Substitution of Benzylic Hydroxyl Groups with Vinyl Moieties Using Vinylboron Dihalides

Haloboration: scope, mechanism and utility

Haloboration, the addition of B–X (X = Cl, Br, I) across an unsaturated moiety e.g., C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Y or C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Y (Y = C, N, etc.), is dramatically less utilised than the ubiquitous hydroboration reaction. However, haloboration of alkynes in particular is a useful tool to access ambiphilic 1,2-disubstituted alkenes. The stereochemical outcome of the reaction is easily controlled and the resulting products have proven to be valuable building blocks in organic synthesis and materials chemistry. This review aims at providing the reader with a brief summary of the historic development and of the current mechanistic understanding of this transformation. Recent developments are discussed and select examples demonstrating the use of haloboration products are given with a focus on the major areas, specifically, natural product synthesis and the development of boron-doped polycyclic aromatic hydrocarbons (B-PAHs).

Haloboration is a mostly overlooked technique, yet it is a powerful way of transforming alkynes stereoselectively into difunctionalised ambiphilic alkenes, which readily undergo a plethora of highly useful subsequent reactions.

Enantioselective addition of boronates to o-quinone methides catalyzed by chiral biphenols

Chiral biphenols were found to catalyze the enantioselective asymmetric addition of aryl- or alkenylboronates to o-quinone methides. Substituted 2-styryl phenols were obtained in good yields (up to 95%) with high enantiomeric ratios (up to 98:2) in the presence of 10 mol % 3,3'-Br(2)-BINOL. A two-step synthesis of (S)-4-methoxydalbergione in good yield and selectivity was achieved.

Arylethyne Bromoboration-Negishi Coupling Route to E- or Z-Aryl-Substituted Trisubstituted Alkenes of ≥98% IsomericPurity. New Horizon in the Highly Selective Synthesis of Trisubstituted Alkenes

The hitherto unprecedented palladium-catalyzed cross-coupling of (Z)-β-bromo-β-arylethenylboranes can be made to proceed satisfactorily through (1) the use of highly catalytically active bis(tri-tert-butylphosphine)palladium or dichloro[N,N-bis-(2,6-diisopropylphenyl)imidazol-2-yl](m-chloropyridine)palladium and (2) conversion of dibromoboryl group to (pinacol)boryl group. Thus, a wide variety of carbon groups can be used to substitute bromine in ≥98% stereo- and regioselectivity, while suppressing the otherwise dominant β-debromoboration. Together with the alkylethyne-based protocols, the alkyne bromoboration-Negishi coupling tandem process has emerged as the most widely applicable and highly selective route to trisubstituted alkenes including those that are otherwise difficult to access.

Highly regio- and stereoselective synthesis of (Z)-trisubstituted alkenes via propyne bromoboration and tandem Pd-catalyzed cross-coupling

Contrary to all previous reports, bromoboration of propyne with BBr(3) proceeds in >or=98% syn-selectivity to produce (Z)-2-bromo-1-propenyldibromoborane (1). Although 1 is readily prone to stereoisomerization, it can be converted to the pinacolboronate (2) of >or=98% isomeric purity by treatment with pinacol, which may then be subjected to Negishi coupling to give trisubstituted (Z)-alkenylpinacolboronates (3) containing various R groups in 73-90% yields. Iodinolysis of 3 affords alkenyl iodides (4) in 80-90% yields. All alkenes isolated and identified are >or=98% Z.

Generation of Cations from Alkoxides: Allylation of Propargyl Alcohols

The reaction of alkoxides with boron trichloride results in the generation of cations in the absence of Brønsted acids. The absence of a Brønsted acid can make a difference in subsequent transformations such as allylation reactions.

Substitution of Hydroxyl Groups with Alkynyl Moieties Using Alkynylboron Dihalides: An Efficient Approach to Secondary Alkylacetylene Derivatives

The reaction of alkynylboron dihalides with benzylic, allylic, and propargylic alcohols provides an efficient route to internal acetylenes. Isomerization of the product alkynes does not occur under the reaction conditions.