Diborylated-arene

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]. 

The double hydroboration of propargyl bromide with 9-borabicyclo[3.3.1]nonane (9-BBN-H) proceeds smoothly to afford the corresponding l,l-diboryl-3-bromo-propane adduct which is quantitatively converted to the B-hydroxy-B-cyclopropyl-9-BBN and / /3-hydroxy-9B BN complexes in equal molar amounts. Addition of this mixture to aryl, alkynyl and alkenyl bromides containing a catalytic (3%) amount of Pd[PPh3]4, after 8-16 hours at reflux temperature, provides the corresponding cyclopropylated arenes, alkynes and alkenes in good to excellent yields.111... 

SCHEME 22.32 Double annulation of 1,2-diboryl reagents with dibromobis(2-bromoethenyl)arenes. 

---