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

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.

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.

The previous post described how the acid catalysed ring opening of propene epoxide by an alcohol (methanol) is preceded by pre-protonation of the epoxide oxygen to form a “hidden intermediate” on the concerted intrinsic reaction pathway to ring opening.

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.

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.

I recently got an email from a student asking about the best way of rationalising epoxide ring opening using some form of molecule orbitals.

This is another in the occasional series of “what a neat molecule”. In this case, more of a “what a neat idea”. The s-triazine below, when coordinated to eg ZnI ₂ , forms what is called a metal-organic-framework, or MOF.

A recent theme here has been to subject to scrutiny well-known mechanisms supposedly involving intermediates. These transients can often involve the creation/annihilation of charge separation resulting from  proton transfers, something that a cyclic mechanism can avoid. Here I revisit the formation of an oxime from hydroxylamine and propanone, but with one change.

Back in the days (1893) when few compounds were known, new ones could end up being named after the discoverer. Thus Feist is known for the compound bearing his name; the 2,3 carboxylic acid of methylenecyclopropane ( 1 , with Me replaced by CO ₂ H). Compound 1 itself nowadays is used to calibrate chiroptical calculations[cite]10.1021/ct300359s[/cite], which is what brought it to my attention.

My previous dissection of the mechanism for ester hydrolysis dealt with the acyl-oxygen cleavage route (red bond). There is a much rarer[cite]10.1039/jr9550001522[/cite] alternative: alkyl-oxygen cleavage (green bond) which I now place under the microscope.