ABSTRACT
2-(Dichloromethylidene)-1,1,3,3-tetramethylindane was "hydrolyzed" by solid KOH in DMSO as the solvent at ≥100 °C through an initial chlorine particle transfer to give a Cl,K-carbenoid. This short-lived intermediate disclosed its occurrence through a reversible proton transfer which competed with an oxygen transfer from DMSO that created dimethyl sulfide. The presumably resultant transitory ketene incorporated KOH to afford the potassium salt of 1,1,3,3-tetramethylindan-2-carboxylic acid (the product of a formal hydrolysis). The lithium salt of this key acid is able to acylate aryllithium compounds, furnishing one-sidedly overcrowded ketones along with the corresponding tertiary alcohols. The latter side-products (ca. 10%) were formed against a substantially increasing repulsive resistance, as testified through the diminished rotational mobility of their aryl groups. As a less troublesome further side-product, the dianion of the above key acid was recognized through carboxylation which afforded 1,1,3,3-tetramethylindan-2,2-dicarboxylic acid. Brominative deoxygenation of the ketones furnished two one-sidedly overcrowded bromoalkenes. Some presently relevant properties of the above Cl,K-carbenoid are provided in Supporting Information File 1.
ABSTRACT
The deceptively simple vinylic substitution reactions Alk2C=CA-Br + RLi --> Alk2C=CA-R + LiBr (A = H, D, or Br) occur via an alkylidenecarbenoid chain mechanism (three steps) without transition metal catalysis. 2-(Bromomethylidene)-1,1,3,3-tetramethylindan (Alk2C=CH-Br, 2a) is deprotonated (step 1) by phenyllithium (PhLi) to give the Br,Li-alkylidenecarbenoid Alk2C=CLi-Br (3). In the ensuing chain cycle, 3 and PhLi (step 2) form the observable alkenyllithium intermediate Alk2C=CLi-Ph that characterizes the carbenoid mechanism in Et2O and is able to propagate the chain (step 3) through deprotonation of 2a, furnishing carbenoid 3 and the product Alk2C=CH-Ph. The related 2-(dibromomethylidene)-1,1,3,3-tetramethylindan (Alk2C=CBr2, 2c) and methyllithium (MeLi) generate carbenoid 3 (step 1), which incorporates MeLi (step 2) to give Alk2C=CLi-CH3, which reacts with 2c by bromine transfer producing Alk2C=CBr-CH3 and carbenoid 3 (step 3). PhCCLi cannot carry out step 1, but MeLi can initiate (step 1) the carbenoid chain cycle (steps 2 and 3) of 2c with PhC[triple bond]CLi leading to Alk2C=CBr-C[triple bond]C-Ph. Reagent 2a may perform both proton and bromine transfer toward Alk2C=CLi-CH3, feeding two coupled carbenoid chain processes in a ratio that depends on the solvent and on a primary kinetic H/D isotope effect.
ABSTRACT
The deceptively simple "cross-coupling" reactions Alk(2)C=CA-Cl + RLi --> Alk(2)C=CA-R + LiCl (A = H, D, or Cl) occur via an alkylidenecarbenoid chain mechanism in three steps without a transition metal catalyst. In the initiating step 1, the sterically shielded 2-(chloromethylidene)-1,1,3,3-tetramethylindans 2a-c (Alk(2)C=CA-Cl) generate a Cl,Li-alkylidenecarbenoid (Alk(2)C=CLi-Cl, 6) through the transfer of atom A to RLi (methyllithium, n-butyllithium, or aryllithium). The chain cycle consists of the following two steps: (i) A fast vinylic substitution reaction of these RLi at carbenoid 6 (step 2) with formation of the chain carrier Alk(2)C=CLi-R (8), and (ii) a rate-limiting transfer of atom A (step 3) from reagent 2 to the chain carrier 8 with formation of the product Alk(2)C=CA-R (4) and with regeneration of carbenoid 6. This chain propagation step 3 was sufficiently slow to allow steady-state concentrations of Alk(2)C=CLi-Aryl to be observed (by NMR) with RLi = C6H5Li (in Et2O) and with 4-(Me3Si)C6H4Li (in t-BuOMe), whereas these chain processes were much faster in THF solution. PhC[triple bond]CLi cannot perform step 1, but its carbenoid chain processes with reagents 2a and 2c may be started with MeLi, whereafter LiC[triple bond]CPh reacts faster than MeLi in the product-determining step 2 to generate the chain carrier Alk(2)C=CLi-C[triple bond]CPh (8g), which completes its chain cycle through the slower step 3. The sterically congested products were formed with surprising ease even with RLi as bulky as 2,6-dimethylphenyllithium and 2,4,6-tri-tert-butylphenyllithium.
