Pinacol borane

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

The variety of C2-bridged PBs was further extended by Muhoro via hydroboration of diphenyl(vinyl)phosphine with catechol- and pinacol-boranes (Scheme 29).56 To compensate for the low Lewis acidity of these boronates, the hydroboration reactions were carried out in the presence of 5 mol% of titanocene bis(catecholborane) as catalyst. The desired products 40g and 40h were obtained with complete anti-Markovnikov selectivity. The spectroscopic data and the crystallographic study performed on 40h showed the expected monomeric open structure. 

Alkynylboration has been achieved in the reaction of alkynyl(pinacol)boranes with alkynes in the presence of nickel catalysts, giving d.v-1 -borylbul-1 -en-3-yne derivatives. 1-Arylalk-l-ynes underwent the alkynylboration regioselectively with the selective introduction of the alkynyl groups at their 1-positions, where the aryl groups were attached.45... 

Silyl(pinacol)borane added to terminal alkenes in the presence of Pt(GH2=CH2)(PPh3)2 to give l-boryl-2-silylalk-anes 247 Platinum-catalyzed silylboration of methylene cyclopropanes provided alkenylboron derivatives via a proximal G-G bond cleavage (Equation (41))250 Nickel(0)-catalyzed silylboration of vinyl cyclopropanes and cyclobutanes provided allylsilane derivatives via analogous G-G bond cleavage (Equation (42)).251... 

A series of novel dithieno[3,2-A2, 3 -r7lphospholes 159 have been prepared in good yields by reacting 3,3 -dibromo-2,2 -dithiophene derivatives with butyllithium and subsequent addition of dichlorophosphines (Scheme 49) <2004AGE6197, 2005EJC4687>. Note that 5,5 -bis(pinacolboryl)-functionalized dithieno[3,2-A2, 3 -r7lphospholes 160 have been prepared by sequential lithiation and addition of isopropoxy(pinacol)borane to the unsubstituted dithienophosphole (Scheme 49) <20060L495>. 

Iridium-catalysed C-H-boronation can be carried out using either pinacol borane or pinacol diborane, both methods giving comparable results. Reaction occurs at a-positions of five-membered rings and is compatible with halogen substituents, as exemplified below. ... 

Schemes, (a) The Bp3-OEt2, mesitaldehyde (b) Bp3-OEt2, CH2(OMe3)2 (c) i. BF3-OEt2, mesitaldehyde ii. DDQ (d) NBS (e) i. Zn(0Ac)2-2H20 ii. pinacol borane, Pd(PPh3)2Cl2, EtsN (f)...

Palladium-catalysed reaction of aryl halides with pinacol borane (351) Preparation of 4-ethoxycarbonylphenylboric acid pinacol-ester (367) 11611... 

A solution of ethyl 4-iodobenzoate (210, 276 mg, 1 mmol), pinacol borane (351, 190 mg, 1.5 mmol, 1.5 eq.) and triethylamine (0.42 ml, 0.30 g, 3 mmol, 3 eq.) in dioxane (4 ml) was flushed with nitrogen, and Pd(dppf)Cl2 (22 mg, 3 mol%) was added. The reaction mixture was stirred at 80 C under a nitrogen atmosphere for 2 h. After being cooled, the reaction mixture was poored into water (20 ml) and extracted with dichloromethane (3x20 ml). Combined organic layers were washed with water and brine, dried (Na2S04), filtered, and evaporated. From the crude product, 218 mg (79%) of pure 4-ethoxycarbonylphenyl pinacolboronate (367) was obtained. 

Scheme 11.4 Synthesis of building blocks for altenuene (734). Reagents and ctmditions a) MeMgBr, THF, —40°C to it, 68% b) pinacol borane, Et3N, 5mol% Pd(PPh3)4, dioxane, 80°C, 2 h, 88%...

The alcohol 41 was converted to the alkoxyalkyne 42 in standard fashion (19) (80%). Rhodium-catalyzed hydroboration (20) with pinacol borane (21) provided the E-vinylboronate 43 (70%). The latter could be... 

Oxidative Heck reactions via Pd(II) C—H functionalization of terminal alkenes with pinacol boranes have been described for the preparation of styrenes and derivatives through electrophilic Pd(II) catalysis (Scheme 3.20). ° Treatment of a functionalized allylic precursor with the Pd(II) catalysts listed facilitated an allylic C—H activation. Subsequent transmetallation of the aryl boronic acid and reductive elimination afforded the desired olefin with excellent stereoselectivity. The scope of the transformation allows for a variety of activating and deactivating substituents on the aryl boronic acid as well as numerous functional groups on the starting alkene. A tandem allylic C—H oxidation/vinylic arylation protocol has also been reported. " ... 

Reaction scope Despite attempts to date to achieve a dearomatization and hydrosilylation of heteroaromatics with magnesium catalysts faiUng to achieve turnover, the hydroboration of pyridine and alkyl and aryl substituted pyridines along with quinoline and isoquinoline with pinacol-borane catalyzed by 5-10 mol% [((ArNCMe)2CH Mg"Bu] has been reported (Scheme 18) [126]. 

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