B-Methoxy-9-BBN

S-Allyl-9-BBN is prepared by the addition of an aluminium derivative of allyl bromide to B-methoxy-9-BBN. 

Since the neat reagent is most effective, the solvent is usually removed before reduction of the ketone. Brown has reported a synthesis using neat a-pinene and solid 9-BBN. The deuterium- or tritium-labeled compound may be prepared by hydroboration with labeled 9-BBN. Alternatively B-methoxy-9-BBN may be reduced with LiAlD4 (see Lithium Aluminum Hydride) in the presence of a-pinene. ... 

Allyllithium gives even higher yields of B-allyl-9-BBN than allylaluminum sesqui-bromide. However, the stoichiometry of the reaction must be controlled to avoid the formation of a diallyl ate complex, and the isolation of product is not simple. Alkylations of B-methoxy-9-BBN with the RMgX reagents meet with limited success. Only the methyl group is quantitatively transferred. Lower yields are obtained with other groups , e.g., n-Bu (42%), Ph (27%). 

Methoxy-3-butene-2-one also undergoes rapid conjugate addition with B-l-alkynyl-9-borabicyclo[3.3.1]noiianes with elimination of B-methoxy-9-BBN to give a 4-fra j-alkynyl-3-butene-2-one in almost quantitative yield. ... 

The inertness of crystalline (9-BBN)2 toward oxygen is due to the unusual stability of the B-H-B bridge in the dimer and is supported by the observation that neat B-methoxy-9-BBN, solid B-chloro-9-BBN, and solid B-hydroxy-9-BBN are all pyrophoric. Moreover, it is found that B-alkyl-9-BBN derivatives are very reactive toward oxygen, more so than the corresponding trialkylbo-ranes [16]. Hence, with the B-H-B bridge no longer present, the exposed boron atom makes these derivatives of 9-BBN unusually reactive toward oxygen. 

Unfortunately, B-ethynyl-9-BBN decomposes on working from -78 °C to room temperature. Evans et al [9] prepared B-[2-(trimethylsilyl)ethynyl]-9-BBN (Scheme 6.9) from (trimethylsilyl)acetylene and B-methoxy-9-BBN. The reagent is isolated under an atmosphere of nitrogen as a solid 1 1 complex with THE in 90% yield. Similar to reagents reported by Brown et al [6, 7], B-[2-(trimethylsilyl)ethynyl]-9-BBN reacts with a variety of aldehydes and ketones... 

On the other hand, the bulkier 2-(triisopropylsilyl)ethanol is prepared only from triisopropyl(a-methoxyvinyl)silane. This is based on Larsons findings that silyl enol ethers [2] undergo hydroboration with 9-BBN, followed by P-elimina-tion of the B-methoxy-9-BBN and subsequent rehydroboration. Consequently, this one-pot procedure [1] of hydroboration-dehydroboration-hydroboration gives the desired alcohol, 2-(triisopropylsilyl)ethanol (Chart 6.10) in 89% isolated yield. As evident in Chart 6.10, (a-methoxyvinyl)silane acts as the key intermediate for the preparation of either 1- or 2-(trialkylsilyl)ethanols. 

Yamamoto and coworkers [1] have developed a methodology for the preparation of stereochemically homogeneous allylic metals (Mg and Ba) from the corresponding allylic chlorides. These allylic metals react in high diastereose-lective-y-allylation with aldehydes in the presence of B-methoxy-9-BBN as an additive. With this method threo homoallylic alcohols are selectively obtained from ( )-allylic metal compounds and erythro isomers from (Z)-allylic metal compounds (Scheme 6.27) in excellent yields [1]. 

The simple workup procedure provides highly pure products (>97%) without the need for further purification of the crude material. The small excess of 4-methoxy-3-buten-2-one utilized in the reaction is conveniently hydrolyzed to water-soluble and/or highly volatile products (Eq. 7.10). The B-methoxy-9-BBN byproduct is oxidized to water-soluble cis-1,5-cyclopentanediol and boric acid. The whole sequence thus provides [18] clean products after simple extraction of the reaction mixture. 

