Iron or Palladium: You Decide

Prakash, G. K. S.; Krishnan, H. S.; Jog, P. V.; Iyer, A. P.; Olah, G. A. A Domino Approach of Heck Coupling for the Synthesis of β-Trifluoromethylstyrenes Org. Lett., 2012, 14, 1146.

Iron(II)-Catalyzed Trifluoromethylation of Potassium Vinyltrifluoroborates Parsons, A. T.; Senecal, T. D.; Buchwald, S. L. Angew. Chem. Int. Ed. Early View Feb 10th 2012

After a bout with a nasty cold and a presentation in front of the department, there is nothing better than returning to blogging. It has been a long while since my last post, which unfortunately (or fortunately?) has been due to all the stuff going on in the Leadbeater lab. While we don’t have any new papers to report on yet, we are certainly getting very close! Over the past couple of weeks we’ve made a lot of headway on our collaboration with Dr. Tilley. We’ve certainly hit a few roadblocks along the way but we’ve found some clever ways around them. DiAndra and I are wrapping up the project she has been focusing on for the past couple of months which we likely will be submitting to either JOC or Org. Lett. In fact, DiAndra also presented this work this week at a departmental seminar and did an excellent job! Both DiAndra and I presented on the same day though on different projects (I focused on some other work I’ve done along with our collaboration with Dr. Tilley). We (and the rest of the Leadbeater group) plan on giving more talks at the CGSS in Buffalo as well as at the fall ACS meeting in Philly. Other than that I don’t think there is much more to report so let’s get to the lit!
This week we have two interesting (if not somewhat related) articles disclosing methods to access trifluomethylated olefins. The first was a article released a few weeks ago by Prakash/Olah and co-workers detailing a clever way to access trifluoromethylated styrenes. Prakash and Olah are, in my opinion, the current leaders in organofluorine chemistry and have been featured before on New Reactions. Prakash is one of the pioneers of making trifluoromethylation practical thanks to his investigations with TMS-CF3. Prakash is a former student of Olah and hence they often work together on projects particularly ones involving fluorine or carbocations. This article is no exception. As part of a broad program to access trifluoromethylated synthons, Prakash and Olah became interested in accessing vinyl trifluoromethylated compounds.

The current methods available rely on two disconnects, at the CF3 group and at the C-C double bond. Neither is very effective for synthesizing these sorts of olefins and have many drawbacks (the former being complicated by the synthesizing the appropriate olefin precursor and the latter requiring the preparation of a trifluoromethylated ylide. Prakash decided to take a somewhat different approach. His disconnect was at the C-C=C of the olefin. Rather than forming the double bond or adding in the CF3 group, he wanted to attach 3,3,3-trifluoropropene to arenes in a Heck-like manner. Now Prakash was not the first to attempt this strategy. Fuchikami and co-workers used 3,3,3-trifluoropropene to prepare trifluoromethylstyrenes. However not only did this require one to work with 3,3,3-trifluoropropene (a gas at room temperature), a autoclave was also needed and the scope of the reaction was very limited. Prakash’s solution was rather simple but ingenious: generate 3,3,3-trifluoropropene in situ and couple it to a iodobenzene derivative via a Heck-reaction. In fact, his group had experience in these sort of domino elimination-heck reaction in the preparation of styrene sulfonate salts. The idea is that under the basic conditions of the reaction, a dehydrohalogenation occurs generating the olefin in situ which can subsequently be coupled by Pd(0).

Prakash and Olah therefore began their investigation using commercially available (and relatively inexpensive) 1-iodo-3,3,3-trifluoropropane as their 3,3,3-trifluoropropene precursor. Rather than using a autoclave for heating and pressure, they opted for something our group is familiar with: microwave irradiation. Prakash and Olah believed that this would dramatically shorten reaction times making this far more practical than Fuchikami approach. Unfortunately they hit a rough start. Their test substrate, 2-iodoanisole, proved to be slow to react under their initial conditions. The authors only got, at maximum, 40 % yield which they attributed to the steric bulk of the methoxy group. They then switched to 3-iodotoluene and were pleased to get far better yield and conversion (83% conversion, 63 % yield). They screened a variety of variables in the process: Pd source, base, solvent, and temperature.

