Allene synthesis

OOERING - LA FLAMME Allene Synthesis Allene synthesis from olefins via gem-dihalocyciopropanes. 

Scheme 2.2 Allene synthesis via SN2 substitution of propargylic acetates 7 with cuprates.

As was the case for allene synthesis by copper-promoted Sn2 substitution reactions, the corresponding 1,6-addition to acceptor-substituted enynes has found sev-... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

Scheme 2.61 Enantioselective allene synthesis with chiral protonating agents.

Scheme 4.23 Chiral allene synthesis via asymmetric aldol reaction and Claisen rearrangement.

The (-)-sparteine-mediated lithiation of achiral propargyl carbamates with wBuLi and allene synthesis has already been described in Section 4.2.7 [84],... 

Scheme 4.62 Asymmetric allene synthesis via isomeration of alkyne 240 with the chiral phase-transfer catalyst 241.

The olefin metathesis reaction apparently has rarely been applied to allene synthesis so far, or at least the potential of this important process has not been exploited... 

The conformationally locked [46] bisallene 127 was first prepared by subjecting the allene dimer 126 to the DMS allene synthesis (Scheme 5.17) [47]. 

For DFT and ab initio quantum mechanical computations of the cydopropylidene ringopening process, the decisive step of this allene synthesis, see H. F. Bettinger,... 

The male bean weevil sex attractant has been made by a new method of allene synthesis from aldehydes and alkenyl aryl sulfoxides by sulfoxide-metal exchange as the key reaction. 

Substitution of a heteroatom substituent for an alkyl group provides an additional method of allene synthesis. For example, metalation of an allenic sulfoxide76-77 or transmetalation of a stannylallene21,78 with methyllithium usually proceeds with complete retention of configuration, hut with reversal of priorities, at the allenic moiety. 

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. 

A. Magnesium Cyclopropylidenes as Intermediates in the Doering-LaFlamme Allene Synthesis... 

The allene synthesis strategy outlined in equation 21 can be rendered asymmetric if non-racemic sulfoxides such as 47a are employed26. The diastereoselective step is an equilibration of the sp3-allylic organometallic species 53 before the -elimination step (equation 23). 

At temperatures above —100 °C the lithiohalocyclopropanes are converted to cyclopropylidenes by formal loss of one molecule of lithium halide. The main route of stabilization of these carbenacyclopropanes consists in the formation of allenes. The process which has been termed the Doering-Moore-Skattebol allene synthesis ( DMS-synthesis ) [66-68], has been developed into the most general method for the preparation of these reactive compounds which especially during the last decade have been used in organic synthesis with growing success [69, 70],... 

Dehalogenation of dihalocyclopropanes with sodium or magnesium gives allenes (equation 47).147 Since the original observation, a number of reagents have been introduced for the allene synthesis via de-... 

Nantz, M. H. Bender, D. M. Janaki, S. A convenient terminal allene synthesis from propargylic acetates. Synthesis 1993, 577-578. 

Light-induced diene migration An allene synthesis. Proc. chem. Soc. [London] 1964, 17. 

---