Hydroboration with 9-borabicyclo nonane

The 9-(3-pinanyl)-9-borabicyclo 3.3.1]nonane reagent may be prepared from either ( + )- or (—)-x-pinene by hydroboration with 9-BBN. The organoborane derived from (+)- -pinene exhibits a negative rotation and is designated as (—)-(/ )-Alpine-Borane. Normally, cx-pinene is 92% enan-tiomcrically pure. Enriched pinene may be prepared10 or obtained from commercial sources. 

Reduction of aldehydes labeled with deuterium or tritium leads to primary alcohols which are chiral by virtue of the label. Alternatively, the label can be introduced by the use of deuterated Alpine-Borane, which can be prepared by hydroboration with 9-borabicyclo[3.3.1]nonane-... 

We then moved to the synthesis of the Cjj-Cig segment (Scheme 10) (29). The alcohol 53 (33) was converted to 55 by a standard C2 homologation procedure in 77% overall yield in 6 steps. Swern oxidation of 55 gave the aldehyde 56 which was submitted to Lewis acid catalyzed crotylstannylation (34). This reaction provided the expected cryfhro-adducts 57a and 57b in a 3 1 ratio which were separated by medium-pressure silica gel chromatography. Protection of 57a as a MOM ether was followed by hydroboration with 9-borabicyclo[3.3.1]nonane to afford the alcohol 58 which was further converted to the sulfone 18 by the procedure shown above. 

Scheme 24) [40], Note that a direct Heck reaction on 105 would lead to the formation of the 5-membered ring instead. Thus the alkene was hydroborated with 9-borabicyclo[3.3.1]nonane (BBN) first after which the Suzuki reaction gave 32 as a mixture of diastereomers in 58% yield. 

Examination of many such addition reactions indicates that triple bonds are, usually, less readily attacked by electrophilic reagents than are double bonds. In concert with this summary statement is the observation that generally, compounds with double bonds and triple bonds will preferentially undergo hydroboration with 9-borabicyclo[3.3.1]nonane (9-BBN) (Scheme 6.70) at the double bond but will preferentially undergo hydroboration with bis(2,4,6-trimethylphenyl)borane (dimesitylborane) at the triple bond. (The former hydroborating reagent is more reactive than the latter, which is more sterically demanding.)... 

The size of the silyl groups in alkynylsilanes 48 effects their regio- and chemose-lectivity in hydroboration with 9-borabicyclo[3.3.1]nonane (9-BBN). Alkynylsilanes... 

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

Since borane BH3 reacts with only one or two equivalents of a sterically hindered alkene, it is possible to prepare less reactive and more selective borane reagents R2BH and RBH2 respectively. In addition to disiamylborane 8 and thexylbo-rane 10, the 9-borabicyclo[3.3.1]nonane (9-BBN) 14 is an important reagent for hydroboration, since it is stable to air it is prepared by addition of borane 2 to cycloocta-1,5-diene 13 ... 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

It has been shown that the cyclization step of 3-halopropylmalonic esters to give cyclobutane-1,1-dicarboxylatcs, e.g. 3. is rate-determining.4 A convenient synthesis of cyclobutyl derivatives via hydroboration has been reported.5 In this case, 4-tosyloxybut-1-yne was bishydroborated with 9-borabicyclo[3.3.1]nonane (9-BBN), and this was followed by treatment with methyllithi-um to give 9-cyclobutyl-9-BBN 4 in 65% yield.5 24... 

It is clear from these representative results that regioselectivity in hydroboration is controlled by steric effects. As a result, nonsymmetric internal olefins usually yield a mixture of regioisomeric alkylboranes when they react with borane. Several hindered mono- and diakylboranes with sterically demanding alkyl groups, however, have been developed for use in selective hydroboration. Disiamylborane [bis(3-methyl-2-butyl)borane, 34], thexylborane (2,3-dimethyl-2-butylborane, 35), and 9-BBN (9-borabicyclo[3.3.1]nonane) are the most frequently used reagents. Improvements of regioselectivities in hydroboration of both... 

Rates of hydroboration of (61 R = Me, Et, Pr, Bu, and Ph) with 9-borabicyclo-[3.3.1]nonane have been elucidated and found to decrease with increasing steric bulk of R no reaction was observed for R = Pr1. The products (62) are potentially valuable for the asymmetric reduction of prochiral ketones.61... 

Transfers of hydride from boron or lithium to carbon usually occur in the context of addition of the complete M—H moiety to polar or non-polar unsaturation. Additions of boranes to alkenes have been extensively reviewed (Brown et al., 1983a), but the experimental characterization of the hydroboration transition state remains problematic. Dialkylboranes, including 9-borabicyclo[3.3.1]nonane (Wang and Brown, 1980), borinane (Brown et al., 1984), and disiamylborane (Chandrasekharan and Brown, 1985) have now been shown to be dimeric in hydrocarbon and ethereal solvents. With unreactive alkenes, their additions are first order in alkene and half order in the dimer. With reactive terminal alkenes, the reactions are first order only in dimer, with intermediate behaviour between these extremes. A reaction scheme (10) involving reaction of monomeric borane with the alkene satisfies the data, with the observed order depending on the ratio k i/k2. 

Hydroboration of terminal acetylenes with boron halides permits the stereospecific synthesis of ( )-l-chloro(or bromo)alk-l-enes919. BBr3 adds to terminal acetylenes in a regio- and stereoselective manner to yield /Lhalovinylboranes, which are very useful syn-thons92a 923. B-bromo- and B-iodo-9 borabicyclo[3.3.1]nonane react similarly924"926. 

Preparative Methods NB-Enantrane may be prepared by hydroboration of nopol benzyl ether (commercially available, or from nopol and benzyl bromide ) with 9-Borabicyclo[3.3.1]nonane (9-BBN) (eq 1). 

Cross-coupling between 1-alkenyl- and arylboron compounds with organic electrophiles have found wide application (see Sect. 1.5.1.1) in organic synthesis [125, 132, 269-271]. The value of this methodology is further realized with the use of alkylboranes, such as B-alkyl-9-borabicyclo[3.3.1]nonanes (B-R-9-BBN), which can be conveniently prepared Ijy hydroboration of olefins [272]. The hydroboration proceeds with high stereoselectivity and chemoselectivity. The choice of phosphines in a catalytic system sometimes affects the chemo- and regioselectivity of the hydroboration. Hydroboration of l-(ethylthio)-l-propyne with catecholborane can be satisfactorily carried out with PdClj (dppf) but the regioselectivity is best for the dppe and dppp complexes of Ni [273]. Notably, PPhj complexes perform poorly. 

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