Alkynes lithium aluminum hydride

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). 

ALKYNES Lithium aluminum hydride. Potassium 3-aminopropylamide. 

Another hydride, magnesium hydride prepared in situ from lithium aluminum hydride and diethylmagnesium, reduced terminal alkynes to 1-alkenes in 78-98% yields in the presence of cuprous iodide or cuprous r rt-butoxide, and 2-hexyne to pure cij-2-hexene in 80-81% yields [///]. Reduction of alkynes by lithium aluminum hydride in the presence of transition metals gave alkenes with small amounts of alkanes. Internal acetylenes were reduced predominantly but not exclusively to cis alkenes [377,378]. 

The conversion of an alkyne to a trans-alkene can be accomplished by heating with lithium aluminum hydride (LAH), by reaction with lithium in liquid ammonia (Li, NH3). Thus all of these reagents (H2/P-2 Ni, LAH, and Li, NH3) are reducing agents for alkynes and give alkenes as the reduced products. 

PhC=CH and PhC=CPh analogs [Eq. (26)] (80). Five coordination is also found in the porphyrin alkyne complex, Mo(TTP)(PhC=CPh) (TTP= mesotetra-p-tolylporphyrin) (81). Reduction of a toluene solution of Mo(TTP)C12 with lithium aluminum hydride in the presence of excess diphenylacetylene produced the violet Mo(TPP)(PhC=CPh) adduct [Eq. (27)]. 

As a class alkynes are much more reactive in hydroalumination than are alkenes. Hence, both terminal and internal alkynes react at feasible rates with both dialkylaluminum hydrides in alkanes and lithium aluminum hydrides (LiAlRnH4-n) in ethers. Selected examples of such additions are presented in Table 2. With alkyl or aryl substituents, it should be noted that R2AIH adds in a kinetically syn manner, (5 equation 2) and (7 equation 3), and LAH yields the anti adduct (14 equation 6). 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives. 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives 56, 57, 104, 105, 106, 107, 108, 109. Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures 59, 60, 61, 62, 63, 64, 65, 85, 104, 106, 107, 108,109,110,113,114,118. 

Reaction of the alkyne adducts with aluminum and boron hydride reagents such as lithium aluminum hydride, sodium boro-hydride, or lithium triethylborohydride, however, leads to the formation of unexpected thiirane derivatives (eq 3). These can be desulfurized to give divinyl sulfides. 

Addition to Alkenes. Phthalimidosulfenyl chloride can be added to alkenes to yield a-chloro thioethers which, after treatment with lithium aluminum hydride, produce episulfides (eq 4). Sodium borohydride is unsuccessful in transforming the alkene adduct to the episulhde. The alkene addition is stereospecifically trans and proceeds through a thiiranium ion intermediate, the opening of which is governed predominantly by steric factors and usually gives a predominance of the anti-Markovnikov addition product. As in the addition of phthalimidosulfenyl chloride to alkynes, the phthalimido group can also be displaced with... 

Lithium aluminum hydride is a very powerful reducing agent it reduces not only the carbonyl groups of aldehydes and ketones rapidly but also those of carboxylic acids (Section 17.6A) and their functional derivatives (Section 18.10). Sodium borohydride is a less reactive and therefore much more selective reagent, reducing only aldehydes and ketones rapidly. Neither reagent reduces alkenes or alkynes to alkanes. 

The carboxyl group is one of the organic functional groups most resistant to reduction. It is not affected by catalytic hydrogenation under conditions that easily reduce aldehydes and ketones to alcohols and that reduce alkenes and alkynes to alkanes. The most common reagent for the reduction of carboxylic acids to primary alcohols is the very powerful reducing agent, lithium aluminum hydride (Section 16.11A). 

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