Formation from allenes

Tlie thioketone 5-oxide 85, generated from allene 86 and SOCk in situ, decomposes to give the alkene 87. A mechanism, involving the transient formation of the 1,2-dithietane 88 (by dimerization of 85 followed by rearrangement), was proposed (85AGE855). 

Fig. 23. Summary of the unimolecular dissociation processes of the ground electronic state ally 1 radical at 115kcal/mol. Two competing pathways, formation of allene and isomerization to 2-propenyl (which subsequently dissociates into allene and propyne), are observed. The experimental (bold) and RRKM (italics) rates are given. The allene formation is the dominating channel. (From Fischer et at.145)...

Biaryl synthesisThis reagent promotes coupling of aryl Grignard reagents to symmetrical biaryls in 70-95% yield with formation of allene and 3-aryl-2-chloropropene as co-products. The reaction is retarded by galvinoxyl and evidently involves an electron-transfer from the Grignard reagent to the dichloropropene. 

Scheme 2.43 Formation of allenes from propargyl mesylates and lithium triorganozincates.

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

Some cases are known in which Diels-Alder reactions of electron-deficient allenes and dienes compete with [2 + 2]-cycloadditions (see also Section 7.3.7) [12, 151, 335, 336]. Recently, a phosphane-catalyzed [4 + 2]-annulation starting from allenic ester 337 and N-tosylaldimines 338 was published [337]. However, the formation of the tetrahydropyridines 339 isolated in excellent yields is explained by a multi-step mechanism and only resembles a Diels-Alder reaction. 

Three zirconium/cycloheptadienyne complexes (231a-c) have been prepared by /3-hydrogen elimination from a mixture of cycloheptatrienyl complexes 269-271 (Scheme 33) and have been used as intermediates for the preparation of a zirconaazulene.87 The alkyne complexes are formed to the exclusion of the allene isomer 268. This is believed to be due to the proximity of the /3-vinyl hydrogen that is a result of both the shorter double bond and its forced coplanarity with the metal. Allene formation from 269 might be induced by blocking the vinyl position (see Sections IV,B and IV,C), but this has not been tested. 

A number of groups have studied one or more of the C3H4 isomers [213, 677—680, 806]. It has been proposed that the loss of H- from the allene ion proceeds via two pathways to give two different (C3H3)+ structures [213] (cf. loss of Cl- from the propargyl chloride ion). A kinetic shift has been determined for formation of (C3H3)+ from allene [806]. Some evidence was found that the cyclopropene and propyne ions isomerised to the allene structure before they decomposed [678]. 

Addition of various hydrogen halides to propyne has been investigated by Griesbaum et al. (1965) (la-c of Table 2). The product distribution parallels that obtained from allene under identical conditions and the most likely mechanism involves formation of a propenyl cation which may react with halide ions to give the expected addition products or cyclodimerize to 1,3-dihalocyclobutane (see discussion in section II, Bib). 

The normal proton abstraction which precedes 3-elimination induces allene formation from II. The allene, being a Michael acceptor then inactivates the enzyme via an alkylative process involving a nucleophilic (Nu) residue on the enzyme. Propargylglycine has since been shown to also irreversibly inactivate glutamate-pyruvate transaminase (8). 

The formation of the propargyl radical indicates that hydrogen abstraction from allene has occurred. However it was shown in a separate experiment that the 1 -methylvinyl radical abstracts hydrogen very easily from allene to form the propargyl radical. (The 1-methylvinyl radical was prepared from 2-bromopropene and sodium and then bombarded with allene). Thus it is unnecessary to postulate that hydrogen atoms can abstract hydrogen directly from allene. 

Nagata, R. Yamanaka, H. Okazaki, E. Saito, I. Biradical formation from acyclic conjugated eneyne-allene system related to neocarzinostatin and esperamicin-calichemicin, Tetrahedron Lett. 1989, 30,4995-4998. 

By contrast, anionic polymerisation of hexafluoro-2-butyne (see Section IIB) proceeds rapidly because elimination of fluoride ion from the propagating anion is difficult, in that it would require the formation of allenes. 

At 750 K, k 2x 10 cm molecule s for these reactions, compared with ksA 2 X for the analogous reaction of alkyl radicals, but is a factor of about 10" greater than for allene formation from CH2CHCH2. 

The formation of propyne and allene by pyrolysis of cyclopropene arises from opposite [1,2]H shifts in diradicals 191 or 192 The substantially larger activation energy (by some 24.5 kJ mol" ) for formation of allene reflects differences in the transition state structures for the two processes. Thus, the propyne-forming reaction requires the migrating hydrogen atom to span a single bond (see 194), whilst in the allene-forming process a double bond is involved and a more strained situation ensues (see 195). The formation of but-2-yne from 3-methylcyclopropene is similarly rationalized but the... 

PtMeL2] proceed in a Markownikov manner by electrophilic attack of Pf thus [Pt(A -2-methallyl)L2] is formed from allene and [PtMe-(acetone)L2], whereas the analogous 1,3-butadiene cation does not lead to a 7r-allylic derivative by Pt—Me insertion. The hydro cation, however, can react by either a Markownikov or an anti-Markownikov mechanism with either Pt+ or attack on the unsaturated ligand. This apparent versatility leads to the formation of Tr-allylic complexes from both allenes and 1,3-dienes with [PtHLg]. ... 

In subsequent work, Bayes (9) found that formation of allene was suppressed by NO and O2, well-known triplet scavengers, at wavelengths above 260 nm. No suppression was observed below 260 nm. From this observation he concluded that C2O in a low-lying singlet state was the photodecomposition product below 260 nm, but that triplet C2O was formed above this wavelength. This behavior with 2 and NO is similar to that found with singlet and triplet methylene. The relative Importance of the two channels of reaction 1 which lead to singlet or triplet C2O is in some doubt near 260 nm, however, since Cundall et al. (13) found that butene-2 suppressed the major part of processes which yield CO at 253.7 nm while Bayes found that 2 and NO had little effect at 254 nm on allene formation. 

Moore, W. R., Ward, H. R. Formation of allenes from gem-dihalocyclopropanes by reaction with alkyllithium reagents. J. Org. Chem. 1962, 27,4179 181. 

Moore, W. R., Hill, J. B. Competitive bicyclobutane and allene formation from phenyl-substituted gem-dibromocyclopropanes. Tetrahedron Lett. 1970, 4553 556. 

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