Diboron, boration with

Bis(pinacolato)diboron reacts with 1-halo-l-lithioalkenes, that is, alkylidene carbenoids, affording 1,1-diboryl-1-alkenes in good yields (Scheme 9).76 The reaction proceeds via formation of a borate intermediate, which is followed by 1,2-migration of the boryl group with elimination of the bromo group. 

The traditional synthesis of organoboron compounds from organic halides is based on the reaction of trialkyl borates with Grignard or lithium reagents. However, the cross-coupling reaction of diboron solves the difficulties associated with the use of Mg/Li compounds. The cross-coupling reaction of diborons... 

Ethylboronic acid can be prepared from chloroethylphenylborane, diethyl ethylboronate, or triethylboroxin." Very impure products resulted from the hydrolysis of the products obtained when trimethyl borate was reacted with bromoethylmagnesium. Ethylboronic acid is best prepared by the reaction of triethylboroxin with water. Triethylboroxin is easily obtained from diboron trioxide with triethylborane or from tetraethyldiboroxane either thermally or by /BH catalysis. ... 

Metalated ferrocenes have served as valuable intermediates for the synthesis of a number of other derivatives. Treatment of lithiated ferrocenes with tributyl borate followed by hydrolysis leads to ferroceneboronic acid (XXXIII) as well as the diboronic acid (73). Ferroceneboronic acid, like benzeneboronic acid, is readily cleaved by cupric bromide or cupric chloride to form the corresponding halo derivatives (XXXIV). Ferrocene-l,l -diboronic acid reacts in the same manner, and either one or two carbon-boron bonds can be cleaved. Further reactions of this type have led to a variety of mixed dihaloferrocenes (73, 75). 

Usually, boronic acids are synthesized by reaction of an aryl magnesium or aryl lithium compound with a borate ester [6]. However, in the case of 4-bromo-benzonitrile, a metal-organic intermediate would react with the nitrile group. Aryl boronic acids could be also synthesized by palladium-catalyzed reaction using diboron reagents [7]. This method would certainly tolerate the cyano func-tionahty, but the diboron reagents are very expensive. 

Shibasaki and co-workers described a Cu-catalysed conjugate boration of p-substituted ot,p-unsaturated cyclic ketones with >w(pinacolato)diboron (Scheme 8.33). 

A mixture of tetramethyl propane-1,3-diboronate and ethyl thioborate heated at 120-140° in vacuo with distillation of the resulting methyl borate tetraethyl propane-l,3-di(thioboronate). Y 88.6%. F. e. s. B. M. Mikhailov and V. F. Pozdnev, Izvest. 1962, 1861 G. A. 58, 9109b. 

Diboron compounds are a part of a relatively unexplored class of compounds. Their main use is for p-boration reactions where a boron centre is rendered nucleophilic with the use of a metal catalyst or a Lewis base to form a sp -sp diboron compound. The reactivity of these... 

In 2008, Yun et al. first reported the asymmetric p-boration of a,p-unsaturated nitriles and esters using Cu catalysts with chiral phosphine ligands [31, 32]. The phebox-Rh acetate complex 5 works as an effective catalyst for asymmetric boration of a,p-unsaturated esters (Scheme 29) [33]. The catalytic reaction of ethyl ( )-cinnamate with bis(pinacolato)diboron (B2pin2, l.leq) in the presence of 1 mol % of the scc-butyl phebox complex 5- Bu and 5 mol% of NaO Bu at 80 C afforded the borylated compound 46 in 86% yield. The product ee value of 97% was... 

Scheme 29 Asymmetric P-boration of cinnamate with diboron...

Since palladium complexes have proven to be efficient in activating diboron compounds throughout transmetalation, a wide range of applications have been considered in the last decade, such as palladium-catalyzed transformation of aUyhc alcohols to allylboronates, borylation of allylic hahdes or aUyhc acetates, and the P-boration of a,P-unsaturated carbonyl substrates.Interestingly, both palladium and nickel showed to be similarly efficient to activate B2pin2 and catalyze the addition to unsaturated substrates. Oshima postulated that Ni(0) species react with substrate a,P-unsaturated esters and amides to generate the r -coordinated complex, which activates the B2pin2 to favor the formation of T -coordinated... 

Miyaura made another proof of concept when he demonstrated the activation of B2pin2 with CuCl in the presence ofKOAc (Scheme 21). Analogously, Ito and Hosomi postulated that [(CuOTf)2-C6H6] itself did not consume bis(catecholato)diboron (B2cat2) in the absence of an a,P-enone. ° The presence of PBU3 enhanced the P-boration of the enone, presumably... 

Additionally, the electron rich B=B double bond in diborenes 62 could be readily oxidized to the corresponding diboron radical cation 93 using tro-pylium salts with bulky non-coordinating borate anions, or with a borole to provide access to the first example of a stable boron-centered radical-anion radical-cation pair 94. ... 

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