Metal induced elimination

An alternative to the above eliminations, which in principle offers a great many advantages, is the metal induced elimination of the elements of ROBr from a 2-halocyclopropyl ether. Thus the sequence of addition of dibromocarbene to an enol ether, reduction to a monobromide and reaction with magnesium in tetrahydro-furan provides a route to cyclopropene itself45). 

Substitution of cyclopropane rings with the alkenyl group permits unique ring transformations based on metal coordination interaction with four -electrons. The transition-metal-induced ring-opening rearrangement also results in the formation of metallacycles. Further elaboration is attained by insertion and reductive elimination. 

In sharp contrast with the remaining transition metals, Group 6 allenylidenes [M (=C=C=CR R )(C0)5] (M = Cr, Mo, W) containing non-donnor substituents at Cy are in general thermally unstable [11-15]. For this reason, most of the reported examples are substituted derivatives bearing heteroatomic Ti-donor groups [9]. In this sense, the first stable Group 6 allenylidenes reported in the literature were the amino-allenylidene complexes 2, prepared by E. O. Fischer and coworkers by a Lewis-acid induced elimination of ethanol from the 3-dimethylamino-l-ethoxy-3-... 

The strain of cycloproparenes is the principal obstacle that must be overcome in their synthesis. Cycloproparenes decompose usually at moderate temperatures, and they undergo ring-opening in the presence of electrophilic or metallic reagents. In contrast, they support even strongly basic conditions quite well. Accordingly, most successful cycloproparene syntheses use some base-induced elimination in the last step. Alternatively, flash vacuum pyrolytic or photochemical extrusion of a neutral fragment, followed by biradical closure or flash vacuum pyrolysis may be used. In these latter approaches the reaction conditions are neutral, and reactive products may be trapped at low temperatures. 

Other C-nucleophiles such as Me , Ph , [C = CPh], likewise add to C5 of 23 forming diynyl metallates. Subsequent Si02-induced elimination of [NMe2] gives new dimethylamino(organyl)pentatetraenylidene complexes (Scheme 3.35) [56]. This substitution route affords a variety of pentatetraenylidene complexes so far not accessible by other routes. 

The alternative strategy for heterogenization has been pursued by Blechert and co-workers, for a polymer-supported olefin metathesis catalyst. A polymer-anchored carbene precursor was prepared by coupling an alkoxide to a cross-linked polystyrene Merrifield-type resin. Subsequently, the desired polymer-bound carbene complex was formed by thermolytically induced elimination of ferf-butanol while heating the precursor resin in the presence of the desired transition metal fragment (Scheme 8.30). 

Thermally induced elimination of fluorine occurs if polyfluorocycloalkanes, -cyclohexadienes or some heterocyclic perfluoro compounds arc passed over a nickel or iron gauze at 500-700cC (examples are summarized in Table 7). Within a certain time the catalytic activity of the metal decreases and it has to be regenerated by passing hydrogen over it at 300-600JC. The yields... 

Suitable candidates for a-elimination reactions are silylmethyl halides (— base-induced elimination of H-Hal), silylmethyl dihalides (— halide/metal exchange followed by elimination of a metal halide) and stable carbenoid-type compounds such as (a-halo-a-silylalkyl)mercury compounds (— thermal elimination of mercury(II) halide). Bis(phenylthio)(trimethylsilyl)methyl lithium (— elimination of LiSPh) represents a borderline case (see Section III.E.8). 

The increased development of transition metal-catalyzed cross-coupling methods to form C-C bonds has served as an impetus to find methods to synthesize 3-halochromones and 3-haloflavones. The synthesis of 3-halochromones and flavones can be achieved with the addition of halogens across the double bond of the pyrone ring by reaction with a halogenating reagent (e.g., Br2) followed by spontaneous, or base-induced, elimination (Scheme 48). Synthesis of these important compounds has been recently reviewed <2003RCR489>. 

Here the substrates are 3-silyloxy-l,5-enynes the concept was to combine a metal-induced 6-endo-trig cyclization with a ring contraction by a sigmatropic rearrangement (Scheme 12.12).27 From the cationic intermediate, elimination to the biaryl product can be observed in addition to the aldehyde. 

Metal-14 anions have been used due to their high basic reactivity. They are able to abstract acidic hydrogen (equation 201)181, induce elimination (equation 202)49 and lead... 

All of the new Pt and Pd complexes were prepared by LDA-induced elimination of HBr from an appropriate vinyl halide, as illustrated for the preparation of a mixture of 222a and 222b [Eq. (40)]. The mechanism of formation of these complexes is not clear. For some time it had been presumed that free cycloheptadienyne was trapped by the metal. However, as discussed earlier for formation of the bis(triphenylphosphine)plati-num(0) complex of cyclopentyne (240), elimination from an initially formed 7r-complex2-82 (258) [Eq. (41)] is also possible. Elimination from either of the two possible insertion products was excluded by their isolation and the finding that neither reacts with LDA to give 222.98... 

The Group 2 thioacetate compounds, M(SOCMe)2L, have been studied as single-source precursors for the formation of binary Group 2 metal sulfides, MS. The rationale to investigate these compounds is that if the reaction pathway involves thermally induced elimination of thioacetic anhydride according to Eq. (32), then the metal sulfide film should be relatively pure. 

The nitrogen source for the aziridination of alkenes, a nitrene or nitrenoid, can be generated in various ways (1) oxidation of a primary amine (2) base-induced -elimination of HX from an amine or amide with an electronegative atom X (X = halogen, O) attached to the NH group or by -elimination of metal halides from metal A-arenesulfonyl-A-haloamides (3) metal-catalyzed reaction of [A-(alkane/arenesulfonyl)imino]aryliodanes (4) thermolytic or photolytic decomposition of organyl azides and (5) thermally induced cycloreversion reactions . 

---