Benzylidene reaction with alkenes

Rate differences observed between the same bromophenylcarbene (241) when prepared by two different routes, diazirine photolysis and the reaction of benzylidene dibromide with potassium r-butoxide, vanish when a crown ether is added to the basic solution in the latter experiment. In this case the complexing potassium bromide is taken over by the crown ether, and selectivity towards alkenes reaches the values of the photolytic runs (74JA5632). 

The Michael reaction of benzylidene acetophenone and benzylidene acetone with ethyl acetoacetate, nitromethane, and acetylacetone was studied by Musaliar and co-workers in the presence of a cetyltrimethy-lammonium bromide-containing aqueous micellar medium.50 The Michael reaction of various nitro alkanes with electrophilic alkenes is performed in NaOH (0.025-0.1 M) in the presence of cetyltrimethylam-monium chloride (CTACI) without any organic solvent (Eq. 10.23).51... 

In addition to copper and rhodium catalysts commonly used in the generation of metal carbene complexes, other transition metals have also been explored in the diazo decomposition and subsequent ylide generation.Che and co-workers have recently studied ruthenium porphyrin-catalyzed diazo decomposition and demonstrated a three-component coupling reaction of a-diazo ester with a series of iV-benzylidene imines and alkenes to form functionalized pyrrolidines in excellent diastereoselectivities (Scheme 20). ... 

Unactivated alkenes can be cyclopropanated if more electrophilic carbene complexes are employed or if the cyclopropanation is carried out in an intramolecular fashion. As an example of the former approach, Casey prepared the highly reactive benzylidene complex (164) in situ and found that it will efficiently cyclopropanate most unactivated alkenes. This is illustrated in Scheme 25 by its reaction with 1-methylcyclopentene. It has since been found that the complex (164) could be isolated and can be... 

Thioketones S=CR2 (R = Ph, C6H40Me-p) react with the benzylidene complex W(CO)5(=CHPh) to form the thiirane derivatives W SC(Ph)HCR2)(CO)5. The thiirane is removed by reaction of these compounds with NEt4Br/CH2Cl2. Results of a mechanistic study on cyclopropane formation from the iron alkyls CpFe (CH3)3X (CO)2 shed light on the selective formation of cis cyclopropanes in the reactions of alkenes with W(=CHPh)(CO)5.227... 

Again, Martins et al. [199] reported the application of a catalytic system BF3.0Et2/triethylamine/[BMlM][BF4] in synthesis of (E)-l-(benzylideneamino)-3-cyano-6-(trifluoromethyl)-lH-2-pyridones 109 by the cyclocondensation reaction of benzylidene cyanoacetohydrazide with 4-alkoxy-l,l,l-trifluoro-3-alken-2-ones at RT (Scheme 59). 

Treatment of 51 with an excess of sodium benzoate in DMF resulted in substitution and elimination, to yield the cyclohexene derivative (228, 36%). The yield was low, but 228 was later shown to be a useful compound for synthesis of carba-oligosaccharides. <9-Deacylation of228 and successive benzylidenation and acetylation gave the alkene 229, which was oxidized with a peroxy acid to give a single epoxide (230) in 60% yield. Treatment of 230 with sodium azide and ammonium chloride in aqueous 2-methoxyeth-anol gave the azide (231,55%) as the major product this was converted into a hydroxyvalidamine derivative in the usual manner. On the other hand, an elimination reaction of the methanesulfonate of 231 with DBU in toluene gave the protected precursor (232, 87%) of 203. 

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

Treatment of methyl 4,6-0-benzylidene-2,3-dideoxy-2,3-epimino-a-D-allopyranoside with nitrous acid gave methyl 4,6-0-benzylidene-2,3-dideoxy-a-D-erythro-hex-2-enopyranoside in high yield (81%). The manno isomer likewise gave the same alkene, in 78% yield. The first step in these reactions is the formation of the unstable, yellow IV-nitrosoepimines, the alio isomer being isolated, and crystallized at —20° it was characterized by u.v. and i.r. measurements and by elemental analysis, and could be stored at —20° for a few days.213... 

Various unsaturated compounds, such as CO2, isocyanates and aldehydes, undergo Pd-catalysed cycloaddition with vinyl epoxides. Reaction of CO2 with 127 affords cyclic carbonates 128 with retention of the configuration at C(3), offering a method of cis hydroxylation of epoxides [66], and has been used for the synthesis of the side-chain unsaturated (—)-exo-brevicomin (129) [67], The tetrahydrofuran 131 was prepared by [3+2] cycloaddition of activated alkenes such as benzylidene malononitrile (130) with vinyl epoxide via Michael addition and allylation [68],... 

A major problem in the reaction of a,/3-unsaturated carbonyl compounds and alkenes proves to be the competition between hetero Diels-Alder and ene reactions. Intramolecular cycloadditions of 1,6- and 1,7-dienes with ester and cyano groups at the double bond yield the ene product nearly exclusively, but with alkylidene- and benzylidene-ketoesters and 1,3-diketones the Diels-Alder reaction is preferred under thermal conditions, however under Lewis acid catalysis also ene reactions occur [12]. 

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