Anti-Selective Vicinal Silaboration and Diboration of Alkynoates through Phosphine Organocatalysis

Abstract

Trialkylphosphine organocatalysts have enabled anti-selective vicinal silaboration and diboration of the C-C triple bond in alkynoates to produce β-boryl-α-silyl acrylates and α,β-diboryl acrylates, respectively. The anti stereoselectivity was complete and robust. A variety of functional groups were tolerated in the alkynoates. The two vicinally installed heteroatom substituents of the β-boryl-α-silyl acrylates and α,β-diboryl acrylates could be differentiated and transformed in a stepwise manner, allowing the synthesis of a diverse array of unsymmetrical tetrasubstituted alkenes.

Mechanism of NHC-Catalyzed Conjugate Additions of Diboron and Borosilane Reagents to α,β-Unsaturated Carbonyl Compounds

“…Phosphine- 21 and NHC-catalyzed boryl additions to tosylimines and alkynes soon followed. 22 Activation of B–B bonds by an external 23 and then a neighboring Lewis basic alkoxide 24 were later exploited to effect diboron additions to unactivated alkenes, and related transformations involving propargyl alcohols were outlined. 25 Metal alkoxides have been shown to catalyze reactions of silylboron reagents with organohalide substrates, affording the corresponding silanes.…”

Opportune gem‐Silylborylation of Carbonyl Compounds: A Modular and Stereocontrolled Entry to Tetrasubstituted Olefins

“…Next, we applied our methodology to the synthesis of (Z)-Tamoxifen from the gem-silylborylated compound 15. [17] To conclude, we have shown that HC(Bpin) 2 (SiMe 3 )( 1)r epresents an ew olefination reagent, efficiently prepared by insertion of (trimethylsilyl)diazomethane into B 2 pin 2 .T he straightforward access to gem-silylborylated products opens the door to the modulars ynthesis of all carbon tetra-substituted alkenes by silicon or boron-based selective transformations. The required trans Ph group was introduced by Suzuki-Miyaura cross coupling between 41 and iodobenzene to form 42 in 98 %y ield (96:4; E/ Z).…”

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

“…Such a broad scope of diboron reagents is in sharp contrast to the P(nBu) 3 catalyst system, 6 in which only 1 served as a diboration reagent. …”

“…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).…”

“…Therefore, we propose a mechanism based on the reaction pathway involving the nucleophilic conjugate addition of the pyridine-based catalyst to 7 as the initial step (Scheme 4), which is related to the mechanism proposed for P(n-Bu) 3 -catalyzed diboration of alkynoates (Scheme 1b, top). 6 The nucleophilic attack of 9 or 10 on 7 forms A, 16 which reacts with diboron to afford B. The transfer of the boryl group gives ylide C, which is then converted to the cyclic intermediate D.…”