Allylidene

Salt-free ylides have been prepared from phosphonium chlorides and bromides by treatment with sodamide in refluxing THF. The sodium halide precipitates and is removed by filtration. Allylidene- and benzylidene-trimethylphosphoranes have been obtained as low melting distillable solids from the phosphonium chlorides and butyl-lithium in ether. The allylidenephosphorane on standing at room temperature slowly decomposed to give methylenetrimethylphosphorane. 

Zanarotti, A. Preparation and reactivity of 2,6-dimethoxy-4-allylidene-2,5-cyclohexa-dien-l-one (vinyl quinone methide). A novel synthesis of sinapyl alcohol. Tetrahedron Lett, 1982, 23, 3815-3818. 

Two examples of [4 + 2] cycloadditions of allylidene cyclopropanes with hetero-enes have been previously reported in Krief s review [31]. Only a few other examples have been studied more recently. 

In all the latter cases the easier dimerization reaction is connected with the particular stability of the intermediate diradical species. This is also the reason for the recently found facile dimerization of the 1-donor substituted allylidene-cyclopropane 136a (Scheme 66) [127]. Allylidenecyclopropane 136a cyclodimer-izes to the expected cyclobutane 467 in very mild thermal conditions, due to the stabilization of the intermediate 466. At higher temperature (120 °C) both 136a and 467 give a more complex mixture of products, with the cyclooctadiene dimer 468 being the prevailing one (Scheme 66) [127],... 

However, the cyclodimer 471 is produced as a side-product during the preparation of the parent allylidenecyclopropane 470 from the corresponding 1-bromo-allylidene derivative 469 by a metalation-protonation procedure (Scheme 67). 

Other sporadic examples of [2 + 2] cycloadditions of olefins on the exo double bond of structurally more complex MCPs, such as methylenecyclo-propenes, allylidene-, and alkenylidenecyclopropanes, have been reported. Thus, dicyclopropylideneethane (2) reacted with TCNE (131) to give the [2 + 2] adduct 164 as a minor product, together with the prevalent [4 + 2] adduct 163 (Scheme 76) [39], The same reaction in a different solvent had been previously reported to furnish exclusively the Diels-Alder product (see Sect. 2.1.2) [5]. 

PTAD reacts with MCP derivatives in a variety of modes of addition to C=C double bonds, depending on the structural features of the reacting MCPs. For example, it reacts with allylidene- and benzylidenecyelopropanes giving [4 + 2] cycloadditions (see Sect. 2.1.2), with alkylidenecyclopropanes giving ene reactions [44], and with BCP giving products probably deriving from an initial [2 + 1] cycloaddition (see Sect. 5.2) [13b]. 

Die Addition von elementarem Schwefel an (3-Phenyl-allyliden)-malonsaure-ethylester-nitril ergibt 2-Amino-3-ethaxycarbonyl-5-thiobenzoyl-thiophen in 40%iger Ausbeute an-stelle dieser Cycloaddition kann man auch direkt Zimtaldehyd mit Malonsaure-ethylester-nitril und Schwefel umsetzen und erhalt so das Thiophen-Derivat in 64% Ausbeute2. 

Given a typical diene, an increased reactivity is expected of the reaction with en-hanciftg polarity alternation of the dienophile. This is easily appreciated in view of an ionic reaction being more exothermic. Thus, acryloxyboranes [172] undergo cycloaddition with cyclopentadiene even at —78 °C. The same principle underlies dienophile activation by replacing an enone with an alkoxyallyl cation [173] or allylidene-metalcarbonyl [174],... 

Table 4. Bieyclic Cyelobutanones and Vinyleyelobutanones from Alkylidene- and Allylidene-cyclopropancs51...

This strategy has already been found useful in natural product synthesis. In the course of a synthesis of V-ATPase inhibitor oximidine HI, John Porco of Boston University has described (Angew. Chem. Ini. Ed. 2004,43, 3601) the cyclization of 7 to 8. In the absence of the pententyl director, the initial complexation of the Ru catalyst was with the 1,3-diene, leading to allylidene complex and so effectively killing the catalyst. In this case, the Hoveyda catalyst 8 provided a cleaner product than G2 did. 

Catalytic hydrogenolysis of 1,1-difluorocyclopropanes occurs at the C2-C3 bond with partial retention of fluorine in the cases studied [49] (equation 38) In contrast, reduction of acetoxy 1,1 difluorocytlopropanes with lithium aluminum hydride occurs with loss of fluoride to give (3-fluoroallyl alcohols, often with high stereose lectivity [50] (equation 39) A further variant involving free radical intermediates produces allylidene difluorides after ring opening [5/] (equation 40)... 

In a similar manner, Lu and Liu have more recently utilized the hetero-Michael addition of lithium propargylic alkoxides to alkylidene malonates in a synthesis of stereodefined allylidene tetrahydrofurans, based on the use of allylic chloride as coupling partner [98]. In this case, the cydization reaction is initiated by a catalytic amount of palladium salt [Pd(OAc)2] rather than by an organopalladium species as mentioned above. 

Reduction of [Mo(CO)(Bu C=CH)2Cp] + BF4 with KBHBu3(s) at — 78°C in an atmosphere of carbon monoxide yields a complex of a vinyl substituted y-lactone linked tj3 t]2 (220). The allylidene ruthenium complex 64, obtained by photochemical addition of one alkyne molecule to a /x-carbene derivative, is transformed into pentadienylidene complexes 65 and 66 on photolysis with more alkyne substrate. These reactions show clearly the stepwise growth of chains in alkyne oligomerizations at dimetal centers [Eq. (31)] (221). Similar reactions are also known for dinuclear iron (222), molybdenum (223), and tungsten (224) complexes. 

Allylidenation of aldehydes.6 This palladium complex in combination with a trialkylphosphine promotes a reaction between phenyl isocyanate and allylic alcohols to provide 1,3-dienes. 

The synthetic applications of halocarbenoids are mainly determined by the framework bearing the carbenoid center. This article describes the different kinds of synthetic transformations that can be achieved by the use of alkylidene, a-heterosubstituted, cyclopropylidene, vinylidene, and allylidene lithium halocarbenoids. Their particuliar value in organic synthesis results from various rearrangement reactions of the primary adducts formed by reaction of the carbenoid with the electrophile. 

---