Rearrangement reactions enynes

Hypothetical (carbene)gold(i) structures of intermediates and reaction coordinates have been calculated (B3LYP/ 6-31G and LAN2DZ levels) for (H3P)Au+-catalyzed cyclization reactions of terminal enynes. The endocyclic skeletal rearrangement reactions were found to proceed exclusively via cyclopropylcarbene complexes.240... 

Shortly after Trost s works, two investigations demonstrated the high reactivity of platinum halide salts for this type of reactions. Blum reported a PtCU-catalyzed rearrangement of allyl propargyl ethers to 3-oxabicyclo[4.1.0]-heptenes (Scheme 79).294 This series of reactions also represented the premiere entry into the versatile formation of cyclopropyl products based on skeletal rearrangements of enynes.295 This intriguing aspect is discussed further. 

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. 

PtCl2 constitutes an efficient and practical catalyst for skeletal rearrangement reaction of enynes. This includes a formal enyne metathesis reaction delivering 1,3-dienes. Skeletal reorganization of enyne 75a having a carbon chain in... 

A mechanism proposed for the skeletal rearrangement of enynes involved the presence of gold carbenes [161]. This proposed mechanism was supported by the capture of intermediate gold carbenoids trapped by reactive alkenes in intermolecu-lar cyclopropanation reactions [162]. 

Alkenes act as nucleophiles with alkynes in the presence of gold catalysts. In the most simple version of the reaction, enynes are converted with gold complexes or salts, and in the absence of nucleophiles, into rearranged dienes, cyclopropanated carbocycles, and/or bicyclic cyclobutenes. Depending on the length of the tether and the nature of the substituents, the olefin attack to the alkyne occurs in an endo or an exo fashion (equation 33). Besides, substitution at the alkene plays an important role on the regioselectivity of the nucleophilic attack. ... 

Coupling of a Fischer carbene complex with an alkene can generate a vinylcarbene intermediate 12 via an insertion-rearrangement reaction, which can then further react with a double bond. For intramolecular reactions of tethered enynes 10, the products formed are bicyclic cyclopropanes 14 intermolecular reactions lead to cycloalkenylcyclopropanes. 

It has been demonstrated that these ruthenium complexes are also effective precatalysts for hydrogenation reactions. In 2001, Louie et al. reported the first example of ruthenium-catalyzed tandem RCM/hydrogenation and RCEM/hydrogenation (Scheme 5.1) [1]. Tandem RCM/isomerization [2], RCM/Kharasch addition [3], RCM/dihydroxylation [4], enyne metathesis/Claisen rearrangement [5], enyne metathesis/cyclopropanation [6], ROMP/hydrogenation [7], and CM/aza-Michael addition [8] have also been developed successfully. 

Enantiomers, preferential crystallization of 59 Endo selectivity 798 Ene reactions 808, 809 Enones, synthesis of 732 Enthalpies of formation 102, 103 Enynes, synthesis of 956 Enzymatic kinetic resolution 829 Epimerization 399 Episulphides, oxidation of 237 Episulphones 650, 775 Episulphoxides, photolysis of 742 a,/J-Epoxysulphones reactions of 811, 812 rearrangement of 685 synthesis of 612 / ,y-Epoxysulphones 781 y,<5-Epoxysulphones 627, 628 Epoxysulphoxides reactions of 613 rearrangement of 744 synthesis of 327, 612 Erythronolides 831... 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

In the reaction of disubstituted alkynes, 1,3-migration of the acetate takes place to give allenyl esters that can be versatile substrates, especially for [3,3]-Cope rearrangements.333 1,5-Enynes have proved to be versatile substrates for the preparation of perfumery agents such as sabinol334 and sabina ketone.335 Transannular systems undergo similar reactions.336... 

The sequential double migratory insertion of CO into acydic and cydic diorganozircono-cene complexes through acylzirconocene and ketone—zirconocene species provides a convenient procedure for preparing acyclic and cyclic ketones (Scheme 5.6) [8], Thus, the bi-cydic enones from enynes can be obtained through CO insertion into zirconacyclopen-tenes followed by a subsequent rearrangement (Scheme 5.7). The scope and limitations of this procedure have been described in detail elsewhere [8d]. This procedure provides a complementary version of the well-known Pauson Khand reaction [9]. 

A variety of palladium-catalyzed dimerizations of conjugated enynes and their additions to diynes and triynes gave rise to styrene and phenylacetylene derivatives, respectively. Inter alia, 1,2,4-cyclohexatrienes have been invoked as intermediates in these reactions [134], 5,6-Diphenyl-l,2,4-cyclohexatriene has been proposed as an intermediate in the rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene to o-ter-phenyl and its possible existence was supported by quantum-chemical calculations [135],... 

Treatment of the propargylic alcohol 144, readily prepared from condensation between benzophenone (143) and the lithium acetylide 101, with thionyl chloride promoted a sequence of reactions with an initial formation of the chlorosulfite 145 followed by an SNi reaction to produce in situ the chlorinated and the benzannulated enyne-allene 146 (Scheme 20.30) [62], A spontaneous Schmittel cyclization then generated the biradical 147, which in turn underwent a radical-radical coupling to form the formal [4+ 2]-cycloaddition product 148 and subsequently, after a prototropic rearrangement, 149. The chloride 149 is prone to hydrolysis to give the corresponding 11 H-bcnzo h fluoren-ll-ol 150 in 85% overall yield from 144. Several other llff-benzo[fc]fluoren-ll-ols were likewise synthesized from benzophenone derivatives. 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

The reaction of enynes with Fischer-type carbene complexes can also lead to the formation of cyclobutanones (Figure 2.23) [315]. The mechanism for this reaction is likely to be rearrangement of the intermediate, non-heteroatom-substituted vinylcarbene complex to a vinylketene, which undergoes intramolecular [2 -i- 2] cycloaddition to form the observed cyclobutanones. 

Ring-closing metathesis of an enyne, which has double and triple bonds in the molecule, is a remarkable reaction which is useful in synthetic organic chemistry. In enyne metathesis, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds. The cleaved alkylidene part is moved to the alkyne carbon. Thus, the cyclized compound formed in this reaction has a diene moiety [Eq. (6.77)]. The reaction is also called skeletal rearrangement and is induced by Pt, Pd, Ga, and Ru catalysts ... 

The allenylidene-mthenium complexes I also catalyze the enyne metathesis to alkenylcydoalkenes with a 1,3-diene stmcture. Initial studies showed the transformation of simple enynes with ether function [37] (Scheme 8.5). This reaction was significantly accelerated by initial catalyst photodiemical activation, which is now understood to favor the rearrangement of the allenylidene- into the active indenylidene-ruthenium moiety and arene displacement. 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

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