Gold allene complex

Hydroamination of Allenes Different related amines can also be cyclized. The use of free amino groups led to long reaction times (several days), but sulfonamides, acetyl or BOc as protecting group led to fast conversion (in the latter case, problems of diastereoselectivity were observed). Optimization studies showed that, although cationic gold (I) complexes were not effective for these conversions, AuCI was a very good catalyst for these reactions. 

Similar to the abovementioned silver nhc coordination compounds, carbene chemistry has also been dominant in the field of gold organometallic chemistry. Noteworthy examples include a Au(PPh3)-compound derived from tetraaminoallene, that can be rationalised in terms of a dicarbene with ylide character and which, owing to the electron-rich character of the central carbon atom, offers the potential for dimetallation products.108 Non-activated allenes and alkynes have been found by Lavallo to be readily aminated by cationic carbene gold complexes.109 For this purpose, a 2,6-diisopropylphenyl functionalized cyclic alkylaminocarbene gold(I) complex... 

Catalytic conversions of allenes are sometimes considered models for catalytic reactions of alkenes, even though allene reactivity is more closely comparable to that of alkynes rather than alkenes. The catalytic hydration of allenes was achieved by means of a cationic gold(I) complex with a carbene steering ligand, (IPr)AuCl/ AgOTf (5 mol%), in dioxane (rt, 4—9 h) in fair yield [180]. Attack of water is selective for the terminal carbons, whereas regioselectivity in nonsymmetric substrates is controlled by steric, electronic, and solvation factors. 

Other cyclizations of allenols have been realized by means of gold-catalysts [186, 187], gold and platinum catalysts [188], and with lanthanum amide catalysts [189]. Intermolecular additions of alcohols to allenes were also catalyzed by cationic gold(I) complexes with carbene [190] or phosphane spectator ligands... 

For alkynes (and in part, allenes), synthetically useful protocols for Markovnikov and anti-Markovnikov selective hydrations, hydroalkoxylations (mainly intramolecular), and hydrocarboxylations are available and find increasing applications in organic synthesis. In the past decade, the research focus on cationic gold(l) complexes has led to new additions to the catalysis toolbox. It can be predicted that a further refining of such tools for alkyne functionalization with respect to catalytic activity and functional group tolerance will take place. 

Intermolecular hydroarylations of terminal allenes with highly nucleophilic methoxyarenes are catalyzed by cationic phosphite gold(l) complexes. The addition takes place regioselectively at the unsubstituted allene terminus and affords ( )-allylated benzene derivatives with moderate-to-high yields. 

Pre-catalyst I and related gold phosphoramidite complexes also promote enantioselective intramolecular [4+2]-cycloadditions of allenic dienes/" This transformation was previously studied by Toste et who obtained the Diels-... 

Toste has described the intramolecular enantioselective hydroamination of y- and 6-aUenyl sulfonamides catalyzed by enantiomerically pure bis(gold) phosphine complexes [42]. For example, treatment of the terminally-disubstituted y-allenyl sulfonamide 59 with a catalytic amount of [(R)-3,5-xylyl-binap](AuOPNB)2 (OPNB =p-nitrobenzoate) formed protected pyrrolidine 60 in 88% yield with 98% ee (Eq. (11.34)). Likewise, treatment of 6-allenyl sulfonamide 61 with a catalytic amount of [(JJ)-Cl-MeObiphep](AuOPNB)2 in nitromethane at 50 °C for 24h formed 2-alkenyl piperidine 62 in 70% isolated yield with 98% ee (Eq. (11.35)). Realization of high enantioselectivity in this protocol required employment of both a terminally disubstituted allene and a sulfonamide nucleophile. 

An important breakthrough was made between 1991 and 2000 when Utimoto et al. and Teles et al. first reported that alkynes could be functionalized by an inter- or intramolecular addition of water, alcohols, and amines in the presence of a gold(III) salt or a eationic gold(I) complex (Scheme 16.2, Eqs. 1-3) [4]. This significant advance was followed by the work of Hashmi et al. [5] who demonstrated in 2000 that AUCI3 could be used as an efficient catalyst for the intramolecular addition of ketones and alcohols to alkynes and for the inter- or intramolecular arylation of alkynes and allenes (Scheme 16.2, Eqs. 4 and 5). 

