Reactions with Allenes

Reversible pyridine dissociation yields the non-Lewis base stabilized imido complex [Cp(NHAr) Ti=NAr] (54). This coordinatively unsaturated species undergoes a [2 + 2] cycloaddition reaction with allene to give an azatitanocyclobutane (55), which is then protonated to the /mamido complex. Elimination of enamine occurs followed by isomerization to the energetically more favorable imine. 

The reaction of ketoximes with monoactivated allenes is either slow or complicated. However, the reaction with allene-l,3-carboxylate 476 afforded addition product ( )-527 highly stereoselectively [238]. 

Selective 1,4-reduction of unsaturated aldehydes and ketones by 6 occurs smoothly in THF between —25 °C and room temperature within a few hours (Eq. 5.7). Particularly noteworthy is the realization that phosphines are noticeably absent from the reaction medium. The analogous combination of CuCl/BusSnH in N-methyl-2-pyrrolidinone (NMP) or DMF does not behave identically [22], failing to react with the hindered substrate isophorone, whereas a 72% yield of the corresponding reduced ketone is formed with reagents XCu(H)Li/Bu3SnH. Nonetheless, a form of CuH is being generated in this more polar medium, effectively utilized by Tanaka to arrive at 3-norcephalosporin 8 upon reaction with allenic ester 7 (Scheme 5.3). 

The addition to a carbon-carbon triple bond results in the formation of cyclo-propene products, and with diazoacetates the catalyst of choice for intermolecular addition is the dirhodium(II) carboxamidate 13 (e.g., Eq. 26). The reactions are general, except for phenylacetylene whose cyclopropene product undergoes [2 + 2]-cycloaddition, and selectivities are high. However, high selectivities have not been reported for reactions with allenes. 

Reaction with allene sulfoxides.1 Allenic sulfoxides are desulfurized by reaction with methyllithium. An example is the transformation shown in equation (I). The propynylcarbinol 1 is treated with benzenesulfenyl chloride to form the allene 2,... 

Trialkylaluminums have shown the capability of entering into nickel-catalyzed reactions with allenic bromides, to afford direct alkylation at vinyl carbon. Inversion of configuration occurs at this carbon to a degree which is greater than with other alkylmetals (magnesium, zinc) (see equation 6O)90. 

Irradiation of a mixture of diphenyl disulfide and diphenyl ditelluride in the presence of vinyl cyclopropane 141 with visible light gives the ring-opened thiatelluration product 142 as an E/Z mixture.208 Similar reaction with alkynes gives the thiatelluration product 143. The product stereochemistry depends on the nature of the alkyne.208 A mixture of diphenyl diselenide and diphenyl ditelluride reacts with alkynes in a similar way (Scheme 78).208 Similar reactions with allenes are less satisfactory.209... 

The reaction of salicylaldehydes with a,P-unsaturated compounds which can lead to chromenes, chromans and other heterocycles has been reviewed <07OBC1499> and details of their reaction with allenic esters and ketones have been published <07CEJ3701>. A proline-catalysed benzoic acid-promoted asymmetric synthesis of chromene-3-carbaldehydes involves a domino oxa-Michael - aldol reaction with a,P-unsaturated aldehydes the ee range is 83-98% (Scheme 11) <07CEJ574>. 

Scheme 8. Pauson-Khand reactions with allenes.

Reactions with alkenes 5.6.4.5 Reactions with alkynes 5.6.4.5 Reactions with allenes 5.6.4.5... 

Chloropalladation occurs with 1,3-dienes and allenes to give 7i-chloroallyic complexes under mild conditions . Two products are obtained from the reaction with allenes, dependent on the polarity of the solvent used (e.g., benzene or methanol). For the chloroallyl complex, treatment with triphenylphosphine releases the allene . 

ReH(n5-C5H5)( J-Ti5-C5H4)Mn(CO)4 formation 10.3.5,3 C14H10M02O4 [Mo(CO)2(CsH5)]2 reaction with acetylenes 9.2.4.7 reaction with allene 9.2.4.7 Ci4HioNb204S2... 

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