After optimizing, they explored the scope (and it turned out to be quite wide). Many were pretty standard (iodonaphthalene, p-iodofluorobenzene, p-iodoanisole etc.). There were some odd balls though. Surprisingly, o-iodoaniline reacted quite well while o-iodobenzoic acid failed to react. They also screened heterocycles, though most attempts were meet with low yields or a complete lack of reactivity. While somewhat of a short article, it was very to the point and thorough and I really enjoyed it.

Only a couple of weeks later, Dr. Steven Buchwald at MIT published a article detailing a somewhat different approach to the synthesis of trifluoromethylated olefins. As with Prakash and Olah, Buchwald and his group have been <a href="; title="featured once before on New Reactions. Buchwald has really become interested in organofluorine chemistry lately, publishing several new methods for the synthesis of some elusive trifluoromethylated moieties. In fact, Buchwald recently disclosed a method to access allyl trifluoromethylated compounds from terminal olefins and Togni’s reagent (a relatively new electrophilic trifluoromethylating reagent). In the processes they encountered a bothersome intermediate, namely the alpha chlorination product. Rather than simply ignoring this side product, Buchwald and his group hoped to capitalize on it as vinyltrifluoromethyl precursor. With that in mind, they decided that vinyltrifluoroborates would make suitable olefins for addition followed by spontaneous elimination.

After a bit of screening, they found that Iron (II) chloride was the most effective catalyst for their desired transformation and gave excellent E/Z selectivity. They found that a variety of vinyltrifluoroborates could be transformed into vinyltrifluoromethylated olefins under their conditions, and most gave fair to excellent yields. In an effort to explain the selectivity of their reaction, they reacted both Z and E vinyltrifluoroborates and obtained near idential E:Z ratios. This seemed to indicate an alternative more traditional pathway (radical or carbocationic) rather than a organometallic reaction. With that in mind, they investigated whether a Lewis acid was a suitable alternative catalyst. Indeed, they found that tin triflate also successfully catalyzed their trifluoromethylation reaction, indicating the reaction is likely proceeding through a carbocation. And that’s exactly where Buchwald leaves you hanging. I bet we will be seeing more of this sort of reaction in the near future.

Well there you have it, two excellent articles detailing new methods to access trifluoromethylated olefins. Hats off to both Prakash’s group and Buchwald’s group for excellent, through jobs! That’s it for now, Ckellz…Signing off…


Organic Chemistry Memes

Hi All,

Just made this facebook group today, hope you all like it. Feel free to like it and post some of your own memes…also new post coming soon!!!


New Blog!

Just ran across a new blog this morning by the Sussex Drug Discovery group over in the UK. It’s a bit more med chem oriented but it looks promising and will likely have a lot of useful information so go take a look!

In Soviet Russia, Catalyst Reacts You!

Kuznetsov, A.; Gevorgyan, V. General and Practical One-Pot Synthesis of Dihydrobenzosiloles from Styrenes Org. Lett. ASAP January 24, 2012

No I’m not dead! Sorry it’s been so long since my last post but I have finally returned to New Reactions after a very very busy 4 weeks. But what a 4 weeks it’s been. So a quick update on the happenings in the Leadbeater lab starting with some bad news. We unfortunately did not get our flow paper into Organic Letters and have subsequently resubmitted to a more appropriate journal, Org. Proc. Res. Dev. where we believe it should get in. But really, that’s about it in terms of bad news. DiAndra and myself continue to make progress with the project we are working on (and its allowed me to get into all sorts of chemistry from making benzofuran to bromination of thiophenes). I’ve never worked with heterocycles as much as I have recent and I have greatly enjoyed it. We are a little behind where I would hope we would be but the reactions are progressing quite well and based on some recent findings, I think this one might be better suited in Organic Letters! As for other projects, we are in the midst of substrate screen for our collaboration with Dr. Tilley and it has been extremely successful! We hope to have that out by the middle of this year. I really am very excited to get that work out and a few other projects we have been working on that are near completion.
Last week was the start of the semester for UConn students meaning the the course that Mike is TAing (Adv. Org. Chem. Lab) has begun. I assisted him in preparing some of the labs as well as giving a joint lecture yesterday on ChemDraw. I think the students will really enjoy the reactions we have in store for them this semester (a RCM, a Sonogashira, Suzuki, Click, and Paal-Knorr to name a few)! I’m also giving a seminar next month on some of the work I’ve done thus far at UConn. I’m also looking into going to the ACS meeting in Philly in the fall to present there as well (hopefully with a bit more accomplished :P). I do find it weird sometimes that I used to be so scared of public speaking but now I really enjoy it. I actually look forward to talks. I still get nervous right before the talk of course, but as soon as I get past that first slide, things just slip into autopilot. Plus I genuinely love just talking chemistry! And with that, let’s get to it.