Indenes can be synthesized via intramolecular hydroarylation of propargyl acetates catalyzed by NHC gold(I) complexes [127, 128]. The formation of the indenes proceeds via a 1,3-shift to form an allene, followed by the hydroarylation (Scheme 1.8). Notably, this reaction requires strictly anhydrous conditions, because conjugated enones and enals are isolated in the presence of water [129]. Related transformations include reaction with propagyl sulphides or dithioacetals. 

Cationic gold(I) complexes favor the formation of six- and seven-membered rings by 6-endo-dig, 6-exo-dig, and 1-exo-dig cyclization. However, indoloazo-cines V are selectively obtained with AuCls via %-endo-dig cyclization. Internal alkynes are also active in the intramolecular process leading to allenes VI and tetracyclic compounds VII (Scheme 1.10). In Scheme 1.10, the proposed mechanism for the formation of the different products is shown. Nucleophilic attack of the indole on the activated alkyne affords intermediate VIII, which arises from a 1,2-shift of the initially formed seven-membered ring iminium cation. Proton loss from VIII forms azocine V, while protonation of intermediate VIII leads to an open intermediate IX, which rearranges to the final allene VI or the tetracyclic compound VII via Michael-type addition of the XH group in intermediate X. 

A variety of other cyclization reactions are also observed with many of the carbon cumulenes. Especially, allenes and ketenes undergo many of these reactions and gold catalysis has achieved a new dimension in selectivity. From bis-allenes, complex natural products, such as 18,19 norsteroids, are generated in one step. 

In 2010, Malacria and coworkers developed a novel Au-catalyzed cyclization of 1,6-enyne 9, furnishing allene-substituted tetrahydrofuran 11 in excellent yield. The proposed mechanism involves an initial enyne cyclization to generate the cationic gold-vinyl complex 10 and subsequent unexpected [l,5]-hydride shift. Deuteration experiments support the [l,5]-hydride shift and rule out the inter-molecular scrambling. This cascade reaction is unusual because the cyclization precedes followed by an uncommon [l,5]-hydride shift (Scheme 12.5) [9]. 

Platinum(II) chloride-catalysed reaction of allenes R R C=C=CHR with N-protected indoles (In) in THF with added MeOH proceeds at 70 C over 20 h and affords products of geminal bisindolylation, that is, R R CH-CH2C(3-In)2R. By contrast, gold(I) complexes catalyse monoindolylation, giving rise to allyl indoles... 

Mechanistic investigation of the Au -catalysed SiO)"-exo-trig hydroalkoxylation of allene (240) revealed a rapid and reversible C-0 bond formation to generate (241), followed by the turnover-limiting protodeauration producing the vinyl tetrahydrofuran (242). This pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex (243). ... 

Among the wide variety of transformations catalyzed by gold(I), the most fundamental transformations have centered on the activation of alkynes, allenes, and alkenes with gold(I) complexes [1-13]. In particular, cationic complexes of gold(I)... 

The cationic gold complex with CAAC ligand 29 can also catalyze the unprecedented hydroamination reaction of alkynes and allenes using ammonia [59, 60]. It was also demonstrated that it can catalyze the simple hydroamination reaction. It... 

At the end of 2007, Widenhoefer et al. reported the first examples of the dynamic kinetic enantioselective hydroamination of axially chiral allenes, catalyzed by a dinuclear complex of gold (Figure 8.1) and silver perchlorate [46, 47]. 

Figure 8.1 Gold complex used as catalyst for the hydroamination of axially chiral allenes.

Reactions by Other Nucleophiles As in the case of the formal cycloadditions of alkenes to allyl cations, the addition of alkenes to gold(I)-activated allenes generates intermediates that determine which cycloaduct formed. Based on this hypothesis, Toste et al. recently developed enantiorich bicycle-[3.2.0] structures by [2+2]-cycloaddition reaction catalyzed by chiral biarylphosphinegold(I) complexes [51]. 

As pointed out earlier, propargylic esters coordinate to gold to form complexes that can undergo 1,2- or 1,3-acyl migrations to form a-acyloxy-o ,jS-unsaturated carbenes or allene-gold complexes (equation 23). ... 

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