A 4,4'-bipyridine-based catalyst system for diboration of pyrazine derivatives was established. The catalyst cycle consists of the following two steps: (1) reductive addition of the boron-boron bond of bis(pinacolato)diboron to 4,4'-bipyridine to form N,N'-diboryl-4,4'-bipyridinylidene and (2) oxidative boryl transfer from the intermediate to pyrazine to give N,N'-diboryl-1,4-dihydropyrazine with regeneration of 4,4'-bipyridine.

An organocatalytic silaboration of terminal alkynes and allenes was established using pyridine-based catalysts. In the presence of 4-cyanopyridine (1-2 mol%), alkyl propiolates underwent regio-and stereoselective addition of silylboronic esters in toluene at 135°C to afford (Z)-3-boryl-2-silylacrylates in good yields. 2,6-Dichloro-4,4'bipyridine and 4-(3,5-dichlorophenyl)pyridine also exhibited high catalyst efficiency for the 1,2-silaboration of ethyl propiolate, whereas 1,1-silaboration was induced by P(n-Bu) 3 , t-BuOK, and ICy to afford ethyl 3-boryl-3-silylacrylates as Z/E mixtures. The silaboration of ethynylbenzenes and terminal allenes was also catalyzed by 4-cyanopyridine to afford (Z)-β-boryl-α-silylstyrenes and β-borylallylsilanes in a regioselective manner.

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...

The mechanism of 4,4′-bipyridine-catalyzed diboration of pyrazines was studied by experimental observation of the intermediates and by theoretical calculations. Intermediary radical species were detected by ESR measurement of the reactions of 2,6-dichloro-4,4′-bipyridines with bis(pinacolato)diboron and identified by simulation. Based on these observations, radical processes involving 4,4′-bipyridine-stabilized boryl radicals were evaluated by DFT calculations combined with single-component artificial force induced reaction (SC-AFIR). The results of calculations indicate that a radical transfer process from 4,4′-bipyridine-stabilized boryl radical to pyrazine is a major pathway in the catalytic reaction. The origin of the high catalytic efficiency of 2,6-dichloro-4,4′-bipyridine is ascribed to the effect of the chlorine atom on the stability of the corresponding N,N′-diboryl-4,4′-bipyridinylidene.

Organocatalytic Diboration Involving "Reductive Addition" of a Boron-Boron -Bond to 4,4'-Bipyridine. -Starting from 4,4'-bipyridines conditions for a scalable reductive two-fold addition of bis(pinacolato)diboron are developed. The obtained N,N'-diboryl-4,4'-bipyridinylidenes are identified to be intermediates in the diboration of sterically demanding pyrazines. Hence, the use of 4,4'-bipyridines as catalysts in the diboration of pyrazines is elaborated. -(OHMURA*, T.; MORIMASA, Y.; SUGINOME, M.; J. Am. Chem. Soc. 137 (2015) 8, 2852-2855, http://dx.doi.org/10.1021/jacs.5b00546 ; Dep. Synth. Chem. Biol. Chem., Grad. Sch. Eng., Kyoto Univ., Nishikyo, Kyoto 615, Japan; Eng.) -F. Schill 31-225