I’ve had the chance to read a number of articles since my last post (many of them quite excellent) and I’d like to share with you a few that stood out to me as cool/interesting/useful:

  • Copper-Catalyzed Oxidative Trifluoromethylation of Terminal Alkynes and Aryl Boronic Acids Using (Trifluoromethyl)trimethylsilane: Describes a relatively effective way to synthesize trifluoromethylated alkynes and arenes. It’s an extension of some of the work done earlier by the Qing group which improves on yields by using a syringe pump and expands their reaction to include boronic acids as substrates.
  • Electron Transfer Reduction of Carboxylic Acids Using SmI2- H2O- Et3N: describes a general procedure for converting carboxylic acids directly to primarily alcohols in one step. I found this to be a very practical, easy approach to avoid using LiAlH4 or boron-based reductants and features a cool mechanism.
  • Fragmentation of β-Hydroxy Hydroperoxides: describes an investigation into the role of vitamins and transition metals (specifically Vitamin E and C in the presence of iron) in the oxidative cleavage of unsaturated fatty acids. While not really synthetically useful, it was a very well done investigation and gave plenty of mechanistic insight.
  • Oxidative Homologation of Aldehydes to α-Ketoaldehydes by using Iodoform,
    o-Iodoxybenzoic Acid, and Dimethyl Sulfoxide
    : describes a convenient and very effective way to make highly reactive ketoaldehydes in situ and trap them as quinoxalines. What I like about this is the shear number of examples done by the authors and the potentially variable approach that can be used by simply switching the phenylene diamine or varying the starting aldehyde.
  • Efficient Palladium-Catalyzed Cross-Coupling of Highly Acidic Substrates, Nitroacetates: an article by the Koziowski group at UPenn which describes a very interesting coupling reaction between highly acidic nitroacetates with various aryl bromides. The article was great and I especially loved the colorful three dimensional graphs!
  • This week’s article comes from, you guessed it, Org. Lett. Its by a Dr. Vladimir Gevorgyan at the University of Illinois at Chicago, whose work I have been following for some time (this being the second article of his I will be featuring). Gevorgyan does chemistry that is near and dear to me, namely small ring synthesis, organosilyl work, C-H bond functionalization, and heterocycle synthesis. He’s sort of a jack of all trades when it comes to his chemistry, but he publishes very good work (in only the top named journals). In this new article, he focuses on a new (or arguably a very recently discovered) class of organosilyl compounds: dihydrobenzosiloles. Prior to the release of this article, no effective synthetic method was available to access these compounds. One example was given by Hartwig in a 2005 paper, but synthesis of these compounds was not the main focus (hence the article gives only one example under very harsh conditions). Dibenzosiloles and biarylbenzosiloles are far better known in the literature.

    Seeking a route to these compounds, Gevorgyan decided to use a two-stage approach to their synthesis. The first step with be β-hydrosilylation of a styrene derivative using diphenylsilane as their hydrosilylation source. That would give the desired phenethyldiphenylsilane compound. With that in hand, a dehydrogenative cyclization could then be used to obtain the elusive dihydrobenzosilole. However, like most things in chemistry, nothing is as easy in practice as it is in theory. Their first step was, simply put, known put somewhat impractical. Hydrosilylation of stryenes using diphenylsilane was known using some late transition metals (gold, rhodium etc.) but none were inexpensive and easy. Hydrosilylation of simple olefins was known using much more affordable nickel based catalysts but it was unknown whether these systems were compatible with stryenes. So rather than finding a new route, Gevorgyan explored some nickel catalysts to mediate the desired hydrosilylation. He ultimately found that NiBr2(PPh3)2 worked best after screen a plethora of nickel catalyst. With the hydrosilylation problem addressed, Gevorgyan turned to the cyclization step. Using a bit of inspiration from some previous findings he was able to use a general method for silylating aromatics to induce his cyclization. Better yet, he found this two step process could be conducted in a single flask in excellent overall yield!

