Allene catalysts

Synthetic strategy Catalytic enantioselective diboration of pro-chiral allenes Catalyst Tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] / (R,R)-phosphoramidite (ligand I)... 

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

Either pure acetoxyallene, on refluxing with the catalyst for 24 hr, equilibrates to an equimolar mixture of the two isomers. The mixture of allenes, if refluxed with acetic acid, rearranges to the 16,20-conjugated enol acetate ... 

Phenylacetylene gives 1-phenyI-l, l-difluoroethane on reaction with a large excess of hydrogen fluoride in ether at 0 C or, in better yield, in the gas phase over a mercuric oxide catalyst [/]. Allene affords 2,2-difluoropropane [/]... 

The polymerization filling was effected by the ion-coordination mechanism [17-19]. The monomers were ethylene, propylene, allene, os-butylene, butadiene. The fillers were mineral materials such as ash, graphite, silica gel, glass fibers. The ultimate aim of filler conditioning prior to polymerization is to secure, on its surface, metal complex or organometallic catalysts by either physical or chemical methods [17-19],... 

Allenes can also be hydrolyzed to ketones, with an acid catalyst. 

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. 

Conjugated dienes can add hydrogen by 1,2 or 1,4 addition. Selective 1,4 addition can be achieved by hydrogenation in the presence of carbon monoxide, with bis(cyclopentadienyl)chromium as catalyst. With allenes catalytic hydrogenation usually reduces both double bonds. 

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

Cyclization of allene 71 using a catalyst system comprising 10 mol % Pd(OAc)2, 20 mol % PPhj and K COj in the presence of amine 72 affords isoquinolone 73 <95CC1903>. 

The weaker Lewis add TMSOTf 20 as catalyst gives, after 2 h at 0°C in CH2CI2, a 20 80 mixture of 805 and 806 in only 23% yield (Scheme 6.8). But this yield will probably increase either on longer reaction time at 0°C or on shorter reaction time at 25 °C On replacing one of the methyl groups in 804 by an acetylene substituent the resulting enyne adds allyltrimethylsilane 82 or anisole in the presence of TMSOTf 20 to give allenes [18]. Substituted allyltrimethylsilanes such as 808 react with the allylic silylether 807 after 70 h at 25 °C in 62% yield to a 41 59 mixture of 809 and 810 as well as 7 [17]. Closely related additions of 82 to allylic ethers or O-acetates are discussed in Refs. 17a-c. 

In 2009, Ritter and coworkers reported a selective Fe-catalyzed hydroboration of 1,3-dienes to produce linear ( )-y-disubstituted allylboranes under mild conditions when a combination of Ll-FeCl2 and magnesium metal as a catalyst was used. Tbe branched ( )-allylboranes were obtained by using L2-FeCl2 instead of Ll-FeCl2 (Scheme 26) [88]. For the synthesis of 2-borylallylsilanes, this method was superior to the previously reported silaboration of allenes [89]. 

Allenes 111 undergo cycloisomerisation in presence of 20 mol% of catalyst 110 (Scheme 5.29) [32]. Interestingly, when the Grubbs first generation catalyst 109 is used in this reaction the ring-closing metathesis is observed [33]. 

Scheme 5.29 Cycloisomeiisation of 1,6-allenes catalysed by Grubbs second generation catalyst...

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]. 

Hydroaminomethylahon of alkenes [path (c)j wiU not be considered [12]. This review deals exclusively with the hydroaminahon reaction [path (d)], i.e. the direct addition of the N-H bond of NH3 or amines across unsaturated carbon-carbon bonds. It is devoted to the state of the art for the catalytic hydroamination of alkenes and styrenes but also of alkynes, 1,3-dienes and allenes, with no mention of activated substrates (such as Michael acceptors) for which the hydroamination occurs without catalysts. Similarly, the reachon of the N-H bond of amine derivatives such as carboxamides, tosylamides, ureas, etc. will not be considered. 

The hydroamination of allene with morpholine or allylamines has been attempted with palladium-based catalysts. Usually, a mixture of 1 1 telomers (hydroamination products) and 1 2 telomers is obtained, the latter being the major [308, 309] or only... 

Although zirconium bisamides Cp2Zr(NHAr)2 do not catalyze the hydroamination of alkenes (see above), they are catalyst precursors for the hydroamination of the more reactive double bond of allenes to give the anti-Markovnikov addition product (Eq. 4.96) [126]. 

Interestingly, significant progress has been made for the hydroamination of more reactive substrates such as styrenes, alkynes, dienes, and allenes. Specifically, highly selective catalysts have been discovered for the synthesis of fine chemicals (pharmaceuticals, natural products, chemical intermediates). In this area however, the problem of catalyst stabiUty can also be questioned in several cases. 

In 1998, Alper et al. reported the hydrothiocarboxylation of allenes using the same catalyst precursor and PPhj to provide 34 (Eq. 7.25) [36]. Again, the regiochemistry... 

When the reaction of 1,1-dimethyl allene with o-BrC6H4SH was carried out in the presence of Pd(OAc)2/dppf/i-Pr2HN/CO in benzene, hydrothiolation of the allene took place (Eq. 7.26) [37]. However, the regiochemistry of the adduct 36 was different from that obtained by the Pd(OAc)2-catalyzed hydrothiolation of mono-substituted allenes (cf. Eq. 7.24), showing that the regiochemistry of the hydrothiolation of allenes can be controlled by the reaction conditions even when the same metal(Pd) catalyst is used. 

Immunohistochemical studies carried out in our laboratories have demonstrated the presence of xanthine oxidase in synovial endothelial cells (Stevens etal., 1991). As expected, the activity of this enzyme per unit weight of tissue is generally higher in synovia taken from RA patients due to their increased vascularity (Allen et al., 1987). In addition, it has also been shown that rheumatoid synoviocytes contain increased levels of iron-saturated ferritin (Morris et d., 1986). Xanthine oxidase (but not dehydrogenase) is able to mobilize iron from ferritin, supplying the necessary transition metal catalyst for the Haber-Weiss reaction and promoting OH formation (Biemond eta/., 1986). 

Establishment of carbon-supported Pt catalysts as a means to achieve higher and more stable dispersion of the precious metal electrocatalyst on an electronically conducting support [Petrow and Allen, 1977]. 

Allenynes 160 were also cyclized chemo- and regioselectively to methylen-eyclopentane derivatives 161 and 162 using Rh(acac)(CO)2 as the catalyst and silanes or alkoxysilanes as the reductant (Eq. 32) [96]. The major product resulted from initial insertion of the internal Jt-bond of the allene into the Rh-Si bond. Only 1,1-disubstituted allenes were used for this reaction others may show less selectivity for the internal Jt-bond of the allene. 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

A ring opening reaction of (1-lactams promoted by methoxide generated nitrogen nucleophiles in situ that subsequently added to proximal allenes producing trisubstituted pyrroles <06CC2616>. In the event, treatment of (3-lactam 3 with MeONa led to pyrrole-2-acetic ester 4 after cleavage of the amide bond, 5-exo-dig cyclization, and loss of methanol. The sequence was notable as no metal catalyst was required. 

The usefulness of allene derivatives has also been revealed in other examples. Thus, the annulated tetrasubstituted furan illustrated in the following scheme was delivered in a moderate yield using the diazoallene as precursor by a two-step reaction in the presence of Rh-catalyst <06S3605>. 

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