musings on music and life

November 17, 2016

Classics in Organic Chemistry, Part VIII

Filed under: Classics in Organic Chemistry — sankirnam @ 3:46 pm

This paper was further down on my list, but I’ve decided to bump it up and cover it today.

Modern practitioners of organic synthesis or medicinal chemistry will no doubt be aware of how hot fluorine chemistry is now; every issue of JACS, JOC, Organic Letters, Angewandte Chemie, or Chemical Science has at least one paper on the development of new fluorination methodologies. But this was not always the case. Fluorine chemistry used to be considered very esoteric, primarily because of the reagents required (F2, HF, SF4, among others), which also necessitated special reaction conditions and apparatus. This limited the accessibility of fluorinated compounds, and research in this area was primarily done by groups in academia (such as Olah, Seppelt, Christe, Bartlett, Rozen, Haszeldine, Barton) or industry (DuPont, 3M) that had the infrastructure in place to carry out this chemistry.

One of the long-standing challenges in organofluorine chemistry was the development of a mild, effective method to introduce the trifluoromethyl (-CF3) group into organic molecules. I had briefly discussed the challenges in isolating the trifluoromethide anion earlier; this is why the development of nucleophilic trifluoromethylation methods only came about recently. It is necessary to use reagents that act as “pseudo-anions”, and can do a transfer of the -CF3 group under certain conditions.

Prakash and Olah were motivated by their desire to study carbocations that had an electron-withdrawing group α to the cation, such as the ones below:coc_8_1

The synthesis of the precursors for these cations is rather interesting – each involves a different type of chemistry. The α-nitro cation above is prepared by ionizing the gem-dinitro compound (which was synthesized from benzophenone oxime and N2O4), while the α-fluoro cation is prepared from gem-difluorodiphenylmethane, which can be prepared from benzophenone and SF4. The α-cyano cation is prepared by ionizing benzophenone cyanohydrin, which can be easily prepared using a procedure developed by Prof. Paul Gassman with TMSCN and ZnI2.

The α-CF3 cation can be prepared from 2,2,2-triphenylacetophenone and phenylmagnesium bromide, but substituted derivatives are more challenging to prepare; you’ll need substituted derivates of 2,2,2,-triphenylacetophenone which are either challenging to synthesize, of limited commercial availability, or expensive. The easier route would be to start from benzophenone and add a -CF3 to the carbonyl. This was elegantly solved by Prakash, Olah, and Krishnamurti in 1989. They demonstrated that the compound TMSCF3 could undergo nucleophilic trifluoromethyl transfer to carbonyls very readily, under fluoride-ion catalysis. TMSCF3 had first been prepared by Prof. Ingo Ruppert (Germany) a few years earlier, but he had not demonstrated any potential reactions with it.

coc_8_2

This is the proposed mechanism; interestingly, fluoride is not necessarily the only catalyst that can initiate this reaction – Dr. Prakash later showed that carbonates and amine-N-oxides can also act as catalysts. I’m not sure if DMF/imidazole can also initiate this reaction (as they do Corey’s TBS protection), but I’m sure that should also work. One big challenge that still has not been solved is to do this transfer asymmetrically; in other words, a facially-selective trifluoromethyl transfer to carbonyls is still lacking.

This has led to a whole slew of developments which are simply too numerous to list here, leading to TMSCF3 being called the “Ruppert-Prakash reagent”, after the chemists who first synthesized it (Ruppert) and demonstrated its synthetic utility (Prakash). The commercial availability of TMSCF3 also opened up trifluoromethylation to all organic chemists (the original synthesis (adapted from Ruppert’s work) uses CF3Br, which is now banned under the Montreal Protocol). Recently, a postdoc in Prakash’s group (who used to work next to me) came up with an improved synthesis of TMSCF3 from CF3H, which is a byproduct of Teflon manufacturing, and therefore much cheaper and more readily available than CF3Br.

Many, many other types of trifluoromethyl transfer reagents have been developed, and almost all of these use TMSCF3 in their synthesis. The electrophilic trifluoromethylating reagent developed by Togni is illustrative of this. Melanie Sanford has also conducted very nice work in organometallic chemistry studying the reductive elimination of -CF3 from Pd(IV); I particularly remember a very interesting set of papers she had published that showed that “F+” reagents were the only compounds capable of oxidizing the Pd(II) to Pd(IV) and selectively inducing the reductive elimination of the -CF3, because the energy of reductive elimination of -F was greater than that of -CF3. It’s not much though; I think it was 5 kcal or less! Of course, all of these trifluoromethylated metal complexes were synthesized with TMSCF3 as the -CF3 source.

Stephen Buchwald (MIT) also published a couple of papers using TMSCF3 and Pd/Cu complexes for doing -CF3 transfer to a variety of systems.

CuCF3 and AgCF3 are also receiving increased interest now; I talked about CuCF3 earlier. Both of these complexes can be generated in situ from TMSCF3 and appropriate metal salts, and can be used for a variety of transformations, including Sandmeyer-type reactions. I remember that I and my labmates had tried to implement this reaction without much success, and when we saw Goossen’s paper, it seems that the copper counterion is very significant; the reaction only works with CuSCN, which we did not have on hand.

As mentioned earlier, the challenge with developing organometallic reagents for nucleophilic -CF3 transfer (such as LiCF3 or CF3MgBr) is that the CF3 anion is kinetically unstable and tends to undergo fast α-defluorination to yield difluorocarbene. This can be a nuisance, but depending on your needs, can also be synthetically useful. Difluorocarbene can also undergo the usual carbene reactions, such as 2+1 additions to olefins to give gem-difluorocyclopropa(e)nes, as well as insertions into weak bonds, such as S-H or Sn-H. Some friends of mine in Prakash’s group were able to use this to develop useful chemistry – one nice example is the insertion of CF2 carbene so generated into the Sn-H bond of Bu3SnH to make Bu3SnCF2H, which proved to be a useful reagent for -CF2H transfer.

coc_8_3

There was a paper published a couple of years ago by a group in Russia describing the synthesis of TMSCF2H from TMSCF3 by a simple reduction using sodium borohydride. This allows improved access to TMSCF2H (which was otherwise difficult to prepare) and related analogues (such as TMSCF2D, TMSCF2Cl, and others). The challenge with TMSCF2H is that it is more difficult to activate compared to TMSCF3 (it is speculated that the reactive species that does the actual -CF3 transfer is a pentavalent siliconate), accounting for the limited substrate scope (with ketones) in this paper by Jinbo Hu.

Anyway, this is a brief overview of trifluoromethylation chemistry, and I hope the huge impact that Dr. Prakash’s initial paper had is evident – TMSCF3 is now the major source of -CF3 in organic chemistry; most research chemists will not think about how it is produced! This is by no means exhaustive, and numerous reviews (such as this one) are being published about this area of chemistry all the time; check those out if you want more details.

Advertisements

Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a free website or blog at WordPress.com.

%d bloggers like this: