Phillip Jochmann

Bis(allyl)calcium

In contrast to the widely employed organomagnesium and organolithium compounds, 2] the s and p complexes of the heavier alkaline earth metals calcium, strontium, and barium have not achieved their full potential. Whereas the chemistry of cyclopentadienyl (Cp) calcium compounds is fairly well developed, that of Cp-free alkyl, alkynyl, aryl, benzyl, indenyl, and fluorenyl calcium compounds has just begun unfolding. Allyl calcium compounds of unknown structure were first mentioned in a patent as polymerization initiators for vinylic monomers. The first structurally characterized calcium–allyl bond was reported by Hanusa et al. for [Ca{h-C3(SiMe3)2H3}2(thf)2] (1), the reactivity of which is suppressed by bulky ligands and stabilization through negative hyperconjugation of the silyl moieties.

Insertion of Pyridine into the Calcium Allyl Bond: Regioselective 1,4-Dihydropyridine Formation and CH Bond Activation†

Selective functionalization of pyridine is currently of interest for the synthesis of pharmacologically important nitrogen heterocycles, such as dihydropyridines (DHPs). 2] Whereas nucleophilic ortho substitution of pyridine with retention of aromaticity is facile, synthesis of DHPs by nucleophilic addition is complicated owing to loss of aromaticity. Besides Hantzsch-type multicomponent reactions, approaches to 1,2and 1,4-DHPs require the use of N-acylpyridinium ions. 4] Grignard or organotin reagents mainly add to give 2-substituted DHPs. 1,4-DHPs with substituents in the 4-position can be obtained with ill-defined organotitanium reagents, lithium dialkylcuprates, and mixtures of Grignard reagents or zinc organyls with cuprous salts. Organometallic reagents of the early transition and f-block metals, on the other hand, typically metalate the ortho C H bond of pyridine to give h-(C,N)-pyridyl complexes. Herein we show that the recently introduced organocalcium complex bis(allyl)calcium (1) selectively transfers its allyl groups to pyridine (py) to give 4-allyl-1,4-DHP, whereas the C H bond of orthoand para-methyl groups in picolines and lutidines are metalated only under thermodynamic control. Stoichiometric amounts of 1 reacted with pyridine in THF upon mixing to give [Ca(NC5H5-4-C3H5)2(L)n] (2 ; L = THF, py) quantitatively. In the presence of excess pyridine, the new calcium amide 2·(py)4 could be isolated as a red powder in almost quantitative yield and be fully characterized (Scheme 1). Variable-temperature NMR spectroscopy in [D8]THF showed that for 2·(py)4, an equilibrium exists, presumably between the cis and trans octahedral isomers. The reaction of electrophiles E Cl (E = CO2CH3, Si(CH3)3) with 2·(py)4 gave the corresponding N-protected 1,4-DHP with concomitant precipitation of CaCl2 (Scheme 1). The solid-state structure of 2·(py)4 features a symmetric octahedral coordination geometry with trans-arranged anionic NC5H5-4-C3H5 ligands. [12] The presence of four pyridine and two dearomatized NC5H5-4-C3H5 (1,4-DHP) rings is apparent from their equal and alternating C C bond lengths, respectively. This structure could be reproduced by computational methods. The only significant difference between the observed and calculated structure is a twist of the trans-arranged six-membered rings, which is attributed to p-packing effects. A solution of 1 in a 1:1 mixture of py and [D5]py led to a product that has proton signals with half the intensity expected for the ring CH groups in 2. This observation indicates the absence of a significant kinetic deuterium effect and an insertion reaction without a rate-determining C H bond-cleavage step. Compound 2 undergoes slow decomposition with first-order kinetics (k = 0.12 d , 0.65m solution in [D5]py) to give an intermediate that, upon heating for several hours, was converted into propene and an unidentified metalation product. The overall mechanism for the reaction of 1 with pyridine was deduced by NMR spectroscopy (Scheme 2). The reaction is initiated by coordination of pyridine at the calcium center to give complex 3. Attack at the ortho position by the nucleophilic allyl group results in the rapid formation of the ortho-allylated product 4 via a six-membered, metalacyclic transition state TS1. Intermediate 4 has a half-life of t1/2 = 10 min at 25 8C. The final 1,4-insertion product 2 is formed by a rate-determining Cope rearrangement (Scheme 2). In this second, six-membered transition state TS2, a lack of conformational flexibility of both the allyl and the pyridine ring fragment disfavors the 1,3-rearrangement. This sequence of allylic rearrangements is analogous to Claisen and subsequent Scheme 1. Formation of the insertion product 2·(py)4 and subsequent reaction to form N-protected 1,4-DHP. py = pyridine.

Allyl strontium compounds: synthesis, molecular structure and properties

The synthesis and attempted isolation of neutral bis(allyl)strontium [Sr(C(3)H(5))(2)] (1) resulted in the isolation of potassium tris(allyl)strontiate K[Sr(C(3)H(5))(3)] (2). In situ generated 1 shows a pronounced Brønsted basicity, inducing polymerisation of THF. Ate complex 2 crystallises as [K(THF)(2){Sr(C(3)H(5))(3)}(THF)](∞) (2·(THF)(3)). The salt-like solid state structure of 2·(THF)(3) comprises a two-dimensional network of (μ(2)-η(3):η(3)-C(3)H(5))(-) bridged potassium and strontium centres. Synthesis of allyl complexes 1 and 2 utilised SrI(2), [Sr(TMDS)(2)] (3) (TMDS = tetramethyldisilazanide), and [Sr(HMDS)(2)] (HMDS = hexamethyldisilazanide) as strontium precursors. The solid state structure of previously reported [Sr(TMDS)(2)] (3) was established by X-ray single crystal analysis as a dissymmetric dimer of [Sr(2)(TMDS)(4)(THF)(3)] (3·(THF)(3)) with multiple Si-HSr agostic interactions. The presence of ether ligands (THF, 18-crown-6) influenced the Si-HSr resonances in the NMR spectra of the amido complex 3.

Soluble Organocalcium Compounds for the Activation and Conversion of Carbon Dioxide and Heteroaromatic Substrates

The effective activation of (hetero)aromatic compounds is of particular interest for the production of tailor made compounds that can serve as key intermediates in the development of alternative combustion fuels. As a sustainable alternative for late transition metals, organocalcium complexes are studied in the context of activation of carbon dioxide and aromatic N- and O-heterocycles. Highly regioselective C–H bond activation and carbometalation reactions have been observed for conversions with pyridine derivatives. Rapid insertion of CO2 into calcium carbon bonds of the obtained products is observed. Furan derivatives are found more inert and the formation of polymeric products is described. Slow isomerization of 2,5-dihydrofuran (2,5-DHF) to 2,3-dihydrofuran (2,3-DHF) is reported.