Intermolecular reactions allenes

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

For the performance of intermolecular reactions of 5, its generation from the sodium salt of tropone tosylhydrazone (517) and from halo-1,3,5-cydoheptatrienes (518) are most suitable, but other precursors can also be employed, as illustrated in Scheme 6.105. In view of the product structures, some reactions of 5 give results that deviate from those of typical allene processes, and this is even valid for the dimerization (Scheme 6.105). 

The use of allene substrates in the intermolecular reaction has also been reported.30 34... 

Intermolecular reactions involving attack of /z-nucleophiles (even F") [262] at n radical cations include reactions at alkenes [263], allenes [264], aromatic systems... 

Photolysis of diazirine 63 included within the cavities of CyDs presumably formed carbene 64 CyD ICs, i.e., 64 CyD (Equation (7)). The lifetime (t) of carbene 64 was expected to be prolonged due to the preclusion of intermolecular reactions (Scheme 14), such as azine 70 formation and solvent insertion, i.e., 64->72. However, interfering innermolecular reactions21 between the host and guest were indicated (vide infra). Therefore, the latent intramolecular rearrangement of carbene 64 to cyclic allene 65, a rare transformation seen under the forbidding, low-temperature... 

Intermolecular reactions of dienes, allenes, and methylenecy-clopropanes with alkenes are mediated by RhCl(PPh3)3, although mixtures of products are usually formed (eqs 50-51). ... 

Wender s group has also developed rhodium-catalyzed intermolecular [5-1-2] cycloadditions. At first, they found the catalysis system of Rh(PPh3)3Cl for the intramolecular reactions was not effective at all for the intermolecular reactions. To effect the intermolecular [5-1-2] cycloadditions, [Rh(CO)2Cl]2 must be used and oxygen substitution of the cyclopropane was necessary (see (17)) [37-39]. Then they successfully expanded the substrate to unactivated vinylcyclopropanes by adjusting the substituents. For monosubstituted alkynes, the substitution on the olefin terminus directs the formation of single isomer that minimized steric hindrance (see (18)) [40]. The [5-1-2] cycloadditions can also be applied to VCPs with allenes (see (19)) [41]. It should be noted that the alkyne substituent did not interfere with the reaction, indicating that allenes as reaction partners were superior to alkyne in the [5-1-2] cycloadditions. Curiously, the authors didn t report the corresponding intermolecular [5-1-2] cycloadditions of VCPs with alkenes. 

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-... 

As with the intermolecular reactions of allenes and furans (cf. Scheme 13.11), intramolecular reactions of allenic dien-ophUes provide access to important chiral architectures upon cycloaddition. For example, exposing 121 to catalytic amounts of Me2AlCl at —20°C produced 122 as a single... 

The intermolecular reaction of allenes with alcohols in the presence of catalytic amounts of PtCl2 was recently reported by Sierra and coworkers [155]. The reaction leads to an unexpected aliphatic acetal formation by attack of two molecules of methanol to the terminal carbon of monosubstituted allene systems with complete reduction of the allene (Scheme 92). 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

As shown in the preceding examples, although intramolecular Pd-catalyzed poly-cyclization is a well-established procedure, some few examples exist of polycycliza-tions where the first step is an intermolecular process. In this respect, the Pd°-cata-lyzed domino reaction of allenes in the presences of iodobenzene reported by Tanaka and coworkers [40] is an intriguing transformation. As an example the Pd-catalyzed reaction of 6/1-60 in the presence of iodobenzene led to 6/1-61 in 49% yield, allowing the formation of three rings in one sequence (Scheme 6/1.14). 

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... 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

The intramolecular and intermolecular deuterium isotope effects in the cycloaddition of acrylonitrile to allene (equation 98) have been studied by Dolbier and Dai231,232. The intramolecular KIEs in the allene-acrylonitrile system were found to be 1.21 0.02 at 206°C and 1.14 0.02 at 225°C. A negligible intermolecular SKIE was found in the reaction of the mixture of tetradeuteriated and undeuteriated allene using a limited amount of acrylonitrile (ku/ku) = 1.04 0.05 at 190-210 °C for D0/D4 allene. An equilibrium deuterium IE of 0.92 0.01 was found at 280-287 5°C (15-45 h reaction time). 

Arylative or silylative cyclizations of allenyl aldehydes or ketones have been reported (Equations (101) and (102)).459,459a The intermolecular process, that is, three-component coupling reaction of aldehydes, allenes, and arylboronic acids, is catalyzed by palladium as well (Equation (103)).46O 46Oa These reactions are proposed to proceed through nucleophilic attack of the allylpalladium intermediates to the carbonyl groups. 

In contrast to the intramolecular process, simple allenes do not participate in the intermolecular [5 + 2]-reaction with VCP 18. However, when a second functionality is incorporated into the allene, an efficient and facile cycloaddition occurs, presumably assisted by a directing effect of the secondary functional group. The [5 + 2]-reaction also works with styrenyl- and cyano-substituted allenes as directing groups (Scheme 12). As would be expected from... 

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