address the kinetic phenomena that controlled allene racemization and chirality transfer.…”

“…Furthermore, we have previously raised concerns with the conclusions drawn from this investigation owing to ambiguity regarding the catalyst resting state. 10,11,18 For these reasons, we sought to build upon our previous investigations of the goldcatalyzed racemization and hydroamination of allenes to investigate the kinetics and mechanism of the gold(I)-catalyzed hydroamination of the axially chiral 1-aryl-1,2-butadienes (R)-1 with aniline nucleophiles. Herein, we provide a full account of this investigation.…”

Dicoordinate Au(I)–Ethylene Complexes as Hydroamination Catalysts

“… 4 Although these processes can be catalyzed by other transition metals 5 and even through metal-free protocols, 6 gold(I) complexes remain one of the most powerful hydroamination catalysts. 3j , 7 , 8 In fact, they can accomplish the intermolecular hydroamination of C≡C triple bonds 9 and even the more challenging C=C double bonds, 10 , 11 in some cases even for inactivated alkenes. For the latter, the Au(I)-catalyzed hydroamination of ethylene, the simplest alkene, has only been reported once.…”

Au(I) as a π-Lewis Base Catalyst: Controlled Synthesis of Sterically Congested Bis(triflyl)enals from α-Allenols

“…A possible pathway for the formation of bis­[(tri­fluoromethyl)­sulfonyl]­ethylated enals 3 from allenols 1 through gold catalysis is depicted in Scheme . Initially, allenols 1 regioselectively form π-activated allenes 1-Au . − In the presence of betaine 2 (latent source of Tf 2 CCH 2 ), allene-gold complexes 1-Au undergo addition of the proximal allenol double bond toward the highly electrophilic bis­(triflyl)­alkene to give metalated zwitterions INT-I . Noteworthy, Au­(I) should function as a π-Lewis base catalyst rather than a π-Lewis acid to activate the allene in η 2 -coordinated complexes 1-Au , facilitating the unusual addition of a carbon nucleophile of the allenol moiety.…”