Rearrangements metal-catalyzed

The vinylcyclopropane rearrangement is of synthetic importance, as well as of mechanistic interest—i.e. the concerted vs. the radical mechanism. A reaction temperature of 200 to 400 °C is usually required for the rearrangement however, depending on substrate structure, the required reaction temperature may range from 50 to 600 °C. Photochemical and transition metal catalyzed variants are known that do not require high temperatures. 

In the presence of transition-metal complexes, organic compounds that are unsaturated or strained often rearrange themselves. One synthetically useful transition-metal catalyzed isomerization is the olefin migration reaction. Two general mechanisms have been proposed for olefin migrations, depending on the type of catalyst employed (A and B) (Scheme 3.8).137... 

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

Metal-catalyzed decomposition of p,y-unsaturated a -diazoketones 400 in the presence of an alcohol affords rearranged y,8-unsaturated esters 407 this process has been termed the vinylogous Wolff rearrangement193 . Full accounts dealing with the... 

Metallocyclobutanes from cyclopropanes have been frequently invoked in transition metal-catalyzed rearrangements of strained ring hydrocarbons, and this body of chemistry is quite rich and diverse, as evidenced in the excellent review by Bishop (72). Because of this diversity, the significance of isolated observations should not be overstated nevertheless, certain reactions outlined by Bishop are closely related to the carbene retroadditions reported by Gassman and co-workers using metathesis catalysts. 

In this section we will survey the use of transition metal-catalyzed additions, at times accompanied by rearrangement processes, that lead to the generation of new carbon-phosphorus bonds. 

As mentioned above, we planned to obtain optically pure styrenyl ethers through Zr-catalyzed kinetic resolution [5] subsequent metal-catalyzed rearrangement would afford optically pure chromenes. However, as shown in Scheme 11, the recovered starting material (40) was obtained with <10% ee (at 60% conversion) upon treatment with 10 mol% (,R)-(EBTHI)Zr-binol (3b) and five equivalents of EtMgCl (70°C, THF). We conjectured that, since the (EBT-HI)Zr-catalyzed reaction provides efficient resolution only when asymmetric alkylation occurs at the cyclic alkene site, competitive reaction at the styrenyl terminal olefin renders the resolution process ineffective. Analysis of the H NMR spectrum of the unpurified reaction mixture supported this contention. Indeed, as shown in Scheme 11, catalytic resolution of disubstituted styrene 49... 

There is much more left to be done in the area of catalytic RCM. It is likely that many elegant and creative uses of catalytic RCM are in the making. Judging by the related developments in recent years, it is also likely that catalytic RCM will influence positively the development of numerous other ongoing metal-catalyzed or uncatalyzed reactions. The advent and utility of complexes la, lb and 2 will undoubtedly inspire organic chemists to devise new and useful transformations, where these transition metal systems are effectively utilized (e.g., styrenyl ether rearrangements) [40]. 

Keyword Carbocycles a Cascade Reactions a Cycloadditions a Combinatorial Chemistry a Domino Reactions a Enantioselective Transformations a Ene Reactions a Eieterocydes a Natural products a Preservation of Resources and Environment a Sigmatropic Rearrangements a Tandem Reactions a Transition Metal-Catalyzed Transformations... 

Twenty chapters cover such new and exciting developments as metal-catalyzed synthesis of allenes, strained cyclic allenes, the numerous applications of different metallated allenes in organic synthesis, as well as the many addition and rearrangement reactions of allenes and allene units in natural products like the remarkable enyne-allenes. 

Interesting direct evidence for the reaction pathway of these metal-catalyzed reactions was obtained from stoichiometric reactions of allenylcarbinols such as 61 with the [j7S-CsHsFe(CO)2]+ (Fp+) complex. From the isobutene adduct of Fp+ and 61 the jt-complex 65 was obtained. The complex 65 was suspected to be the product of an acid-catalyzed rearrangement indeed, in the presence of N,N-dimethylaniline as a base, 63 was isolated (Scheme 15.14) [30],... 

Two main pathways of metal-catalyzed skeletal rearrangement have been distinguished bond shift mechanism and C5 cyclic isomerization (7, 8). 

In the past few years, new approaches for the enantioselective synthesis of / -benzyl-y-butyrolactones appeared in the literature. Some of these approaches involve the asymmetric hydrogenation of 2-benzyl-2-butenediols (j [34]), the radical mediated rearrangement of chiral cyclopropanes (r [35]), the transition metal catalyzed asymmetric Bayer-Villiger oxidation of cyclobutanones n [36]), or the enzymatic resolution of racemic succinates (g [37]). 

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cyclo-addition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

Intermediate metallacyclopentanes are also implicated in transition metal-catalyzed alkene cycloadditions to form cyclobutanes and the corresponding cycloreversions, e.g. dimerization of norbomadiene (73JA597) and rearrangements of cubane and other cyclo-butanoid hydrocarbons (78JA2573). 

Kinetic Studies Provide Only Limited Mechanistic Information. While such studies are invaluable and frequently indicate the nature of pre-rate-determining steps, they provide almost no information concerning such vital fast steps as electron transfers and rearrangements. For example, despite extensive studies of the kinetics of acetaldehyde and vinyl acetate syntheses, it is clear only that olefin, nucleophile, and palladium combine in a complex. The nature of the rate-determining step as well as the details of post-rate determining product forming steps remains uncertain (7,94). In some cases—e.g., the metal-catalyzed autoxi-dation of thiols to disulfides—re-oxidation of metal to its catalytically... 

Ketocarbenes (1) are usually generated from the corresponding diazo compounds (3).s Other sources which are occasionally used are a,a-dibromo compounds (4),9 sulfur ylides (5)10 and iodonium ylides (6 Scheme 2).11 The thermal or photochemical decomposition of diazo compounds in the presence of ir-systems is often complicated by indiscriminate side reactions, such as Wolff rearrangements,12 C—H insertions and hydride migrations. To avoid such problems, the use of metal-catalyzed decomposition of diazo compounds is generally preferred.1 2... 

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