Oxidative addition of carbon−halogen bonds at transition metals typically follows either a two-electron pathway (concerted M−R/M−X formation) or a radical chain pathway (stepwise M−R/M−X formation). When the reactive metal species is generated slowly, however, both mechanisms can compete to yield unexpected reactivity paths. The present report highlights the synthesis of rhodium methylidenes from chloroalkanes (e.g., CH 2 Cl 2 and CHCl 3 ) at POP-pincer frameworks (e.g., POP = 4,6-bis(ditert-butylphosphino)dibenzo[b,d]furan) via a cascade of halide abstraction and electron transfer steps. Experimental and computational studies are reported that support the proposed mechanism, including characterization of important reaction intermediates. The overall transformation represents a route toward reactive metal alkylidenes using milder and less-reactive carbenoid precursors than what is presently used.

<div><div><div><p>Oxidative addition of carbon-halogen bonds at transition metals typically follow either a two-electron pathway (concerted M-R/M-X formation) or a radical chain pathway (stepwise M-R/M-X formation). When the reactive metal species is generated slowly, however, both mechanisms can compete to yield unexpected reactivity paths. The present report highlights the synthesis of rhodium methylidenes from chloroalkanes (e.g. CH2Cl2 and CHCl3) at POP-pincer frameworks (e.g. POP = 4,6-bis(di-tert- butylphosphino)dibenzo[b,d]furan) via a cascade of halide abstraction and electron transfer steps. Experimental and computational studies are reported that support the proposed mechanism, including characterization of important reaction intermediates. The overall transformation represents a route towards reactive metal alkylidenes using milder and less-reactive carbenoid precursors than what is presently used.</p></div></div></div>

Oxidative addition of carbon-halogen bonds at transition metals typically follow either a two-electron pathway (concerted M-R/M-X formation) or a radical chain pathway (stepwise M-R/M-X formation). When the reactive metal species is generated slowly, however, both mechanisms can compete to yield unexpected reactivity paths. The present report highlights the synthesis of rhodium methylidenes from chloroalkanes (e.g. CH2Cl2 and CHCl3) at POP-pincer frameworks (e.g. POP = 4,6-bis(di-tertbutylphosphino)dibenzo [b,d]furan) via a cascade of halide abstraction and electron transfer steps. Experimental and computational studies are reported that support the proposed mechanism, including characterization of important reaction intermediates. The overall transformation represents a route towards reactive metal alkylidenes using milder and less-reactive carbenoid precursors than what is presently used.Transition metal alkylidene complexes are profoundly important entities, owing to their use in a variety of chemical transformations including olefin metathesis, 1-5 cyclopropanation, 6 and C-H insertions. 7-10 Although a large number of transition metal complexes bearing metal-carbon double bonds have been reported, mononuclear complexes of the simplest carbene (i.e. terminal methylidenes, LnM=CH2) are considerably rarer due to their high reactivity. Due to their importance as fundamental models of catalytic intermediates, metal alkylidenes have been the subject of intense synthetic effort and computational study. Over the past half-century, these studies have yielded dividends in the form of new alkylidene synthons for catalytic transformations.Several synthetic methods are available for the preparation of reactive transition metal alkylidenes, such as the use of phosphonium or sulfonium ylides, 11,12 but diazo derivatives are among the most common in carbenoid catalysis. [13][14][15][16][17] Although many such reagents are kinetically stabilized by the addition of electron-donating groups, 18 these species are notoriously toxic, reactive, and poorly-stabilized derivatives can explode unexpectedly. Polyhalogenated precursors have proven to be sources of carbene fragments in stoichiometric reactions involving polarized M-CR2-X intermediates, most notably in the Simmons-Smith reaction. [19][20][21] Recently Baker and Ozerov have independently shown that a mixture of CsF and Me3SiCF3 (i.e. the Ruppert-Prakash reagent) 22,23 provides a straightforward route to difluoromethylidene transition metal complexes of Co, Rh, Ni, Such species are also accessible from 'standard' haloalkanes, as Goldman has reported the isolation of (PNP)IrCF2 from initial C-H activation of HCF3. 28 To date, however, the use of halogenated alkanes for the synthesis of nonhalogenated terminal metal carbenes has not been demonstrated.One of the research areas of our group has focused on the generation of transition metal electrophiles for heterolytic C-H activation chemistry, 29,30 most recently exploring late-metal pincer systems. In an effort to minimize...