Allenes diazomethane

Computational studies have compared substituent effects on the stability of ketenes, allenes, diazomethanes, diazirines, and cyclopropenes. Ketenes belong to the first generation of reactive intermediates along with carbocations, carbanions, radicals, and carbenes, and are intensively studied members of the cumulene family, with many useful synthetic applications. Ketenes were first recognized in 1905, when diphenylketene, a stable and isolable example, was obtained from the dehalogenation of the a-bromodiphenylacetyl bromide (Scheme 7.37). The most characteristic reaction of ketene is cycloaddition, as in the formation of p-laclams. 

Diazomethane in the presence of palladium acetate gave with allenes 613 a similar mixture of methylenecyclopropanes 614 and spiropentanes 615. In contrast to Simmons-Smith reagent, diazomethane prefers to add to the less substituted allenic double bond (Scheme 87) [162]. 

An asymmetric synthesis of the spiropentanes 630, albeit with low enantiomeric excess, was achieved by the reaction of allenes 629 with diazomethane in the presence of an optically pure copper (II) chelate complex (R) or (S)-631 (Scheme 94) [170],... 

The addition of allene to an ethereal solution of diazomethane gave 4-methylene-l-pyrazoline [81]. 

Abstraction Reactions. Many of the reactions of CH2 show indications of partial free-radical character. Evidence for the presence of CH3 radicals in the reactions of CH2 with H2 and CH4 has been discussed in Sects. IV-A and IV-B. Also, ethane and other by-products are reported in the reaction of CH2 with ethylene,46,120 ketene,20 diazomethane,42 allene,43 butene-2,120 and isobutene.39 The results of Doering and Prinzbach26 (see Sec. IV-E-3) provide very convincing evidence of partial free-radical participation in the reaction of CH2 with the C—H bonds of isobutene, especially in the gas phase. 

Ketenes easily add water,alcohols, amines and thiols to give derivatives of phosphorylated carboxylic acids ( -7). Phosphorylated ketenes undergo reactions of cyclo"aadition to vinyl ethers,diazomethane,styrene, cyclopentadiene, etc. From phosphorylated ketenes other phosphorylated cumulenes can be obtained,for example allenes,isocyanates,ketenimines... 

Palladium-catalyzed methylene transfer from diazomethane has proved effective for the cyclopropanation of 1-alkenylboronic acid esters allylic alcohols and amines 1-oxy-l,3-butadienes and allenes " Readily accessible iron complex (CO)2FeCH2S Me2 BF4 35 undergoes direct reaction with a range of alkenes to give cyclopropanes (equation 67) The salt is sensitive to steric effects and the reaction proceeds... 

Additions of diazoalkanes to the cumulated double-bond systems of ketenes and allenes yield cyclopropanones (Houben-Weyl, Vol. 4/3, pp 65 and 66) and alkylidenecyclopropanes respectively. For example, methylenecyclopropane (53) was made by the addition of diazomethane to allene. The intermediate 4-methylene-4,5-dihydro-3//-pyrazole is exeeptionally sensitive to tautomerism and poor yields were obtained under all but strictly neutral conditions. 

A further addition of diazomethane to the strained exocyclic double bond of methylenecyclopropane gave a mixture of 4,5-dihydro-37/-pyrazoles. Separation of the 4-spiro-sub-stituted isomer by preparative GC and pyrolysis gave spiropentane 56.Note the formation of the methylenecyclobutane. In this case, the alkene-forming side reaction is in part driven by the relief of ring strain. Alternatively, spiropentane derivatives were obtained by treating an allene with an excess of diazoalkane and deazetization of the bisadducts 57 formed, e.g. formation of 1,1,4,4-tetramethylspiropentane (58). ... 

Huisgen has reexamined the reaction of diazomethane and tetracyanoethylene <87CB153> there was an old report by Castells (quoted in <87CB153 indicating that this reaction affords a bicyclic compound (243), but in Huisgen s hands only the A -pyrazoline (244) was isolated. From 2,5-dihydrothiophene (245) the bicyclic A -pyrazohne (246) was prepared <92T8loi> this pyrazoline was the precursor of pyrazoles discussed in (see Section 3.01.6.2.2). The reaction of diazomethane with allene afforded a mixture of 4-methylene-A -pyrazoline and 3-methylene-A -pyrazoline <9475561 >. 

