Alkynes, halo carbonylation

The alkenylation reaction of ct-halo carbonyl compounds with alkenylindium in the presence of triethylborane proceeds via a radical process. Unactivated alkene moieties as well as a styryl group can be introduced by this method. The geometry of the carbon-carbon double bonds of the alkenylindium is retained. Preparation of alkenylindium via hydroindation of 1-alkyne followed by the radical alkenylation establishes an efficient one-pot strategy (Scheme 113).384... 

A couple of years ago we have disclosed a new mode of alkyne activation towards isomerization as a detouring outcome of the Sonogashira coupling. As a result of coupling electron deficient (hetero)aryl halides (or a,p-unsaturated p-halo carbonyl compounds) 11 and aryl propargyl alcohols 12 a new access to 1,3-di (hetero)aryl propenones 13, i.e., chalcones, was established (Scheme 9) [77, 78]. The scope for electron deficient (hetero)aromatic halides 11 is fairly broad and even organometallic complexes like 13c can be synthesized by this sequence. 

Scheme 6.69. A representation of a potential pathway for the reaction between an alkyne and hypohalous acid (X = Cl, Br). Although it is possible that an a-halo carbonyl derivative (aldehyde or ketone) might form, the result of enol-keto equilibrium will insure that the a,a-dihalo carbonyl derivative results (as long as sufficient hypohalous acid is present).

The reaction of allyl halides with terminal alkynes by use of PdClifFhCNji as a catalyst affords the l-halo-l,4-pentadienes 297. 7r-AlIylpalladium is not an intermediate in this reaction. The reaction proceeds by chloropalladation of the triple bond by PdCh, followed by the insertion of the double bond of the allyl halide to generate 296. The last step is the regeneration by elimination of PdCh, which recycles[148]. The cis addition of allyl chloride to alkynes is supported by formation of the cyclopentenone 299 from the addition product 298 by Ni(CO)4-catalyzed carbonylation[149]. 

The amide ions are powerful bases and may be used (i) to dehydrohalogenate halo-compounds to alkenes and alkynes, and (ii) to generate reactive anions from terminal acetylenes, and compounds having reactive a-hydrogens (e.g. carbonyl compounds, nitriles, 2-alkylpyridines, etc.) these anions may then be used in a variety of synthetic procedures, e.g. alkylations, reactions with carbonyl components, etc. A further use of the metal amides in liquid ammonia is the formation of other important bases such as sodium triphenylmethide (from sodamide and triphenylmethane). 

The chemistry of haloalkynes with low-valent metal complexes has been reviewed.142 Two competing reactions of anionic metal carbonyls [M(CO) ] with halo-alkynes are (i) abstraction of X+ to give MX(CO) and (ii) nucleophilic displacement of X- to give M(C=CX)(CO) and M(CO) 2(ji-C=C). A recent study of reactions of haloalkynes with [M(CO)3Cp ]- (M = Cr, Mo, W Cp = Cp, Cp )] has clarified the reaction conditions, which are determined by the nucleophilicity of the anionic metal carbonyl and the electron-withdrawing power of any substituent on the C=C triple bond. Cross-coupling of the anion with a chloroalkyne is catalyzed by palladium.143... 

Compared with chiral nonracemic a-amino carbonyl compounds - which are not suitable substrates for MBH reaction, mainly due to their racemization under normal conditions after prolonged exposure times to catalyst or due to poor diastereoselectivity " a-keto lactams, enantiopure 3-oxo-azetidin-2-ones 168, readily react with various activated vinyl systems promoted by DABCO to afford the corresponding optically pure MBH adducts 169 without detectable epimerization (Scheme 1.69). " However, the Lewis acid-mediated reaction of electron-deficient alkynes with azetidine-2,3-diones 168 as an entry to p-halo MBH adduets was not very sueeessful the coupling product 170 was achieved with concomitant acetonide cleavage as a single ( )-isomer in low yield, in the presence of trimethylsilyl iodide under BF3 OEt2-induced catalysis (Scheme 1.69). 

An alkyne is hydrolyzed to an enol, which tautomerizes to a carbonyl, and halo-genation of a carbonyl followed by elimination gives an alkyne. 

During the course of some work aimed at the selective reduction of a substituted alkyne to the c/5-alkene, Olsen et al have discovered the novel synthesis of a rare ring system. Lithiated alkynes were found to react with the halo-substituted phthalimide (360) at the carbonyl group rather than at the halogen-bearing carbon as expected. The intermediate cyclized to this position to give an oxazolo[2,3-a] isoindole (361). 

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