1,3-Substituted propadienes

Stereospecific polymerization of substituted conjugated dienes. Stereoregular polytactic polymers have been obtained from a number of substituted dienes, including one optically active 1,3-substituted propadiene, and various 1- or 1,4-substituted butadienes. (R)-penta-2,3-diene has been polymerized by means of 7T-allyl-Ni-iodide to an optically active polymer, to which an interesting stereoregular structure has been attributed (Scheme 26) (224). Some of the stereoregular polymers... 

Figure 11.2 Schematic presentation of FMOs of selected 1-substituted propadienes [15].

It has not been possible to correlate FMO energies and orbital coefficients with the observed reactivities and selectivities, in particular for thiyl radical additions to a-substituted propadienes (Figure 11.2) [15]. Therefore, it must be concluded that the addition is guided by thermodynamic factors, and hence the relative stabilities of adduct radicals. 

Full details of reactions between (CFs)2N-substituted propadienes and trifluoro-... 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

The stereoselective elimination reaction of suitably substituted allylic compounds is a reasonable approach to the construction of the propadiene framework. Central chirality at the allylic position is transferred to axial chirality of the allene by stereoselective /3-elimination (Scheme 4.53). 

In contrast to the limited success with vinyl sulfides as components of [2 + 2] cycloadditions, allenyl sulfides show wide applicability. As illustrated in Scheme 8.91, Lewis acid-catalyzed [2 + 2] cycloadditions of l-trimethylsilyl-l-methylthio-1,2-propadiene (333) with a variety of electron-deficient olefins 336 provide cycloadducts 337 with excellent regioselectivity but with moderate stereoselectivity [175c], Nara-saka and co-workers reported the first Lewis acid-catalyzed asymmetric [2 + 2] cycloaddition of C-l-substituted allenyl sulfides 319 with a,/3-unsaturated compounds 338 using a chiral TADDOL-titanium catalyst. The corresponding cycloadducts 339 were obtained with 88-98% ee, but a low level of trans/cis selectivity (Scheme 8.92) [169,175d[. 

IN3 can also react with propadiene to afford 3-iodovinylic azide 27 [20], which further reacted with IN3 to afford the 2 1 adduct 28. In this reaction the regioselectiv-ity is unique. In contrast, the reaction of a substituted allene, i.e. 2,3-pentadiene, afforded the normal product 29. 

If photolyzed with light of the intensity I, HBr adds to propadiene (la) in the gas phase with a rate given by v=kexp[HBr]I<). This transformation affords within the detection limit (GC) 2-bromo-l-propene (5a) as sole reaction product (Table 11.1). The conversion of methyl-substituted allenes, such as lc and If, under these conditions follows the same kinetic expression [37]. Results from competition experiments indicate that the reactivity of an allene towards HBr increases progressively with the number of methyl substituents from propadiene (la) (= 1.00) to 2,4-dimethylpenta-2,3-diene (If) (1.65). In all instances, Br addition occurs exclusively at Cp to furnish substituted allyl radicals, which were trapped in the rate determining step by HBr. 

Aryl- and alkylsulfonyl radicals have been generated from the corresponding iodides and added to, e.g., propadiene (la), enantiomerically enriched (P)-(+)-propa-2,3-diene [(P)-(lc)] and (P)-(-)-cyclonona-l,2-diene [(P)-(lk)] [47]. Diaddition of sulfo-nyl radicals may compete considerably with the monoaddition [48,49]. Also, products of diiodination have been purified from likewise obtained reaction mixtures, which points to a more complex reactivity pattern of these substrates towards cumulated Jt-bonds. An analysis of regioselectivities of arylsulfonyl radical addition to allenes is in agreement with the familiar trend that a-addition occurs in propadiene (la), whereas alkyl-substitution at the cumulated Jt-bond is associated with a marked increase in formation of /3-addition products (Scheme 11.7). 

The a-selectivity for carbon radical addition to propadiene (la) is retained on substituting chlorine or fluorine for hydrogen in radicals of the type CX3 (X=F, Cl), no matter whether the reaction is conducted in the liquid or in the gas phase (Table 11.4) [14, 49-51]. /3-Selective addition to allenes becomes progressively more important for the CC13 radical with an increase in number of methyl substituents [14, 47]. For example, treatment of optically active (P)-(+)-2,4-dimethylpenta-2,3-diene [(P)-(lc)] with BrCCl3 affords a 59 41 mixture of a- and /3-monoadducts [47]. The a-addition product consists of a 20 80 mixture of E- and Z-stereoisomers, whereas the product of /3-addition exclusively exhibits the Z-configuration. The fraction of 2,4-dimethylpenta-2,3-diene (P)-(lc) that was recovered from this reaction mixture had completely retained its optical activity. These results indicate that the a-and the /3-CCl3 addition proceed under kinetic control. If one of the addition steps were reversible, at least partial racemization would inevitably have taken place. 

