Hydroboration of allenes

The hydroboration of allenes or certain conjugated dienes leads to the synthesis of allylic boranes that have immense synthetic importance [1-3]. The chemistry of allylic boranes differs markedly from that of saturated analogs. Both vinylic and allylic boranes react readily with many substrates toward which trialkylbo-ranes are inert [4]. Mikhailov in his book [5] and review [6] has documented the synthetic applicability of allylic boranes, with cautions of high thermal reactivity with respect to allylic rearrangement, e.g., l-methyl-2-propenyl dialkylborane rearranges spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 5.34). 

In the case of hydroboration of allenes with 9-BBN, four types of organobo-ranes vinylboranes (I) allylboranes (II), 1,2-dibora (III), and 1,3-dibora (IV) species are expected (Chart 5.22) [7]. 

Among these species, Mikhailov [2] and Brown [8] have shown that allylboranes, especially those with low-molecular-weight, nonsterically hindered allyl groups are readily hydrolyzed and produce olefins rather than expected allylic alcohols under usual alkaline hydrogen peroxide oxidation conditions. Consequently, it is appropriate to convert the allylborane to a derivative that would give an unambiguous product on oxidation. Such a derivative is prepared by the reaction of allylborane with ketone, such as acetone, prior to oxidation [9]. The other potential adducts (I, 111, and IV) are inert to acetone under these conditions. After oxidation of this derivatized mixture, the quantity ofhomoallylic alcohol determines the amount of the attack at the terminal carbon of the allenic system (allylborane), the quantity of ketone determines the extent of internal attack (vinylborane), and the quantity of unreacted allene reveals the amount of dihydroboration (0% allene = 0% dihydroboration, 50% allene = 100% dihy-drob oration). The positions of the diol reveal the point of attack in dihydroboration. 

Hydroboration of propadiene leads to dihydroboration, and oxidation of the reaction mixture produces 1,3-propanediol (48%) and residual propadiene (50%) [7, 10]. It has been observed that rate of addition of the second 9-BBN is faster than is the rate of initial hydroboration. Moreover, this reveals that the attack of boron is exclusively at the both ends of the unsaturated chain in both steps, since no trace of 1,2-propanediol or acetone is detected. 

The presence of an alkyl group at one end of allene chain, significantly, alters the course of reaction as hydroboration of 1,2-butadiene with 9-BBN results in the formation of mainly allylboranes (Eq. 5.35) [7]. 

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Hydroboration of allenes 65 with pinacolborane in the presence of Pt(DBA)2 and a trialkylphosphine provides either the allyl boronate 66 or the vinyl boronate 67 regioselectively, depending on the stereoelectronic factors of the phosphine employed (Equation 2) <1999CL1069>. Allyl and vinyl boronates are synthetically important because of their ability to undergo nucleophilic addition to carbonyl compounds as well as transition metal-catalyzed cross-coupling. 

Hydroboration of allenes. With few exceptions, the 9-BBN hydroboration of allenes affords B-allylic-9-BBN derivatives. In contrast, the reactions of allenes with disiamylborane or dicyclohexylborane afiord predominantly vinylic boranes. With unsymmetrical allenes, 9-BBN binds to the less substituted carbon atom. Allene itself affords a 1,3-dibora derivative. The B-allylic-9-BBN derivatives are useful reagents for the allylic boration of carbonyl compounds. ... 

Allylic-9-BBN derivatives are also available by hydroboration of allenes with 9-BBN-H. Unlike disiamylborane, 9-BBN-H shows a marked preference for attachment of boron to one of the termini of the allene system rather than to the central carbon atom. Allene itself is dihydroborated by 9-BBN-H, but substituted allenes give rise predominantly to allylboranes (e.g. equation 34). ... 

Allylboron compounds can sometimes be obtained via hydroboration of allenes or 1,3-dienes with dialkylboranes (e.g. see Section 3.10.4.2). Allylboron compounds are particularly readily hydrolyzed, and hydrolysis is accompanied by transposition of the double bond. For example, tricrotylborane reacts with water at room temperature to give 2 mol of 1-butene and 1 mol of crotyldihydroxyborane (Scheme 13). 

Energetically this reaction can be expected to be dipole-HO-controlled ( Lumo - Fhomo = 15.5 eV), but this should have little regiocontrol since the LUMO of the allene is so little polarised. The LUMO(dipole)/ HOMO ene) interaction ( lumo - homo = 16.75 eV) might come into play, since the allene HOMO is polarised, but the orbitals do not match up to explain the high level of regioselectivity, since the larger coefficient in the LUMO of diazomethane is on carbon. Finally, the hydroboration of allenes gives mainly the product with the boron on the central carbon 6.407.895 With boron as the electrophilic atom, frontier... 

HB-bicyclo[3.3.1]CgHi4 Hydroboration of allenes 5.3.2.5.1 Hydroboration reagent 5.3.2.5.1 Isomerization 5.3.2.6.2 Reactivity toward functional groups ... 

Hydroboration and silylboration. Hydroboration of allenes with (dba)2Pt as the catalyst shows regioselectivity dependence on the phosphine ligands. ... 

Hydroboration. The nature of the phosphine additive plays a critical role in the hydroboration of allenes with pinacolatoborane under the catalysis of (dba)2Pt. Thus, different regioisomeric adducts can be prepared by variation of such additive. 

Hydroboration of allenes with pinacolborane provides the allylboronate or vinylboronates regioselectively depending on the bulk and basicity of the supporting phosphine ligand (eq 12). 

The B-allyl-9-BBN derivatives formed by hydroboration of allenes are utilized for addition of the allyl group to the carbonyl moiety of aldehydes, ketones, and other carbonyl derivatives (allylboration). Since allenes are easily prepared from corresponding olefins [12, 13], the route the olefin-allene-allylborane-carbonyl addition adduct is a general allylation procedure, which provides an attractive alternative to Grignard-based sequences for the syntheses of complex structures. 

Wang and coworkers [ 19] extended this methodology for the synthesis of various terminal 1,3-butadienes from the readily available variety of trimethylsilyl-substituted terminal allenes [15,16]. Hydroboration of allenes 7-12 with 9-BBN gives the corresponding allylboranes 13-18 (Scheme 24.7). Allylboranes 13-17 are predominantly E isomers ( Z > 91 9), while 18 favors Z geometry. ( Z = 28 72). Subsequent condensation of these allylboranes with hexanal, benzalde-hyde, acetaldehyde, or crotonaldehyde takes place smoothly to yield the corresponding 1,3-butadienes, after either basic or acidic workup (Table 24.14) [19aj. 

Hydroboration of allenes has long been known to give the corresponding allylbor-anes, which are useful for the preparation of allyl alcohols [346-348]. When 1-... 

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