Improved Protocols for Molybdenum‐ und Tungsten‐Catalyzed Hydrostannations

Abstract

A series of (isonitrile)tungsten carbonyl complexes of type W(CO) m A C H T U N G T R E N N U N G (CNR) n has been synthesized and evaluated as hydrostannation catalysts. The results are compared with those obtained by the previously reported tri(tert-butylisonitrile)molybdenum tricarbonyl catalyst, Mo(CO) 3 A C H T U N G T R E N N U N G (CN-t-Bu) 3 (= MoBI 3 ). The yields and selectivities strongly depend on the isonitriles used, and with certain substrates better results are obtained compared to the molybdenum catalyst. No side products are observed in hydrostannations under microwave irradiation or when the reactions are carried out under an atmosphere of carbon monoxide. Based on these findings, a mechanistic rational is given, explaining the different pathways responsible for the formation of hydrostannation or distannation products.

Regioselective Formation of (E)‐β‐Vinylstannanes with a Topologically Controlled Molybdenum‐Based Alkyne Hydrostannation Catalyst

“…[3,4,12,14,15] Previously,wereported the six-coordinate complex MoI 2 -(CO) 2 (CNAr Dipp2 ) 2 (1;A r Dipp2 = 2,6-(2,6-(i-Pr) 2 C 6 H 3 ) 2 C 6 H 3 ; Figure 1), [17] which features two sterically encumbering m-terphenyl isocyanide ligands in a trans orientation. [6,[21][22][23][24][25] Herein, we show that the more sterically encumbered complex 1 also serves as ah ighly efficient alkyne hydrostannation catalyst precursor.H owever, owing to its increased steric profile, 1 catalyzes the regioselective formation of (E)-b-vinylstannanes for aw ide range of terminal and unsymmetrically substituted internal alkynes without the need for sterically biased substrates.Precatalyst 1 can be readily activated under mild conditions to aw ell-defined and stabilized 14 e À zero-valent active species,inwhich the rigid and encumbering trans topology of the m-terphenyl isocyanide ligands is maintained. [6,[21][22][23][24][25] Indeed, Mo(CN-t-Bu) 3 (CO) 3 possesses relatively unencumbered ligands,a nd correspondingly,i ts use as an alkyne hydrostannation precatalyst results in excellent regioselectivity for a-vinylstannanes.…”

“…Accordingly,w es ought to develop an encumbered alkyne hydrostannation system where an overall octahedral metal coordination geometry is conserved throughout the hydrometalation process.Such amodification would allow for two additional axial ligands to exert mutually cis steric pressure on a C-alkenyl unit in am anner not achievable with systems that generate square-planar intermediates (Scheme 1, bottom). [6,[21][22][23][24][25] Indeed, Mo(CN-t-Bu) 3 (CO) 3 possesses relatively unencumbered ligands,a nd correspondingly,i ts use as an alkyne hydrostannation precatalyst results in excellent regioselectivity for a-vinylstannanes. [18][19][20] Notably,d iiodide 1 is related to the six-coordinate cis-Mo(C-alkenyl)(SnR 3 )L 4 intermediates proposed by Kazmaier and co-workers for alkyne hydrostannation with the zerovalent tris-isocyanide tricarbonyl catalyst precursor Mo(CN-t-Bu) 3 (CO) 3 ).…”

“…Accordingly,a t2 .0 mol %l oading, complex 1 is effective for the hydrostannation (30). [6,24] Fore xample,u nder these conditions, 1 converts phenylacetylene into the corresponding (E)b-vinylstannane with 13:87 a/(E)-b regioselectivity after 20 min in C 6 D 6 solution (entry 1). [6,24] Fore xample,u nder these conditions, 1 converts phenylacetylene into the corresponding (E)b-vinylstannane with 13:87 a/(E)-b regioselectivity after 20 min in C 6 D 6 solution (entry 1).…”

Insights into the Reaction Behaviour of Stannylated Allylic Substrates

“…The stannylated phenyl ether 1g could be obtained either through hydrostannation of phenylpropargyl ether [9] or by allylic alkylation. Pd(PPh 3 ) 4 (6 mg, 5.0 μmol, 2 mol-%) was dissolved in dry DMF (2 mL) in a Schlenk flask and the mixture was stirred under nitrogen at room temperature for 15 min.…”

“…Best results are obtained under CO, under which the formation of side products can be suppressed almost completely. [9] With these building blocks to hand, we have been able to show that the proposed allylic alkylation/Stille approach can be applied successfully in amino acid and peptide chemistry. With chelated glycine enolates the corresponding stannylated allyl glycines are formed.…”