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Henry Rzepa's Blog

Henry Rzepa's Blog
Chemistry with a twist
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This is a follow-up to comment posted by Ryan, who asked about isocyanide’s role (in the form of the anion of tosyl isocyanide, or TosMIC): “In Van Leusen, it (the isocyanide) acts as an electrophile”. The Wikipedia article (recently updated by myself) shows nucleophilic attack by an oxy-anion on the carbon of the C≡N group, with the isocyanide group acting as the acceptor of these electrons (in other words, the electrophile). In the form shown

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The title of this post comes from a comment posted by Ryan, who asks about isocyanide’s role (in the form of the anion of tosyl isocyanide, or TosMIC) in two named reactions, Van Leusen and Ugi FCR.  “In Van Leusen, it (the isocyanide) acts as an electrophile: however, in Ugi, it acts as a nucleophile”. Here are some valence bond forms for this species;

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Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. Niels Bohr[cite]10.1007/BF01326955[/cite] was one such and four decades later Robert Woodward wrote “ The conservation of orbital symmetry ” (Chem. Soc. Special Publications (Aromaticity), 1967 , 21 , 217-249;

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In the preceding post, I introduced Dewar’s π-complex theory for alkene-metal compounds, outlining the molecular orbital analysis he presented, in which the filled π-MO of the alkene donates into a Ag + empty metal orbital and back-donation occurs from a filled metal orbital into the alkene π* MO. Here I play a little “what if” game with this scenario to see what one can learn from doing so. Firstly, I will use

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The period 1951–1954 was a golden one for structural chemistry; proteins, DNA, Ferrocene (1952) and the one I discuss here, a bonding model for Zeise’s salt ( 3 ). In “A review of π Complex Theory”, Bull. Soc. Chim. Fr. , 1951 , 1 8 , C79 (it is not online) M. J. S. Dewar sets out his theory of the role of π-complexes in (mostly) organic chemistry.

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Lukas, who occasionally comments on this blog, sent me the following challenge. In a recent article[cite]10.1021/jo3021709[/cite] he had proposed that the stereochemical outcome ( Z ) of reaction between a butenal and thioacetic acid as shown below arose by an unusual concerted cycloaddtion involving an S-H bond.

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In a previous post on the topic, I remarked how the regiospecific ethanolysis of propene epoxide[cite]10.1021/ja01208a047[/cite] could be quickly and simply rationalised by inspecting the localized NBO orbital calculated for either the neutral or the protonated epoxide. This is an application of Hammond’s postulate[[cite]10.1021/ja01607a027[/cite] in extrapolating the properties of a reactant to its reaction transition state.

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A few posts back, I explored the “benzidine rearrangement” of diphenyl hydrazine. This reaction requires diprotonation to proceed readily, but we then discovered that replacing one NH by an O as in N,O-diphenyl hydroxylamine required only monoprotonation to undergo an equivalent facile rearrangement. So replacing both NHs by O to form diphenyl peroxide (Ph-O-O-Ph) completes this homologous series.