Insertion involving dienes

Nolan and co-workers also showed that alkenes could react with NHCs in what appeared to be a net migratory insertion, although in this case an inter-molecular attack was proposed. Reaction of diene adduct 17 with an excess of IPr led to 18 (Equation (2.3)). Calculations (R = H) showed that 18 formed via initial addition of an NHC ligand to the Pt centre followed by intermolecular attack of a second NHC moiety. The intermolecular attack occurred with an activation barrier of +7.6kcalmol whereas intramolecular migratory insertion involving the Pt-bound NHC would entail a barrier of -f 40kcalmoH ... 

Both MeMn(CO)5 and PhMn(CO)5 react with acetylenes to yield vinyl ketone tetracarbonyl complexes, most likely via a pathway involving CO insertion [Eq. (18)] 14, 36). Reactions of these same alkyls with 1,3-dienes may proceed similarly 16, 95, 96). The chelating ligand o-styryldiphenyl-phosphine (L—L) converts MeMn(CO)j into two products 25) whose structures (XXII and XXIII) were elucidated by X-ray crystallography 24). An unusual migration of COMe onto L—L occurs subsequently to the initial insertion step. 

Two type la syntheses of (3-hydroxypyrroles have appeared. An aza-Nazarov cyclization of l-azapenta-l,4-dien-3-ones produced (3-hydroxypyrroles including 2,2 -bipyrroles <06EJO5339>. A second approach to a (3-hydroxypyrrole involved an intramolecular N-H insertion into a rhodium carbene derived from the decomposition of a diazoketone <06JOC5560>. On the other hand, the photochemical decomposition of the diazoketone led to pyrrolidin-2-ones. 

Under the influence of nickel catalysts, 1,5- and 1,6-dienes undergo isomerization and cyclization, preferably to five-membered ring compounds. The cyclization takes place probably via an intramolecular insertion reaction ( , ) involving a ir-5-alken-l-ylnickel complex such as 33, Table III, and 34, Table IV formed by Ni — C, and Ni — C2 additions... 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

The proposed mechanism involves the usual oxidative addition of the aryl halide to the Pd(0) complex affording a Pd(II) intermediate (Ar-Pd-Hal), subsequent coordination of allene 8 and migratory insertion of the allene into the Pd-C bond to form the jt-allylpalladium(II) species 123. A remarkable C-C bond cleavage of 123 leads by decarbopalladation to 1,3-diene 120 and a-hydroxyalkylpalladium species 124. /8-H elimination of 124 affords aldehyde 121 and the H-Pd-Hal species, which delivers Pd(0) again by reaction with base (Scheme 14.29). The originally expected cyclization of intermediate 123 by employment of the internal nucleophilic hydroxyl group to form a pyran derivative 122 was observed in a single case only (Scheme 14.29). 

Complexes M(CH2C CC=CMe)(CO)nCp [325 M = Mo, n = 3 M = Fe, = 2 (Scheme 74)] were obtained from the carbonyl anions and l-chlorohexa-2,4-diyne. Subsequent chemistry involves protonation (HBF4) to cationic allene or diene complexes, or addition of MeOH to give allylic derivatives, which are formed with concomitant insertion of CO. The latter can also be obtained from the cationic species and NaOMe. The allene-iron cation reacts with NHEt2 to form an ynenyl complex. The luminescent complex Re(CO)3(5,5 -Bu 2-bpy) 2 (At-C=CC6H4C CC=CC6H4C=C) has been reported. ... 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Isocyanides, which are better candidates to react with dienes in a 1,4-fashion, were shown to cycloadd to 1-azadienes. Thus, the formation of isoindole derivative 15 as the major product (ca. 28% yield), upon treatment of benzoquinone 13 with two equivalents of p-tolyl isocyanide [81AG(E)982] was reported the reaction involves the insertion of the isocyanide carbon atom into the C—H bond of 13 leading to the 1-azadiene derivative 14, which in turn undergoes a [4 + 1] cycloaddition with a second isocyanide molecule (Scheme 4). 

Another reaction is reductive cyclization. 1,6-Diynes and 1.6-enynes undergo reductive cyclization using hydrosilanes as a hydrogen source in AcOH. The 1,6-diynes 91 and 95 are converted into the 1,2-dialkylidenecyclopentane derivatives (1,3-dienes) 94 and 96. Triethylsilane is used as a hydrogen donor for the reaction[48]. The reaction involves the formation of a vinylpalladium bond in 92 via the insertion of an alkyne into the Pd—H bond, followed by the alkyne insertion to give 93, which is hydrogenolyzed with Si—H to give the 1,3-dienes 94 and 96. 

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. 

The conjugation process 1,4-diene — 1,3-diene goes by interconversion of 7r-Pd complexes I and III via the intermediate 7r-allyl-Pd hydride complex II and involves 1,3-hydrogen shifts. Conversion of cis,trans to the thermodynamically favored trans,trans-conjugated dienes occurs, but both of these isomers would be carboxylated the same way. CO insertion via a Pd carbonyl hydride (allyl or olefin) intermediate gives unsatu-... 

Cycloadditions to the cycloproparenyl n framework with dienes and carbenes that likely proceed via norcaradiene intermediates have been discussed (Section V.A.3). Presented here are those processes that involve insertion into the strained three-membered ring a bond. 

The insertion of CO into 7r-allylpalladium complexes has been used to prepare various P/y-unsaturated carboxylic acid derivatives. The process can be initiated from preformed allyl complexes,227-235 catalyti-cally from appropriate ir-allyl precursors228,229 236 237 or from diene-Pd complexes.238 239 Particularly mild conditions, involving low pressures and temperatures, to accomplish this insertion have been discovered.235 The key reagent in allowing these mild conditions to be employed was found to be carboxy-late anions.235 For unsymmetrical allyl complexes, CO insertion occurs preferentially at the less-substituted allyl terminus (equations 71-73). 

A number of cheletropic reactions also appear to be anomalous, including the best known of all cheletropic reactions, the stereospecific insertion of a carbene into a double bond, as in the reaction of dichlorocarbene 2.173 with alkenes. Here we have a reaction involving only four electrons, which is known to be suprafacial on the alkene, preserving the geometry of the substituents in the starting alkenes in the cyclopropanes 2.174 and 2.175. Furthermore, the [2+2] reaction takes place even with a diene, which could. undergo an allowed [4+2] reaction, but chooses not to. 

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