Stereoselective Synthesis of γ-Substituted (Z)-Allylic Boranes via Kinetically Controlled Hydroboration of Allenes with 10-TMS-9-borabicyclo[3.3.2]decane

Abstract

The hydroboration of allenes is a potentially useful but relatively undeveloped method for synthesis of allylic boranes. [1][2][3][4][5][6][7] The hydroboration of monosubstituted allenes with di (isopinocampheyl)borane [(Ipc) 2 BH] generally gives (E)-allylic boranes 4 with excellent selectivity. 3,4,8 It is inferred, based on work of Wang who studied the hydroborations of 1-alkyl-1-trimethylsilylallenes with 9-BBN and dicyclohexylborane, 5,6 that the hydroboration of allenes 1 with (Ipc) 2 BH proceeds via thermodynamically controlled isomerization 4,9 of the kinetically formed (Z)-allylic borane 2 by way of the methallylborane isomer 3 (Figure 1). 10,11,12 Thus, allene hydroboration has not proven generally useful 8 for synthesis of the (Z)-allylic boranes 2 owing to the facile 1,3-isomerization of 2 that leads to 4 (Figure 1). 4,9,13,14 In connection with an ongoing research problem, we needed to develop a stereocontrolled synthesis of (Z)-allylic borane 2 [R = B(OR') 2 ] and explored the hydroboration of allene 1 [R = B(OR') 2 ] with (Ipc) 2 BH. Because attempts to suppress the 1,3-allylic isomerization of 2 [R = B(OR') 2 ] by performing the hydroboration of 1 with (Ipc) 2 BH even at -78 °C in the presence of transition metal catalysts 15 were unsuccessful, 16 we sought to identify a chiral dialkylborane that would hydroborate allenes 1 with high (Z)-stereoselectivity but without competitive 1,3-isomerization of the initially formed (Z)-allylic borane 2. Based on the unusual thermal isomeric stability of (E)-and (Z)-crotyl-10-TMS-9-borabicyclo[3.3.2]decane reagents, 17,18 we targeted the use of and report herein that 10-TMS-9-borabicyclo[3.3.2]decane 7 19,20 is highly effective as a reagent for kinetically controlled allene hydroboration leading to (Z)-allylic boranes 2. 10 Importantly, of the Soderquist borane 7 has enabled us to synthesize several (Z)-allylboranes that are inaccessible by alternative synthetic methods. 1 Non-racemic borane 7 [10-TMS-9-BBD-H] is easily prepared from pseudoephedrine complex 5 by using Soderquist's procedure (Scheme 1). 19,20 Both enantiomers of 5 are commercially available but also are easily prepared in two steps from B-OMe-9-BBN. 17 We found, however, that generation of 7 from ate complex 6 is best performed in the presence of the allene, owing to the instability of 7. 21 Conditions for the kinetically controlled allene hydroboration were optimized by using 8 as the substrate; stereoselectivity was assessed by subjecting the derived allylboranes to an allylboration-oxidation sequence 3 with benzaldehyde (Table 1). A 1:1 mixture of syn-1,2-diol 11a and the 1,2-anti diastereomer 12a was obtained when the hydroboration of 8 was performed at 20 °C for 2 h (entry 1). Decreasing the hydroboration temperature from 20 °C to 0 °C led to a significant improvement in the reaction diastereoselectivity (87 : 13 favoring 11a, entry 2). The optimal balance between product yield and diastereoselectivity was obtained by performing the allene hydroboration at -10 °C for 5 h (75% yield, 11a:...

Cobalt‐Catalyzed Regio‐ and Stereoselective Hydroboration of Allenes

“…However, compared with the well‐established catalytic hydroboration of alkenes, alkynes, and 1,3‐dienes, the hydroboration of allenes has remained underdeveloped . In 2009, a one‐pot two‐step reaction to form homoallylic alcohols involving non‐catalytic hydroboration of alkyl‐substituted terminal allenes with a chiral borane reagent was reported, however, the stereoselectivity is rather poor (Scheme a) . In 2013, Tsuji and co‐workers reported a copper‐catalyzed hydroboration of mono‐substituted terminal allenes to form linear E ‐allylic boronates as the major products, although the E / Z selectivity is subject to the steric effect of the R group (Scheme b) .…”

Enantioconvergent Hydroboration of a Racemic Allene: Enantioselective Synthesis of (E)-δ-Stannyl-anti-homoallylic Alcohols via Aldehyde Crotylboration

“…The face selectivity of this step is consistent with the known enantioselectivity of hydroboration of ( Z )-olefins by ( d Ipc) 2 BH, 8b,14 as well as by the preference of allene hydroboration to occur anti to bulky substituents at the distal position. 5, 15h, 15i The hydroboration of ( P )- 1 by ( d Ipc) 2 BH is thus stereochemically matched. The resulting allylborane ( R ) d - Z - 7 can undergo a stereochemically controlled, stereospecific, suprafacial 1,3-boratropic shift, 15 to give ( S ) d - E - 3 .…”

A Versatile Bis‐Allylboron Reagent for the Stereoselective Synthesis of Chiral Diols

“…[8][9][10][11][12][13][14] Several types of 1,2-, 1,4-, and 1,5-diols can thus be selectively obtained. By varying the electronic environment of the boron atom, these reagents allow the intermediate product, derived after the first addition, to be used in other types of C-heteroatom or C À Cb ond-forming transformations,s uch as the addition of as econd aldehyde.R eagents bearing a1 ,2-and 1,3-diboron relationship are designed to decouple the first allyl transfer event from the second.…”