Alkynylboronic esters

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. 

Palladium(0)-catalysed coupling of an orf/to-halophenolic ether (thioether) with a terminal alkyne (or with an alkynylboronic ester ) and ring closure promoted with an electrophile - iodine has been most often used - is an excellent method to make both benzothiophenes °° ° and benzofurans. ortfto-AIkynyl-phenols can be comparably closed with palladium catalysis in the presence of copper(II) halides to give the corresponding 3-halo-benzofurans, ° and ortfto-alkynyl pyridin-2- and -3-yl acetates likewise ring close with iodine, generating furopyridines. ... 

A route to alkynylarenes and enynes from 1-alkynes involves formation of alkynylboronic esters [lithioalkynes -I- (/-PrOl B] and Suzuki coupling in situ. Suzuki cross-coupling using thallium(I) ethoxide as promoter is superior to TlOH because of its stability, commercial availability, and ease of use. A convenient method for assembling conjugated polyenes is assured. 

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

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. ... 

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. 

Though conceptually simple, photochemical E to Z isomerization of double bonds is not an efficient approach for accessing geometrically pure alkenylboronic esters [253, 258]. Alkynylboronic esters, however, can be selectively hydrogenated over Lindlar s catalyst [276]. 1,4-Dioxane was found to be the optimal solvent for providing (Z)-l-... 

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). 

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 ... 

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)). 

Transmetallation of alkenyl- and alkynylboronic acid esters with ArMgX reagents provides an access to mixed alkenyl-, aryl-, and alkynylborinic acids. In contrast, treatment of alkynylboronic acid esters with alkyl RMgX compounds displaces the alkynyl group and alkylboronic acid ester is formed ° ... 

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). 

Similarly, the cycloaddition reaction of nitrile oxides to alkynylboronates has been exploited to provide isoxazoleboronic esters with good levels of regiocontrol. These synthetic intermediates participate efficiently in Suzuki cross-coupling reactions (Scheme 9.44) [96, 97]. 

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]. 

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. 

Gandom, V., Leca, D., Aechtner, T., Vollhardt, K.P., Malacria, M. and Aubert, C. (2004) Synthesis of fused arylboronic esters via cobalt(0)-mediated cycloaddition of alkynylboronates with ct,w-diynes. Organic Letters, 6(19), 3405-3407. 

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