Gold carbenoid intermediate

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). 

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). ... 

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],... 

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]. 

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. 

In gold catalyzed transformations, the carbenoid intermediates such as 31 arising from a 5-exo-dig cyclization can also be either oxidized in the presence of Ph2SO leading to carbaldehyde 32 or trapped by an alkene to give bis-cyclopropanic derivatives 33 (Scheme 5.12) [55-57]. 

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. ... 

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). ... 

The reaction is thought to operate through a gold(I) carbenoid intermediate that is generated by 1,2-shift of the propargylic ester function, as shown in Fig. 10.52. 

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