Organoboranes participate in either ionic or radical reactions [1]. However, there are a few reports on the apphcations of organoboranes as initiators for radical reactions [2, 3]. It is reported [4] that in the presence of a catalytic amount of 9-BBN or B-hexyl-9-BBN, or to some extent B-methoxy-9-BBN, initiates the radical addition of alkanethiols to alkenes under very mild conditions to provide the corresponding dialkylsulfides almost in quantitative yields (Scheme 14.1). On the other hand, the radical reactions of alkenes with thiols are generally initiated by thermal decomposition of peroxides or azo compounds, by UV radiations, or by radiolysis [5]. The reaction initiated by 9-BBN is completely inhibited in the presence of galvinoxyl, a radical trapping agent. This confirms that 9-BBN participates in the initiation of radical addition (Table 14.1) [4]. 

B-l-Alkynyl-9-BBN derivatives are prepared as 1 1 THF complexes in excellent yields. B-Methoxy-9-BBN in THF on treatment with an alkynyllithium reagent at -78 °C results in the formation of an adduct. This adduct is stable and may be isolated as a complex with THF. However, the treatment of this ate complex with 1.3 equiv of BFj-OEtj at -78 °C, followed by warming to 25 "C results in the formation of trimethylborate, lithium tetrafluoroborate, and the desired B-l-alkynyl-9-BBN THF complex. This complex is easily isolated by evaporation of the THF solvent, extraction with pentane, and crystallization. The B-1-alkynyl-9-BBN THF complexes are stable crystalline solids that can be stored at room temperature for up to 1 year with no apparent decomposition (Chart 23.9 Table 23.6) [11]. 

The ate complex prepared from B-methoxy-9-BBN and Bu3SnLi [24] adds under copper(I)-catalyzed reaction to 1-alkyne to yield exclusively or predominantly lithium [2-tri-n-butylstannyl-(Z)-l-alkenyl]-l-borates [25]. These borates are selectively coupled in the presence of organopalladium or organo-cuprate with a variety of electrophiles (Chart 24.5), exclusively at the vinylcar-bon-boron bond to form a carbon-carbon bond (Table 24.8) [25]. The process is extremely versatile, as various functional groups are tolerated. However, an alkene with opposite regiochemistry (entry K, Table 24.8) [25] is obtained when BF,-Et,0 is added to the initial reaction, which is then refluxed. 

Further, it is found that B-OMe-9-BBN is for more powerful than any other species examined (Table 25.13). Another advantage of this reagent is its easy removal from the product. B-Methoxy-9-BBN forms the hydroxy derivative, which is soluble in sodium hydroxide as the ate complex. Consequently, it provides a practical method for the reduction of esters in the presence of reducible groups such as chloro and nitro (Table 25.13) [39]. 

Table 25.13 Reduction of esters by lithium borohydride in refluxing ether in the presence of B-methoxy-9-BBN or methyl borate [39]...

An unprecedented electronic preference for the meta product in Diels-Alder reaction of [(trimethylsilyl)ethynyl]-9-BBN has been reported [36]. [(Tri-methylsilyl) ethynyl]-9-BBN-THF is prepared (Eq. 30.3) [36] in 97% yield by the reaction of (trimethylsilyl) ethynyllithium and B-methoxy-9-BBN. The reagent [(trimethylsilyl) ethynyl]-9-BBN-THF is crystalline but is air sensitive. However, the reagent is indefinitely stable in a freezer under an inert atmosphere. 

Lithiation of 19 with t-butyllithium followed by B-methoxy-9-BBN and 4/3 BF3 OEtj produced in situ the allenylboranes 2 (Scheme 4) 8,9). Subsequent condensation with 9 furnished 20 with high diastereoselectivity, allowing stereoselective transformations to the ( )-enyne-allenes 21 and the (Z)-enyne-allenes 22. 