The regiochemistry of the cycloadditions of allenes is not easily explained by these frontier orbitals. Penta-2,3-diene and acrylonitrile give the adducts Z- and -6.404 in which the central carbon of the allene, with the smaller HOMO coefficient, has bonded to the / carbon of the Z-substituted alkene.894 Equally unexplained is the regiochemistry of the reaction between allene itself and diazomethane, which gives more of the adduct 6.405 than of its regioisomer 6.406.893... 

Energetically this reaction can be expected to be dipole-HO-controlled ( Lumo - Fhomo = 15.5 eV), but this should have little regiocontrol since the LUMO of the allene is so little polarised. The LUMO(dipole)/ HOMO ene) interaction ( lumo - homo = 16.75 eV) might come into play, since the allene HOMO is polarised, but the orbitals do not match up to explain the high level of regioselectivity, since the larger coefficient in the LUMO of diazomethane is on carbon. Finally, the hydroboration of allenes gives mainly the product with the boron on the central carbon 6.407.895 With boron as the electrophilic atom, frontier... 

In the context of the ethynyl sulfones 6.30 Padwa s cycloadditions of phenylsulfonyl-substituted propa-1,2-dienes like 6.31 with diazomethane should be mentioned briefly (Padwa et al., 1993a, with many references on allenes used as dienophiles). The example in (6-22) demonstrates the regiospecificity of the cycloaddition to the more reactive electron-deficient 71 bond. The ready reactivity of cumulated double bonds is based on the relief of strain when the 1,2-propadiene... 

Electron density distributions on the CNDO/S level in diazocompounds are only available for diazomethane (42) (54). Electron densities of substituted diazocompounds have been evaluated on the INDO level (95). As there exists no correlations of INDO and CNDO/S electron densities (tested for the phenylallenes 16, 38-40 in Table 1 and diazomethane (42)), the substituent effects on the (INDO) electron density distributions in diazo-compounds (95) cannot be compared with the substituent effects on the CNDO/S electron densities in ketenes and allenes. 

Substituent effects on one-bond carbon-proton coupling constants in diazocompounds seem to be similar to those in allenes. For instance, y( CH) in diazo acetic acid ester (254) is increased relative to the value in diazomethane (42) by about 8 Hz (6c = 46.3 ppm (896), y( CH) = 203.3 Hz for 254 (946)). 

Whereas in allenes the AOs of the methylene hydrogen atoms may become part of the molecule s ir system, in planar ethylene, ketene (43), diazomethane (42), and butatriene (223) the methylene proton 1j AOs are always part of the molecular skeleton on the CH2 two-bond proton-proton couplings in these last cumulenes should be determined by electron populations of the H2CX units which are determined by the antisymmetric or symmetric group orbitals of the hydrogen 1j AOs ( sh I h ) (162). 

Two-bond proton-nitrogen-15 coupling constants in diazocompounds may be assumed to follow qualitatively trends observed for VC Ca-Ha) in allenes. Such an example is found for C NH) in diazomethane (42) (158) and ethyl diazoacetate (254) (94b). In 254 V( NH) is more positive than in 42, comparable to the situation in allene and allenecarboxylic acid ester. 

Allenes are also successfully cyclopropanated if a large excess of diazomethane is employed, which preferentially takes place at the less substituted double bond. Nevertheless, the bisadducts are formed in many cases as well (Table 7). ... 

The reactive species, trimethylenemethane, has been observed only by photolysis of an adduct of allene and diazomethane in a matrix of hexafluorobenzene at — 185°C. The e.s.r. spectrum shows that it has two unpaired electrons (a triplet ground state) which is consistent with a planar structure with 3-fold symmetry (Djh). This species can, like cyclobutadiene, be stabilized by coordination to tricarbonyliron. The parent compound, a diamagnetic pale yellow solid, m.p. 28°C can be obtained by the two routes shown. 

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