Diboration of allenes

Miyaura and co-workers reported the platinum-catalyzed diboration of allenes with bis(pinacolato)diboron (Scheme 16.52) [57]. The catalytic cycle involves a sequence of oxidative addition of bis(pinacolato)diboron to Pt(0), insertion of an allene into the B-Pt bond and reductive elimination of an allylic boronate, re-producing the Pt(0) species. (Z)-Allylic boronates are formed stereoselectively in the reaction with monosubstituted allenes, which strongly suggests a pathway via a vinylplatinum species rather than a Jt-allylplatinum species. 

A stable ds-Pt(B02C2Me4)2(PPh3)2 complex has been isolated and characterized by X-ray analysis, whereas the corresponding Pd(0) complex is not known. Compatibly, no Pd-catalyzed diboration is known. However, Yang and Cheng reported for the first time a palladium-catalyzed diboration of allenes using an aryl, alkenyl iodide or I2 as a co-catalyst (Scheme 16.53) [58]. 

The catalytic asymmetric diboration of allenes provides a-substituted 2-boronyl allylic boronates of type 25 (see Eq. 32). One of them, 91, adds to ben-zaldehyde, albeit with a slight erosion of stereoselectivity (Eq. 65). The major P-hydroxy ketone stereoisomer, isolated after an oxidative work-up, originates from the putative chairlike transition structure 92. 

A new efficient route for the synthesis of bis(diboranes) catalyzed by palladium complexes is diboration of allenes (Scheme 22).75 The stereochemistry of these bisboronic products was studied and the Z-isomer was also identified as the major product (93-95%). 

The diboration of allenes afforded another series of allylboron compounds that each have a boryl group at the vinyl carbon. The addition had a strong tendency to occur at the internal double bond of terminal allenes such as 1,2-heptadiene (Equation (33)).237 However, steric hindrance in both allenes and phosphine ligands forced the addition toward the terminal double bond of dimethylallene (Equation (34)).237 On the other hand, addition to the terminal double bond occurred selectively for both monosubstituted and 1,1-disubstituted allenes when Pd(dba)2 was used in the presence of a co-catalyst (RI) such as I2, Arl, and iodoalkenes (Equation (35)).238 The role of co-catalyst was attributed to in situ formation of I-Bpin intermediate, which undergoes oxidative addition and insertion leading to 2-boryl-7r-allylpalladium intermediate. 

Diboration of allenes. 2,3-Bis(pinacolatoboryl)-l-propene is formed in the Pt(0)-catalyzed reaction of allene with bis(pinacolato)diboron. ... 

Yang and Cheng observed an unusual variant of allene diborylation where an aryl or alkenyl halide can act as a co-catalyst with a phosphine-free palladium catalyst [51]. As shown with the formation of allylboronate 29, the addition is completely regiose-lective and highly stereoselective (Equation 17). Examples with aryloxy-substituted allenes were also successfully demonstrated. Notably, Pd(dba)2 does not promote the diboration of allenes without the co-catalyst, and a new mechanism involving oxidative addition of an I-B bond to the palladium center was proposed to rationalize these observations. 

It seems that palladium complexes with high oxidative states and basic ligands can be involved in oxidative addition as weU as in o-bond metathesis. Cheng has demonstrated that phosphine-free Pd complexes together with alkenyl or aryl iodides are very efficient catalysts for 1,2-diboration of allenes. This reaction is completely regioselective and highly... 

Scheme 18 1,2-Diboration of allenes with B2pin2 by phosphine-free Pd complexes.

Morken and coworkers extended their previously developed palladium-catalyzed asymmetric diboration of prochiral allenes [54] to the subsequent a-aminoallylation of the in situ generated allyl and vinylboronate 62, as versatile intermediate for allylation reactions (Scheme 4.30) [55]. 

Synthetic strategy Catalytic enantioselective diboration of pro-chiral allenes Catalyst Tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] / (R,R)-phosphoramidite (ligand I)... 

Diboration of Unsaturated Hydrocarbons. B2pin2 adds to unsaturated hydrocarbons in the presence of a catalytic amount of a Pt complex to afford diborated products in high yield with excellent regio- and stereoselectivity. The reaction is recognized to proceed through a catalytic cycle, which involves (a) oxidative addition of the B-B bond to Pt , (b) insertion of the unsaturated hydrocarbon into the B-Pt bond, and (c) reductive elimination of the product to regenerate Pt (eq 1). The diboration of alkynes, allenes, conjugated dienes, methylenecyclopropanes, anda, -unsaturated carbonyl compounds is efficiently catal) ed by phosphine-based Pt complexes (eqs 2-6), whereas phosphine-free Pt complexes are favorable for the reaction of simple alkenes because of the low coordination ability of the alkene over phos-... 

Diboration of 1,3-Dienes, Allenes, Enones, and Other Unsaturated Molecules. As described previously in the original article, the Pt[0] catalyzed addition of 1,3-dienes with B2pin2 affords various allylboronic esters (eq 4). Reactions using phosphine-based catalysts such as Pt(PPh3)4 stereoselectively produce els -1,4-addition products with Z-configuration for aliphatic and cyclic dienes. The reaction mechanism suggests... 

NHC-Cu-catalyzed diborations of styrene and phenylacetylene were first studied by the groups of Perez and Fernandez on the basis of initial studies on the borocupration of unactivated alkenes made by the group of Sadighi. Since these first developments, numerous methods have been developed. It has been shown that carbon n-systems such as alkenes, alkynes and allenes could be variously functionalized by trapping the organocopper species intermediately formed with an external electrophile. 

All]rlboronates from the Transition-metal Catalyzed Diboration and Silabora-tion of Dienes and Allenes... 

The Miyaura group similarly applied the diboration process to allenes, and found that the regioselectivity depended highly on the specific conditions used, in particular the presence or absence of a bxxlky phosphine ligand (Equation 16) [50]. 

Palladium-catalyzed diboration can be achieved with bis(pinacolato)diboron for strained 1,2-disubstituted alkynes. ° Analogously, palladium-catalyzed addition reactions of diboron compounds provide 1,2-bisborylalkanes from l-alkenes ° or cis-l,4-bisboryl-2-alkenes from 1,3-dienes.2,3-Bisboryl-l-propenes are accessible from allenes (Scheme 5-174). The asymmetric allene diboration was examined by Burks et ai, who showed that monodentate phosphoramidite ligands gave excellent enantioselectivities (up to 97% ee) (Scheme 5-174). The (asymmetric) metal-catalyzed reactions of diboron, silylboron and disilane compounds have been reviewed.Recently, the addition across two triple bonds to give dienes has been explored by Singidi et... 

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