Allene catalysts, palladium complexes

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C=CH2 moiety of the allene in the anti-Markovnikov mode from the opposite side of the substituent. This addition gives a 7t—allyl palladium complex with the (Z)-configuration,18 which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 

Primary propargylic formates decarboxylate in the presence of Pd(acac)2 and Bu3P at room temperature to give mainly allenic products (Eq. 9.115) [91]. Initial formation of a propargylic palladium complex, which rearranges to the more stable allenylpalladium species, accounts for this transformation. Under similar conditions, a terminal allenyl formate afforded a 99 1 mixture of allene and acetylene product (Eq. 9.116) [91]. However, a mixture of enyne elimination products was formed when a secondary propargylic carbonate was treated with a palladium catalyst (Eq. 9.117). 

Catalytic enantioselective synthesis of 4,4-dimethyl-l-phenyl-l,2-pentadiene from 4,4-dimethyl-1,2-pentadiene and iodobenzene using 0.4 to 1 mol % of palladium complexes containing chiral phosphane ligands as the catalyst for the enantioselective cross coupling134 is the only example of substoichiometric transition metal catalyzed enantioselective allene synthesis. 

Several catalytic systems have been investigated for hydroamination of unsaturated bonds [16]. Takahashi et al. reported the telomerization of 1,3-dienes in the presence of an amine leading to octadienylamine or allylic amines when palladium catalysts are used in association with monodentate or bidentate phosphine ligands, respectively [17]. Dieck et al. demonstrated the beneficial effect of addition of an amine hydroiodic salt in the hydroamination reaction of 1,3-dienes in which the allylic amines are produced via an intermediate Jt-allyl palladium complex [18]. Coulson reported the Pd-catalyzed addition of amines to allenes where dimerization is incorporated [4]. This reaction presumably proceeds via a cyclic palladium intermediate in which the Pd activates the olefinic bond for nucleophilic attack the reactions are therefore different from pronucleophilic additions. 

The first examples of a Pd-catalyzed addition of terminal alkynes to allenes have been reported by Trost and Kottirsch (Scheme They observed that different palladium complexes efficiently catalyze the addition of terminal alkynes I to 3-substituted allenecarboxylates 23 to produce a mixture of the a,)8-unsaturated ester 24 and the two isomeric /S.-y-unsaturated esters (Z)- and (E)-25 (Scheme 9). Remarkably, the type of Pd catalyst used played a crucial role in determining the regiochemistry of addition to the 1,3-disubstituted allenes 23. Thus, with the Pd(OAc)2 TDMPP system (cat. A) a 76 24 ratio of 24 and E/Z)-25 was produced, whereas with a combination of tetrakis(carbo-methoxy)palladacyclopentadiene (TCPC) and TDMPP (cat. B) these products were obtained in a ratio of 9 91 (Scheme 9). ° ... 

An asymmetric version of the intramolecular hydroarylation of alkynes (286) to produce fused rings (287) has been developed. With the cationic Pd(II)/(5)-(285) catalyst, the resulting 4-aryl 2-quinolinones were obtained with <98% ee. Asymmetric addition of boronic acids R B(OH)2 to activated cumulenes R C=C=C=C(R )C02Et, catalysed by an NHC-palladium complex, has been shown to yield allenic products R C=C=C(R )CH(R )C02Et with moderate to good enantioselectivities. ... 

Axially chiral allene can be racemized by addition-ehmination of AB, opposite to the other process above mentioned in this section (Schemes 5.28-5.41). Allenic alcohol was transformed to its butyrate in 83% yield with 89% ee by the combined system of NHC-palladium bromide complex together with hpase (Scheme 5.42) [115]. Although allenic esters are also racemized by this palladium complex, the half-life time of racemization is about fivefold longer than that of allenic alcohol. In the case of PdBr2(CH3CN)2, instead of NHC complex, the selectivity of the racemization was lost. The choice of palladium catalyst is important for this DKR system. 

Palladium-catalyzed annulation reactions of conjugate acceptors and allenyl boronic ester provide substituted cyclopentenes in high yields and diastereose-lectivities (Scheme 6.24). These reactions are hypothesized to commence by the conjugate addition of a nucleophilic propargyl-palladium complex. Transmetalation of allenylboronic acid pinacol ester with a Pd(II) catalyst proceeds via an SE2 mechanism to provide the propargyl-palladium complex, which on conjugate attack on the electrophile furnishes an allene intermediate. Finally, endo carbopalladation of the pendant allene and protodepalladation generates the cyclopentene [28]. 

It seems that palladium complexes with high oxidative states and basic ligands can be involved in oxidative addition as weU as in o-bond metathesis. Cheng has demonstrated that phosphine-free Pd complexes together with alkenyl or aryl iodides are very efficient catalysts for 1,2-diboration of allenes. This reaction is completely regioselective and highly... 

A stable ds-Pt(B02C2Me4)2(PPh3)2 complex has been isolated and characterized by X-ray analysis, whereas the corresponding Pd(0) complex is not known. Compatibly, no Pd-catalyzed diboration is known. However, Yang and Cheng reported for the first time a palladium-catalyzed diboration of allenes using an aryl, alkenyl iodide or I2 as a co-catalyst (Scheme 16.53) [58]. 

Another difference between the two mechanisms is that the former involves 1,2-and the latter 1,3-shifts. The isomerization of 1-butene by rhodium(l) is an example of a reaction that takes place by the metal hydride mechanism, while an example of the 7i-allyl complex mechanism is found in the Fe3(CO)i2-catalyzed isomerization of 3-ethyl-l-pentene. A palladium catalyst was used to convert alkynones RCOCSCCH2CH2R to 2,4-alkadien-l-ones, RCOCH=CHCH=CHCHR. The reaction of an en-yne with HSiCls and a palladium catalyst generated an allene with moderate enantioselectivity (see p 148 for chiral allenes). ... 

Active methylene compounds can be added to polar double bonds such as those in acrylate esters and methyl vinyl ketone as has been described in the previous section. Active methylene compounds can also be added to carbon-carbon multiple bonds in allenes and alkynes with the aid of the transition metal complexes as the catalyst. The addition of methylmalononitrile to 3-phenyl-l,2-butadiene takes place in the presence of Pd2(dba)3-CHCl3 to give the corresponding addition product with E-stereochemistry (Eq. 67) [137 a]. The C-C bond formation occurs exclusively at the terminal position of the allenes. Trost et al. independently reported the similar results with respect to palladium-catalyzed addition of C-H bonds in active methylene compounds to allenes [137 b. ... 

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