Diboryl

The chemistry and stereochemistry of aminoboranes containing the siLicon—nitrogen—boron linkage have been the subject of numerous studies. Many of these compounds are useful precursors to other B—N systems including diboryl-amines (45) and B—H substituted aminoboranes (46). A series of... 

Diboration of terminal alkenes has also been studied with other d " metals (Fig. 2.12) including the Ag and Au complexes 75-77 and the Pt" complexes 78-79. Styrene is diborylated with 100% selectivity and good conversions in THF (46% for 75 and 94% for 77 at 5 mol%, 60 h) using equimolecular amounts of (Bcat)j. The difference in activity between the Ag and Au complexes has been ascribed to the increased lability of the Ag-NHC bond, which may lead to catalyst decomposition under the reaction conditions, hi both catalytic systems it is believed that the active species involves only one coordinated NHC ligand. Complex 77 is less active than 74 and 75, possibly due to steric reasons. The enantioselectivity of 77 in the diboration of prochiral alkenes is very low [63]. 

Diboration of terminal and internal alkynes by (Bcat) [B(cat)2 alkene = 1 1] was also achieved by using complexes 78 and 79 at room temperature in THF. Best activity was observed with 78 containing the less electron donating triazolyhdene carbene hgand. Terminal alkenes can also be diborylated under the same conditions, however the selectivity for the diborylated product was much lower (up to 65%) [64]. 

The complexes [Cu(NHC)(MeCN)][BF ], NHC = IPr, SIPr, IMes, catalyse the diboration of styrene with (Bcat) in high conversions (5 mol%, THF, rt or reflux). The (BcaO /styrene ratio has also an important effect on chemoselectivity (mono-versus di-substituted borylated species). Use of equimolecular ratios or excess of BCcat) results in the diborylated product, while higher alkene B(cat)j ratios lead selectively to mono-borylated species. Alkynes (phenylacetylene, diphenylacety-lene) are converted selectively (90-95%) to the c/x-di-borylated products under the same conditions. The mechanism of the reaction possibly involves a-bond metathetical reactions, but no oxidative addition at the copper. This mechanistic model was supported by DFT calculations [68]. 

Figure 23 Charge transfer coordination polymers (33) obtained from diborylated ferrocenes with 4,4 -bipyridine (bipy) or 1,2-bis(4-pyridyl)ethane (pyetpy) bridges. (Adapted from ref. 55.)...

Similar polymers (34) (Fig. 24) of diborylated ferrocenes linked by pyrazine units were developed that had an unusual dark-green appearance indicative of charge-transfer interactions between the iron centers and the electron-poor pyrazine adduct bridges.56... 

The reaction with 2 mol of diphenylboric acid ester gives rise to a diborylated product (117) [Eq. (76)] (89IZV946). The reaction proceeds in a similar manner when triethylamine is replaced by metallic sodium [Eq. (77)] (90IZV886). 

Diborations of 1,3-dienes are carried out in the presence of platinum(O) catalyst. In the Pt(PPh3)4-catalyzed dibora-tion of 1,3-dienes, 1,4-addition takes place to give (Z)-l,4-diboryl-2-alkenes stereoselectively (Equation (10)).62 In contrast, the Pt(dba)2-catalyzed reaction of 1,3-pentadiene affords 1,2-addition product, in which more substituted C=C bond is left intact (Scheme 5). The use of Pt(dba)2 as catalyst also enables the diborative dimerization of isoprene to occur (Equation (ll)).48... 

Diboration of a,/3-unsaturated esters is catalyzed by the platinum(0)/diimine catalyst, giving a,/3-diboryl esters, that is, 3,4-addition products (Equation (13)). Although the a,/3-diboryl ester products are hydrolytically more stable than the corresponding 1,4-addition products bearing a boron ester enolate moiety, they gradually undergo hydrolysis... 

Reaction of bis(pinacolato)diboron to methylenecyclopropanes proceeds with cleavage of the proximal G-C bond of the cyclopropyl ring, giving 2,4-diboryl-l-alkenes (Equation (14)).74... 

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. 

Although simple alkyl esters (ethyl propionate) fail to enolize with the boryl triflate reagents under normal conditions, the more acidic acyloxyboranes 66 readily form the diboryl enediolates 67 (eq. [52]) (6a,66). Several interesting trends are noted in the data included in Table 23. Since previous studies have demonstrated that enolate geometry strongly correlates with product stereochemistry, enediolate 67 has been employed to directly compare the reactivities... 

