Allenes, halogenation

Allenes react with other typical electrophiles such as the halogens and mercuric ion. In systems where bridged-ion intermediates would be expected, nucleophilic capture generally occurs at the allylic position. This pattern is revealed, for example, in the products of solvent capture in halogen additions and by the structures of mercuration products. ... 

The conclusion of Brown et a/.346,347 viz. that the increased reactivity along a series of alkyl halides is due to the increased polarisation of the carbon-halogen bond has been challenged by Allen and Yats348, who found constant meta para isomer ratios for methylation, ethylation, and isopropylation of toluene, and since this ratio reflects the selectivity and hence reactivity of the electrophile they con-... 

Halogen-allene bilden mit Lithiumalanat je nach Reaktionstemperatur in verschie-denem Verhaltnis Acetylene und deren Reduktionsprodukte5. So erhalt man z. B. aus Brom-allen in Bis-[2-athoxy-athyl]-ather mit 1,2 Mol-Aquivalenten Hydrid bei 100° 9% d.Th. Propin und 37% Propan, bei 25° dagegen 60% d.Th. Propin6 ... 

Halogenation of alkenyl organometallic compounds Addition of hydrogen halides to triple bonds Halogenation of alkynes or allenes Addition of alkyl halides to triple bonds Addition of acyl halides to triple bonds... 

The addition of HC1, HBr, and HI to allene has been studied in some detail.145 In each case a 2-halopropene is formed, corresponding to protonation at a terminal carbon. The initial product can undergo a second addition, giving rise to 2,2-dihalopropanes. The regiochemistry reflects the donor effect of the halogen. Dimers are also formed, but we have not considered them. 

Fig. 15 The experimental geometries of allene- -HC1 and allene- -C1F, drawn to scale. The n-electron model for allene is also shown. The angles 2- - H and 2- - Cl, respectively, where is the centre of the C C bond, are both close to 90°, as required by rule 2. The hydrogen and halogen bonds both show small non-linearities. See Fig. 1 for key to the colour coding of atoms... 

The Lewis dot formalism shows any halogen in a molecule surrounded by three electron lone pairs. An unfortunate consequence of this perspective is that it is natural to assume that these electrons are equivalent and symmetrically distributed (i.e., that the iodine is sp3 hybridized). Even simple quantum mechanical calculations, however, show that this is not the case [148]. Consider the diiodine molecule in the gas phase (Fig. 3). There is a region directly opposite the I-I sigma bond where the nucleus is poorly shielded by the atoms electron cloud. Allen described this as polar flattening , where the effective atomic radius is shorter at this point than it is perpendicular to the I-I bond [149]. Politzer and coworkers simply call it a sigma hole [150,151]. This area of positive electrostatic potential also coincides with the LUMO of the molecule (Fig. 4). 

Transition metals 172 a-bonded to cyclopropanes, substituted on the a-carbon with a halogen atom, are interesting intermediates for cyclopropylidene complexes 173 or allene ones 174 [88]. The former complexes are also supposed to be precursors of the above-mentioned nickel enolates. (Scheme 65)... 

Nitronate(47a) is not the only oxazete derivative. For example, sterically hindered nitroalkenes (42b-d) can be prepared by nitration and halogenation of readily available allenes (48). Compounds (42b-d) are rather smoothly isomerized into the corresponding four-membered cyclic nitronates (47b-d) by the first-order reaction equation (168). Storage of nitronate (47c) is accompanied by its slow transformation into acid chloride (47e) from which amide (47f) can be easily synthesized. 

F. H. Allen, J. P. M. Lommerse, V. J. Hoy J. A. K. Howard, G. R, Desiraju, The halogen—O(nitro) supramolecular synthon in crystal engineering a combined crystallographic database and ab initio molecular orbital study , Acta CrystaUogr, Section B, 1997, 53,1006-1016. 

Whereas deprotonation and halogen-lithium exchange represent the most common methods to access allenic and propargylic lithium intermediates, several less direct routes to more functionalized analogues have also been reported. Additions of various lithium acetylides to acylsilanes followed by Mel or EtI afforded alkylated allenyl silyl ethers (Table 9.4) [10]. The adducts were analyzed after hydrolysis to the related enones. 

Sulfenyl chlorides and halogens react with 1,2-alkadienylphosphonic acids to afford phosphorus-containing heterocydes [72], However, the electrophilic addition of dialkyl 4-methyl-2,3,5-hexatrien-2-yl phosphonates with sulfenyl or selenyl chloride afforded 2-thienyl methylphosphonates or the seleno analogues [73, 74]. The conjugate addition of sulfenyl or selenyl chloride with the 2,4-diene moiety in the starting allene leads to the formation of the five-membered skeleton (Scheme 10.69). 

Halogen-substituted allenes undergo [2 + 2]-homodimerization under relatively mild conditions to afford l,2-bis(halomethylene)cyclobutene derivatives in good yields [9]. 

In principle, three basically different types of reaction modes are applied for cross-coupling reactions of allenes. First, cross-couplings of allenes with suitable halogen or metal substituents at one of the sp2-hybridized carbons furnish products still bearing the intact cumulene it-system. On this basis, numerous reactions for conversions of precursor 1 or 3 into substituted allenes 2 have been developed (Schemes 14.1 and 14.2). 

Limitations of the Pd(II)-catalyzed cydoisomerization/dimerization are strongly coordinating groups such as pyridyl groups, amines, thioethers or halogens on the allene [61]. With 96, AuC13 delivered products 102, which are constitutional isomers of the dimers of the Pd(II)-catalyzed reactions (Scheme 15.27) [62]. Compounds 102 are always accompanied by the monosubstituted furans 99. Furthermore, these catalysts were highly reactive and allowed a much faster reaction than Ag(I) or Pd(II) under the same conditions. 

Several acetylenic compounds undergo successful coupling with halogen compounds in which halogen is linked to sp2-carbon (vinylic, allenic, aromatic and heteroaromadc halogen compounds). In all cases a stoichiometrical amount of a copper(I) salt appears to be necessary, which makes the syntheses less attractive for performance on a larger scale [196]. 

Allenes have the 1,2-diene structure (I), where R1, R2, R3, R4 = H, alkyl, aryl, halogen, heterocyclic, ether, etc. Since the terminal methylene groups lie... 

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