Dienes, catecholborane

Hydroboration of conjugated dienes proceeds without a catalyst to give 1,2-adducts. However, the less reactive catecholborane reacts with isoprene with catalysis by Pd(PhiP)4, yielding the 1,4-adduct 73[66]. 

NaOH-HiOz) or to carboxylic acids (with w-chloroperbenzoic acid).-190 Double bonds can be hydroborated in the presence of triple bonds if the reagent is 9-BBN.191 On the other hand, dimesitylborane selectively hydroborates triple bonds in the presence of double bonds.192 Furthermore, it is often possible to hydroborate selectively one particular double bond of a nonconjugated diene.191 When the reagent is catecholborane, hydroboration is catalyzed by rhodium complexes, such as Wilkinson s catalyst.194 Enantioselective hydro-boration-oxidation has been achieved by the use of optically active rhodium complexes.195... 

An alternative procedure is to convert the carbonyl compound into the toluene-p-sulphonylhydrazone,12 followed by reduction with either sodium borohydride in acetic acid,13 or with catecholborane, followed by decomposition of the intermediate with sodium acetate or tetrabutylammonium acetate.14 The former method is illustrated by the conversion of undecan-6-one into undecane (Expt 5.6), and the latter by the conversion of acetophenone into ethylbenzene (Expt 6.4, Method B). A feature of these methods is that with a, / -unsaturated ketones, migration of the carbon-carbon multiple bond occurs thus the tosylhydrazone of isophorone gives 3,3,5-trimethylcyclohex-l-ene, and the tosylhydrazone of oct-3-yn-2-one gives octa-2,3-diene. 

Hydroboration proceeds without a catalyst, but hydroboration with less active catecholborane (225) is accelerated by catalysts. Usually 1,2-addition to conjugated dienes takes place, but the Pd-catalysed reaction of catecholborane (225) gives the 1,4-adduct 226. This reaction is not catalysed by Rh complexes [98], Hydroalumination of conjugated dienes catalysed by Cp2TiCl2 affords the allylic aluminium compounds 227 by 1,4-addition. The Pd-catalysed hydrostannation of isoprene with HSnBu3 affords the (Z)-2-alkenylstannane 228 with high regio- and stereoselectivities [99],... 

Catalytic hydroboration is a new methodology of great synthetic potential. The reaction is usually carried out with catecholborane in the presence of rhodium, palladium, iridium and ruthenium compounds.2 In contrast to olefins, very little is known on catalytic hydroboration of conjugated dienes and enynes. Our earlier studies on the uncatalyzed monohydroboration of conjugated dienes,6 reports on the hydroboration of 1-decene with catecholborane catalyzed by lanthanide iodides,7 and monohydroboration of 1,3-enynes in the presence of palladium compounds,8 prompted us to search for other transition metal catalysts for monohydroboration of conjugated dienes and enynes 9 10... 

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. 

Hydroboration of isoprene with catecholborane at 1 1 molar ratio in tetrahydrofliran in the presence of 1 molar % of NiCbCdppe) at room temperature was completed in 6 h. An interesting observation is a lower reactivity of 1-decene compared with isoprene under these conditions. It is the first case of higher reactivity of 1,3-diene than 1-alkene in the hydroboration reaction. It is also an indirect indication that borane is not involved in the reaction, since acyclic conjugated dienes are less reactive than 1-alkenes toward borane. Hydroboration of representative conjugated dienes in the presence of NiCUCdppe) was carried out on a preparative scale and the results are shown in Table 2.9 The mono-hydroboration products were isolated by distillation and oxidized to the corresponding unsaturated alcohols. 

Table 2 Monohydroboration Products of Conjugated Dienes and Enyrtes with Catecholborane Catalyzed by NiCltfdppe) in THF at 20 °C...

This synthesis can be accomplished in two steps. Hydroboration of 1 -hexyne with catecholborane forms a vinylborane. Coupling of this vinylborane with (Z)-2-bromostyrene gives the desired 1,3-diene. The E configuration of the vinylborane and the Z configuration of the vinyl bromide are both retained in the product. 

Related Reagents. Bis(bicyclo[2.2.1]hepta-2,5-diene)rho-dium Perchlorate [l,4-Bis(diphenylphosphino)butane](norbora-diene)rhodium Tetrafluroborate Catecholborane (1,5-Cycloocta-diene)[ 1,4-Bis(diphenylphosphino)butane]iridium(I) Tetrafluoro-borate (1,5-Cyclooctadiene)(tricyclohexylphosphine)(pyridine) iridium(I) Hexafluorophosphate Octacarbonyldicobalt Palladium (II) Chloride Tetrakis(triphenylphosphine)palladium(0) 2-Amino-3-picoline Benzylamine. 

In 1989, Suzuki and co-workers ° reported the reaction of conjugated dienes, snch as isoprene, with catecholborane in the presence of Pd and Rh complexes. In this reaction, Pd complexes were significantly more effective than Rh complexes (Table 6). 

One facile way to incorporate boryl moieties into diene-containing polymers is via hydroboration. The addition of a B—H bond to C=C bonds via hydroboration is not stricdy a metal-mediated process, but catalyzing it with transition metal complexes provides a number of benefits. For example, transition metal catalysts can alter selectivity of hydroboration and provide alternatives for manipulating regio-, stereo-, and chemoselectivity. Additionally, they can help accelerating the addition of boryl substituents when the reaction rates are very slow—e.g., addition of catecholborane. 

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