All examples mentioned so far correspond to reactions between two aromatic groups, however, couplings in which one or both partners are alkyl groups can be achieved using electron-rich boron-based nucleophiles. Fiirstner has reported the use of B-alkyl or 5-allyl methoxy-9-BBN anions for the efficient coupling with some aryl chlorides using an in situ prepared IPr HCl/Pd(OAc)j system [118], Some of the results obtained with these easy-to-handle borate-based nucleophiles are shown below (Scheme 6.34). 

More recently, Brown and co-workers have demonstrated that six-membered boracyclanes are likewise capable of ring contraction. As an example, the light-induced reaction of bromine with 9-methoxy-9-borabicyclo[3.3.1]nonane (61) in the presence of water affords the boronic acid 62 which is in turn readily oxidized to the alcohol in 65 % overall yield.11S) The structurally related B-alkyl-9-BBN derivatives react under comparable conditions but in the dark to deliver analogous products.116)... 

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. 

BF3 Et20 to remove the methoxy moiety. The procedure has also been used for the preparation of cA-vinyl-9-BBN derivatives since the normal route to such derivatives based upon the hydroboration of 1-haloalkynes, followed by hydride-induced rearrangement gives ring expansion products competitively with (Z)-l-halovinyl-9-BBNs. " Similar behavior has been observed for the reaction of a-methoxyvinyllithium with B-alkyl-9-BBNs (see 1-Methoxyvinyllithium) ... 

Soderquist and coworkers [17] have reported the synthesis of stable c/s-vinyl-OBBDs. TheB-methoxy-9-BBN on selective oxidation with anhydrous trimeth-ylamine-iV-oxide (TMANO) (85%, CHClj, 0°C) affords the corresponding borinic ester. The borinic ester on alkynylation, followed by demethoxylation gives the stable alkynylborinate. The hydroboration of alkynylborination with dicyclohexylborane (Chx BH) affords cleanly the 1 1 gem-diboryl adduct This is selectively protiodeborylated with acetic acid at 0 °C and gives the corresponding cis-B-vinylborinate. The reaction sequence is outlined in Scheme 20.5 [17]. 

Deprotonation of l,3-bis(trimethylsilyl)propyne with Bu Li and subsequent addition of carbonyl compounds in the presence or absence of an additive such as MgBr2 has been reported to furnish the enyne products with moderate to good Z-selectivity (Scheme 2.84) [235, 236]. It has been reasonably postulated that the reaction proceeds via a kinetically controlled pericyclic process as illustrated by intermediate 134, where steric factors and the interaction between the counter ion and the oxygen atom play major roles. Other additives are also employed in Z-selective enyne formation reactions, such as Ti(OPr )4, B(OMe)3, and B-methoxy-9-borabicydo[3.3.1]nonane (B-MeO-9-BBN) [225, 226, 237-240]. 

Yamamoto et al. ° have examined the addition of allyl organometallic reagents to a-alkoxyaldimines (40) derived from (S)-2-methoxy(methoxy)propionaldehyde and (/ )- and (S)-l-phenylethylamine (equation 10). The results are summarized in Table 10. Chelation control with allyl-AlEtaMgCl, -MgCTl and -ZnBr (entries 1-3, Table 10) and nonchelation (Cram) control with allyl-Ti(Pi O)3, -B(OMe)2 and -9-BBN (entries 4-6, Table 10) parallels that observed in the allyl metal-a-alkoxyaldimine additions (involving aldimines that lack a chiral nitrogen substituent) shown in Table 6. The chirality of the a-alkoxy... 

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]. 

B-l-Alkynyls-9-BBN easily and quantitatively prepared from lithium methyl al-kynyldialkylborinate [12] undergo a facile condensation with readily available 4-methoxy-3-buten-2-one in hexane at room temperature to provide, in excellent yield, conjugated enynones (Eq. 7.9) [18]. 

Table 7.18 Conversion of alkynes into 4-alkynyl-3-buten-2-ones by the reaction of the corresponding B-l-alkynyl-9-BBN [18] derivatives with 4-methoxy-3-buten-2-one[18]...

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