Dienes cyclic hydroboration

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 double bonds in a conjugated diene are hydroborated separately, that is, there is no 1,4 addition. However, it is not easy to hydroborate just one of a conjugated system, since conjugated double bonds are less reactive than isolated ones. Thexylborane °(48) is particularly useful for achieving the cyclic hydroboration of dienes, conjugated or nonconjugated, as in the formation of 53." ... 

Stereoselective cyclic hydroboration. The hydroboration of l,4 and 1,5-dienes with thexylborane generally results in boracycles.2 If the intial hydroboration produces an asymmetric center, the second intramolecular hydroboration can be effected with remote asymmetric induction. Thus hydroboration of the diene 1 with thexylborane (1.25 equivalent) followed by oxidation results in a 6 1 mixture of the diastereomers 2 and. 3. An even higher induction is obtained in the case of the 1,4-... 

Cyclic hydroboration (10, 397). Two laboratories1,2 have reported stereoselective syntheses of the Prelog-Djerassi lactonic acid (4), both of which involve cyclic hydroboration of a slightly different 1,5-diene. One of these syntheses is outlined in... 

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

The use of thexylborane (1,1,2-trimethylpropylborane) as the hydroborating agent permits (a) the synthesis of mixed trialkyboranes, and (b) cyclic hydroboration of dienes. When followed by carbonylation, these hydroboration-carbonylation sequences generate a variety of unsymmetrically substituted ketones and cyclic ketones, respectively. Since the thexyl moiety exhibits a low migratory aptitude in carbonylation reactions, it serves as an anchor group. 

Brown, H. C., Negishi, E. Cyclic hydroboration of dienes. Simple convenient route to heterocyclic organoboranes. Pure Appl. Chem. 1972, 29, 527-545. 

A tandem carbocyclization/hydroboration sequence has been disclosed by Molander [83], The carbocyclization of a,co-dienes proceeds smoothly in the presence of the divalent samarocene Cp 2Sm(THF) and the metal-alkyl intermediate is trapped by a l,3-diaza-2-boracyclopentaneto afford the cyclic hydroboration product (29). 

Octadiene and higher a,co-dienes with 1 1 HjB-THF leads to polymeric material. Low yields of cyclic products are also obtained in the hydroboration of mixed dienes, e.g., 4-vinylcyclohexene or limonene (see Table 7), but 1,5-cyclooctadiene undergoes cyclic hydroboration in high yield. A mixture of 9-borabicyclo[3.3.1] nonane (9-BBN) and 9-borabicyclo[4.2.1]nonane is formed. Other intermediates are also observed by B NMR. Upon heating, the latter compound is transformed " to the more thermodynamically stable 9-BBN ... 

The difunctionality of monoalkyl- and monoarylboranes suits them well for cyclic hydroboration of dienes. The B-alkyl- and B-arylboracycloalkanes can be formed in excellent yields ... 

The cyclic hydroboration of a,oj-dienes with monoalkylboranes leads to boracycles, often producing these ring systems in high isomeric purities. An early example of this process is represented by the cyclic hydroboration of divinyl ether (1) with t-butylborane-trimethylamine complex (Equation (1)) <61JA2541>. 

Diols from dienes via stereospecific cyclic hydroboration C = C CHC(OH)... 

Borinane is earlier prepared via the cyclic hydroboration of the corresponding diene (Eq. 17.1) [2] in 40% yield. 

Brown HC, Negishi El (1972) The cyclic hydroboration of 40. dienes. A simple convenient route to heterocyclic oigtmoboranes. 

Cyclic hydroboration of acyclic non-conjugated dienes has been explored as a stereoselective approach to acyclic diols in principle the diastereomeric relationship of the chiral centres produced depends on the overall conformation of the transition state leading to the boracycle. As an example, 2,6-dimethyl-hepta-1,5-diene leads to a 6 1 ratio of diastereomeric 1,5-diols (Scheme 29), the major product deriving from a boat-like transition state (64), which has the non-hydrogen substituent equatorial and the carbon-carbon ir-bond eclipsing a B—H bond in the favoured arrangement for formation of a boracycle. 

Brown, H.C. and Negishi, E. (1967) Carbonylation of the organoboranes from the cyclic hydroboration of dienes with thexylborane. Simple procedure for the conversion of dienes into cyclic ketones. Journal of the American Chemical Society, 89,5A11-5A7S. 

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

Dienes. The bifunctionality of dienes makes their hydroboration more complex than that of simple alkenes. Competing hydroboration of the two double bonds may lead to mixtures of products arising from mono- or diaddition. Additionally, cyclic or polymeric organoboranes may be formed. Differences in the reactivity of the two double bonds and the use of appropriate hydroborating agents, however, may allow selective hydroboration.29,330... 

More remote stereoselcction is achieved in the hydroboration of 1,4- and 1,5-dienes with thexylborane. The first stereogenic center is created by intermolecular hydroboration of the terminal double bond. This then directs the intramolecular addition to the second double bond proceeding via a cyclic transition state and leading to high levels of selectivity46. 

The hydroboration of simple, open-chain dienes can be somewhat complicated, leading to mixtures of regioisomers, cyclic and open-chain structures, and oligomeric structures of different sizes, all depending on reaction conditions and/or reactant proportions. - However, by control of stoichiometry, thermal isomerization of product mixtures, control of temperature during the hydroboration stage, subsequent treatment of a trialkylborane with further borane, or similar tricks , it is often possible to arrive at an unique product. Several important cyclic dialkylboranes can be made in this way, as shown in equations (6), (7) " and (8). Similar treatment of trienes can lead to bicyclic systems (e.g. equation 9). -24..to... 

Hydroboration of acyclic, symmetrical, nonconjugated dienes with one equiv. of 9-BBN-H produces almost statistical mixtures of mono- and di-hydroborated species, but cyclic analogs may show substantial deviations from the statistical mixture. For example, monohydroboration of 1,5-cyclooctadiene occurs to the extent of 85% using 1 1 stoichiometry (equation 31), ° whereas disiamylborane gives predominantly dihydroboration product under such conditions. 

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. 

Table 7. Synthesis of Cyclic Dialkylboranes via Hydroboration of Dienes with...

---