A 3-alkylidene

Simple addition to carbonyl compounds occnrs nnder mild acidic conditions. Examples given illns-trate reaction with acetone, an aldol-like reaction, and conjngate addition to methyl vinyl ketone, a Michael-like reaction. The first-formed alcohol products in aldol-like reactions usually dehydrate to give a 3-alkylidene-3//-indolium cation. 

A 1+2 addition of dimesitylsilylene to 1-silaallenes is believed to occur during the simultaneous photolysis of the precursors for both of these transients149,150 The isolated product is a 3-alkylidene-l,2-disilacyclopropane. 

The easy displacement of the amine (ammonium) group proceeds by way of an elimination, involving loss of the indole hydrogen, and thus the intermediacy of a (3-alkylidene-indolenine which then readily adds the nucleophile, regenerating the indole. This mechanism has been verified by observing (i) very much slower displacement with a corresponding 1-methylgramine, and (ii) racemisation on... 

The betaines 3 have been claimed to give the trithiepine 4 on standing,330b but an X-ray structural investigation showed the product to be a 3,5-bis(alkylidene)-l,2,4-trithiolanc.330a... 

More recently, a new metathesis catalyst involving a ruthenium-alkylidene complex with a sterically bulky and electron-rich phosphine ligand has been synthesized and applied to RCM in aqueous media (Figure 3.5).197 This catalyst has the benefit of being soluble in almost... 

A new domino lithium acetylide addition/rearrangement procedure on trans-1,2-dibenzoyl-3,5-cyclohexadiene furnished 3-alkylidene-2,3-dihydrofurans via an intriguing mechanism involving three bond formations and two bond cleavages in one single operation <06SL1230>. The reaction of dimedone with meso-diacetoxycyclohexene in the presence of a palladium catalyst led to the formation of the tricyclic product as depicted below <06S865>. 

Alkylidene-methylene coupling affording a coordinated olefin... 

The isomerization of a-alkylidene cyclic carbonyl compounds to a,/3-unsaturated cyclic carbonyl compounds was achieved by using catalytic amounts of RhCl(PPh3)3 and Et3SiH (Scheme 48).87... 

The ratio of cross-/self-metathesis products, with respect to the alkyl-substituted alkene, was generally poorer (typically 3 1) than the analogous reactions with styrene or acrylonitrile, probably due to the absence of a good alkylidene stabilising substituent on either alkene and the closer nucleophilicities of the two substrates. 

In 2006, Chatani and coworkers published a rhodium vinylidene-mediated alkyne cyclodimerization incorporating allylamine (Table 9.13) [26]. Although RhCl(PPh3)3 alone was a competent catalyst for the stereospecific formation of ( )-3-alkylidene-3,4-dihydro-2H-pyrroles (69), the addition of an ammonium salt helped to suppress undesired alkyne homocoupling. Under optimized conditions, a variety of functionalized alkynes underwent cyclodimerization with allyl- or crotylamine in good yield. 

Another radical 1,4-addition was reported by Giese and Roth. In the photoaddition of a 5-alkylidene-l,3-dioxan-4-one with pentyl iodide, mediated by Bu3SnH and di-/r /t-butyl peroxide (DTBP), a 95 5 mixture of diastereoisomeric dioxanones was obtained in 63% yield (Equation 31) <1996JBS243>. 

With 2-diazopropane, two stereoisomers of the resulting tricyclic product 116 (cis, anti, cis and cis, syn, cis) were obtained (183). Formation of 116 can be rationalized by N2 extrusion from the cycloaddition product 114 and a subsequent [34 + 22 ] cycloaddition of the resulting 3-alkylidene-l,2,3-diazaphosphole 115 with the remaining heterophosphole 110. When an excess of 2-diazopropane was used, 115 was trapped by 1,3-dipolar cycloaddition across the exocyclic P=C bond. 

A highly efficient synthesis of l-alkylidene-13-dihydroisobenzofurans and 3-alkylidene isobenzofuran-l(3//)-ones (phthalides) through Pd-Cu-catalysis using acetylenic carbinols as synthons was reported <99SL456>. 

The first preparative use of intramolecular C—H insertion in organic synthesis was based on the observation that on flash vacuum pyrolysis, a conjugated alkynyl ketone such as 1-(1-methyl-cyclopentyl)-2-propynonc is smoothly converted to a mixture of the cyclizcd enones 1 and 223. This elegant reaction apparently proceeds via isomerization of the alkyne to the corresponding alkylidene carbcne, followed by subsequent intramolecular C-H insertion. It should be noted that despite a 3 2 statistical predominance of primary C-H bonds over secondary C—H bonds, a marked preference for insertion into the latter (methylene) is observed. 

---