Unexpected Generation of Diastereomers by Double Diboration of a Dialkyne

Abstract

During the last two decades, boron compounds have become established as versatile reagents in organic synthesis, and different methods have been developed for their preparation. Along with hydroboration, [1][2][3][4] transition-metalcatalyzed diboration [5] has become useful for the functionalization of unsaturated organic compounds. [6,7] This reaction is widely applicable to substrates such as alkynes [8,9] arynes, [10] alkenes, [11][12][13] carbenoids, [14,15] as well as diazo [16] and carbonyl compounds. [17][18][19][20] Although bis(catecholato)diborane(4) and bis(pinacolato)diborane(4) represent by far the most commonly employed diborane(4) species, the reaction is not restricted to their use. For instance, several diboranes(4) with interesting properties are derived from [2]borametalloarenophanes. The first example of this class of organometallic compound, the [2]boraferrocenophane 1, was prepared by Herberhold, Wrackmeyer and co-worker, [21] while the molecular structure along with an alternative synthesis was published by our group.[22] Subsequently, a series of [2]borametalloarenophanes has appeared. [23-28b] These ansa complexes show tilted molecular structures, thus featuring significant molecular strain. In 2006, we successfully exploited this ring strain in oxidative addition reactions of [2]borametalloarenophanes to low-valent platinum centers, affording[3]diboraplatinametalloarenophanes. In addition, we demonstrated the suitability of [2]borametalloarenophanes as diboranes(4) in the platinum-mediated diboration of alkynes to form [4]diboradicarbametalloarenophanes.[29] By expansion of this strategy, we also achieved the diboration of diazo compounds [16] and isocyanides.[30]Herein, we report on the two diastereomeric [4]diboradicarbaferrocenophanes 4 and 5, which were obtained by double diboration of the conjugated dialkyne 3 either by a platinum-mediated route employing the [2]boraferrocenophane 1 in the presence of [PtA C H T U N G T R E N N U N G (PEt 3 ) 3 ], or stoichiometrically by using the [3]diboraplatinaferrocenophane 2, directly (Scheme 1). In contrast to related transformations of monoalkynes, [29] monitoring the reaction progress by multinuclear NMR spectroscopy revealed the gradual disappearance of the starting materials, and the formation of two distinct compounds in benzene solution at elevated temperatures. Both species feature similar resonances in the 1 H, 11 B, and 13 C NMR spectra, which we rationalized by the generation of diastereomeric species. Complexes 4 and 5 were eventually isolated after workup as orange, moderately moisture-sensitive powders. The NMR spectroscopic data of both diastereomers are fully consistent with the presence of doubly bis(borylated), conjugated triene systems bearing two ferro-

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

“…The B–B bond in [2]-borametalloarenophanes has been cleaved by addition to alkynes using palladium and platinum catalysts, inserting the carbon–carbon triple bond into the B–B bridge. − Braunschweig and co-workers were also able to isolate heterobimetallic compounds resulting from oxidative addition of the B–B bond to a platinum(0) compound (Scheme , right). ,,, Reaction with B 2 pin 2 or B 2 cat 2 resulted in boryl exchange and loss of the B–B bridge (Scheme , left) . Additionally, these complexes were found to be active catalysts in ethylene polymerization and ethylene/1-hexene copolymerization. , …”

[n]Borametalloarenophanes (n = 1, 2): Strained Systems with Uncommon Reactivity Patterns

“…Recently, the diboration chemistry of [ n ]boraferrocenophanes 33 and 48 has been successfully expanded to a variety of differently substituted dialkynes with varying electronic and steric properties 42,43. Dialkynes studied are depicted in Scheme and include common derivatives RCC–CCR ( 59 : R = Me; 60 R = Ph) and spacer‐bridged species (SiMe 3 )CC–X–CC(SiMe 3 ) ( 61 : X = E ‐C 2 H 2 ; 62 : X = 1,4‐C 6 H 4 ).…”

“…(ii) Single diboration reactions with additional coordination of the pendant alkyne C–C triple bond to a [Pt(PEt 3 ) 2 ] complex fragment resulted in the formation of dinuclear complexes [(3,4‐η 2 ‐Fe{η 5 ‐C 5 H 4 –B(NMe 2 )} 2 –{R}C=C–CCR)Pt(PEt 3 ) 2 ] ( 67 and 68 ), if [3]diboraplatinaferrocenophane 48 was treated with spacer‐free dialkynes 59 and 60 under stoichiometric reaction conditions 43. (iii) Double diboration was eventually achieved for the sterically less congested dialkynes 61 and 62 applying both stoichiometric and homogeneous catalysis conditions 42,43. Reactions were conducted employing 2 equivalents of either 33 or 48 and enabled the isolation of dinuclear bis[4]diboradicarbaferrocenophanes of the type [1,4‐(Fe{η 5 ‐C 5 H 4 –B(NMe 2 )} 2 –{Me 3 Si}C=C) 2 X] ( 69 and 70 ).…”

“…The Gibbs energies of activation for these isomerisation processes were determined to Δ G # = 102.5 kJ mol –1 ( 69 ) and Δ G # = 72.7 kJ mol –1 ( 70 ) by means of kinetic NMR studies. A plausible isomerization pathway for the interconversion of the diastereomers 69 was eventually proposed on the basis of DFT calculations 42,43…”

Ansa‐Complexes of [Mn(η5‐C5H5)(η6‐C6H6)]: Preparation, Characterization, and Reactivity of [n]Manganoarenophanes (n=1, 2, 3)

“…[18] Reactivity studies on these metalloarenophanes revealed their potential for further transformations, such as ringopeningp olymerization [7,[10][11][12] to produce metallopolymers or the functionalization of unsaturated organic substrates. [19][20][21][22][23][24][25][26][27][28] Herein, we focus on the manganese-containing complex [Mn(h 5 -C 5 H 5 )(h 6 -C 6 H 6 )],w hichi si soelectronic to ferrocene. Alongside [Cr(h 5 -C 5 H 5 )(h 6 -C 6 H 6 )] it represents ar are example of as ystem with ab enzene and ac yclopentadienyl ligand that has successfullyb een employed for the preparation of ansacomplexes.…”