Versatile building blocks with a dihydroaromatic vinylic boron substructure can be generated efficiently by cobalt‐catalyzed Diels–Alder reactions and used in Suzuki coupling reactions for the synthesis of functionalized polycyclic compounds (see scheme).

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[Reaction: see text] A cobalt(I)-catalyzed Diels-Alder reaction of a boron-functionalized enyne is the key step in a two-step reaction cascade interconnecting four simple starting materials to obtain polycyclic multifunctionalized products in good yields and with a very high degree of diastereoselectivity.

The cobalt-catalyzed Diels-Alder reaction of alkoxysubstituted 1,3-butadienes with terminal and internal alkynes is described. While the reaction of 1-alkoxy derivatives gave the aromatic hydrocarbons upon elimination of alcohol from the alkoxysubstituted dihydroaromatic intermediates, the reactions with 2-alkoxy derivatives generated stable dihydroaromatic enol ethers in good to excellent chemical yields and good to high regioselectivities for unsymmetrical starting materials. The enol ethers can be easily hydrolysed to the corresponding b,g-unsaturated ketones in a one pot reaction sequence or used in cyclopropanation or other subsequent chemical transformations.Transition metal catalyzed cycloadditions have been investigated for a long time and important contributions have been made in the field of Lewis-acid promoted cycloaddition reactions. 1 Especially successful are applications of Lewis-acids in the Diels-Alder reaction with normal electronic demand, while in Diels-Alder reactions with neutral electronic demand only few contributions can be found in the literature. 2 In particular, the use of low valent cobalt-complexes for such neutral Diels-Alder reaction has been described by us and other research groups in the past. 2,3Herein we describe our investigations into the use of oxygen-functionalized 1,3-dienes as substrates for the cobalt(I)-catalyzed Diels-Alder type reaction with terminal and internal alkynes. For the present investigation, we used a mixture of CoBr 2 (dppe)/ZnI 2 /Bu 4 NBH 4 as catalyst system. 4 1-Alkoxy-functionalized 1,3-ButadienesWhen 1-methoxy-1,3-butadiene (R 3 = Me) or 1-trimethylsilyloxy-1,3-butadiene (R 3 = SiMe 3 ) was used as diene in the cobalt(I)-catalyzed neutral Diels-Alder reaction with alkynes, the 1:1 adduct was formed in quantitative conversion (GC) under mild reaction conditions. However, the dihydroaromatic intermediates could not be observed, and instead the corresponding aromatic products, which are formed upon in situ elimination of R 3 OH, were detected and isolated in good chemical yields (see Scheme 1).Since the synthesis of the 1-trimethylsilyl derivative is easier than the corresponding methyl enol ether and the yields for the generation of biphenyl (Table 1, entries 1, 2) were not significantly lower, subsequent reactions were conducted with the TMS derivative. 5 Besides terminal alkynes, internal alkynes and 1,3-diynes could also be reacted with the silyloxy-substituted butadiene derivative under mild conditions (generally 5-10 mol% Co-catalyst, CH 2 Cl 2 , r.t., 0.5-16 h) 6 to yield the corresponding monoand disubstituted aromatic compounds. We also found that for the reaction with a diyne substrate (entry 5), even with an excess of the diene and at elevated temperatures (60°C in a sealed tube) no conversion to the desired tetraphenyl derivative could be detected. Scheme 1 R 1 R 2 R 1 R 2 R 1 R 2

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