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Reactivities of zero-valent group 10 complexes toward organic isocyanates: Synthesis of metallacycles containing dimeric isocyanate units, isocyanate cyclotrimerization, and computational chemistry

New Journal of Chemistry, ISSN: 1369-9261, Vol: 43, Issue: 39, Page: 15614-15625
2019
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Article Description

The reactions of [Pd(olefin)(PR)] (PR = PMe, PMePh) with two equivalents of an aryl or alkyl isocyanate afford cis-[Pd{-N(R)C(O)N(R)C(O)-}(PR)] (R = 1-naphthyl, 4-phenoxyphenyl), which are five-membered palladacycles bearing dimeric isocyanate units, or cyclic tetramers as assemblies of four five-membered palladacycles, [Pd{C(O)N(R′)C(O)N(R′)}(PMe)], (R′ = 3-methylbenzyl, 4-methylbenzyl or 4-methoxybenzyl), depending on the alkyl substituent on R-NCO. Interestingly, these reactions afford cyclic trimers as catalytic products when two equivalents or excess amounts of benzyl isocyanate are used. In contrast, reactions of [Pt(olefin)(PR)] with two equivalents of an alkyl or aryl isocyanate afford only the five-membered platinacycle, namely cis-[Pt{-N(R)C(O)N(R)C(O)-}(PMe)] (R = 3-methylbenzyl, 4-methylbenzyl, 4-fluorobenzyl, 4-methoxybenzyl, (S)-(+)-(1-naphthyl)ethyl, (R)-(-)-(1-naphthyl)ethyl, 4-phenoxyphenyl and 2,6-difluorophenyl). Aided by theoretical calculations, we propose mechanisms for the formation of the five-membered palladacycle or platinacycle, the cyclic tetramer, and the cyclotrimerization of the organic isocyanate. In addition, the ligand-exchange reactions between a five-membered platinacycle bearing a chiral substituent such as (S)-(+)-(1-naphthyl)ethyl or (R)-(-)-(1-naphthyl)ethyl) moieties and 1,2-bis(diethylphosphino)ethane (DEPE), a chelating phosphine, clearly afford the corresponding platinacycle bearing a DEPE ligand with retention of chirality. On the other hand, reactions of [Ni(COD)] with various organic isocyanates in the presence of tertiary phosphines only afford the corresponding cyclic trimers. In contrast, similar reactions in the presence of N-heterocyclic carbenes (NHC) such as 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidine (SIPr) afford unexpected adducts between R-NCO and the NHC ligand.

Bibliographic Details

Young Sung Han; Yong Joo Kim; Kyoung Koo Baeck; Kang Yeoun Jung; Gang Min Lee; Soon W. Lee

Royal Society of Chemistry (RSC)

Chemical Engineering; Chemistry; Materials Science

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