Tt-Allylpalladium intermediate

Aryl- or alkenylpalladium comple.xcs can be generated in situ by the trans-metallation of the aryl- or alkenylmercury compounds 386 or 389 with Pd(Il) (see Section 6). These species react with 1,3-cydohexadiene via the formation of the TT-allylpalladium intermediate 387, which is attacked intramolecularlv by the amide or carboxylate group, and the 1,2-difunctionalization takes place to give 388 and 390[322]. Similarly, the ort/trt-thallation of benzoic acid followed by transmetallation with Pd(II) forms the arylpalladium complex, which reacts with butadiene to afford the isocoumarin 391, achieving the 1,2-difunctionalization of butadiene[323]. 

Butenoic acid and 4-pentenoic acid (42) react with alkenyl halides or tri-flates to afford 7-alkenyl-7-lactones and the ( -alkenyl-5-valerolactone 44 via the TT-allylpalladium intermediate 43 formed by the elimination of Pd—H and its readdition in opposite regiochemistry using a phosphine-free Pd cata-lyst[43]. 

The dienyne 394 undergoes facile polycyclization. Since the neopentylpalla-dium 395 is formed which has no hydrogen /J to the Pd after the insertion of the disubstituted terminal alkene, the cyclopropanation takes place to form the tt-allylpalladium intermediate 396, which is terminated by elimination to form the diene 397(275]. The dienyne 398 undergoes remarkable tandem 6-e. o-dig. 5-cxo-trig. and -exo-trig cyclizations to give the tetracycle 399 exclu-sively(277]. 

Pd(Ph3P)4, dimedone, THF, 88-95% yield. The catalyst is not poisoned by the presence of thioethers such as methionine. Diethyl malonate has also been used as a nucleophile to trap the TT-allylpalladium intermediate and regenerate Pd(O)." ... 

Tomita and Endo have shown that three-component coupling of bisallene 163, aryl dihalide, and a malonate nucleophile takes place with a palladium catalyst. Arylpalladium species derived from the halide attacks the central carbon of allene to form a Tt-allylpalladium intermediate, which is then attacked by the malonate anion to form C-C... 

Palladium-catalyzed substitutions of allylic esters 93, which proceed via intermediate jz-allylpalladium complexes such as 94, are well established [178]. In contrast, the reactions proceeding through TT-allylpalladium intermediates, generated by addition of the palladium interaiediates from haloarenes, haloalkenes, or the corresponding triflates, to allene and substituted allenes 95 (Scheme 3-26), have scarcely been tapped [79]. 

Type III reaction proceeds by an attack of a nucleophile at the central sp carbon of the allenylpalladium. In contrast to facile Pd(0)-catalyzed reactions of allylic esters with soft carbon nucleophiles via TT-allylpalladium intermediates, propargylic esters are less reactive towards soft carbon nucleophiles. No reaction of soft carbon nucleophiles occurs with propargylic acetates. However, soft carbon nucleophiles such as -keto esters and malonates react with propargylic carbonates under neutral conditions using dppe as a ligand [43]. 

Formation of the lactone 90 from 89 is explained by the intramolecular acyloxypalladation mechanism. On the other hand, the lactones 90 and 92 are formed by two paths. Two different mechanisms cooperate to form the desired lactone 90 in 60 % yield and 92 in 30 % yield. The formation of 90 and 92 can be rationalized by generation of the TT-allylpalladium intermediates by allylic C—H bond activation as an alternative pathway [45]. 

Different regioselectivities were observed in the reaction of 3,4-pentadien-l-ols 387 with aryl iodides. The expected products 390 and 391 from the tt-allylpalladium intermediates 389 were not formed [151]. Exclusive formation of the dihydrofuran 388 is explained by concerted inter- and intramolecular exo-oxypalladation as shown by 392 to give 7r-allylpalladium 393, and reductive elimination gives rise to the dihydrofuran 394, showing that the intramolecular oxypalladation is faster than the intermolecular carbopalladation with Ar-Pd-I. 

Efficient domino reactions of the alkenyl iodide 453 with CO, aUene, and piperidine proceeded to give 456 in very high yield (80 %) after six-step reactions [172], CO is the most reactive, and CO insertion is followed by olefin insertion to generate 454. Then allene insertion occurs after the second CO insertion. Finally, the TT-allylpalladium intermediate 455 is trapped by the amine to yield 456. 

