Borylations 3-unsaturated carbonyl compounds

Ishiyama, T., Takagi, J., Kamon, A., Miyaura, N. Palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron with vinyl trifiates -substituted by a carbonyl group efficient synthesis of -boryl- -unsaturated carbonyl compounds and their synthetic utility. J. Organomet. Chem. 2003, 687, 284-290. 

The ready availability of arylboronates by an aromatic C-H borylation provides a synthetic link to the well-established palladium-catalyzed cross-coupling reactions, rhodium-catalyzed 1,4-addition to a,p-unsaturated carbonyl compounds, and other bond forming reactions using arylboronic esters (Scheme 2.12). Borylation of 1,3-dichlorobenzene with pinacolborane is followed directly by a cross-coupling reaction with methyl p-bromobenzoate for the synthesis of a biaryl product in 91% yield [60]. Pinacol esters of arylboronic acids react much slower than the free acids [62], but both derivatives achieve high isolated yields and comparable enantioselectivities (91% ee) in asymmetric 1,4-addition to N-benzyl crotonamides [63]. Borylation of arenes followed by oxidation of the C-B bond is synthetically equivalent to an aromatic C-H oxidation to phenols [64]. Oxidation of the resulting arylboronates with Oxone in a 1 1 acetone-water solution is completed within 10 min at room temperature. 

A variant of these reactions was optimized based on the Hosomi-Miyaura borylation of a,P-unsaturated carbonyl compounds with nucleophilic boryl copper, providing 2-alkoxycarbonyl (as well as 2-acyl- and 2-cyano) allylboronates such as 40 in high stereoselectivity (Equation 22) [60]. The resulting 2-alkoxycarbonyl reagents react with aldehydes to provide a-methylene-y-butyrolactones. 

Clearly, the platmum(II) boryls display very different chemical reactivity toward acyclic a,p-unsaturated carbonyl compounds when compared with the copper (1) boryls. The different reactivity has been explained through a bond polarization difference between a Cu-B bond and a Pt-B bond, i.e., a Cu-B bond is polarized toward B while a Pt-B bond is polarized toward Pt since the electronegativity of B is greater than Cu but smaller than Pt (Scheme 12) [106]. This bond polarization argument is also supported by the calculated NBO (natural bond orbital) atomic charges of Cu, Pt, and B atoms in the related boryl complexes (Scheme 12) [106], It should be noted here that the M-B bond polarization was calculated for various boryl moieties and was used to understand their nucleophilic and electrophilic character [107]. 

The expansion of this chemistry came by the work ofYun, since the copper borylation of a,P-unsaturated carbonyl compounds could be achieved by the activation of B2pin2 with CuCl in the presence of base and MeOH to favor the tr-bond metathesis. Remarkably, the use of chiral diphosphines to modify the Cu(I) salt was the key to dehver the Bpin moiety with asymmetric induction. A more detailed mechanism for the borylation of a,P-unsaturated carbonyl compounds was described by Lin with the aid of DFT calculations. The tr-bond metathesis step has also been elucidated, experimentally and theoretically, in the copper-catalyzed borylation of aryl hahdes with B2PUI2 by Marder and Kleeberg. 

The activation of B2PUI2 could also be carried out in the presence of Cu(ll) salts to promote P-boration of a,P-unsaturated carbonyl compounds in water as solvent. Santos has proposed a mechanism to activate the B2pin2 where amine/H20 could contribute to the preactivation of the diboron to form the Cu(lI)-boryl species (Scheme 23A). In a similar context,... 

Kobayashi has postulated a a-bond metathesis step between B2pin2 and Cu(OAc)2 to form a Cu(II)-boryl species by assistance of a chiral 2,2 -bipyridine hgand (Scheme 23B), which resulted in a very efficient catalyst for the asymmetric P-boration of a,P-unsaturated carbonyl compounds in water as solvent. ... 

Dang L, Lin Z, Marder TB. DFT studies on the borylation of a,P-unsaturated carbonyl compounds catalyzed by phosphine copper(I) boryl complexes and observations on the interconversions between O- and C-bound enolates of Cu, B, and Si. Organometallics. 2008 27 4443-4454. 

A copper-catalyzed nucleophilic borylation of a,/3-unsaturated carbonyl compounds yielding /3-borylated carbonyl compounds in good yields has been simultaneously reported by Hosomi and coworkers [123] and Miyaura and coworkers... 

In addition to enol silyl ethers, an optically active boryl enolate underwent the highly anri-stereoselective aldol reaction with a wide variety of aldehydes in the presence of TiCU (Eq. 34) [120]. The vinyl sulfides shown in Eq. (35) reacted with a,fi-unsaturated ketones via the 1,4-addition pathway in the presence of a titanium salt, but the reaction was followed by the cleavage of a carbon-carbon bond in the cycloalkane to give open chain products in a stereoselective manner [121]. The 1,2-type addition was observed, if the olefinie acetal was used instead of the corresponding carbonyl compound, as shown in Eq. (36) [121], The successive scission of the carbon-carbon bond took place analogously to give the same type of products as shown in Eq. (35). 

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... 

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