Hydroboration of 1,3-butadiene

Substituted diboranes derived from the hydroboration of 1,3-butadiene l,2-tetramethylenediborane(6), 1 and l,2-bis(tetramethylene)-diborane(6), 2 undergo symmetrical and unsymmetrical cleavage reactions 12). 

The hydroboration of 1,3 butadiene by diborane(6) yields polymeric material 113> which can be thermally cracked 114> to produce a variety of organoboranes and alkyl substituted derivatives of diborane(6) the structures of which have been in dispute. In particular, Koster and... 

The conjugation causes the deactivation toward the first hydroboration step. However, the formation of a homoallylic double bond in the hydroboration of 1,3-butadiene produces B-(3-buten-l-yl)-9-BBN, whose homoallylic double bond has reactivity comparable to that of 1-butene and second hydroboration, is much faster and produces 1,4-dibora product (Eq. 5.32) [3],... 

The hydroboration of 1,3-butadiene is carried out by adding the diene to the slurry of 9-BBN (2 molar equiv) in THF at -78 °C. The reaction mixture is slowly warmed to 25 °C, and BMS is added and redistribution is carried out at 25 °C for 14-18 h. The initially formed unstable [14] boracyclane, borolane, undergoes a rapid ring opening to form 1,6-diboracyclodecane (Scheme 32.2) [13]. 

Brown HC, Negishi El, Burke PL (1971) Organoboranes. 12. Hydroboration of 1,3-butadiene with borane in tetrdiydrofuran in a ratio equal to or smaller than 1 1— interconversion between B-alkylborolanes and 1,2-tetramethylenediboranes. J Am Chem Soc 93(14) 3400. doi 10.1021/ja00743a017... 

The significance of disproportionation is shown by the 1 1 reaction of 1,3-butadiene with HsB THF (THF = tetrahydrofuran), which produces a polymer best represented by I. The results can be rationalized by a mechanism involving hydroboration followed by a series of disproportionations (Scheme 2). 

The hydroboration of 1,3-dienes has also been reported, and these reactions generate Z-allylic boronic ester products. These reactions have been reported with palladium catalysts and are thought to occur through Tr-allyl intermediates. Reactions with butadiene and isoprene, followed by addition of the product to benzaldehyde, are shown in Equation 16.47. This equation also shows the presumed mechanism that proceeds by generation of a palladium allyl from the combination of diene and palladium hydride formed by oxidative addition of the borane. 

Caution. The reaction should be carried out in a well-ventilated hood and away from flames, since BMS, 1,3-butadiene, organoborane by-products, and methyl sulfide are flammable, odoriferous, and possibly toxic substances. BMS and the initial hydroboration product are sensitive to air and moisture. The inert atmosphere technique8 is therefore recommended. Peroxide-free tetrahydro-furan (THF) should be distilled from a small quantity of lithium tetrahydro-aluminate according to the procedures in Reference 8. BMS is a very concentrat-... 

Asymmetric hydroboration of 1-phenyl-1,3-butadiene (95) catalyzed by Rh-BINAP gave the corresponding optically active 1,3-diol 155 with 72% ee [89,90] (Scheme 2.15). Palladium-MOP complex also exhibited catalytic activity for the asymmetric hydroboration of but-l-en-3-yne (156), giving an optically active allenyl borane 157 [91]. 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

A similar example is seen in the [Pd2(dba)3]-catalyzed hydroboration of 2-methyl-l-buten-3-ynes [274]. While PPhj and PPh2(CgF5) favor the 1,4-addition product allenylborane 100 all diphosphines yield the 1,2-addition product ( )-dienylborane 102 exclusively (Table 1-13). This remarkable difference in selectivity is explained based on an 1,3-enyne monophosphine complex 103 and an alkynyl diphosphine complex 104 as intermediates. Dppf exhibits the best product yield among the phosphines tested. Similar observation was noted in the asymmetric hydroboration (Scheme 1-44) [275]. The action of catecholborane on 1-phenyl-1,3-butadiene also proceeds regioselectively to give, after oxidation, anti-l-phenyl-l,3-butanediol... 

Cyclic dialkylboranes are prepared by hydroboration of dienes with 1 1 HjB THF. Trifunctional boranes give with difunctional dienes polymeric organo-boranes. Fortunately, these products can be transformed into monomers or dimers by further reaction. Thus, the hydroboration product obtained from 1,3-butadiene is polymeric " . The initial products, which are dumbbell-shaped organoboranes, are cleaved by the remaining borane to give the polymer. 

These results for 1,3-butadiene, 1,4-pentadiene and 1,5-hexadiene allow the courses of thermal isomerization in other cases to be predicted. Hydroboration of 1,5-cycloocta-diene yields quantitatively a 72 28 mixture of 9-borabicyclo[3.3.1]nonane (9-BBN) (IX) and 9-borabicyclo[4.2.1]nonane (X), both of which exist as dimers. In refluxing THF, the latter can be isomerized to 9-BBN within 1 h. ... 

