Alkene derivatives intermolecular reactions

In order to safely identify k0 with intramolecular carbenic reactions (e.g., k and the formation of alkene 4 in Scheme 1), product analysis should demonstrate that the yield of intramolecular products exceeds 90%, while dimer, azine, and solvent-derived (intermolecular) carbene products should be absent or minimal. If these conditions are not met, mechanistic interpretation is often ambiguous, a result that is well illustrated by the saga of benzylchlorocarbene (see below, Section IV.C). Less desirably, k0 can be corrected for competitive intermolecular carbenic reactions. Bimolecular reactions like dimerization and azine formation can be minimized by working at low carbene precursor concentrations, and careful experimental practice should include quantitative product studies at several precursor concentrations to highlight potential product contamination by intermolecular processes. 

Radical species derived from reduction of ketones are also trapped by alkenes in an intermolecular reaction. Reaction only occurs with terminal alkenes of the type... 

The inclusion of a separate chapter on catalysed cyclopropanation in this latest volume of the series is indicative of the very high level of activity in the area of metal catalysed reactions of diazo compounds. Excellent, reproducible catalytic systems, based mainly on rhodium, copper or palladium, are now readily available for cyclopropanation of a wide variety of alkenes. Both intermolecular and intramolecular reactions have been explored extensively in the synthesis of novel cyclopropanes including natural products. Major advances have been made in both regiocontrol and stereocontrol, the latter leading to the growing use of chiral catalysts for producing enantiopure cyclopropane derivatives. 

With the exception of propynoic acid derivatives, all alkynes undergo the reaction. On the other hand, generally, only strained olefins react efficiently in the intermolecular PKR whereas electron deficient alkenes give the reaction only in limited examples. With respect to regioselectivity, the bulkier substituent of the alkyne is placed adjacent to the carbonyl in the cyclopen-tenone product. Unsymmetrical olefins usually give mixtures of regioisomers... 

The protic acid and Lewis acid-catalyzed [4 + 2] cycloaddition reactions of electron-rich alkenes with imines derived from anilines and aryl aldehydes constitute an extensively explored class of 2-azadienes capable of providing the products of a formaJ Diels-Alder reaction (equation 9).i5.27.i77 a useful extension of these studies and in efforts to increase the rate of the Att participation of simple 2-aza-1,3-buta-dienes in [4 + 2] cycloaddition reactions, Mariano and coworkers have examined the Lewis acid-catalyzed intermolecular reactions of (l ,3 )-l-phenyl-2-aza-l,3-pentadiene with electron-rich dienophiles, including enol ethers. Reductive work-up of the cycloaddition reactions provided the pro-... 

Instead of relying on ionic reactions to execute the crucial assembling of two glycosidic units, one may profitably consider radical reactions [12,16]. An anomeric radical, being an alkoxyalkyl radical, will behave as a nucleophile, and its high-lying SOMO will better interact with the LUMO of an electron-poor alkene, typically a, 8-unsaturated carbonyl compounds or similar derivatives. In other words, in intermolecular reactions, the radical acceptor should be activated enough to compete efficiently with an H-donor. 

Fused cyclopropanes are obtained by intramolecular cyclopropanations or by intermolecular reactions with cyclic alkenes or aromatic derivatives. The phosphoryl carbenes are the only phosphorus-substituted carbenes used in these reactions. 

The reaction is thought to proceed by co-ordination of the alkene with the organopalladium(II) species, followed by carbopalladation. Subsequent p-hydride elimination regenerates an alkene and releases palladium(II). This is reduced (reductive elimination) to palladium(O) in the presence of a base, to allow further oxidative addition and continuation of the cycle (1.211). The carbopalladation and p-hydride elimination steps occur syn selectively. Excellent regioselectivity, even for intermolecular reactions, is often observed, with the palladium normally adding to the internal position of terminal alkenes (except when the alkene substituent is electron-rich as in enamines or enol derivatives), thereby leading to linear substitution products. 

Both inter- and intramolecular [2-1-2] cycloaddition reactions have been used in synthesis. In intermolecular reactions, a common problem is that mixtures of regioisomers (sometimes referred to as head-to-tail and head-to-head products) in addition to more than one stereoisomer may be formed. In general, the head-to-tail regioisomer is the major product using an electron-rich alkene, whereas the head-to-head regioisomer is favoured using an electron-deficient alkene. For example, the first step in Corey s synthesis of caryophyllene involved addition of cyclohexenone to isobutene to give predominantly the trans-cyclobutane (head-to-tail) derivative 151 (3.107). 05... 

Ni(COD)2/PiPr3 catalyses intramolecular addition of pyridone derivatives across tethered alkenes in the presence of AlMCs. The reaction proceeds mainly in an exo-trig fashion to form bicyclic products in good yields. The intermolecular reaction only occurs when a vinylarene is employed and results in branched all lated pyridones. However, a modified catalyst system can promote intermolecular reaction between pyridone derivatives and aliphatic alkenes. Moreover, the reaction selectively gives linear allq lated pyridones (Scheme 14.74). The reaction occurs at the C6 position of pyridones exclusively. If the C6 position is occupied by a substituted group, the alleviation can occur at the C4 position. For example, l,6-dimethyl-2-pyridone undergoes alkylation at the C4 position when the reaction was run at 100... 

Scheme 14.74 Ni-catalysed intramolecular and intermolecular reaction of pyridone derivative with alkenes.

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

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