Boronates alkynylboronates

Allenylboranes can be prepared by treatment of propargylic acetates with butyl-lithium and a trialkylborane [19]. The reaction proceeds by initial formation of an alkynylboronate followed by migration of an alkyl group from boron to carbon (Eq. 9.17). 

The reactivity profiles of the boronate complexes are also diverse.43 For example, the lithium methyl-trialkylboronates (75) are inert, but the more reactive copper(I) methyltrialkylboronates (76) afford conjugate adducts with acrylonitrile and ethyl acrylate (Scheme 16).44 In contrast, the lithium alkynylboronates (77) are alkylated by powerful acceptors, such as alkylideneacetoacetates, alkylidene-malonates and a-nitroethylene, to afford the intermediate vinylboranes (78) to (80), which on oxidation (peracids) or protonolysis yield the corresponding ketones or alkenes, respectively (Scheme 17).45a Similarly, titanium tetrachloride-catalyzed alkynylboronate (77) additions to methyl vinyl ketone afford 1,5-diketones (81).4Sb Mechanistically, the alkynylboronate additions proceed by initial 3-attack of the electrophile and simultaneous alkyl migration from boron to the a-carbon. 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

In contrast to the thermal reactions of 1-alkynylboronic esters, the cycloaddition smoothly takes place under very mild conditions in the presence of metal catalysts (Scheme 28). A cobalt(i) complex catalyzed the [4+2]-cycloaddition of alkynyl boronates with 1,3-dienes to give cycloalkenyl boronates 266431 and with ct,cu-diynes giving arylboronates... 

The radical addition to 1-alkenyl or 1-alkynylboronic esters or amides took place extremely smoothly because the boron atom stabilizes the resulting ct-radical intermediates. Bu3SnH and PhSH predominated the trans-addition products 287 in the addition to 1-alkynylboronic amides at 90 °G, whereas Bu3SnH and Ph2Ph produced the m-addition products 288 at 0°C (>98% Equation (82)).455 Intramolecular addition to 1-alkenylboronic esters has been demonstrated in boron-tethered radical cyclization that provided 1,3- or 1,4-alkanediols 290 via oxidative workup (Equation (83)).456 Inter- and intramolecular additions of alkyl radical457 and sulfonyl radical458 have also been studied. 

An intramolecular version of olefin cross-metathesis has been demonstrated in cyclization of a,u -alkadienes 293462 (Equation (85)), cyclization of enyne to provide 1,3-dienylboronic esters 297406 (Equation (86)), and in cyclization of boron-tethered enynes obtained from 1-alkynylboronates and allylic alcohols (298 463 (Equation (87)) or allyl boronates and propargyl alcohols.464... 

A novel class of quinone boronic esters 301465 was synthesized by utilizing a highly regioselective benzannulation of Fischer carbene complexes 300 with 1-alkynylboronates (Equation (88)). 

The transmetallation of (R 0)3B with R—M (M=Li, MgX) at low temperature (usually at -78 C) proceeds by initial formation of a relatively unstable teracoordinat-ed complex [RB(OR0i]M, which is in equilibrium with RB(OR )2 and R OM. If the monoalkyl(trialkoxy)borate can be cleanly formed, and if equilibrium favors this complex, the boronic ester will be formed selectively. Otherwise, successive steps will give rise to the di-, tri-, or tetraalkylborates (eq (7)). Triisopropyl borate is shown to be the best of the available alkyl borates to prevent such side reactions thus allowing the syntheses of a number of alkyl, aryl. 1-alkenyl [9]. and 1-alkynylboronates [10] in high yields, often over 90% (eq (8)). 

Lithium 1-alkynylboronates are attacked by many electrophiles at the position P to the boron atom. The subsequent rearrangement gives a variety of functionalized 1-alkenylboronanes (eq (28)). The stereochemistry can be either " or Z, or a mixture of the two [15]. The reaction with chloro(diethyl)borane or chloro(trimethyl)silane and -stanane stereoselectively produces c /5-bimetallic compounds [41]. 

A wide range of isoxazole-4-boronates and some 5-boronates can be prepared by dipolar cycloaddition of nitrile oxides with alkynylboronates."""... 

Boron Conjugate addition of alkenylboronic acids PhCH=CHB(OH)2 and alkynylboronic esters to ArCH=CHCOMe (Ar = unprotected indoles), carried... 

Scheme 10.99 Carbozirconation of alkynylboronates synthesis of tetrasubstituted alkenyl-boronates [82].

