Alkynes boron halides

Borylated alkynes that are stabilized through Lewis base coordination or r-donation from electron-donating amino and alkoxy substituents on boron are well studied. Their synthesis typically involves either the reaction of a bis(dialkylamino)haloborane with an acetylide to form a diaminoborylacetylene (equation 31), or the treatment of stannylated alkynes with boron halides. However, while alkynylboranes are readily available, in the absence of steric protection or electronic stabilization they tend to decompose thermally. 

The situation is further complicated when these isomers react with more alkyne. This reaction may then proceed via either transfer of X, or of the alkenyl group. The products of BX3 (X = Cl, Br, I) with PhC=CH are reformulated from the original assignments . The formation of products depends critically on conditions which may not always be reproducible, but the addition of boron halides to carbon-carbon triple bonds is an important step to heterocyclese.g. ... 

Insertion reactions of alkynes into palladium-element bonds is an important route to palladium(ii)-alkenyl complexes. Insertion of alkynes into palladium-boron bonds arising from oxidative addition of boron halides gives complexes 38 (Scheme 8). An X-ray structure was reported for = COeBu Alkenyl- and dienylpalladium(ii) compounds 39... 

Boron Halide-Mediated Coupling of Aromatic Aldehydes with Alkynes. 

Boron trihalide-mediated coupling reactions of aryl aldehydes and alkynes have been reported (Scheme 23.21). ° Interestingly, the stereochemistry of the diene product depends on the boron halide used. If boron trichloride is used, the reactions afford (Z, )-l,4-pentadienes as major products, while (Z,Z)-l,4-dienes are obtained using boron tribromide. In order to avoid dihalogenation reactions of the aryl aldehydes, reactions must be carried out at low temperatures. 

SCHEME 23.21 Boron halide-mediated aryl aldehyde-alkyne coupling. 

Allenyllithium reagents are commonly prepared through lithiation of propargylic halides or by deprotonation of alkynes or certain allenes (Eq. 9.1). Lithiated allenes often serve as precursors to stable allenylmetal compounds such as stannanes or silanes. They can also be employed for the in situ synthesis of allenylzinc, -titanium and -boronate compounds, which can be further transformed to substitution products not accessible from their allenyllithio precursors. 

Extension of this reaction to electrophiles other than aldehydes was unsuccessful [22, 23], However, propargylic boronates were found to react with allylic halides and various carbonyl compounds [23], The boronates were prepared by lithiation of a methyl-substituted alkyne with t-butyllithium followed by treatment with a trialkylborane. The propargylic boronate preferentially reacts with the electrophile at the y-position to yield propargylic products (Eq. 9.20). The methodology has also been applied to alanates with comparable results. 

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

An example of addition of sulfenyl halides to an alkyne is shown in Scheme 1928 where the initial trans adduct of 1,4-dichlorobut-2-yne is further transformed by treatment with base or aluminum amalgam. Another example involves boron trifluoride-catalyzed addition of an arenesulfenamide to alkynes in the presence of acetonitrile, giving (3-acetamidinovinyl sulfides (Scheme 20),29 which can be hydrolyzed to (3-keto phenyl sulfides.30... 

Boron-sulfur bonds, addition, to alkynes, 10, 778 Boron trihalides, in boron compound synthesis, 9, 146 Boron-zinc exchange and copper-catalyzed substitutions, 9, 518 for organozinc halide preparation, 9, 89 Borostannylation, enynes, 10, 334... 

The coupling of terminal alkynes with aryl or vinyl halides under palladium catalysis is known as the Sonogashira reaction. This catalytic process requires the use of a palladium(0) complex, is performed in the presence of base, and generally uses copper iodide as a co-catalyst. One partner, the aryl or vinyl halide, is the same as in the Stille and Suzuki couplings but the other has hydrogen instead of tin or boron as the metal to be exchanged for palladium. 

Water-soluble palladium(O) complexes have also been used as homogeneous catalysts in aqueous-solution alkylation reactions. The particular complex that has been used is Pd(TPPMS>3. Aryl or heteroaromatic halides can be coupled with aryl or vinyl boronic acids, alkynes, alkenes, or dialkyl phosphites with this palladium(0) complex. This complex in aqueous solution can also be used for the coupling of alkynes with unprotected iodonucleotides, iodonucleosides, and iodoamino acids (133). 

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