Gold carbenoids

A mechanism proposed for the skeletal rearrangement of enynes involved the presence of gold carbenes [161]. This proposed mechanism was supported by the capture of intermediate gold carbenoids trapped by reactive alkenes in intermolecu-lar cyclopropanation reactions [162]. 

The solvent effect on the regioselectivity of the AuCls-catalyzed cycloisomerization of a bromoallenyl ketone was evaluated by density functional theory calculations. Upon the generation of the gold carbenoid intermediate from cyclization of the aUene precursor, the tetrahydrofiiran solvent can act as a proton shuttle to assist the 1,2-H migration to afford the 2-bromofuran product (14S2149). 

A variety of N-(furan-3-ylmethylene)benzenesulfonamides were obtained by a gold (1)-catalyzed cascade reaction from easily accessible starting materials. The reaction pathway involved a rarely observed 1,2-alkynyl m jation onto a gold carbenoid (14CEJ14868). 

Recently, Chen and coworkers reported an efficient new method to construct a series of densely functionalized 4,5-dihydro-IH-azepine products 41 from the intermolecular reaction of alkyl azides with propargylic esters. In this approach, sequential reaction of vinyl-gold carbenoids 42 with allqrl azides and formation of vinyl imine intermediates 43 may be involved fScheme 4.14). Then a subsequent formal [4+3] cycloaddition with another molecule of vinyl-gold carbenoid 42 afford the desired azepine 41. 

This effect was first described by Toste and coworkers who developed a gold (I)-catalyzed oxidative rearrangement of enynes based on the trapping of the gold carbenoid intermediate by diphenylsulfoxide (see Scheme 9.17) [36]. 

Scheme 16.18 Examples of transformations involving gold carbenoid intermediates.

A gold-catalysed transformation of a-(2-indolyl) propargylic alcohols with imines in the presence of the oxidant 8-isopropylquinoline A-oxide gives dihydro-y-carbolines by intermolecular trapping of an a-carbonyl gold carbenoid intermediate, followed by cyclization and a gold-assisted 1,2-acyl migration (Scheme 129). ... 

As previously stated with Pt catalysts, the cycloisomerization involves an intramolecular cyclopropanation to give a cyclopropyl gold-carbenoid species, which is followed by a 1,2-// shift. Interestingly, in the presence of a quaternary center on the propargylic position in 55, a 1,2-alkyl shift is observed, allowing a... 

Scheme 11.10 Generation of gold carbenoid via internal redox of homopropargylic sulfoxides.

Yeom et al. demonstrated that the gold-catalyzed redox cyclization reaction of (Z)-isomer of o-alkynylaryl ketoximes (Z)-95 produced isoindole 96 (Scheme 27.34) [43]. The reaction proceeds through N—O bond redox reaction to form gold carbenoid intermediate 98 and subsequent cyclization and elimination of the gold catalyst, furnishing the isoindole 96. In contrast, reaction of the -isomer (E)-95 afforded isoqunoline oxide 100 [44],... 

An efficient catalytic C-H functionalization by means of gold carbenoids for the one-step synthesis of 3-acyloxindole derivatives 50 has been explored by Zhang and Qian in 2012 (Scheme 12.24) [28]. 3-Acyloxindole derivatives were prepared in good yields by a gold-catalyzed tandem oxidation/C-H functionalization reaction of electron-deficient Af-arylpropiolamides 49 using 2-bromopydine... 

Gold carbenoids are frequently invoked intermediates of catalytic cycles. Their rendition as gold-stabilized earbocations or gold carbenes has been debated [eqn (4.7)]. On the basis of experimental and computational studies, it now seems that the earbene depiction is irrelevant, but the discussion is not closed. ... 

Scheme 4.32 Au-catalyzed cyclopropanation of cfs-stilbene 115 with cyclopropene 114 via gold carbenoids of type 117.

Figure 4.14 Calculated bond distances (A) for L-gold carbenoids.

The authors rationalized the selectivity on the basis of electronic effects. With the n-acidic phosphite, Au-C backdonation is limited thus, the gold carbenoid reacts as a carbocation. With the strongly o-donating NHC, Au-C backdonation is more efficient and the gold carbenoid behaves as a carbene. Other theoretical studies on [(NHC)Au]-stabilized carbocations, including non-classical ones, have been reported, yet no specific role of the NHC was highlighted. ... 

The generation and reactivity of a-oxo gold carbenoids by the action of gold(I) catalyst in the presence of aromatic A-oxides have been detailed. ... 

The gold(I)-catalysed oxidative rearrangement of propargyl alcohols has been reported as a selective route to 1,3-diketones (Scheme 123). a,j0-Unsaturated carbonyl compounds have been reported to be formed from gold-catalysed oxidative rearrangement of homopropargylic ether with a gold carbenoid and oxonium ylide proposed as intermediates (Scheme 124). ... 

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