The cyclic dialkoxyborane 4,4,6-trimethyl-l,3,2-dioxaborinane (HBhg), the vinylboronic ester 2-ethenyl-4,4,6-trimethyl-l,3,2-dioxaborinane (vinylBhg) and the alkynylboronic ester 2-ethynyl-4,4,6-trimethyl-l,3,2-dioxaborinane were first reported in 1966-67 by Woods and coworkers. The authors stressed the high stability of HBhg towards disproportionation, as compared to the parents 1,3,2-dioxaborinane and 1,3,2-dioxaborolane, and its potential for the hydroboration of alkynes. In addition, the first reported o-metal-boron bond (rhodium) was prepared from HBhg. ... 

In contrast to arylboronic acids, early reports document the great stability of alkyl-boronic acids under aqueous acidic solutions. For example, various simple alkyl-boronic adds were unaffected by prolonged heating in 40% aqueous HBr or HI [40]. Like arylboronic acids, however, deboronation is observed in hot basic aqueous solutions [76]. Alkenylboronic esters undergo protonolysis in refluxing AcOH [85], and alkynylboronic acids were reported to be quite unstable in basic aqueous solutions (Section 1.3.5). 

Harrity and co-workers described the application of 2-substituted 1-alkynylboronic esters in the Dotz cycloaddition of Fisher chromium carbene complexes, affording in a highly regioselective fashion a novel class of hydroxy-naphthyl boron pinacolates (entry 18, Table 1.3) [217]. These reaction products also provided, upon treatment with ceric ammonium nitrate, the corresponding quinone boronic esters. 

Like their aryl and alkenyl counterparts, alkynylboronic adds can be made by displacement of magnesium or lithium acetylides with borate esters. For example, Mat-teson and Peacock have described the preparation of dibutyl acetyleneboronate from ethynylmagnesium bromide and trimethyl borate [294]. The C-B hnkage is stable in neutral or acidic hydrolytic solvents but readily hydrolyzes in basic media such as aqueous sodium bicarbonate. Brown and co-workers eventually applied their organo-lithium route to boronic esters to the particular case of alkynylboronic esters, and in this way provided a fairly general access to this class of compounds [295]. 

Gobalt-catalyzed Diels-Alder reactions offer an interesting alternative since the cycloadditions are carried out under mild conditions. With isoprene, alkynylboronic esters predominantly gave the regioisomer in which the methyl group of the diene and the boronic ester functionality are meta related [64], Subsequent Suzuki coupling reactions can be performed without isolation of the intermediates (Scheme 9.28). 

A novel class of quinone boronic esters has been prepared via a Dotz annulation of Fischer carbene complexes with alkynylboronates. The origin of the high regioselec-tivity is discussed in terms of steric and electronic effects. Additionally, these compounds undergo Pd-catalyzed coupling reactions to give substituted quinones and hy-droquinones (Scheme 9.49) [102]. 

Platinum(0)-catalyzed diboration of 1-alkynylboronates with bis(pinacolato)dibo-rane yielded the corresponding trisboronated alkene products in high yields (Scheme 9.58) [120]. Catalyzed diboration of styrylboronate esters in the presence of either [Rh(PPhj)3Cl] or [Rh(COE)2(li-Cl)]2 and two equivalents of P(o-tol)j yielded predominantly the corresponding alkenyl tris(boronate) [121]. 

In a similar sense, cycloaddition of 1,2,4,5-tetrazine with alkynylboronate esters 4 yielded pyridamines 5 and 6 (eq 2). The resultant boronic ester in 5 was then capable of further Suzuki coupling reactions to generate l ... 

SYNTHESIS OF ALKYNYLBORONATES AND RELATED COMPOUNDS THROUGH THE FORMATION OF BORON-CARBON(sp) BONDS... 

The same year, Gandon et al. reported the synthesis of fused arylboronic esters 36 via cobalt(0)-mediated cycloaddition of alkynylboronates 33 with diynes 35 (Scheme 1.11) [22], The boronate is first reacted with Co2(CO)g at room temperature for 4 h to generate the corresponding dicobaltatetrahedrane 34. The diyne is then added and the mixture is refluxed for 2 h. To show the utility of the products, one of them was treated with phenyl iodide under Pd catalysis to give 37. Complementary to these investigations, Ru-catalyzed [2 - - 2 - - 2] cycloaddition of tethered alkynyl-boronic esters with alkynes was reported [23]. In this case, the borylated arene could not be isolated but was converted directly in situ by Suzuki-Miyaura coupling. 

The cobalt-catalyzed Diels-Alder reaction of alkynylboron units could also be applied to the synthesis of boron-functionalized biphenyl derivatives such as 46. In... 

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