Palladium‐Catalyzed Synthesis and Isolation of Functionalized Allylboronic Acids: Selective, Direct Allylboration of Ketones

Abstract

Allyl boronates are very important reagents in advanced organic synthesis for the allylation of carbonyl compounds [1] and in cross-coupling [2] reactions. A practically unrivalled property of allylboronates is their highly regio-and stereoselective addition to carbonyl compounds to afford homoallylic alcohols. [1a-d, 3] This high selectivity is mainly based on two factors: 1) Allylboronates are configurationally stable, and unlike many allyl metal compounds [4] (such as allylGrignard reagents, allyl lithium compounds or allylboranes) do not undergo metallotropic rearrangement and E/Z isomerization at ambient conditions. 2) The six-membered ring transition state (TS) of the allylboration of carbonyl compounds is highly conformationally constrained, which leads to a strong differentiation in the stereoselection process. [3a, 5] Method development for broadening the number of accessible functionalized allyl boronates is one of the most important challenges in organoboron chemistry.In allylboration reactions with allylboronic esters, allyl pinacolborate (allyl-Bpin) reagents are most frequently applied as the allyl source, [1a-d] as allyl-Bpins can be easily handled and the functionalized derivatives can be obtained by metal-catalyzed borylation of the allyl precursors with bis(pinacolato)diboron (B 2 pin 2 ) as the boronate source. [6] Problems in synthetic applications arise from the relatively low reactivity of allyl-Bpin compounds towards most carbonyl compounds. Although aldehydes easily react with allyl-Bpin without catalysts, ketones do not. There are very few attempts at the direct allylation of ketones by functionalized allylBpins or their ester analogues reported in the literature.

Palladium‐Catalyzed Three‐Component Coupling Reaction of Allyl Carboxylates, Norbornenes and Diboronates Involving Sequential Olefins Insertion and Borylation Reaction

“…Li et al On the basis of the above-mentioned results and previous reports, [7,8,[14][15][16][17]20,21] we proposed a tentative mechanism (Scheme 4). Firstly, oxidative addition of allyl carboxylate to Pd 0 formed π-allyl Pd(II)-intermediate I-1, followed by coordinating with norbornene 2a and a syn-addition of I-1 to carbon-carbon double bond to produce σ-alky Pd(II)-intermediate I-2.…”

“…[6] And several reports have elegantly demonstrated that σ-alkyl Pd(II)-intermediates can be readily trapped by B 2 pin 2 . [7] Meanwhile, π-allyl Pd(II)-intermediates are susceptible to take coupling reactions with nucleophiles to form C-C, [8] C-Si, [9] C-N, [10] C-O, [11] C-P, [12] C-S, [13] C-B [14] bonds (Scheme 2a). Based on our investigation into the insertion of norbornene derivatives in a palladium-catalyzed reaction system [2,3,15] and boration of alkenes, [16] we anticipated that norbornenes can coordinate with π-allyl Pd(II)-intermediates to form σ-alkyl Pd(II) intermediates and they were stable enough to take subsequent conversion.…”

Iterative reactions of transient boronic acids enable sequential C–C bond formation

“…30 As noted in Table 1, allylic boronic acid intermediates could be easily generated from simple commercially available boronic acids and allylic diazo compounds. These allylic boronic acid intermediates showed high reactivity in allylation reactions with different aldehydes.…”

Catalytic Borylative Opening of Propargyl Cyclopropane, Epoxide, Aziridine, and Oxetane Substrates: Ligand Controlled Synthesis of Allenyl Boronates and Alkenyl Diboronates

“…According to the deuterium-labelling experiments this g-proton also arises from tBuOH [Eqs. (2) and (4)]. …”

“…[2] Recently,t he synthesis and application of alkenyl diboronates,inwhich one of the carbon-boron bonds is in the vinylic position and the other is in the allylic position, has attracted al ot of attention. [3] Ther eason for the attraction is that the two types of carbon-boron bonds may undergo either orthogonal functionalization or consecutive functionalization, thus creating molecular complexity in as ingle reaction step with high stereoselectivity.Another emerging area is allenylboration of carbonyl compounds.…”