Alkenylboronic Esters Research Articles

Rhodium catalysed enantioselective hydroboration of alkenylboronic esters with catecholborane

Alkenylboronic esters such as ( E)-2-(2-phenylethenyl)-1,3,2-dioxaborolane were subjected to catalytic hydroboration with catecholborane and with use of neutral and cationic rhodium complexes modified by various diphosphine ligands. The resulting 1,2-diboryl intermediate was oxidised with alkaline hydrogen peroxide to give the corresponding 1,2-diol with enantioselectivities up to 79 % e.e.

Synthesis and Reactions of Enediynes and Enyne‐Allenes

AbstractThe thermal cycloaromatization reactions of (Z)‐3‐hexene‐1,5‐diynes (enediynes) and (Z)‐1,2,4‐heptatrien‐6‐ynes (enyne‐allenes) provide easy entries to a variety of carbon biradicals. Several new synthetic routes to these highly unsaturated compounds were developed by using multifunctional reagents properly substituted with combinations of boron, silicon, and tin appendages. Condensation of γ‐(trialkylsilyl)allenylboranes 1 and 2 with conjugated acetylenic and allenic aldehydes followed by the elimination step of the Peterson olefination reaction furnished enediynes and enyne‐allenes with high geometric purity. Convenient procedures for the synthesis of enediynes and enyne‐allenes were also developed by using alkenylboronic ester 28 and the trimethyltin‐substituted alkenylboranes 34 for cross‐coupling reactions. On heating, acyclic enyne‐allene 22 underwent a sequence of intramolecular transformations through biradical intermediates to form 26, providing a new example of a one‐step 0 → ABCD ring construction of the tetracyclic‐steroidal skeleton.

Synthesis of a new class of bidentate Lewis acids based on boronic esters and zirconocene

Hydrozirconation of various alkenylboronic esters 1 with H(Cl)ZrCp 2 produces a new class of bidentate Lewis acids as crystalline solids or oils, 2, in 81-96% yield. Products 2 were characterized by IR, NMR, and in selected cases elemental analysis.

A convenient procedure for the synthesis of enyne-allenes

Bromoboration of 1-hexyne with BBr 3 followed by treatment with 2-propanol produced alkenyl boronic ester 3. Subsequent Pd(0)-catalyzed cross-coupling with acetylenic zinc chlorides and iodination furnished enyne iodides 6, which were then converted to enyne-allenes 8 by a second Pd(0)-catalyzed reaction with allenic zinc chlorides.

Reactions of N-halosuccinimides with 1,1-bimetallics based on zirconocene and boronic esters: A new synthesis of α-haloboronic esters

Rapid hydrozirconation of alkenyl boronic esters by zirconocene hydrochloride, followed by addition of N-halosuccinimides leads to the corresponding α-halo boronic esters in high yields.

The use of alkenylboronate esters in [2+2] enone-olefin photocycloadditions

The participation of alkenylboronate esters in intermolecular [2+2] enone-olefin photocycloadditions has been demonstrated using the E-1-hexenyl, Z-3-hexenyl, and Z-2-butenyl derivatives. The major photoadducts are those which result from head-to-head addition. Oxidative fragmentation of the boron-substituted cyclobutanes could be accomplished under relatively mild conditions.

A novel route to 1,3-dienyl-2-boronic esters providing simple syntheses of conjugated [E,E]-dienes and conjugated [E]-alkenones.

Abstract A highly stereo- and regioselective synthesis of [ E ]-2-(1′-[ E ]-alkenylsubstituted-1′-alkenyl)-l,3,2-dioxaborinanes has been achieved for the first time via the nucleophilic substitution of [ Z ]-(1-bromo-1- alkenyl)boronic esters with [ E ]-l-lithio-l-alkenes. These are novel boron intermediates, which are converted into conjugated [ E,E ]-dienes and conjugated [ E ]-alkenones in good yields and in excellent stereochemical purities.