B- -l-alkenyl-9-BBN

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]. 

The B-( )-l-alkenyl-9-BBN derivatives exhibit exceptional stereospecific reactivity [12] and are prepared via the regio- and stereospecific monohydrobora-tion [13] of 1-alkynes with 9-BBN or dehydroboration [14] of the 1,1-diboryl adduct. For the synthesis of B-(Z)-l-alkenyls-9-BBN, the hydroborylation of 1-haloalkynes with 9-BBN, followed by treatment with tert-butyllithium afford [15] a mixture of two products, due to indiscriminate migration of both the hydride and cyclooctyl ring. The controlled catalytic hydrogenation of B-l-al-kynyl-9-BBN derivatives under a variety of experimental conditions also results in a complex mixture of products [16]. 

A stereospecific synthesis of B-( )-l-alkenyl-9-BBN derivatives that are not available by hydroboration reactions has been developed. The 9-BBN dimer has featured in thermal isomerization and thermolysis studies and similar studies on dimesityl-3-hexyl-borane show that, in comparison to the diphenyl compound, it is quite resistant to thermal isomerization and an explanation is offered. ... 

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. 

B-l-Alkenyl-9-borabicyclo[3.3.1]nonanes (1). These substances are prepared by hydroboration of alkynes with 9-BBN (2, 31 3, 24-29 4, 41 5, 46-47 6, 62-64). -Allylic alcohols. Usually reaction of organoboranes with carbonyl groups results in reduction. However, B-l-alkenyl-9-borabicyclo[3.3.1.]nonanes (1) add to simple aldehydes to give, after oxidation, allylic alcohols, with retention of configuration (equation I). The reaction is a Grignard-like synthesis and should be a useful route to allylic alcohols, even to those containing functional groups. 

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]. 

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]. 

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] ... 

B-trans-l-Alkenyl-9-BBN undergoes facile reaction with the a-halo carbanion generated from chloroacetonitrile in the presence of potassium 2,6-di-tert-butyl-... 

The B-(Z)-l-alkenyl-9-BBN, however, are prepared from (Z)-l-lithio-l-al-kene [17] and B-OMe-9-BBN. (Z)-l-lodo-l-alkene [18] on reaction with t-butyllithium in pentane affords the corresponding (Z)-l-lithio-l-alkene. The treatment of a solution of (Z)-l-lithio-l-alkene in diethylether-n-pentane mixture at -78 °C with B-OMe-9-BBN in THF affords the ate complex. Treatment of the ate complex with 1.33 equiv of borontriffuoride-diethyl etherate at -78 °C, followed by warming results in the formation of the desired B-(Z)-l-alkenyl-9-BBN (Chart 23.10 Table 23.7) [19]. 

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-... 

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) ... 

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. 

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... 

The stereodefined alkenyl halides are of prime importance due to the developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides, catalyzed by transition metal compounds [1-4]. 1-Halo-l-alkenes are conveniently synthesized via hydrobo-ration-halogenation reaction [5]. However, with these methods it is not possible to synthesize 2-halo-1-alkenes. Although the halometalation reaction is considered to be a powerful tool for the synthesis of 2-halo-1-alkenes, the reaction has not been adequately developed for such purpose [6]. Lappert and coworkers [7-10] are perhaps the first to report the systematic haloboration reactions of unsaturated hydrocarbons with BX,. However, there is no systematic approach to the haloboration with B-X-9-BBN to organic synthesis [11]. The desired B-X-9-BBN is prepared [12] by the reaction of 9-BBN with phosphorous pentachlo-ride, bromine, or HI. Alternatively, B-X-9-BBN are very conveniently obtained... 

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. 

Singleton and coworkers [40] have studied the regioselective behaviors of six 2-alkenyl- and c/s-1-alkenylboranes toward dienophiles for Diels-Alder reaction. The boranes are generated in situ from the reaction of BBr, BClj, 9-BBN or B-chloro-3,6-dimethylborepane [41] with 2-propenyltributyltin or cis-l-pro-penyltributyltin. 

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