(N-Heterocyclic Carbene)₂-Pd(0)-Catalyzed Silaboration of Internal and Terminal Alkynes: Scope and Mechanistic Studies

Abstract

Pd(ITMe) 2 (PhCCPh) acts as a highly reactive precatalyst in the silaboration of terminal and internal alkynes to yield a number of known and novel 1-silyl-2-boryl alkenes. Unprecedented mild reaction temperatures for terminal alkynes, short reaction times, and low catalytic loadings are reported. During mechanistic studies, cis-Pd(ITMe) 2 (SiMe 2 Ph)(Bpin) was directly synthesized by oxidative addition of PhMe 2 SiBpin to Pd-(ITMe) 2 (PhCCPh). This represents a very rare example of a (silyl)(boryl)palladium complex. A plausible catalyst decomposition route was also examined.

Cobalt‐Catalyzed Alkyne Hydrosilylation and Sequential Vinylsilane Hydroboration with Markovnikov Selectivity

“…[1] Alkyne hydrosilylation is the most atom-economic method to access such compounds. [6] Whereas several methods are available for the synthesis of vicinal borosilanes, [7] there are only few approaches towards geminal borosilanes, [8] and methods that provide access to derivatives with at etrasubstituted carbon center are particularly rare. [4] In the early 2000s,the groups of Tr ost [4a,b] and Yamamoto [4c] independently reported the Markovnikov hydrosilylation of terminal alkyl alkynes to form a-vinylsilanes with pentamethylcyclopentadiene ruthenium catalysts.…”

“…[2] One important issue in the hydrosilylation of terminal alkynes is the control of the regio-and stereochemistry because the reaction can generate (E)-b-, (Z)-b-, and a-vinylsilanes.I ndeed, although many catalysts can be used for anti-Markovnikov selective hydrosilylation to form b-vinylsilanes, [3] reactions of alkynes without ad irecting group that proceed with Markovnikov selectivity (a-selectivity) are rare. [6] Whereas several methods are available for the synthesis of vicinal borosilanes, [7] there are only few approaches towards geminal borosilanes, [8] and methods that provide access to derivatives with at etrasubstituted carbon center are particularly rare. Cossy and co-workers showed that Ru alkylidene complexes also catalyze the hydrosilylation of alkyl alkynes to form a-vinylsilanes with ah igh degree of regioselectivity.…”

“…[4,5] Them etal-catalyzed hydroboration of vinylsilanes can generate borosilanes,which are likely to be valuable synthetic intermediates for the construction of complex molecules. [6] Whereas several methods are available for the synthesis of vicinal borosilanes, [7] there are only few approaches towards geminal borosilanes, [8] and methods that provide access to derivatives with at etrasubstituted carbon center are particularly rare. [9] Driven by our interest in developing base-metal catalysts for alkene/alkyne functionalizations, [10,11] we herein report am ethod for the Markovnikov hydrosilylation of terminal alkynes with ap yridinebis(oxazoline) Co catalyst.…”

4,4′-Bipyridine-catalyzed Stereoselective trans-Diboration of Acetylenedicarboxylates to 2,3-Diborylfumarates

“…8 However, even those organocatalytic diborations have been hardly applicable to acetylenedicarboxylates 7, which have been recognized as reactive substrates in related catalytic element element additions such as bis-silylation, 10 silastannation, 11 and silaboration. 12 Recently, we reported the diboration of substituted pyrazines with 1, where 4,4¤-bipyridines are effective catalysts to activate the boronboron σ bond of 1.13 This study triggered the development of diboron-based borylations by pyridine-based catalysts.14 Herein, we describe the first diboration of acetylenedicarboxylates 7 to afford 2,3-diborylfumarates in the presence of 4,4¤-bipyridine as an effective catalyst (Scheme 2).The reaction of dimethyl acetylenedicarboxylate (7a) with 1 was carried out under conditions established for the diboration of other alkynes (Entries 24, Table 1). In the presence of Pt(PPh 3 ) 4 (5 mol %), 7a underwent addition of 1 in DMF at 110°C to give 8a in a low yield as a mixture of stereoisomers (38%, E:Z = 14:86, Entry 2).…”

“…8 However, even those organocatalytic diborations have been hardly applicable to acetylenedicarboxylates 7, which have been recognized as reactive substrates in related catalytic element element additions such as bis-silylation, 10 silastannation, 11 and silaboration. 12 Recently, we reported the diboration of substituted pyrazines with 1, where 4,4¤-bipyridines are effective catalysts to activate the boronboron σ bond of 1.…”