Addition of the dicyanomethyl radical to propadiene (la) occurs exclusively at Q (not shown in Scheme 11.8) [60]. On the other hand, methyl-substituted allenes, e.g. Id, undergo /3-selective reactions with 2-bromomalodinitrile (15). The significant /3-selectivity has been associated with the steric demand of the incoming radical 16, which favors addition to the sterically least hindered site at the diene Id to provide allylic radical 17. However, it seems likely that a stabilization of an intermediate allylic radical, e.g. 17, by methyl substituents contributes significantly to the observed regioselectivity of product formation. Trapping of intermediate 17 with bromine atom donor 15 proceeds at the least substituted carbon to afford allylic bromide 18. 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

Allenes with substitution pattern A2XY, though more complicated than the systems reviewed above, still avoid the multiplicity of options available to optically active propadienes. 

The transformation into propadiene derivatives also oceurred when tris(/er/-butylsulfanyl)cy-clopropenylium ion 15 was treated with various arylmagnesium bromides. In some cases the substituted indenes 17 were formed as the major product. ... 

The lack of a plane of symmetry as a criterion for potential optical activity allows us to identify other molecules that are chiral. For example, if one hydrogen on each terminal carbon of propadiene is substituted for then the resulting molecule is chiral. 1,3- Difluoropropadiene, for example, has a twofold axis of symmetry but no plane of symmetry. The enantiomers are shown in Figure 6.15. Construct models of these and convince yourself that they are non-superimposable. 

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... 

Insertion of 1,2-propadiene, allene, into the transition metal-carbon bond gives methylene-substituted polymers (Scheme 1.31) [90],... 

Dibromobutyne was used in the preparation of cyclic trithiocarbonates with a propadiene side-chain. From potassium trithiocarbonate, a double substitution occurs in the presence of 18-c-6. 59 Sonication speeds up the reaction by a factor of 10. Without sonication, only polymers are formed. 

Yamamoto has also demonstrated the intermolecular hydroamination of mono-substituted arylallenes with morpholine [40]. For example, treatment of l-p-tolyl-1,2-propadiene with morpholine and a catalytic 1 1 mixture of [PPh2(o-tolyl)]AuCl and AgOTf in toluene at 80 °C for 24h led to isolation of the allylic amine 56 in 83% yield as a single regio- and stereo-isomer (Eq. (11.31)). 1-Alkylallenes, 1,1-, and 1,3-disubstituted allenes also underwent gold(I)hydroamination with morpholine, albeit with diminished efficiency and/or regioselectivity. 

If the alkene is a cumulative diene, that is, allene (1,2-propadiene [CH2=C=CH2]) or a substituted allene, addition can occur once and/or twice (Equation 9.24),... 

The decay behavior of allenes (1,2-propadienes) is quite different from that of the conjugated 1,3-dienes. Figure 8 shows the decay of ArS in cyclohexane for the reaction with methyl-substituted allenes [46]. By adding allene, the decay of ArS is accelerated even in the degassed solution, suggesting that the reaction proceeds irreversibly. Such irreversibility occurs when the incipient C atom-centered radical becomes a resonance stable allyl-type radical by rotation of the C-C bond, as shown in Scheme 9. In the aerated solution, the decay of ArS is further accelerated, indicating that the irreversibility due to the rotation is not completely established the addition of O2 further shifts the equilibrium to the peroxy radical side by trapping the incipient short-lived C atom-centered radicals. 

With the exception of propadiene, the addition of sulfonyl halides and seleno-sulfonates to allenes can be totally regioselective (equation (57)) [116], The attack of sulfony] radical on the central carbon atom, which leads to a stabilized ally] radical, is probably less reversible, if at all, than the addition to the terminal carbon. The ensuing atom or group transfer occurs at the less substituted end of the allyl radical. Therefore, the reaction results in 1,2-addition to the less substituted double bond. Subsequent oxidation of the adducts when X = SePh gives rise to allylic alcohols since the [2, 3]-sigmatropic rearrangement of selenoxide is much faster than the elimination of PhSeOH. 

---