Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?

The Birch reduction is a classic method for partially reducing e.g. aryl ethers using electrons (from sodium dissolved in ammonia) as the reductant rather than e.g. dihydrogen. As happens occasionally in chemistry, a long debate broke out over the two alternative mechanisms labelled O (for ortho protonation of the initial radical anion intermediate) or M (for meta protonation).

A dichotomy is a division into two mutually exclusive, opposed, or contradictory groups. Consider the reaction below*.* The bicyclic pentadiene on the left could in principle open on heating to give the monocyclic [12]-annulene (blue or red) via what is called an electrocyclic reaction as either a six (red) or eight (blue) electron process. These two possibilities represent our dichotomy;

Not a few posts on this blog dissect the mechanisms of well known text-book reactions. But one reaction type where there are few examples on these pages are reductions. These come in three types; using electrons, using a hydride anion and using di-hydrogen. Here I first take a closer look at the third type, and in particular di-hydrogen as delivered from di-imide.

A game chemists often play is to guess the mechanism for any given reaction. I thought I would give it a go for the decomposition of the tris-peroxide shown below.

The text books say that cyclohexenone A will react with a Grignard reagent by delivery of an alkyl (anion) to the carbon of the carbonyl ( 1,2-addition ) but if dimethyl lithium cuprate is used, a conjugate 1,4-addition proceeds, to give the product B shown below.

When methyl manganese pentacarbonyl is treated with carbon monoxide in e.g. di-n-butyl ether, acetyl manganese pentacarbonyl is formed. This classic experiment conducted by Cotton (of quadruple bond fame) and Calderazzo in 1962[cite]10.1021/ic50001a008[/cite] dates from an era when chemists conducted extensive kinetic analyses to back up any mechanistic speculations. Their suggested transition state is outlined below.

Following on from our first mechanistic reality check, we now need to verify how product A might arise in the mechanism shown below, starting from B . This pathway backtracks the original one in reversing the final arrow of that process (shown in red in previous post and in magenta here for the arrow in reverse), to go uphill in energy to reach the secondary (unstabilised) carbocation.

The reaction described in the previous post (below) is an unusual example of nucleophilic attack at an sp ² -carbon centre, reportedly resulting in inversion of configuration[cite]10.1021/ja00765a062[/cite]. One can break it down to a sequence of up to eight individual steps, which makes teaching it far easier. But how real is that sequence?

The reaction below plays a special role in my career. As a newly appointed researcher (way back now), I was asked to take tutorial groups for organic chemistry as part of my duties. I sat down to devise a suitable challenge for the group, and came upon the following reaction[cite]10.1021/ja00765a062[/cite]. I wrote it down on page 2 of my tutorial book, which I still have.