Diene and thexylborane

As an alternative method for the preparation of polyalcohols from organoboron polymers, dichloromethyl methyl ether (DCME) can be used (23). Various organoboron polymers prepared by hydroboration polymerization of dienes and thexylborane were reacted with DCME at 0°C in THF followed by treatment with lithium triethylmethoxide. After oxidative treatment with NaOH and H2O2, the corresponding poly(alcohol)s were obtained. The... 

High levels of asymmetric induction have been achieved in the hydroboration of 1,3-, 1,4-, and 1,5-dienes with thexylborane (482,483,489,490). The first chiral center is formed by an intermolecular reaction. In the second step, the organoborane intermediate undergoes an intramolecular hydroboration, creating the second chiral center with high diastereoselectivity. 

Generally, trialkylboranes are useful intermediates in the field of organic synthesis with versatile reactivity. The polymers prepared by polyaddition between diene monomers and thexylborane are polymer homologues of trialkylboranes, which can be converted to poly(alcohol)s, poly(ketone)s, and other polymers having some functional groups (scheme 4).8-12... 

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-... 

An excellent, general procedure for the stereoselective synthesis of conjugated 13,13 dienes utilizes 1-halo-l-alkynes and thexylborane (36). We prepared the compound XXXX (which is the 13,13 isomer of the Lobesia botrana pheromone XXIX) in >98% stereochemical purity by this procedure (Figure 19). 

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. 

Syntheses of /(-(-i-)-myoporone (em-12) and / -(-H)-10,l 1-dehydromyoporone (ent-14) have been published recently (22). These rely on the use of methyl 5-citronellate as the source of the asymmetric carbon (Scheme 3). Of more general interest and application is an approach (23) which leads to the stereospecific synthesis of 9-dihydromyoporone, which contains two asymmetric centres. This synthesis formally constitutes a synthesis of myoporone. Still and Darst (23) have shown that hydroboration of -2,6-dimethylhepta-1,4-diene with thexylborane leads, with high diastereoselectivity, to the diastereomeric 1,4-diols (Scheme 4). Appropriate transformation of the yn-diol to the electrophilic iodo compound, followed by alkylation of the P-keto sulphoxide, gives... 

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. 

Organoboron copolymers were prepared by polyaddition of a mixture of dienes with thexylborane [5]. When a mixture of dienes such as 1,2-diallyloxyethane and p-diallylbenzene was polymerized with thexylborane, the peaks in GPC using both UV and RI detectors were shifted to higher molecular weight regions with the increase of the amount of thexylborane. The molecular weight distribution of the... 

Hydroboration of terminal diyne gives organoboron polymer having branched structure due to the further hydroboration reaction toward the initially formed vinylborane structures. That is, terminal diynes are taken to have a potential as a multifunctional monomer. When the polymerization between 1,7-octadiene and thexylborane was carried out in the presence of a small amount of 1,7-octadiyne, the molecular weights of organoboron polymers were found to be increased when the ratio of diyne/diene was increased [5]. 

Organoboron copolymers were prepared by polyaddition of a mixture of dienes with thexylborane (19). When a mixture of dienes such as 1,2-diallyloxyethane and p-diallylbenzene was... 

As a typical example for hydroboration polymerization of internal diyne, the reaction of 3,9-dodecadiyne and thexylborane was carried out in THF at 0°C under nitrogen (20). The polymerization behavior was quite similar to hydroboration polymerization of dienes. The organoboron polymer obtained was a colorless wax and soluble in common organic solvents such as benzene, THF or chloroform. An IR spectrum of the polymer showed the absorption band assignable to C=C bond, which was also supported by H- and B-NMR spectra. 3,8-Undecadiyne and 3,10-tridecadiyne were also used in hydroboration polymerization with thexylborane to produce the corresponding organoboron polymers. 

Cis-olefins or cis./rjns-dienes can be obtained from alkynes in similar reaction sequences. The alkyne is first hydroborated and then treated with alkaline iodine. If the other substituents on boron are alkyl groups, a cis-olefin is formed (G. Zweifel, 1967). If they are cir-alkenyls, a cis, trans-diene results. The reactions are thought to be iodine-assisted migrations of the cis-alkenyl group followed by (rans-deiodoboronation (G. Zweifel, 1968). Trans, trans-dienes are made from haloalkynes and alkynes. These compounds are added one after the other to thexylborane. The alkenyl(l-haloalkenyl)thexylboranes are converted with sodium methoxide into trans, trans-dienes (E. Negishi, 1973). The thexyl group does not migrate. 

The polymers prepared from thexylborane and diene monomers were reacted with carbon monoxide at 120°C, followed by treatment with NaOH and H202 to produce a polyalcohol (scheme 5).13 This conversion includes migration of the polymer chain and thexyl group from the boron atom to carbon, as shown in scheme 5. When this reaction was carried out under milder condition, poly(ketone) segments were included in the polymer backbone due to an incomplete migration of the thexyl group. 

Thexylborane is more reliable than borane for permitting clean formation of ring compounds from dienes. Under kinetic control there is a preference for formation of five- or seven-membered rather than six-membered rings (equations 14 and IS). - This regioselectivity can be altered by modification of the substitution pattern or by thermal isomerization after the initial hydroboration. 

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. 

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... 

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... 

A pressure of 70 atmospheres of CO is required since hindered boranes, especially thexylborane, react sluggishly at atmospheric pressure. However, thexylborane reacted with cyclopentene to generate 188, and subsequent reaction with O-acetyl-hept-6-en-l-ol gave the trialkylborane (189). Carbonylation in the presence of water followed by oxidation gave the mixed ketone (190) in 73% yield. 33 In a similar manner, dienes can be transformed into the corresponding cyclic ketones. 34,135... 

Brown s ketone synthesis (2, 148 149). Brown s method for conversion of dienes into cyclic ketones via carbonylation of trialkylboranes has been used in a synthesis of ( )-3-methoxy-l,3,5(10)-estratriene (4). The key step is hydro-boralion of the diene 1 with thexylborane to form 2, which is immediately treated with carbon monoxide and then oxidized to give the ketone 3. This is cyelized to the A - -estratetraene and then reduced to give 4. 

---