Hydroboration cyclic, with dienes

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-... 

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 ... 

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. ... 

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... 

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... 

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. ... 

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...

Two equivalents 1-alkyne with thexylborane produces the thexyldialkenylboranes . Thexylalkylalkenylboranes can be obtained either by sequential hydroboration of an alkene-alkyne pair with thexylborane [Eq. (ct)] or via hydroboration of an alkene with thexylchloroborane, followed by reduction and hydroboration of an alkyne [Eq. (cw)]. Cyclic trialkylboranes are synthesized via hydroboration. Thus, HjB-THF reacts 2 3 with simple acyclic dienes to give predominantly the dumbbell-shaped organoboranes. When heated, they undergo isomerization to the more thermodynamically stable isomers. These empirical rules predict their thermal behavior (a) the thermal... 

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>. 

Studies relating to the stereoselective hydroboration of polyfunctional alkenes continue to appear. Various vinyl substituted pyridines, thiophenes and furans have been reacted with representative hydroborating agents,11" and further aspects of the hydroboration of cyclic dienes have been disclosed.11 The asymmetric hydroboration of cyclic enol ethers and enamines (heterocyclic alkenes) provides access... 

Unsymmetrical cyclic ketones can be formed by the hydroboration of dienes followed by the oxidation sequence just described. In a typical example, Bryson reacted 176 with thexylborane but sodium cyanide was present (in situ) and this mixture generated 177. When this product was treated with trifluoroacetic anhydride... 

Cyclization of dienes. Reaction of dienes with diborane in THF at 0 and then with alkaline silver nitrate at 20° results in cyclic products. This reaction is an extension of the coupling of olefins via hydroboration (2, 368). ... 

Full details have been published on the hydroboration of acyclic oteo-dienes, using monochloroborane etherate, and on the conversion of the cyclic boranes so obtained into carbocycles. Thus hexa-1,5-diene was treated with monochloroborane to give a mixture of chlorodialkylboranes which were thermally depolymerized and treated with methanol (Cl - OMe) and dichloromethyl methyl ether to give a mixture of cycloheptanone and 2-methylcyclohexanone (74% total yield) in a 90.5 9.5 ratio, respectively. 2,5-Dimethylhexa-1,5-diene was similarly converted into 3,6-dimethyl-cycloheptanone. ° Hepta-1,6-diene has been converted into cycloheptane (67%) by treatment with diborane and then alkaline AgNOj in a new cyclization procedure. Diketones (35 n = 1, 2, or 3) cyclize to give acylcyclopentenes rather than cyclo-heptenones. ... 

Hydroboration of cyclic 1,3-dienes also results into three major types of or-ganoboranes [4] allylboranes, homoallylboranes, and dibora species. As usual oxidation of cyclic allylboranes also produces olefins rather than allylic alcohols, the allylboranes are indirectly estimated by reacting them with acetaldehyde to derivatized product, before oxidation. Homoallylic and dibora species are inert to acetaldehyde. The oxidation of derivatized product affords homoallylic alcohols, which estimate the amount of allylborane. The amount of underiva-tized homoallylic alcohol indicates the percentage of homoallylborane, and the amount of unreacted dienes gives the amount of dihydroboration (0% diene = 0% dihydroboration, 50% diene = 100% dihydroboration). 

Brown HC, Bhat KS (1986) Hydroboration 76. Hydroboration of cyclic dienes with representative hydroborating agents. J Org Chem 51(4) 445 49. doi 10.1021/jo00354a005... 

Hydroboration of dienes to cyclic boranes defines the stereochemistry of the diols obtained on treatment with sodium hydroperoxide in this way high stereoselectivity has been obtained in the conversion depicted in Scheme 12. ... 

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. 

A detailed study has been made of the hydroboration of cyclic dienes with representative reagents and has shown that high yields of monohydroborated products are obtained with six-menibered diene systems. As ring size increases,however,the proportion of this... 

When compared with C2 symmetric ligands, Ci symmetric and bidentate NHCs induced the highest ee values (<99% ee) with unsaturated C-C bonds and [B(pin)]2 in MeOH. For instance, in the p-boration of a- and p-unsatu-rated aldehydes and esters, which are more reactive than other C-C unsaturated substrates, a 213-derived system is more enantioselective than the 214-derived complex (74% ee vs. 59% ee, respectively) (Scheme 13.8). Improved results were obtained by Hoveyda and co-workers, using bidentate 217-220 with acyclic and cyclic internal alkenes, alkynes, 1,6-dienes and various allenes ee s of <98% were achieved (Scheme 13.8). For the regio-and enantioselective hydroboration of a,p-unsaturated carbonyl compounds... 

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