A mild one-pot procedure based on a platinum-catalyzed diborylation of 1,3-butadienes (see Eq. 30) gives doubly allylic boronate 144, which adds to an aldehyde to form a quaternary carbon center in the intermediate 145 (Eq. 105). The use of a tartrate auxiliary in this process leads to good levels of enantiose-lectivity in the final diol product, which is obtained after oxidation of the primary alkylboronate intermediate. Although examples of aliphatic, aromatic, and unsaturated aldehydes have been described, enantioselectivities vary widely (33 to 74% ee), and are good only for aliphatic aldehydes. An intramolecular variant of this interesting tandem reaction is also known. ... 

Quantum chemical calculations on different levels have been reported for 2//-1,2,3-diazaphospholes (G) <83X1507), for 4,6-diamino-l,3,5-triaza-2-phosphapentalenes derived from 1,3,2-diazaphospholes (I) <86CB3213), for the tetrazaphospholium ion (S) <93CB1513), for 1//-1,2,3-azadiphosphole (U) <89NJC309), and for l//-l,2,3-benzazadiphosphole <93PS(76)45). Planar and nonplanar structures of 2/7-1,3,2-diazaphosphole and its 2-boryl derivative as well as of 4//-1,2,4-diazaphosphole and its 4-boryl and 3,5-diboryl derivatives are compared by correlated ab initio calculations <95JPC586). 

Tri-fi m-butylazadiboriridine, 187, was observed to react with ethyne in a manner sensitive to the ratio of the reactants. When the reactants were combined in a 1 1 ratio, the heterocyclopentene, 188, resulted. Under normal conditions, 188 did not react further with excess 187 at the G=C bond. On the other hand, [l,2,5]azabiborolo[3,4-f][l,2,5]azadiborole 76, the product of a double diborylation of ethyne, was prepared by reaction of 187 and ethyne in a 2 1 ratio in the presence of a catalytic amount of Bp3-OEt2 (Equation 18) <2001ZEA1815>. 

Yet another approach to a l,2-dihydro-l,2-azaborine was elaborated by Matteson et al., in which the nitrogen atom was introduced last. The reaction between triborylmethide anions and aldehydes was used to give the four-carbon molecule (126). Treatment of this diborylated ethylene with liquid ammonia caused ring closure to give the pyrimido fused azaborine (127 Scheme 22) (78JOC950). 

Suggest experimental techniques for assessing the affinity of a host for a particular anion in solution. Which of these techniques might be appropriate for (a) podand (4.40), (b) diboryl hosts such as hydride sponge (4.69) and (c) zwitterions such as (4.35). 

The highly strained spirobicyclic boracyclobutene 63 isomerizes at 10°C to an isolable diborabicyclo[l,l,l]pentane 64, which undergoes a further cycloreversion reaction at room temperature, ultimately providing the structurally fascinating diboryl allene 65 (Equation 23) <1995AGE657, 1995AGE1340>. Stannylated boracyclobutene 66 reacts... 

The reaction is the basis for the metal catalyzed diborylation of alkenes.48... 

Platinum diboryl complexes (PPh3)2Pt(BCat)2 react with alkynes to give cis-diborylation products, via a Pt-vinyl intermediate.199 Alkynes are also known to insert into M—Si, M—N, and M—Cl bonds.200... 

Phosphine dissociation from platinum boryl intermediates (PPh3)2Pt(BCat)2 allows the coordination of an alkyne prior to the boryl transfer step. The diborylation of alkenes requires phosphine-free catalysts.45,46 Boron—boron bonds react with a,j8-unsaturated ketones with 1,4-addition.47... 

Diboryl compounds do not yield aldehydes with alkaline hydrogen peroxide. Instead there is r d hydrolysis, presumably via a boron-stabilized carbanion (see Volume 1, Ch ter 2.6) which is protonated and then oxidized to the alcohol (equation 16). ... 

The transition-metal-catalyzed diboration of alkynes can be directed to give highly selective di-addition yielding 1,2-diborylated olefins (equation 24). Depending on the catalyst. 

Diborylated organoboranes with a BCB backbone such as (17) are usually prepared by double hydroboration of terminal aUcynes (equation 29). " The treatment of the diborane B2pin2 with diazomethane derivatives provides an interesting alternative that allows for the synthesis of species pinB-CR2-Bpin (18, R=H, Ph equation 30), which are not accessible via hydroboration reactions. ... 

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