B-alkynyl-9-BBN

The reaction of B-alkynyl-9-BBN compounds with propionaldehyde are summarized in Table 6.11 [6]. 

Unlike alkynyllithium, B-alkynyl-9-BBN compounds are prepared and stored until needed. These compounds thus add to carbonyl moiety in a Grignard-like fashion. 

In addition, B-alkynyl-9-BBN compounds exhibit high chemospecificity between less and more sterically hindered aldehydes. B-l-(3,3-dimethylbutynyl)-9-BBN reacts with propionaldehyde in less than 15 min, whereas its reaction with pivalaldehyde requires 5 days for completion under identical conditions. Similarly, its reaction with cyclohexanone is complete in 10 h at room temperature, whereas cyclopentanone requires 16 h at 65 °C. The introduction of Pitzer strain in going from sp to sp carbon center plays an important role in the rate of reaction ofB-alkynyl-9-BBN and cyclic ketones. Of special significance is the chemoselectivity of B-l-(3,3-dimethylbutynyl)-9-BBN with relatively reactive... 

A similar reaction with B-alkynyl-9-BBN compounds gives 2-en-4-ynones. The elimination of the /3-alkoxy group may be assisted by the Lewis acid character of boron and may take place through a cyclic transition state. 

The synthetic utility of the alkyne/B-X-9-BBN adducts has also been explored.152 (Z)-l-Alkynyl-2-halo-1-alkenes are the result of lithium alkynide action on these latter boranes (Scheme 76).157 The adducts resulting from the treatment of alkynes with B-X-9-BBN have been shown to be labile under protic conditions. However the decomposition has been harnessed and is an efficient preparation of 2-halo-1 -alkenes.158... 

The conjugate addition of B-(l-alkynyl)-9-borabicyclo(3.3.1)nonanes (B-l-alkynyl-9-BBN) to a,P-unsaturated ketones provides a convenient procedure for the preparation of y,8-acetylenic ketones81) (Eq. 35). Cuprous methyltrialkylborates react... 

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.

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. ... 

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... 

The rate of reduction with B- -octyl-9-BBN ofp-substituted benzaldehydes correlates with (p + 1.03) [12]. Relative rates ofp-substituted benzaldehydes with B-3-pinanyl-9-BBN gave a of + 0.49. Electron-withdrawing groups increase the rate of reduction of substituted benzaldehydes. This effect is presumably also important for alkynyl ketones [7], where the small steric size of the acetylene and its negative inductive effect can combine to make the reduction possible. 

B-l-Alkynyl-9-BBN compounds prepared [6-8] from the corresponding 1-alkynes and B-MeO-9-BBN, readily undergo addition to aldehydes and ketones, and afford the propargylic alcohols in very high isolated yields (Scheme 6.8) [9]. They are not basic and are safely used in the presence of compounds such as sulfoxides and diethylmalonate. 

B-l-alkynyl-9-BBN compounds are exceptionally mild reagents and are essentially inert at 25 °C toward alkyl halides, acid chlorides, amides, anhydrides, esters, nitriles, acetals, and ketals. 

B-l-Alkynyl-9-BBN, easily and quantitatively prepared by the reaction of boron trifluoride-diethyletherate with the corresponding lithium methyl alkynyldial-kylborinate [12], undergoes a smooth 1,4-addition in pentane at room temperature to methyl vinyl ketone (MVK) and ketones capable of adopting cisoid conformation [13]. The reaction proceeds through srx-membered cyclic transition state to afford the enol borinate intermediate, which on hydrolysis afford y,6-acetylenic ketones, in high yields (Scheme 7.8). 

Table 7.15 Conversion of alkynes into 4-alkynyl-2-butanones by the reaction of the corresponding B-l-alkynyl-9-BBN derivatives with MVK[13]...

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]. 

The B-alkyl, alkenyl, alkynyl, or aryl-9-BBN derivatives not available via hydroboration are obtained by alkylation of 9-BBN or B-C1-9-BBN or B-MeO-9-BBN with organolithium, Grignard reagent, organocadmium, lithium dialkylcuprates, (phenylthio)alkylcuprates, andhydroboration-carbonylation-reduction. 

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 reaction is extended to the synthesis of arylacetylenes and stereodefined enynes. The reactions sequence involves the preparation of thermally stable lithium complexes from B-OMe-9-BBN and alkyllithium in THE at -78 C. These complexes undergo Pd-catalyzed Suzuki coupling to both aromatic and olefinic substrates to produce a variety of alkynyl derivatives and enynes, with complete retention of the double bond geometry (Eq. 31.4) [4a]. 

Midland and others reported that B-isopinocampheyl-9-borabicyclo[3.3.l]no-nane [Alpine-Borane (7 )-79] is an effective reagent for the highly asymmetric reduction of alkynyl ketones to afford the propargylic alcohol 8030 (Scheme 4.3z). The reagent (R)-19 is prepared from (+)-a-pinene and 9-borabicyclo[3.3.1]no-nane (9-BBN) and often represented as 19banana. The levels of asymmetric... 

Organoboranes typically used in these reactions are Et3 B, i-PrsB, and alkylated 9-BBN derivatives. Interestingly, multiple organoboration steps may take place with di-, tri-, and tetra-1-alkynyl element compounds. Intermediates such as the... 

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]. 

B-Alkynyl derivatives of 9-BBN act as mild sources of nucleophilic acetylenic groups. Reaction occurs with both aldehydes and ketones, but the rate is at least 100 times faster for aldehydes." ... 

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