Diboronates, formation

Scheme 1. Formation of a macrocyclic complex between a diboronic acid and a saccharide...

The reaction proceeds through the formation of an (acetoxo)palladium(II) species (110) prior to transmetalation with diboron (Scheme 1-39). The presence of a base... 

The synthesis of 1-alkenylboronic acids from l-alkenylmagnesiums or -lithiums suffers from difficulty in retaining the stereochemistry of 1-aikenyl halides, but the palladium-catalyzed coupling reaction of diboron 82 with 1-aikenyl halides or tri-flates directly provides 1-alkenylboronic esters (Scheme 1-43) [157, 158]. Although the reaction conditions applied to the aryl coupling resulted in the formation of an... 

A stoichiometric reaction of tetrakis(triphenylphosphine)platinum(0) with bis(pinacolato)diboron gives cis-diborylplatinum(n) complex in high yield (Scheme 3).38 The diborylplatinum complex then reacts with an alkyne, giving m-diboration product.40,41 These results indicate that the diboration proceeds through the general mechanism shown in Scheme 1 (E1 = E2 = Bpin), which involves the formation of diborylplatinum(n), insertion of an alkyne into the B-Pt bond, and reductive elimination. 

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 formation of cyclic boriozirconocenes 63 has been observed by treating l-alkynyl-6-alkenyl diboronates 64 with Negishi s reagent (Scheme 7.20) [133]. Hydrolysis of 63 with HC1 in Et20 gave the 1-alkenyl-l-alkyl diboronate 65 in 80% yield. 

Fig. B10.4.1. Formation of non-cyclic and cyclic complexes of saccharides with the diboronic receptor S-la,b. ...

Although the [Ir(Ti -C5Me5)(NHC)] complex was initially tested in the diboration of styrene, this did not lead to the formation of any diboronate esters when bis(catecholato)diboron (B2cat2) was added to a solution of the catalyst and styrene in tetrahydrofuran (THF) under argon (Scheme 7.23). However, the addition of... 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

An alternate approach to the formation of pyridylboronic acids is the cross-coupling of a halopyridine with a diboronate ester (usually bis(pinacolato)diboron, 7.7.)9 The analogous reaction of 2-chloropyridine led to pyridine formation through protodeboronation. The product of the reaction, either after hydrolysis to the boronic acid or in the ester form, can be further reacted with another aryl halide to give a biaryl. In certain cases the reaction might also be carried out in a one-pot manner.10... 

The analogy between oxygen and nitrogen could not be extended to the formation of 2//-benz[e ][ 1,2]oxaborins from styrenes treatment of 2-hydroxystyrene with boron halides gave only tars. However, tolan-2,2 -diboronic acid was cyclized in the presence of alkali to give a product whose structure was suggested to be (198) (59TL(14)21). 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

The starting material bis(pinacolato)diboron is a poor Lewis acid and 1 B-NMR of KOAc and B2bin2 in DMSO-d6 shows no evidence of the coordination of the acetoxy anion to a boron atom leading to a tetrahedral activated species. However, the formation of an (acetato)palladium(II) complex after the oxidative addition of the halide influences the reaction rate of the transmetalation step. The Pd-O bond, which consists of a hard Lewis base with a soft Lewis acid, is more reactive than a Pd-X (X=Br, I) bond. In addition, the high oxophilicity of boron has to be considered as a driving force for the transmetalation step, which involves an acetato ligand. 

The diboronic acids 23 and 24 form rigid cyclic complexes with chiral monosaccharides and disaccharides this is the origin of the CD observations. The induced chirality upon formation of rigid, chiral complexes was monitored by CD spectroscopy, and this rigidification process can be utilized in the design of spectroscopic... 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

SCHEME 9.2 Formation of boroxine frompara-diboronic acid. 

Bis(pinacolato)diboron 8 also reacts with 1-halo-l-lithioalkenes 10 (available from 1,1-dihaloalkenes or 1-haloalkenes by halogen-metal exchange or metalation reaction, respectively) to afford 1,1-bismetalated dienes 11. These are readily converted into polysubstituted dienes through various transition metal-catalyzed carbon-carbon bond formations (Scheme 4) [29]. However, these seemingly attractive methods must not hide the problems of the stereochemistry and regiospecificity of the subsequent cross-coupling reactions. 

The C-H coupling of benzo[, ]furan 82 with bis(pinacolato)diboron 103 was carried out in octane with [IrCl(COD)]2-(4,4 -di-/( tt-butyl-2,2 -bipyridine) as a catalyst (3mol%), leading to the formation of the 2-borylated product 104, as can be seen in Equation (93) <2002TL5649>. 

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