Allenes, protonation

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

Additional allene homologues were prepared by using this methodology with a variety of electrophiles (EX, Table 9.2) [6], For reactions requiring removal of a secondary allenic proton the base of choice was tBuLi. Only allenic products were formed except in the reaction with cyclopentanone, in which a small amount of the homopropargylic alcohol product was produced (last entry). 

Carbamate substituents have also been found to permit the direct removal of allylic, propargylic and allenic protons by organolithium reagents [32, 33]. In the latter case, the resulting lithioallenes can be converted to the allenyltitanium reagents with ClTi(OiPr)3 (Eq. 9.28) [8]. As illustrated, subsequent addition to acetaldehyde proceeds with only modest diastereoselectivity. 

In conclusion, the most important result is that the use of permethylated cyclodextrin as chiral solvating agent for NMR spectroscopy not only affords a simple and practical way for the determination of the stereochemical purities of trisubstituted allenes, but also allows one to simultaneously determine their absolute configuration. Indeed, TRIMEB induced only positive complexation shifts of all the allene protons, which are greater for the (S )-enantiomer than for the (R)-enantiomer, independent of the structure of the allene. This empirical correlation seems to be reliable since it has been satisfied by a large number of trisubstituted allenes. 

TRIMEB/allene molar ratio. Data relative to the allene proton of llOa-f are summarized in Table 43. 

TABLE 43. Unequivalence (A<5" at 300 MHz, CD3OD) induced in the allene proton of trisubstituted allene (40 mM) in the presence of TRIMEB, as a function of the temperature and of the aUene/TRIMEB molar ratio... 

Alcohols will add to allenes in the presence of trace amounts of acids to give vinyl ethers (or acetals) or allylic ethers.401 Analogous to the hydration of allenes, protonation occurs on the terminal carbon of the allenic functionality in 1,2-propadiene, 3-alky 1-1,2-propadienes and 1,3-dialkyl- 1,2-propadienes (equation 248).402 Addition of an alcohol to the resulting vinylic cation produces a vinylic ether, which may on further reaction form an acetal of the corresponding ketone. 

The experimental conditions for the carboxylation of allenyllithium are governed by the consideration that introduction of the COOLi group renders the allenic protons more acidic (resonance stabilization). If the normal order addition is used (introduction of C02 into the solution of allenyllithium), the allenic carboxylate primarily formed may easily be deprotonated by the strongly basic allenyllithium. The new species (either LiCH=C=CHCOOLi or H2C=C=C(Li)COOLi) may react also with C02. To avoid this situation the solution of allenyllithium is gradually added to a strongly cooled solution of carbon dioxide in THF. As usual, liberation of the acid occurs by adding mineral acid. In view of the possibility of an acid-catalyzed cyclization to a lactone too strongly acidic conditions should be avoided ... 

The reaction conditions for the metallation of propadiene show that this compound is readily metallated. Introduction of alkyl substituents causes a marked decrease of the kinetic acidity of the allenic protons. This is reflected in the higher temperature in the lithiation of 1,1-dimethylallene. 

Dimethyl-penta-1,2-diene I-methyl-I -isopropyl-allene) Proton chemical shift 350. 

Electrophilic Additions to Allenes.—Protonation of tetramethoxyallene, the bis(dimethylacetal) of carbon suboxide, takes place at the central carbon atom the resulting cation reacts with nucleophiles either by addition or by demethylation (Scheme 16). The tetramethoxyallyl cation (356), produced by... 

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