1,2-alkylidene shift

However, benzylidene derivatives show a strong bathochromic shift in comparison with alkylidene derivatives. Thus absorption is a result of the whole conjugated system that is comparable to that of the quinoid dyes. The color of this type of compound is sensitive to acids and bases. 

Finally, although strictly outside the scope of this chapter (and book), we mention a study of the transmission of electronic effects in iV-alkylidene-sulfenamides, -sulfinamides and -sulfonamides, XC6H4SO N=CHC6H4OH-2, through measuring the PMR chemical shifts of the o-OH148. The p values lie in the order S < SO > S02 (cf. above) which is attributed to a complex interplay of factors. 

A pair of reactions of 1,4-dihydropyridines with electron-accepting alkenes (Scheme 31) shows experimental evidence for the mechanistic spectrum between the pseudoexcitation and transfer bands. Acrylonitrile undergoes an ene reaction [143] (Scheme 31a). This is a reaction in the pseudoexcitation band. A stronger acceptor, alkylidene- and arylmethylydenemalonitriles are reduced [144] (Scheme 31b). This is a reaction in the transfer band, where a hydride equivalent shifts without bond formation between the ti bonds of the donors and acceptors. 

The initial site of protonation in these reactions has not been unambiguously determined. Alkyl ligand formation by protonation at the metal followed by a rapid 1,2-proton shift to the alkylidene ligand is equally as plausible as direct protonation at Ctt. As the metal electron density... 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

Vinylcyclobutanes substituted by an alkylidene group in the 3-position undergo [1,3] and [3,3] shifts (see Table 2).25 31 138 139 Sometimes the products of both rearrangements are identical, but if the substitution pattern allows a distinction, usually mixtures are found. If the substituents R1 and R2 differ strongly, just one product may be formed selectively. 

A reversible vinylidene insertion was proposed to explain die formation of (55) on flash vacuum pyrolysis of the anthracene derivative (56) at 1100 °C.65 The expected loss of HC1 followed by 1,2-H shift and 1,5-CH insertion of the resulting vinylidene species would give rise to the strained paracyclophane (57). This is proposed to ring open to the alternative alkylidene (58) before proceeding to the observed product (55). 

A photochemical 1 1 addition of bis(alkylidene)silacyclobutanes to Cgo was reported (equation l)24. The structure of adducts 9a and 9b were identified by various means including 29Si NMR spectra (9a S = -14.30, 4.20 9b -11.47, 8.12). A Si-H HMBC heteronuclear shift correlation NMR spectra of 9a was also taken. A photochemical addition of octaarylcyclotetrasilane to Cgo was also reported, together with full 29Si NMR... 

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

An increase in the number of methyl substituents on the alkylidene carbon atom of the hydrazones 63 (R4 = R5 = H < R4 = H R5 = Me < R4 = R5 = Me) displaces the equilibrium in favor of the open-chain tautomer. The steric demands of the open-chain 63A and the cyclic 63B tautomers clearly differ from those in the systems 59 and 61, where an opposite effect was detected. An increase in the temperature of the solutions shifts the equilibrium toward the open-chain tautomer. In most cases, an equilibrium shift in the same direction was observed on increase of the solvent polarity (83ZOR2310). /V-(3-Aminoprophyl)-A-methylhydrazones 65 (R = H, Me) exist as stable open-chain isomers and do not exhibit any tendency to seven-membered-ring closure (83ZOR2310). 

A shift of the equilibrium 92A 92B in favor of the cyclic tautomer was observed by means of IR and -NMR spectroscopy (88AKZ385) in nonpolar solvents (CC14, CDC13) for alkylidene hydrazides of cyanoacetic... 

The even more strained alkylidene cycloproparenes gave rise to the same kind of G-complex intermediate with silver ion. In the presence of alcohol, trapping of this intermediate occurred, leading to alkoxystyrene derivatives. Water could also act in the same way, yielding arylmethylketones after keto-enol equilibration. However, if a proton was present on the alkylidene moiety, H shift occurred, leading to an arylalkyne. No dimerization was observed in this case, probably due to steric constraints in such a process (Scheme 3.17).31... 

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