Synthetic applications of quintuple bonds?

Wow. Wow, wow, wow. I was only vaguely aware that there were organometallic compounds containing quintuple bonds. They do exist, sure, but is there anything synthetically sound you can use them for?

I just came across this article in Chemistry World. A very recent paper of Kempe’s group (Universität Bayreuth, Germany) in Chem. Commun. shows some potentially interesting application for these somewhat exotic highly bonded complexes. They can activate small stable molecules such as CO2 and SO2… Great! But how can you make this quintuple-bonded wonder?

(from ChemistryWorld)

The answer lies in a previous paper of the group in Nature Chem. and it will seem odd to many readers I guess… you have to mix the Cr(II) precursor with two equivalents of KC8, a very powerful reducing agent used in inorganic chemistry (i’ts basically a potassium-graphite mixture). So far, not too practical for synthetic applications, but later maybe… we’ll have to wait for new developments.

A deacetonative Sonogashira coupling of aryl chlorides and propargyl alcohols

While sp-sp2 couplings are now classics, whether using terminal alkynes, zinc, magnesium, boron or silicon alkynides, it is the first time I see a propargyl alcohol used as a partner in this type of coupling. The group of Yang and Wu (Zhengzhou University, China) reported last year a deacetonative “Sonogashira” coupling between aryl chlorides and tertiary propargyl alcohols:

13aIf you know your Name Reactions really well, it should remind you of the retro-Favorskii reaction, named after the Favorskii reaction (or Favorskii-Babayan reaction, see Scheme below), not to be confused with the Favorskii rearrangement.

13bReference: J. Org. Chem. 2013 10506-10511 link

Mechanism Monday – The Synthesis of furo-fused oxazinanes via gold asymmetric catalysis

Recently, the team of Prof. Zhang (East China Normal University, Shanghai) reported a new asymmetric heterocycle synthesis using gold catalysis (yes, you have understood by now, I am quite a big fan of gold catalysis). I propose two examples to solve, they are directly extracted from the paper. For the sake of the exercise, determining the stereochemistry of the transition states are optional.

12And this one for a little twist (R and R1 are aromatic):

12b

Reference: Angew. Chem. Int. Ed. 2014, 4350-4354. link

On NHC-gold complex-catalysed aryl protodecarboxylation

Here the group of Prof. Nolan (University of St Andrews, UK) revisits the classic protodecarboxylation of arenes with a twist, a golden twist. No need for strong basic conditions anymore, a dash of NHC-gold complex, heat, and it’s done!

10 Reference: Chem. Eur. J. 2013, 14034-14038. link

Base-controlled enantiodivergence in α-aminonitrile acylation.

The groups of Prof. Takeda (Hiroshima University) and Dr. Otani (The University of Tokyo) have come across a rather surprising outcome in the α-acylation of aminonitriles: under otherwise identical conditions, the choice of base (LDA vs. MHMDS) had a dramatic influence on the retention or inversion of the stereocentre.

11Reference: Angew. Chem. Int. Ed. 2013, 12956-12960. link

9. Mechanism Monday – The Endgame of Overman’s Total Synthesis of Actinophyllic Acid

Well, this is what I consider a classic. I have proposed it a couple of times in group meetings and it has never ceased to amaze me.

This, Ladies and Gentlemen, is what I call a spectacular endgame! Four reactions for the price of one!

Have a close look at it, give it a try, and get back to the original paper once you have found a satisfactory mechanism.

9Reference: J. Am. Chem. Soc. 2010 4894-4906 link

8. Mechanism Monday: the Gold-catalysed spiroannulation of enynes and cyclopropenones by Matsuda et al.

Though the field is not that new and was explored in much details by many, I still find gold catalysis has a strong appeal. A good reason for this is that I have found gold-catalysed reactions to occur following an original mechanism that would seem queer in other Lewis-acid or transition-metal-catalysed reactions.

The following reaction between an enyne and a cyclopropenone was reported earlier this year by Matsuda and Sakurai from the Tokyo University of Science. Trying to find the mechanism is trickier than it looks at first and I found it to be a good exercise on gold catalysis .

8.a

Reference :

J. Org. Chem. 2014, 2739-45. link