Acylpalladium was generated by the oxidative addition of acyl chloride 98 to Pd(0), and reacted with 1,1-dimethylallene (99) to give tt-allylpalladium intermediate 100. Then transmetallation of 100 with diborane and reductive elimination provided 2-acylallylboronate 101. Thus highly regio- and stereoselective acylboration of allenes occurred using ligandless Pd catalyst [44]. 

Dihydro-l,4-benzodioxines 139 and 140 are prepared by the reaction of propargyl carbonates 136 with catechol (137) by attacking either terminus of the TT-allylpalladium intermediate 138. 

Treatment of the propargyl benzoate 162 with Pd2(dba)3 and DPPF in the presence of A -alkyltosylamide generated the allenylpalladium 163, and the TT-allylpalladium intermediate 164 was generated by the attack of the amino group. 

The TT-allylpalladium intermediate 130, formed by the intramolecular carbopalladalion of 129, could be trapped with the enolate of diethyl malonate, with BusSnPh, with piperidine, or with phenol to afford a variety of cycloocta-1,3-diene derivatives 131-134 (Scheme 43).[ ii... 

In all of the reactions discussed above, the carbopalladation of the allene occurred regioselectively in such a way that the substituent of the first formed organopalladium intermediate would be attached to the central carbon atom of the allene moiety to form a tt-allylpalladium intermediate. However, Grigg et al. and later Oppolzer et al. observed that the intramolecular carbopalladation of the first formed allene 165 proceeded in a completely different manner (Scheme 53). Instead of the formation of a tt-allylpalladium conplex, a reverse regioselectivity to form a Csp2—Pd intermediate was observed. 

TT-Allylpalladium intermediates described in this section can undergo either a /3-H elimination or a nucleophilic substitution reaction. Enantioselective nucleophilic substitution can afford the corresponding chiral compounds with high enantiomeric excesses. 

Asymmetric carbopalladation can also be combined with other reactions to give domino asynunetric processes. A group selective example from a synthesis of ( capnellene 216 features the cyclization of the alkenyl triflate 210 (Scheme 32). The resultant TT-allylpalladium intermediate has effectively been captured with a variety of nucleophiles including acetate anion for the synthesis. A domino Suzuki coupling/in-tramolecular Heck reaction converts ditriflate 212 into tricycle 213 in modest yield with 85% The transformation accomplises an annnelation, two carbon-carbon bond... 

The reaction of 2,3-dimethylbutadiene with 2-methylacetylacetate 14 gave an excellent yield of a single 1,4-addition product (Eq. (8)). It was suggested that the reaction proceeds via a tt-allylpalladium intermediate formed by Pd-H addition to the diene, followed by nucleophilic attack by the carbanion (cf. Scheme 11.1). It is Hkely that the reaction proceeds along path A (Scheme 11.1) that is, via an external nucleophilic attack by the carbanion. 

Bis-TT-allylpalladium intermediates are involved in the dimerization-addition reaction of 1,3-dienes with nucleophiles. 

It has been suggested that the active catalytic species in the formation of the crucial tt-allylpalladium intermediate is most likely a coordinatively unsaturated Pd° complex of either the form PdL2 or Pdl X, where L is typically a phosphine or phosphite and X a... 

The most prominent use of ketenes is for [2 + 2] cycloaddition with imine for the construction of /3-lactam skeleton. When the Y group in Scheme 1 is vinyl or aryl group, the deprotonation of the activated a-proton is highly facilitated. In this context, the carbonylation of some allylic derivatives, for example, allyl bromide, allyl acetate, allyl phenyl ether, allyl methyl carbonate, allyl phenyl sulfone, and allyl phosphate, documented to form TT-allylpalladium intermediates is examined. It is interesting to note that only phosphate undergoes the cycloaddition to produce /3-lactam. The characteristic dependency of the stereochemistry on the reaction conditions, being contrary to the results in the usual base-induced cycloaddition is also intriguing. Scheme 2 presents the... 

Intramolecular carbopalladations, as in the Heck reaction, with subsequent reduction by a hydride source have established themselves as an indispensable tool for the construction of various carbocyclic and heterocyclic skeletons. Such a reaction comes about when the i yn-addition of an aryl- or alkenylpalladium species to a multiple bond leads to an intermediate that does not or cannot undergo a rapid 5yn-/3-hydride elimination such as certain cycloalkyl- (Scheme 21, Eq. neopentyl- (Eq. Eq. Eq. or TT-allylpalladium intermediates (Eq. 5). The most efficient reagents for the reduction are formic acid in the presence of secondary or tertiary amines and alkali metal formates such as sodium formate. In some cases tin hydrides have successfully been employed (Scheme 33, Eqs. 3-5). 

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