Alkynylboronic 4+2 cycloadditions

Metal carbonyl monoanions, with trivalent Group 13 element halides, 3, 347, 3, 357 Metal carbynes, Fe-containing, 6, 325 Metal-catalyzed cycloadditions, alkenyl- and alkynylboron compounds, 9, 180 Metal cation receptors characteristics, 12, 466 ferrocene-based, 12, 466 Pt- and Au-based receptors, 12, 471 types, 12, 473 Metal chalcogenides... 

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... 

Scheme 27 1,3-Dipolar cycloaddition to 1 -alenyl- or 1 -alkynylboron compounds.

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

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. 

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). 

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]. 

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 ... 

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. 

It is worthy of note that polyborylated cyclohexadienes can also be formed by cobalt-catalyzed [2- -2-1-2] cycloaddition between alkynylboronates and alkenes after oxidation [25]. In this way, alkenes can be considered as substitutes of alkynes [26]. Liquid enol ethers might also advantageously replace acetylene gas in [2 -I- 2 -I- 2] cycloadditions without oxidation [27]. This strategy has been applied to synthesis of the diborylated arenes 57 and 58 (Scheme 1.14) [28]. 

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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