1.4- Pentadiene hydroboration

The (partial) description of the synthesis and coupling of the five fragments starts with the cyclohexyl moiety C —C. The first step involved the enantio- and diastereoselective harpless epoxidation of l,4-pentadien-3-ol described on p. 126f. The epoxide was converted in four steps to a d-vinyl d-lactone which gave a 3-cyclohexenecarboxylate via Ireland-CIaisen rearrangement (cf. p. 87). Uncatalysed hydroboration and oxidation (cf. p. 131) yielded the desired trans-2-methoxycyclohexanol which was protected as a silyl ether. The methyl car-... 

The hydroboration of 1,4-pentadiene is similar. The product is 70 30 polymeric organoborane and borinane along with a trace of 2-methylborolane. In contrast, the... 

Consequently, the hydroboration of 2,4-dimethyl-1,4-pentadiene, which is also representative for 1,5-hexadienes, is ... 

Convenient procedures are available for isomerically uncontaminated borinane which can be obtained by the hydroboration of 1,4-pentadiene in a 3 2 ratio, followed by thermal isomerization and exchange with borane ... 

The 1 1 hydroboration of 1,4-pentadiene with H3B>THF gives, after oxidation, a 45 55 mixture of 1,4- and 1,5-pentanediols. Upon heating this kinetic, 1 1 hydroboration... 

Table 3. Thermal Treatment at 170°C of the 1 1 Hydroboration Product Derived FROM 1,4-Pentadiene and Borane...

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. ... 

Hydroboration of a.,oi-dienes. Cyclic hydroboration of ,cu-dienes can be effected with this reagent. The initial product is often polymeric, but six- to eight-membered cyclic B-chloroboracycloalkanes can be obtained after depolymerization by distillation in good yields. The B-chloroboracycIoaikanes (2) and (3) from 1,4-pentadiene (1) were transformed into the cyclic ketones (6) and (7) by reaction with a,a -dichloromethyl methyl ether (5, 200 203) followed by oxidation. ... 

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]. 

Internal allenes on hydroboration with 9-BBN again give the allylboranes as the major product, as is seen in the case of 2,3-pentadiene (Eq. 5.39) [7]. Di-cyclohexylborane, on the other hand, gives mixture of both vinyl- and allylboranes. 

In sharp contrast to the behavior of 9-BBN, hydroboration of an internal allene with disiamylborane and dicyclohexylborane reveals preferential attack of the boron at the central carbon of the allenic chain, forming vinylboranes. For example, 2,4-dimethyl-2,3-pentadiene is converted [7] essentially, quantitatively, to the vinylborane with dicyclohexyl- and disiamylboranes, but exclusively to the allylborane with 9-BBN (Chart 5.24). In sharp contrast to acyclic allylborane, these allylborane products of 9-BBN do not react with acetone however, it undergoes usual alkaline hydrogen peroxide oxidation to afford the corresponding allylic alcohol. No protonolysis is observed on oxidation. This anomalous behavior of the allylborane is due to the steric bulk surrounding the boron atom, which prevents the coordination of oxygen of the acetone or water, necessary for allylboration or protonolysis [2, 8]. 

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