Dienes, acylation intermediates

On the basis of these findings, the reaction of acyl imines with methanesulfony 1 chloride-triethylamine is not expected to proceed via a sulfene intermediate as previously proposed [99]. Again, a carbanion intermediate accounts nicely for the experimental facts. The electrophihcity of the hetero-l,3-diene is exdemely high, therefore the carbanion, formed on reaction of triethylamme with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero-1,3-diene faster than sulfene formabon by chloride elimination. 

Trapping reactions of benzoylmethyleneoxophosphorane 39 a with carbonyl compounds dispel any remaining doubts as to the existence of acylated phosphenes. Unlike the diphenylmethyleneoxophosphorane 9, whose P/C double bond participates in cycloadditions, compound 39 a acts as a hetero-1,3-diene and undergoes [4 + 2]-cycloaddition with aldehydes and ketones 10 I7,35> it may again be assumed that the reaction is a two-step process involving 55 as intermediate. 

The most plausible mechanism proceeds through oxidative addition of the aldehyde to an active Ru(0) species to form (acyl)(hydrido)ruthenium(ll) complex 155. Insertion of the less-substituted double bond of the 1,3-diene into the Ru-H bond occurs to generate an (acyl)( 73-allyl)ruthenmm(ll) intermediate of type 156. Successive regioselective reductive eliminations between the acyl and the 73-allyl ligands provide the desired product with regeneration of the... 

In complexes where the carbomethoxy or acyl group is adjacent to the metal-complexed acyl group (e.g., 145.a), photolysis affords a 1 1 mixture of isoprenic diene complexes (E-152 and Z-152) directly. The formation of an intermediate allylketene complex (153) may be demonstrated by methanol trapping, followed by aerial oxidation to quantitatively yield the diester 154. 

The intermediate generated in pyrolysis of IV-acylacetyl-iV-phenylhydroxylamines 48 can form a 3-aza-4-oxa-l,5-diene system (50) (equation 16). A homolytic cleavage of the O—H bond with subsequent rearrangement to the aniline radical, followed by recombination with the hydrogen radical to give the corresponding 0-acyl hydroxylamine 49,... 

Anomalous isomerizations have been noted during the photolytic and thermal rearrangements of 3-acyl-2-methoxy-3//-azepines (2 R -acyl, R2 = OMe) and 3-acyl-3H- azepin-2-ones (69T5217). Irradiation in methanol solution produces mixtures of 3-azabicyclo[4.1.0]hepta-2,4-dienes (28 R1==acyl and H, R2 = OMe, R3 = H) (or -4-ene-2-ones) and 3-phenacylpyridines (or pyridones), albeit in poor yields. Detailed, but tentative, arguments involving azanorcaradiene and/or diradical intermediates are presented to explain the formation of these unusual products. 

Carbanions, such as that from malonodinitrile, generate dienes (66) which ring-close to iV-acyl-l,2-dihydropyridines (67). Subsequently these products may rearrange to 2-amidopyridines (Scheme 21) (75BCJ73). However, if substituted malonodinitriles are employed in this reaction the intermediate 6H-1,3-oxazines can be isolated for now deprotonation and ring opening are inhibited. 

PdLX complex undergoing insertion of the coordinated double bond into the a-Pd-carbon bond to form a Pd-alkyl intermediate. With 1,4-penta-diene and 1,5-hexadiene, cyclic keto esters are formed in MeOH and a similar cyclic mechanism is suggested involving insertion of the coordinated double bond into the acyl Pd complex intermediate (16). Although CO pressures of 1000 atm were used, these cyclic ketones were produced also at 250 atm in the presence of p-toluene sulfonic acid, but no details were reported. 

Pathways involving alkyl-acyl rearrangements are proposed to explain the carbonylation of a-bonded alkoxy complexes (17). The stereochemistry of the products indicates that the ester group replaces Pd with retention of configuration at the carbon to which Pd is o-bonded. In all these studies with unconjugated dienes the nature of carbonylation products to be expected is clearly influenced by the geometry of the intermediate Pd complexes. 

The second possibly useful reaction in this group is the tetracarbonylcobalt anion-catalyzed conversion of alkyl halides with a base, CO, and conjugated dienes into acylated dienes (24). In this reaction the alkylcobalt intermediate... 

A lithium-induced cyclization of the imine-diene 267, followed by N-alkylation (e.g., with EX = EtBr) or iV-acylation (e.g. with EX = PhCOCl), to form the 4,5-dihydroazepines 268 in poor to fair yields has been reported (Equation 36) <1995TL7065>. By using the different imine-dienes 269 but the same conditions as for 267 to form the anionic intermediate for the 1,7-electrocyclization, the N-substituted 2,3-dihydroazepines 270 were obtained (Equation 37) <1996T14801>. NMR spectroscopy was used to monitor the progress of these reactions. 

Nucleophilic attack occurs at C(2) of the diene. The 1,3-cyclohexadiene complex 66 is converted to the homoallyl anionic complex 67 by nucleophilic attack, and the 3-alkyl-1-cyclohexene 68 is obtained by protonation. Insertion of CO to 67 generates the acyl complex 69, and its protonation and reductive elimination afford the aldehyde 70 [20]. Reaction of the butadiene complex 56 with an anion derived from ester 71 under CO atmosphere generates the homoallyl complex 72 and then the acyl complex 73 by CO insertion. The cyclopentanone complex 74 is formed by intramolecular insertion of alkene, and the 3-substituted cyclopentanone 75 is obtained by reductive elimination. The intramolecular version, when applied to the 1,3-cyclohexadiene complex 76 bearing an ester chain at C(5), offers a good synthetic route to the bicyclo[3.3.1]nonane system 78 via intermediate 77 [21]. 

Indeed, cyclobutanones constitute challenging synthetic intermediates for a whole host of applications including cyclopentanone, cyclohexanone and cyclooctanone formation, olefin and diene synthesis, geminal alkylation, reductive acylation, among others,... (vide infra). 

Benzofurazans show greater thermal stability but may be cleaved photochemically. Irradiation of benzofurazan in benzene and in methanol gives the azepine (26) and the urethane (27), respectively in the presence of triethyl phosphite (Z,Z)-l,4-dicyanobuta-1,3-diene is formed. The proposed mechanism (Scheme 3) involves nitrile oxide, oxazirene, acyl nitrene and isocyanate intermediates, and is supported by spectrophotometric studies (76HCA2727) and by trapping of the nitrile oxide as its isoxazole cycloadduct with DMAD (75JOC2880). 

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