Gas-Phase Synthesis of Charged Copper and Silver Fischer Carbenes from Diazomalonates:  Mechanistic and Conformational Considerations in Metal-Mediated Wolff Rearrangements

Abstract

J. Am. Chem. Soc. 2003, 125(15), 4478-4486. IntroductionThe gas phase synthesis of metallo-carbenoid compounds has a surprisingly long history. 1 In early studies, the metallo-carbenoids were created by high energy methods utilizing electron impact ionization. The resulting ions were then used in bimolecular reactions with various neutral molecules to study the chemistry of and determine metalcarbon bond strengths for these charged Fischer carbenes. The importance of studying gas phase species in determining the mechanistic pathways for olefin metathesis was also realized early on, 1b and more recent studies have taken advantage of electrospray ionization (ESI) to continue this work. 2 Gas phase experiments have also enabled mechanistic studies with other highly reactive metallo-carbenoid species. 3 116The utility of diazo compounds as carbene and metallo-carbenoid precursors has been heavily exploited in solution phase chemistry. 4 The use of diazo compounds as precursors in gas phase mass spectrometry (MS) remains less explored. We have recently reported that diazo compounds are excellent carbene precursors for MS. 5 In these experiments, a carbene is produced through the low-energy collision activated dissociation (CAD) 6 of a diazo compound. The resulting loss of N 2 and generation of the carbene is achieved under conditions sufficiently mild that non-covalently bound complexes are not dissociated in the process. The highly reactive carbene then inserts into the guest and converts the noncovalent complex into a covalently bound molecule.All of the molecules in this previous study were based on the diazomalonate core (with at least one 18-crown-6 crown ether attached). These reagents have been appropriately named "molecular mousetraps." 5Diazomalonates 7 (such as 6.1 and 6.2 above) and related diazo ketones 8 are known to undergo Wolff rearrangement in the gas phase as shown in Scheme 6.1. Since its discovery in 1902, the Wolff rearrangement 9 has been the subject of numerous studies. 10 In the present work we use the benefits of gas phase experiments to study the mechanism of multiple, consecutive Wolff rearrangements observed in diazomalonates. 7 The effects that various coordinated metal ions and other charged groups have on Wolff rearrangements are discerned from gas phase MS experiments. Theory is used to quantitatively assess each intermediate for the proposed mechanism. Although the solution phase synthesis of stable copper (I) and silver (I) Fischer carbenes has been 117 known for some time, 11 here we report the first gas phase synthesis of copper (I) and silver (I) Fischer carbenes. The results for several intermolecular reactions of these carbenes with various adduct molecules are presented.Scheme 6.1 Experimental MethodologyAll mass spectra were acquired on an LCQ Classic instrument from Finnigan.Solutions of the reagents in the ~30-80 µM range were electrosprayed from a ~80/20(v/v) solution of methanol/water with a minimum of 0.1% MeCN added. Soft ionization settings were used to maximize th...

Silver Carbenoids

“…In a combined mass spectrometry and computational study, Beauchamp and Stoltz have demonstrated the formation and rearrangement of a silver carbene species derived from dimethyl diazomalonate. 9 In this study, the first stable species observed was the b-diketo complex 2 (for comparison see complex 21 in Fig. 8.2), which, on activation using low-energy collision-activated dissociation (CAD), loses the acetonitrile ligand, producing 3 (Scheme 8.1).…”

“…Additional support for this decomposition pathway was provided through the use of labeled substrates (deuterium or 13 C), although these experiments were performed with the corresponding copper system, which underwent similar chemistry. 9 Chavan and coworkers provide evidence that the Wolff rearrangement is facilitated by the formation of silver nanoclusters, which initiate electron transfer to the diazo compound providing 8. While the precise fate of this species remains to be firmly established, they suggest a multicycle process involving the intermediacy of a silver carbene 10 (Scheme 8.2).…”

3.19 The Wolff Rearrangement

“…124 However, if the diazo compound is derived from malonate, having an additional ester functional group, the products formed will be an olefin 125 126 or a Fischer carbene 127 if copper(I) or silver(I) are present whereas divalent metals do not promote the carbene formation (Scheme 34). 126 The vacuum pyrolysis 127 or matrix photolysis 128 of cyclic diazo compounds was observed to provide the decarbonylated product after ring contraction reaction.…”

Historical Perspectives in the Study of Ion Chemistry by Mass Spectrometry: From the Gas Phase to Solution

“…Thus, isolated ions are available for structural analysis by tandem mass spectrometry using techniques such as CID, providing information about organometallic complex stoichiometries. Table 2.1 lists some of the mechanistic studies of organic and organometallic reactions reported in the literature by ESI-MS. All sorts of reactions have been successfully explored in the gas phase, such as the Baylis-Hillman reaction [211][212][213], C-H or N-H activation [214][215][216][217][218][219], cyclopropanation reaction [220], Diels-Alder reactions [221], displacement reactions [222], electrophilic fluorination [223,224], Fischer indole synthesis [225], Gilman reaction [226,227], Grubbs metathesis reaction [228][229][230][231], Heck reaction [194], methylenation [232], oxidation [233,234], Petasis olefination reaction [235], Raney Nickel-catalyzed coupling [236], ruthenium (II)-catalyzed asymmetric reduction [237], Stille reaction [238], sulfoxidation [239], Suzuki reaction [240], Wittig reaction [241], Wolff rearrangement [242], Ziegler-Natta olefin oligomerization [243], Pd-catalyzed CÀC bond formation [244][245][246][247][248]...…”