Complex with an Allene

In all of the stoichiometric intermolecular reactions of a 7r-allylpalladium complex with an allene to form a new Tr-allylpalladium intermediate, the type of insertion reaction was established by H NMR spectroscopy as well as X-ray single crystal structure analy-However, it is interesting to note that the corresponding intramolecular car-... 

Three-component assembly of allenes, organic halides and arylboronic acids has been reported in which Suzuki coupling of a Jt-allylpalladium complex with an orga-noboronic acid is utilized (Scheme 16.26) [31], Addition of phosphorus ligands to the reaction mixture greatly decreases either the product yields or E/Z ratios. The decrease in E/Z ratio may be explained based on the fact that donor ligands readily promote anti-syn rearrangement of a Jt-allylpalladium species via a cr-allylpalladium intermediate. 

An intriguing iso-osmabenzene with an allenic (1,2,4-cyclohexatrienic) structure was reported its X-ray crystallographic determination confirmed the presence of an -hybridized carbon atom in the ring [298], Martin and coworkers surveyed the criteria for judging the aromaticity of various metallabenzenes (resonance energy, molecular structure, electronic structure, magnetic resonance, chemical reactivity), concluding that whereas aromaticity is undoubtedly present, metallabenzenes show specific reactions such as the formation of a cyclopentadienyl semisandwich metal complex [299],... 

The organopalladium species generated from coupling reaction of ArB(OH)2 with an allene is readily trapped by a properly distanced carbonyl group. Accordingly, 5,6-alkadi-enals are transformed into cw-2-(a-styryl)cyclopentanols. Adding (5)-SEGPHOS to complex the Pd salt has the desirable effect of asymmetric induction. ... 

Note 2. The further conversion into the yneamine CH3CEC-Morph is stopped almost completely by the addition of HO-tert.-Ci,Hg which forms the 1 1 complex with KO-tert.-CgHg. If the isomerization with KO-tert.-CgHg is carried out in DMSO, an equilibrium mixture of about 80 of the allenic amine and 20% of the yneamine is formed after 1-2 min at 30°C. 

In the first method a secondary acetylenic bromide is warmed in THF with an equivalent amount of copper(I) cyanide. We found that a small amount of anhydrous lithium bromide is necessary to effect solubilization of the copper cyanide. Primary acetylenic bromides, RCECCH Br, under these conditions afford mainly the acetylenic nitriles, RCsCCHjCsN (see Chapter VIII). The aqueous procedure for the allenic nitriles is more attractive, in our opinion, because only a catalytic amount of copper cyanide is required the reaction of the acetylenic bromide with the KClV.CuCN complex is faster than the reaction with KCN. Excellent yields of allenic nitriles can be obtained if the potassium cyanide is added at a moderate rate during the reaction. Excess of KCN has to be avoided, as it causes resinifi-cation of the allenic nitrile. In the case of propargyl bromide 1,1-substitution may also occur, but the propargyl cyanide immediately isomerizes under the influence of the potassium cyanide. 

Oxidative cleavage of the complex 549 with CuCri affords 2,3-bis(chloro-methyl)-1,3-butadiene (550) and regenerates PdCri. Thus the preparation of this interesting dimerization product 550 can be carried out with a catalytic amount of PdCl2 and two equivalents of CuCb in MeCN[495], Similarly, treatment of allene with PdBr2 affords the dimeric complex 551. Treatment of this complex with 2 equiv, of bromine yields the dibromide 552. The tetra-bromide 553 is obtained by the reaction of an excess of bromine[496]. Similarly,... 

Asymmetric hydrosilylation can be extended to 1,3-diynes for the synthesis of optically active allenes, which are of great importance in organic synthesis, and few synthetic methods are known for their asymmetric synthesis with chiral catalysts. Catalytic asymmetric hydrosilylation of butadiynes provides a possible way to optically allenes, though the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (2S,4S)-PPM turned out to be the best catalyst for the asymmetric hydrosilylation of butadiyne to give an allene of 22% ee (Scheme 3-20) [59]. 