One approach to the borolane ring is by hydroboration of an appropriate butadiene. The triphenylphosphine- -hexylborane adduct (88) reacted with 2,3-dimethylbutadiene in the presence of benzyl iodide to give the borolane derivative (89) (Equation (18)) <81AG1098>. Hydroboration of l-(trimethylsilyl)-1,3-butadiene with BH3 SMe2 in ether, methanolysis, and thermal isomerization, in one pot, provided racemic B-methoxy-2-(trimethylsilyl)borolane (90) (Equation (19)) <89JA1892>. 

However, in contrast to compounds with either isolated or cumulative double bonds, electrophilic addition reactions with alkenes that contain conjugated n-systems routinely produce products resulting from the interaction of both double bonds. While hydroboration appears to be an exception, with each double bond reacting separately (and the second more rapidly than the first), conjugated dienes, such as 1,3-butadiene (CH2=CH-CH=CH2), suffer addition across each double bond ( 1,2-addition ) as well as across the entire conjugated system ( 1,4-addition ). Commonly, the products ( 1,2-addition and 1,4-addition ) are formed concurrently. 

The hydroboration of symmetrical nonconjugated dienes, such as 1,5-hexa-diene with 9-BBN, in a 1 1 molar ratio proceeds in an essentially statistical manner, giving approximately 25% residual diene, 50% monohydroboration product, and 25% dihydroboration product. On the other hand, conjugated dienes, for example, 1,3-butadiene, behaves differently and affords equal amounts of residual diene and 1,4-dihydroboration product. Conjugation markedly decreases the reactivity of diene toward hydroboration. Consequently, nonconjugated dienes like 1,4-hexadiene are selectively hydroborated with 9-BBN in the presence of 1,3-pentadiene (Scheme 5.6) [3]. 

Trimethylsilyl-substituted allenes [15, 16] undergo ready hydroboration with 9-BBN to afford the corresponding allylboranes. For example, hydroboration of l,2-bis(trimethylsilyl)-2,3-butadiene (2), readily prepared from 1 (40% yield) [15], with 9-BBN [17] affords the corresponding allylborane (3). Condensation of 3 takes place smoothly with aldehydes and ketones, and after basic or acid workup affords the [ ]- or [Z]-2-[(trimethylsilyl) methyl]-1,3-butadienes, respectively (Scheme 24.6 Table 24.13) [18]. 

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. 

Diels-Alder reaction of cyclopentadiene and butadiene affords a mixture of exo-5-vinyl-2-norbornene (la) and e do-5-vinyl-2-norbornene (lb) [1], Preparative GC separation [1] of these isomers encounters difficulties in obtaining the individual isomer in pure form and in large quantities. An alternative approach of separation via thermal isomerization [2], in which lb gets transferred to 4,7,3a,7a-tetrahydro-lff-indene, whereas la remains unchanged, is also not successful. This is because it is difficult to prevent la being contaminated by unreacted lb. As no other method is available for their separation, Inoue has reported [3] that hydroboration of 1 with 9-BBN, followed by oxidation with alkaline hydrogen peroxide results in the formation of alcohols 2a and 2b. The iodoether cyclization only of the endo isomer takes place. The sequence of approach is delineated in Scheme 29.1. 

Hydroboration of 5 with 9-borabicyclo[3.3.1]nonane (9-BBN-H) furnished the Y-(trimethylsilyl)allylboranes 1 (Scheme 1) (2-5). Condensation of la with a variety of aldehydes produced 6 in situ. Subsequent treatment with sodium hydroxide to induce a syn elimination and concentrated sulfuric acid to induce an anti elimination furnished the terminal 1,3-butadienes 7 and 8, respectively. The high geometry purity of the resulting 1,3-butadienes could be attributed to high diastereoselectivity during condensation. Similarly, all four geometric isomers of representative internal 1,3-butadienes were synthesized from lb and related compounds with high isomeric purity. 

Hydroboration of 1-trimethylsilylallenes with 9-borabicydo[3.3.1]nonane (9-BBN) affords the corresponding y-silyl allylboranes 199, which have predominantly the -geometry. Subsequent condensation of 199 with aldehydes proceeds to give the corresponding 1,3-butadienes with high diastereoselectivity after work-up (Scheme 2.125) [349-351]. This method has been applied to the conversion of 1-alkoxy- or 1-phenylthio-l-trimethylsilylallenes to the corresponding 1,3-dienes in both an E- and a Z-stereoselective manner, while the use of titanium reagents allows Z-selectivity or only low -selectivity [352, 353]. 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

B-3,3-Dimethylallyldiisopinocampheylborane (2). This related borane is prepared by hydroboration of 3-methyl-1,2-butadiene with either (-I-)- or (— )-diisopinocampheyl-borane. It reacts with aldehydes to give, after oxidation, products 3 with an irregular... 

Monohydroboration of allenes with 9-BBN places the boron atom exclusively at the terminal position. Thus, 3-methyl-1,2-butadiene gives B-3-methyl-2-butenyl-9-BBN as the only product (Eq. 43) However, such a procedure for the preparation of allylic boranes by the hydroboration of conjugated dienes and allenes is no t a general... 

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