Wolff Rearrangement of Oxidatively Generated α‐Oxo Gold Carbenes: An Effective Approach to Silylketenes

Abstract

Gold-catalyzed oxidationso fa lkynes by N-oxides offer direct access to reactive a-oxo gold carbene intermediates from benign and readily available alkynes instead of hazardous diazoc arbonyl compounds.D espite various versatile synthetic methods developed based on this strategy,one of the hallmarks of a-oxo carbene/carbenoid chemistry,t hat is,t he Wolff rearrangement, has not been realized in this context. This study discloses the first examples that show the Wolff rearrangement can be readily realized by a-oxo gold carbenes oxidatively generated from TBS-terminated alkynes (TBS = tert-butyldimethylsilyl). The thus-generated silylketenes can be either isolated pure or subsequently trapped by various internal or external nucleophiles in one pot to afford a-silylated carboxylic acids,t heir derivatives,o rT BS-substituted allenes.The Wolff rearrangement [1] is one of the classical transformations of a-diazo ketones [2] and has been extensively studied and widely employed in organic synthesis (Scheme 1A). [3] Thek etene generated in this powerful reaction is ar eactive intermediate of exceptional synthetic value.T he main drawback of the reaction, however, is the use of diazo ketone substrates,w hich are hazardous and potentially explosive,b ut necessary to access highly reactive a-oxo carbenes/carbenoids or undergo concerted conversion (Scheme 1A).In 2010, [4] we reported that highly electrophilic a-oxo gold carbene intermediates could be generated by intermolecular oxidation of benign alkynes instead of from hazardous diazo ketones (Scheme 1B). [5] Thesafety benefit of this strategy and the new approach to a-oxo gold carbenes and their novel chemistry spurred ad iverse range of subsequent studies by us [4][5][6] and others, [7] and established that the gold carbene intermediates,orinsome cases their precursors,can undergo insertions into O À H, [4,6a] N À H, [7c] B À H, [7j] and even unactivated CÀHb onds [6f,g, 7a,d, 8] and cyclopropanate alkenes, [6d,e, 7g,h] including electron-deficient ones, [6e, 7g] as well as undergo electrophilic arene substitutions [6c, 7e] or Buchner reactions, [6h] and the formation of ylides. [7f, 9] Despite these advances,n o Wolff-type rearrangement involving these oxidatively generated a-oxo gold carbenes has been reported to date.H erein, we disclose af irst example,w hich offers one-step access to stable yet versatile silyl ketenes from readily available TBSterminated alkynes (Scheme 1C).In 2015, we reported that acyl/acyl-substituted gold carbenes generated upon gold-catalyzed oxidation of enones undergo insertions into unactivated C À Hb onds. [6f] In our effort to expand the C À Hinsertion chemistry,weexamined as substrates silyl-terminated alkynes including silylated 3phenyl-1-propyne (1). As shown in Table 1, entry 1, with the silyl group of 1 as TBS,IPrAuNTf 2 (5 mol %) as the catalyst, and 8-isopropylquinoline N-oxide as the oxidant, the goldcatalyzed oxidation occurred smoothly overnight to deliver three identifiable products.A mong them, the silylated indano...

Synthesis, Characterization, and Comparative Theoretical Investigation of Dinitrogen-Bridged Group 6-Gold Heterobimetallic Complexes

“…C 6 D 6 , C 6 D 5 Cl, and THF- d 8 (purchased from Eurisotop) were degassed by freeze–pump–thaw cycles, dried over molecular sieves, and stored in the glovebox. The Mo/W dinitrogen complexes 2 M – 4 M , 28 15 N– 3 W , 28 [Au(NTf 2 )(Me 2 Idipp)] ( 6b NTf 2 ), 29 and B(C 6 F 5 ) 3 ( 1 ) 30 were prepared according to reported procedures and stored in the glovebox. The [Au(Cl)(NHC)] complexes 6a – e Cl, dimethylphenylphosphine (PMe 2 Ph), 1,2-bis(diphenylphosphino)ethane (dppe), 1,2-bis(diethylphosphino)ethane (depe), and NaB(C 6 H 3 -3,5-{CF 3 } 2 ) 4 were purchased from TCI, Sigma-Aldrich, or Strem Chemicals and used as received.…”