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

Insertion of an allene molecule into the Pd-H bond, with regeneration of the starting allene complex 8 which then re-enters the catalytic cycle. In the absence of allene, extensive Pd black formation was observed. 

Terminal allenes.1 A synthesis of 1,2-dienes (3) from an aldehyde or a ketone involves addition of ethynylmagnesium bromide followed by reaction of the adduct with methyl chloroformate. The product, a 3-methoxycarbonyloxy-l-alkyne (2), can be reduced to an allene by transfer hydrogenolysis with ammonium formate catalyzed by a zero-valent palladium complex of 1 and a trialkylphosphine. The choice of solvent is also important. Best results are obtained with THF at 20-30° or with DMF at 70°. 

In 1963, an asymmetric synthesis of chloroallenes was reported by the SNi reaction of propargyl alcohols with thionyl chloride [34]. Since then, rearrangement of pro-pargylic precursors has been one of the most useful methodologies for the synthesis of allenes [35]. Treatment of 84, obtained by asymmetric reduction with LiAlH4-Dar-von alcohol complex, with thionyl bromide gave 86 as the major product via 85 (Scheme 4.21) [36],... 

A rather different allene cyclization that takes place in tandem with an alkylative process has been described by Trost and Urabe [66] (Eq. 13.52). Exposure of ketoal-lene 169 to divinylmethylaluminum 170 in dichloromethane solution leads to 171 in 60% yield. The mechanism of this alkylative cyclization involves complexation of the Lewis acidic aluminum to the carbonyl oxygen atom, followed by ring closure, leading to zwitterion 172. Intramolecular transfer of a vinyl group from aluminum to carbon completes the process. 

An a-allenic sulfonamide undergoes Pd-catalyzed carbonylative cyclization with iodobenzene, affording a mixture of isomeric heterocycles (Scheme 16.12) [17]. The coupling reaction of an allene with a PhCOPdl species takes place at the allenyl central catrbon to form a 2-acyl-Jt-allylpalladium complex, which is attacked by an internal sulfonamide group in an endo mode, affording a mixture of isomeric heterocycles (Scheme 16.13). 

In sharp contrast, a vinylallene bearing two methyl groups at the allenic termi-nusl is treated with [RhCl(PPh3)3] to give ( 4-vinylallene)rhodium(I) complexes as an isolable mixture of endo and exo isomers. The endo isomer is found to isomerize thermally to the exo isomer (Scheme 16.39) [39, 40],... 

The intermolecular PKR of an allenic compound with an alkyne Co2(CO)6 complex takes place under very mild conditions when N-methylmorpholine oxide (NMO) is used as a promoter, giving rise to ( )-4-heptylidene-2,3-dipropyl-2-cyclo-pentenone in 69% yield (Scheme 16.48) [50-53],... 

Silaboration of 3-substituted 1,2-dienes takes place smoothly at the internal double bond in the presence of the catalytic Pd(acac)2-2,6-xylyl isocyanide complex and the boryl group is regioselectively introduced to the central carbon atom of an allene (Scheme 16.55) [59, 60]. The same regioselectivity is observed with the catalytic system Pd2(dba)3-P(OCH2)3CEt [59]. 

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

An allene is a very promising unsaturated partner in cobalt-mediated [2 + 2 + 2]-cycloaddition reactions. Exposure of an allenediyne to a stoichiometric amount of CpCo(CO)2 in boiling xylenes under irradiation for 5 h furnishes red-brown complexes in 42% isolated yield (Scheme 16.76) [84—87]. Treatment of a 7 3 mixture of the two diastereomers thus obtained with silica gel provides an oxygen-sensitive cobalt-free tricydic compound. 

It has been shown that an allene acts as a jt-nucleophile and attacks a Jt-allylpalla-dium complex on the face opposite to that of palladium (Scheme 16.90) [98]. An alle-nic allylic pivalate, cis-isomer, undergoes a smooth cyclization in the presence of catalytic amounts of Pd(dba)2 in toluene to give a cis-fused 5,6-ring system. The reaction with the trans-isomer gives no cyclization product and the starting material is recovered. 

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