Borane, derivatives alkenyl

Coupling reactions of alkyl boranes, formed by hydroboration of alkenes, with unsaturated halides (or triflates or phosphonates) is possible, and this reaction is finding increasing use in synthesis. For example, coupling of the alkyl borane derived from hydroboration (with 9-borobicyclo[3.3.1]nonane, 9-BBN) of the alkene 200 with the alkenyl iodide 201 gave the substituted cyclopentene 202, used in a synthesis of prostaglandin Ei (1.205). This type of B-alkyl Suzuki coupling reaction is very useful for the synthesis of substituted alkenes. 

J3-[(Z)-l-Bromo-l-alkenyl]borane derivatives offer potential for the conversion of 1-bromo-alkynes into ketones. Treatment of the adducts from the acetylene and dialkylboranes or alkylbromoboranes with sodium methoxide gives alkenyl-borinate or -boronate esters, either of which may be oxidized to the ketone (Scheme 14). When using dialkylboranes, thexylalkylboranes may be used with advantage. ... 

Palladium-mediated cross-coupling of 2-pyrone-5-boronate (63), prepared by coupling of 5-bromo-2-pyrone with pinacol borane (30), with a range of androster-one-derived alkenyl triflates 64 affords bufadienolide type steroids 65 in high yields (Scheme 3.29) [47]. 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

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 trialkylborane is derived from monoalkyl-(l,l,2-trimethylpropyl)borane by hydroboration. Care must be taken in the oxidation step leading to the a-chiral ketones and buffering is necessary to avoid racemization. However, even with these precautions, 2-6% racemization is observed in some cases9,20. The intermediate alkenyl-(l,l,2-trimethylpropyl)borinates are relatively resistant to oxidation thus prolonged reaction times or higher temperatures are sometimes required to complete the reaction via the haloalkyne route. 

Boron Derivatives. Diethyl(3-pyridinyl)borane 162 can be alkenylated under Suzuki conditions (Scheme 67). ... 

Allylic Alcohols. The allylic oxidation of alkenes mediated by selenium dioxide produces reduced selenium by-products which can be a disadvantage in the practical separation of the allyl alcohol products. A new procedure uses only catalytic amounts of selenium dioxide in the presence of t-butyl hydroperoxide to reoxidize and thus recycle the reduced selenium species, and leads to clean allylic oxidation with yields comparable to the stoicheiometric method. B-Alkenyl derivatives (27) of 9-BBN (3) react in a Grignard-like fashion with aldehydes to give allyl alcohols (Scheme 7) in which the alkene stereochemistry is retained from the vinyl-borane. 

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]. 

Functionalized alkenyl diamino- and dialkoxyboranes have been produced regio-and stereoselectively through addition of carbon- or heteroatomgenerated from bromotrichloromethane, thiols, phosphines and tributyltin hydride) to ethynylbis(diisopropylamino)boranes. The synthetic utility of these reactions was illustrated by the preparation of stereodefmed (Z)- or ( )-alkenylboronic esters via palladium-catalyzed crossStille reaction and a Suzuki coupling under basic conditions can be further conducted (Scheme 9.14) [33]. 

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. ... 

Organoboration studies of a variety of substituted stannyl alkynyl derivatives have led to a new fused heterocycle, chiral alkenyl-(N-azolyl)boranes,boron substituted slla and germa cyclopentadienes, two isomeric highly substituted boroles ... 

Carboxylic Acids and Acid Derivatives Carboranedicarboxylic acid (XXXVIII) and 1-alkyl (or alkenyl)car-borane-2-carboxyhc acids are most satisfactorily prepared by carbonation of the corresponding lithio- or sodiocarborane (Section III, C, 4) 327,334, 337,349). In an attempt to prepare carboranecarboxylic acid by carbonation of lithiocarborane the major acid obtained is carboranedicarboxylic acid 334). A lithium exchange [Eq. (148)] has been proposed which takes into account the influence of the carboxyl group. 

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