Lithium alkenyl-borate

Before preparation of any a-halo boronic ester is undertaken, it should be noted that two sets of model conditions are outlined in what follows. If the group R1 of the boronic ester 1 is a typical alkyl group, then the rearrangement of the derived borate complex 2 requires a number of hours at 20 C, but if R1 is aryl or alkenyl, then shorter times and lower temperatures are required in order to avoid epimerization of the product 3 catalyzed by zinc chloride and lithium... 

Lithium triethyl(l-methylindolyl-2)borate has been introduced as a convenient source of indolyl residue for carbonylative cross-coupling with aryl iodides, alkenyl iodides, or triflates. The reaction requires elevated CO pressure and high loading of catalyst (5mol.%) (Equation (15)). Aryl and alkenyl bromides, as well as aryl iodides... 

Alkynyl(methoxy)borates prepared in situ from an alkynyllithium or sodium and 9-methoxy-9-BBN coupled with 1-alkenyl and aryl halides (Equation (210)).899-902 Addition of triisopropylborate to lithium acetylide yielded an air stable and isolable ate complex that couples with aryl and alkenyl halides (Equation (211)).903 904 Air and moisture stable alkynyltrifluoroborates were probably the most convenient reagents that allow handling in air and coupling reactions in basic aqueous media (Equation (212)).46... 

Alternatively, a similar borate intermediate can be obtained by the reaction of organoborate with chloromethyl- or (dichloromethyl)lithiums (eq (22) and (23)). When a chiral diol ester reacts w ith (dichloromethyl)lithium, an optically active a-chloroalkylboronate is produced with an excellent enantiomeric excess [34] (eq (23)). The C-Cl bond is then displaced readily w ith alkyl, aryl, or 1-alkenyl nucleophiles with inversion of configuration. The procedure has been extensively used for the preparation of chiral boronates, which have been used in organic syntheses [30]. 

The boron compound can be a borane (R jB), a borate ester (R -B(OR)2), or a boric acid (R -B(OH)2), where R is an allqrl, alkenyl, or aryl group. Boranes are made using hydroboration of alkenes or allgmes. Borates are made from aryl or alkyl lithium compounds and trimethyl borate. 

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. 

The lithium[2-tri- -butylstannyl-Z-l-alkenyl]-l-borates, prepared by copper catalyzed addition of Bu3SnB-(OCH3)-9-BBNLb to 1-alkynes (vide supra), are selectively coupled via organopalladium or organocuprate with allylbromide, exclusively at the vinyl boron bond. The process results into the exclusive or predominant formation of 5-tributylstannyl-l,4-(fi)-dienes (Chart 24.8) [38]. 

Like their aryl and alkenyl counterparts, alkynylboronic adds can be made by displacement of magnesium or lithium acetylides with borate esters. For example, Mat-teson and Peacock have described the preparation of dibutyl acetyleneboronate from ethynylmagnesium bromide and trimethyl borate [294]. The C-B hnkage is stable in neutral or acidic hydrolytic solvents but readily hydrolyzes in basic media such as aqueous sodium bicarbonate. Brown and co-workers eventually applied their organo-lithium route to boronic esters to the particular case of alkynylboronic esters, and in this way provided a fairly general access to this class of compounds [295]. 

G. Zweifel and S. J. Backlund. Synthesis of 1,4-disubstituted ( ,Z)-1,3-dienes from lithium dicyclohexyl(rrans-l-alkenyl)(l-aIkynyl)borates. J. Organometallic Chem., 1978, 156, 159. 

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