B-Alkenyl-9-BBN

B-Alkenyl-9-borabicyclo(3,3,l)nonane (B-alkenyl-9-BBN) adds across the carbonyl group of a simple aldehyde to afford in good yields, the corresponding allylic alcohol S2). The reaction proceeds with complete retention of vinylic borane stereochemistry resulting in the /raw-disubstituted olefin linkage in the final product. The reac-... 

B-Alkenyl-9-BBN A borane/borinate exchange takes place at remarkably low temperature (0° vs. ca. 100° for borates and boranes) when alkenyldicyclohexylboranes and S-methoxy-9-BBN are mixed together in THF. The S-alkenyl-9-BBN are not directly accessible by hydroboration of alkynes because formation of 2 1 adducts ( e/n-bisboryl derivatives) predominates. 

The reaction involves the formation of carbanions that are stabilized by boryl groups . Similar carbanions are prepared from 9-methyl-9-borabicyclo[3.3.1]nonane (B-methyl-9-BBN), 9-alkenyl-9-borabicyclo[3.3. l]nonanes(B-alkenyl-9-BBN) , or alkenyldisiamylboranes with 2,2,6,6-tetramethylpiperidine (Li-TMP) in tetrahydro-furan (THF) ... 

The monohydroboration of internal alkynes is achieved with a 100% xs of the terminal alkyne to prevent the competing dihydroboration . The B-alkenyl-9-BBN... 

Carbonyl compounds are an important class among organic molecules. Literature records several methods for their synthesis. However, there are very few methods to convert carbon-carbon unsaturation to carbonyl compounds. Hydroboration of acetylenes, followed by oxidation provides a novel method for carbonyl synthesis. It has been noted that regioselectivities achieved in the monohydroboration of internal acetylenes with thexylborane [1], disiamylbo-rane [1], dicyclohexylborane [1], and catecholborane [2] are similar to, but less pronounced than, that realized by 9-BBN [3]. The B-alkenyl-9-BBN derivatives undergo oxidation to the corresponding ketones or aldehydes under aprotic conditions with trimethylamine N-oxide [4, 5] or under protic conditions by inverse addition to buffered hydrogen peroxide [3]. The inverse addition, i.e., the slow addition of the B-alkenyl-9-BBN in THF to the buffered H O, suppresses the otherwise undesirable protonolysis reaction and favors the oxidation pathway to the desired aldehyde or ketone. 

Excellent yields of the ketones are achieved from internal alkynes (Eq. 7.5 Table 7.8) [1] via hydroboration with 1 equiv of 9-BBN. The boron of 9-BBN adds to the less hindered carbon of carbon-carbon triple bond, and oxidation of the resulting B-alkenyl-9-BBN affords the corresponding ketones. 

B-Alkenyl-9-BBN p,y-Unsaturated ketone Yield (%) Isomeric purity (%)... 

B-Alkenyls-9-BBN prepared by the hydroboration of acetylenes with 9-BBN are transferred in a stereospecific manner to a,p-unsaturated carbonyl compounds in a 1,4-fashion (Scheme 7.6) [10]. The hydrolysis of the enolborinate provides the corresponding y,6-enones. 

Tabla 7.14 Conversion of alkynes into 4-alkenyl-2-butanones by the reaction of the corresponding B-alkenyl-9-BBN derivatives with methylvinyl ketone [ 10]... 

The conjugated cisoid ketones react satisfactorily with B-alkenyls-9-BBN, whereas transoid conjugated ketones, like cyclopentenone and cyclohexenone, afford a complex mixture of products. The reaction proceeds thermally and does not appear to involve radicals. The alkenyl group of borane is transformed with strict retention of configuration. As the reaction occurs only with cisoid enones, and thus suggesting a cyclic transition state (Chart 7.8) [10]. 

The terminal alkynes react with 2 equiv of 9-BBN and affords, quantitatively, the corresponding 1,1-diboraylalkanes. Soderquist has reported that this trialkyl-borane reacts with 1 equiv of benzaldehyde or 1-NaphCHO in 2 h at 25 °C, and quantitatively form B-ArCH20-9-BBN and B-alkenyl-9-BBN, exclusively with tram configuration [18]. The frans-B-alkenyl-9-BBN undergoes selective oxidation [18] with 1 equiv of anhydrous trimethylamine-N-oxide (TMANO) [19] and affords almost quantitatively the corresponding stable fraws-B-vinyl-9-oxa-10-borabicyclo[3.3.2]decane derivatives (traws-B-vinyl-OBBD). tram-B-Yinyl-OBBD derivatives are inert to atmospheric oxygen and are unreactive toward protonolysis (HOAc, 25 °C, 8 h) or insertion process (PhCHO, neat, 80 C, 6 h). The reaction sequence for the synthesis of fraws-B-vinyl-OBBD is outlined in Scheme 20.6 [18]. 

