1-Chloro-l-alkyne

The currently available methods for the synthesis of the title compounds are confined to the preparation of homo-1,1-dihalo-1-alkenes 180 while only a few reports are available for mixed 1,1-dihalo-1-alkenes of defined stereochemistry 18u. As the hy-droboration reaction proceeds in a stereospecific manner, the hydroboration-oxi-dation-bromination-debromoboration sequence of 1-chloro-l-alkynes produces selectively (Z)-l-bromo-l -chloro-l-alkenes (Eq. 116),82>. The oxidation with anhydrous trimethylamine oxide of the alkenylborane prior to the addition of bromine is necessary to avoid the competing transfer of one of 1,2-dimethylpropyl group from boron to the adjacent carbon atom. Similar reaction sequence provides 1,1-dibromo-l-alkenes (Eq. 117)182). 

Hydroalumination of 1-chloro-l-alkynes. Lithium aluminum hydride adds to 1-chloro-l-alkynes (1) regio- and stereoselectively to form the a-chlorovinyl alanates 2, which are moderately stable at 0°C. On methanolysis they are converted into (E)-l-chloro-l-alkenes (3). They can also be converted into (Z)-l-bromo-l-chloro-l-alkenes (5) and into (Z)-l-chloro-l-iodo-l-alkenes (6). 

Chloro-l-alkynes (e.g., CICsCR R = n-Bu, n-C6H13, n-CgH17) polymerize with Mo catalysts in high yields. These monomers do not polymerize at all with W catalysts. As an example, Table 12 includes some results on the polymerization of 1-chloro-l-octyne 45). Both MoCls—cocatalyst and Mo(CO)6—CCI4—hv give polymers with Mw of 5 x 103-1 x 10 , while MoCls alone is less efficient. 

Villieras, J., Perriot, P, and Normant, J.F., Simple route from aldehydes to alkynes and 1-chloro-l-alkynes. Synthesis, 458, 1975. 

Dichloro-l-alkenes can also be converted into 1-chloro-l-alkynes by reaction with lithium diethylamide in ether-THF (equation 11) ... 

Aliphatic and aromatic aldehydes react with carbon tetrabromide and triphenylphosphine to yield 1,1-dibromoalkenes 156, which are converted into alkynes 157 by the action of butyllithium or lithium amalgam. A convenient modification of the second step is the use of magnesium metal in boiling THF. 1-Chloro-l-alkynes 159 (R = Bu, hexyl, heptyl etc.) are produced from aldehydes and carbon tetrachloride/triphenylphosphine/magnesium, followed by dehydrochlorination of the products 158 with potassium hydroxide in the presence of the phase-transfer agent Aliquat 336. ... 

With catalyst 26, 1-chloro-l-alkynes with different alkyl lengths also undergo living polymerizations. Consequently, the sequential addition of 1-chloro-l-hexadecyne (A), 1-chloro-l-hexyne (B) and 1-chloro-l-hexadecyne (A) in this order provides an A-B-A-type triblock copolymer. Similarly, one can obtain a B-A-B-type triblock copolymer. These are the first examples of block copolymers from substituted acetylenes. 

Many monomers with simple structures, including phenylacetylene, t-butyl-acetylene, 1-phenyl-1-propyne, 2-octyne, and 1-trimethylsilyl-l-propyne, are commercially available. These monomers are usually purified by distillation in the presence of suitable drying agents prior to use. On the other hand, monomers that are more complex, such as ort/zo-substituted phenylacetylenes, A-pro-pargylcarbamates, ring-substituted diphenylacetylenes, and 1-chloro-l-alkynes, must be synthesized. Derivatization of simple alkynes rather than formation of the acetylenic moiety, is frequently applied to synthesize such monomers. These are then purified by vacuum distillation or column chromatography. 

