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

Abstract

Acetylenedicarboxylates undergo trans-addition of tetraalkoxydiboron in THF at 60°C in the presence of 4,4¤-bipyridine (5 mol %) as a catalyst to give 2,3-diborylfumarates in good yields with high stereoselectivity.Keywords: Organocatalyst | Diboration | Organoboron compoundsCatalytic diboration of alkynes with diboron reagents such as bis(pinacolato)diboron (1) provides stereoselective accesses to diborylalkenes.1 Since the first report by Ishiyama, Miyaura, and co-workers in 1993, 2a transition metal catalysts have been used for stereoselective cis-1,2-diboration of a broad range of terminal and internal alkynes.2,3 A recent report on cobaltcatalyzed 1,1-diboration makes diboration-based organic transformations more valuable. 4 However, it is interesting to note that the transition-metal-catalyzed diboration has never been applied to alkynes in which the carboncarbon triple bond is conjugated with carbonyl groups, except for nonstereoselective diboration of alkynoates 2a and 2b using heterogeneous gold catalysts (Scheme 1a). 3a,3b,5 This particular limitation of transition metal catalysts has been overcome by transition-metal-free catalysts. 69Sawamura and co-workers have reported P(n-Bu) 3 -catalyzed diboration of 3-aryl-and 3-alkylpropiolates 2, leading to the stereoselective formation of (E)-α,β-diborylacrylates (E)-3 through trans-1,2-addition of 1 (Scheme 1b, top).6 Santos and co-workers have also reported trans-1,2-diboration of alkynoic amides 4, which is promoted by NaH/15-crown-5 (Scheme 1b, bottom).7 The 1,1-diboration of propiolates has also been accomplished using a catalytic amount of t-BuOLi. 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). No reaction took place when the reaction was carried out either with P(n-Bu) 3 (5 mol %) in THF at 80°C 6 or t-BuOLi (5 mol %) in CH 3 CN at 60°C 8 (Entries 3 and 4), although these conditions have been effective for the diboration of alkynoates (Schemes 1b and 1c). ICy, which was used as a catalyst for 1,4-addition of 1 to ethyl crotonate, 15 did not promote the diboration of 7a (Entry 5).We...

Inter‐Element Boration Reactions of Carbon‐Carbon Multiple Bonds via Lewis‐Basic Activation of Boron Reagents

“…A related diboration reaction of acetylenedicarboxylates was reported by Suginome and Ohmura [32] . Pyridine‐based catalysts are uniquely effective with this class of unsaturated substrates and dimethyl acetylenedicarboxylate was diborated in 88% yield with high trans ‐selectivity with the aid of 4,4’‐bipyridine (Scheme 13).…”

Pyridine‐Based Organocatalysts for Regioselective syn‐1,2‐Silaboration of Terminal Alkynes and Allenes

“…Cl‐substituted 4,4′‐bipyridines 5 c and 5 e (entries 4 and 6) showed higher catalyst efficiency than Me‐substituted 5 b and 5 d (entries 3 and 5). In sharp contrast, no catalyst activity was observed with unsubstituted 4,4′‐bipyridine ( 5 a ) (entry 2), although it showed high catalyst performance in the diboration of acetylenedicarboxylates . The reaction was also promoted by 4‐arylpyridines 6 a – 6 c , where Cl‐substituted 6 c showed high catalyst performance (entries 6–8).…”

“…We have recently reported on transition‐metal‐free catalytic addition reactions of diboron reagents to substituted pyrazines and acetylenedicarboxylates, where the B−B bond of diboron is activated efficiently by 4,4′‐bipyridines used as catalyst. Based on our findings and on the related borylation by pyridine‐based catalysts, we envisioned that pyridines, including 4,4′‐bipyridines, would also be efficient catalysts for activation of the B−Si bond of silylboronic esters.…”

Mechanistic study on the reaction of pinB-BMes2with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent

“… 5 Two seminal reactions, alkoxide-catalyzed diboration of alkenes 6 and phosphine-catalyzed β-boration of electron-deficient alkenes, 7 induced the next evolution of the diboration of alkynes using a Lewis base as a catalyst. 8,9 These reactions involve the formation of sp 2 –sp 3 intermediates via the coordination of the Lewis base to the diborane(4) reagent. 10 Apart from the development of the base-catalyzed diboration of alkynes, two electrophilic diboration reactions of alkynes were recently reported ( Scheme 2 ).…”