The other direct evidence comes from the polymerization of a borane containing a-olefin (B-alkenyl-9-BBN) as shown in Equation 1. 

Coupling of aryl or alkenyl halides with trialkylboranes.1 This reaction can be carried out in refluxing THF with a base (NaOH or NaOCH3,1 equiv.) catalyzed by PdCl2(dppf). A B-alkyl-9-BBN is more useful than a trialkylborane, since only one alkyl group is utilized. 

Most recently, B-halo-l-alkenyl-9-BBN s have revealed to undergo a 1,4-addition reaction to ae,P-unsaturated ketones such as methyl vinyl ketone. Hydrolysis of the initially formed intermediates produces the corresponding (Z)-5-halo-y,5-unsaturated ketones in good yields stereospecifically (>98%),88). In the reaction with methyl vinyl ketone, the major products isolated after hydrolysis are not the expected halo-enones but the aldol adducts, which are readily converted into the halo-enones by treatment with sodium hydroxide, as depicted in Scheme 3. No aldol adducts are formed in the case of enones other than methyl vinyl ketone. 

Catalytic coupling reactions of alkyl- [121,122] and 1-alkenyl- [123 —125] boronates usually proceed with a base. Cross-coupling between R3B or B-alkyl-9-BBN (BBN = borabicyclo[3.3.1]nonane) with aryl or 1-alkenyl halides, for example. 

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. 

Cyclopropyl arenes, alkynes and ALKENES from the IN srru GENERATION OF B CYCLOPROPYL 9 BBN AND THE SUZUKI-MIYAURA COUPLING OF ARYL, ALKYNYL AND ALKENYL BROMIDES... 

The double hydroboration of propargyl bromide with 9-borabicyclo[3.3.1]nonane (9-BBN-H) proceeds smoothly to afford the corresponding l,l-diboryl-3-bromo-propane adduct which is quantitatively converted to the B-hydroxy-B-cyclopropyl-9-BBN and / /3-hydroxy-9B BN complexes in equal molar amounts. Addition of this mixture to aryl, alkynyl and alkenyl bromides containing a catalytic (3%) amount of Pd[PPh3]4, after 8-16 hours at reflux temperature, provides the corresponding cyclopropylated arenes, alkynes and alkenes in good to excellent yields.111... 

Table 4.2 The cyclopropylation of aryl, alkynyl and alkenyl bromides with B-cyclopropyl-9-BBN generated in situ.

Hydroboration of the byproduct alkene gives isomeric B-alkyl-9-BBN products. Generally, hydrocarbon solvents are preferable to ether or THF for this process because the greater stability of the intermediate methoxyborate complexes (i.e. Li[R(MeO)-9-BBN]) at —78°C in these solvents prevents the product from being formed and competing with B-MeO-9-BBN for the alkyl-lithium reagent prior to its complete consumption. The complex is stable for alkenyl and alkynyl derivatives which require... 

B-trans-l-Alkenyls-9-BBN prepared in situ in THF react with a-halo carbanions generated from phenacyl bromide to afford the ( )-p,y-unsaturated ketones in good yields (Eq. 7.7 Table 7.13) [9]. 

Table 7.13 p.y-Unsaturated ketones from the reaction of B-frans-alkenyl-9-BBN derivatives with phenacyl bromide under the influence of 2,6-di-ferf-butylphenoxide [9]... 

The unknown B-(Z)-l-alkenyls-9-BBN have been prepared by the reaction of (Z)-l-lithioalkens with B-OMe-9-BBN followed by the treatment of the ate complexes with borontrifluoride-diethyletherate. The B-(Z)-l-alkenyl-9-BBN reacts in a stereospecific manner with a methylvinylketone to afford the corresponding (Z)-y,5-enones in good yields (Scheme 7.7) [11]. 

Conjugated dienones are found in nature [14], and both conjugated tram, trans-dienoes [15] and conjugated cis-trans dienones [16] are prepared easily in high stereospecific manner but in modest yields. Brown etal[ 7] have developed the conjugate addition-elimination reaction of B-l-alkenyl-9-BBN with the commercially available 4-methoxy-3-butene-2-one to provide the corresponding conjugated tram, tram-dienones in essentially the quantitative yields (Eq. 7.8 Table 7.17) [17]. 

Table 9.4 p.y-Unsaturated esters from a base-promoted a-alkenylation of ethyl bromoacetate with B-frans-l-alkenyl-9-BBN derivatives [11] ... 

---