The change in charge polarization across the C C reflected in the C NMR data for haloalkynes indicates that polarization decreases in the order I > Br > Cl. This results in an increase in the amount of negative charge or electron density at C-2 and decrease in that at C-1, as one proceeds from the iodo- to bromo- to the chloro-substituted compounds. This thus correlates with the rate of hydroboration (which occurs at C-1) in the order I > Br > Cl. Further, the reason for the nucleophilic attack at the C-2 position [23] in 1-chloro-l-alkynes is the decrease in the electron density at that position caused by the -M effect of chlorine. Both regioselectivity and rate of hydroboration depend on the electron availability at specific sites in the molecule, and the above reason explains why hydroboration does not occur at the 2 position. 

The stereoselective synthesis of the title compounds has been achieved. Thexylborane on reaction with 2 molar equivalents of 1-iodo-l-alkyne at 0 °C proceeds to near completion (88% for 1-iodo-l-hexyne) to form fully substituted organoborane (33), which upon treatment with 2 molar equivalents of sodium methoxide at 0 °C readily produces trans-1,2,3-butatrienes (Eq. 102) 157). The same reaction, however, with either 1-chloro or 1-bromo-l-alkynes is sluggish to form the thexyl-l-halo-l-alkenyl-borane31). 

Chloro-l,3-dienes are stereoselectively formed via c/.v-halocarborative addition of al-kenes to alkynes in the presence of catalytic amounts of palladium(II) chloride, lithium chloride and oxygen46. The reaction proceeds via cis chloropalladation of the alkyne and c/.v-orientated alkene insertion, followed by /(-hydrogen elimination. Alternatively, 1.4-dienes are obtained if, for steric reasons, a. nn-periplanar /(-hydrogen atom is not available to form the conjugated diene (e.g., after m-orientated insertion of cyclohexene)46. 

Hydroboration of 1-bromo-l-alkynes with chloro(thexyl)-borane leads to the synthesis of alkynyl ketones in 61-63% yields by the sequence of reactions shown in eq 5. Sequential treatment of alkenylchloro(thexyl)boranes, which are formed by the reaction of chloro(thexyl)borane with alkynes, with lithium chloro-propargylide and aldehydes affords 1,3-enynols or 1,2,4-trienols depending on the reaction conditions. ... 

Chlorine substitution in 1-hexyne reduces the C-l-C-2 C NMR shift difference from 15.75 in the parent to 12.51 ppm. This reveals that the effect of chlorine on 1-hexyne makes C-1 more positive and C-2 more negative. This effect of chlorine on the alkynes agrees with the -i-M effect analyzed in alkenes. An identical situation is observed in 1-chloro-l-octyne and in chlorophenylacetylene. 

The comparative data of C NMRfor 1-hexyne and the 1-halo-l-hexyne describe the importance of the -M effect, which in haloalkynes decreases in the order I > Br > Cl. In fluoroalkynes, no -M effect is possible as no d orbitals are available. Only a +M interaction is possible in the 1-fluoro-l-alkynes, and one expects their hydroboration at the C-1 position would (1) proceed at a slower rate than the corresponding chloro analogs, or (2) switch to C-2 position in case the electron density at C-1 decreases enough. Indeed, in 1-fluoro-l-alkynes the nucleophilic attack occurs at the C-1 position, whereas the same occurs at C-2 position in 1-chloro-1 alkynes [22a]. Their comparative hydroboration studies are not conducted [21], as fluoroalkynes are difficult to prepare and handle, and often very unstable [22]. 

Table 9.1). The rate of dehydrobromination from the intermediate bromoalkenes follows the pattern 2-bromoalkenes > Z-l-bromoalkenes > E- -bromoalkenes the corresponding chloro derivatives react more slowly. For optimum yield, the reaction temperature should be <100°C to reduce decomposition of the catalyst, and the concentration of base should be kept low to prevent isomerization of the resulting alkynes. [3-Elimination of HBr from 1,2-dibromo-1 -phenylethane can be controlled to yield 1-bromo-l-phenylethene in 83% yield [15]. The addition of alcohols and diols have a co-catalytic effect on the elimination reaction, as the alkoxide anions are transferred more effectively than the hydroxide ions into the organic phase [13]. 

---