    After screening a variety of styrene derivatives, he found that only meta styrene substrates proved problematic. They lead to regiochemical isomers (whose ratio was influenced by sterics). m-fluorostyrene was the worst offender of this giving a 2:1 regiochemical ratio. Gevorgyan then investigated α-phenyl and α-methyl stryenes to see if the alkene substitution patterned played any role and in fact it did. These substrates were far more difficult to hydrosilylate and hence the addition of a LA catalyst was necessary. By adding B(C6F5)3, 3-methylbenzosilole and 3-phenyldehydrobenzosilole could successfully be accessed.

    Gevorgyan finally got to my favorite part of discovering new reactions: mechanism proposal. After some study, he was able to determine that electron-withdrawing groups on the styrene accelerated the reaction dramatically. Moreover, kinetic isotope studies showed that the second cyclization step was likely the rate-determining step. Bearing this information in mind, the following mechanism was proposed (and its cool!):

    Finally all good methods papers end with an application. Gevorgyan used his dihydrobenzosiloles to not only synthesize benzosiloles by DDQ oxidation but also to access dihydrobenzofuran derivatives by peroxide oxidation of the silyl moiety followed by Mitsunobu-mediated cyclization.

    Overall I found this to be a excellent article by Gevorgyan and co-workers. Dihydrobenzosiloles are very unusual and interesting compounds (though I’m a bit biased since I came from Dr. Tilley’s group and therefore I love silicon-related molecules). I look forward to future articles from the Gevorgyan group. That’s all for now…Ckellz…signing off…

    2011 No More!

    HAPPY NEW YEARS!!!! Thank you all for making this year a wonderful year of blogging for me and I hope you continue to visit in 2012!

    Moar Oxidation! MOAR!

    Eddy, N. A.; Kelly, C. B.; Mercadante, M. A.; Leadbeater, N. E.; Fenteany G. Access to Dienophilic Ene-Triketone Synthons by Oxidation of Diketones with an Oxoammonium Salt Org. Lett. ASAP December 29, 2011

    So that moment that you all have been waiting for (well lets be real, mostly the moment I have been waiting for) has arrived. I can finally share with you the project we worked on with Dr. Fenteany’s group this past year. After our group became interested in green oxidations using, surprise surprise, Bobbitt’s salt, we found that a graduate student upstairs had discovered something very unique. If 2,2 dimethyl-1,3-cyclohexadione was exposed to Bobbitt’s salt for an extended period, very little of the α-oxidation product was obtained. The student, Nick Eddy, was really interested in obtaining this another project he was working.

    However, he noticed that another unusual product was obtained because some of the starting material had been consumed. After careful NMR analysis, he determined that the product was in fact an overly oxidized derivative of his starting material, an ene-triketone. Now at that point, it was fundamentally interesting but was it useful? After doing a bit of reading, he soon found out that the compound he has prepared normally took four steps to make and at 15% overall yield. No other ene-triketone had been reported in the literature, likely because of the difficulty of their synthesis. After talking with us about it, we agreed that if we could optimize the reaction to give a high yield of the ene-triketone and turn this into a project, it would likely be of high interest to the organic community. So we spent some time optimizing the reaction, ultimately finding that reaction only occurred at slightly elevated temperatures in highly polar solvents (DMF, MeCN). Moreover, it required a large loading of salt (3.8 equiv)! On a large scale, this meant greater than 60 grams of salt. So we became experts at preparing Bobbitt’s salt (which, as I’ve said, is very easy).
    We also found that the quality of the salt needed to the be the highest it possibly could be. It needed to be absolutely free from impurities and water (as did the solvent). We needed to not only recrystallize the salt from boiling water but also rigorously dry it using not a desiccator but with a Abderhalden over KOH and ethanol! This boosted our yield from 20-30% using the normal powder version of the salt to 60-80% using the highly purified salt. With that accomplished, we then prepared a range of 2,2 disubstituted cyclohexadiones. Now you may wonder why it was necessary to block that 2 position (in between the two ketones). That is to prevent enolization at that position (very stable enol) and oxidation to give the 1,2,3 triketone which has been reported. Therefore we tried a variety of substituents with mixed results. Most of the ones we tried worked (and the syntheses of these substrates were amazingly fun)! We also got some meaningful mechanistic information out of the substrates we tried. The following two results:

    told us that the driving force of the reaction was that first ene-like reaction and sterics (which was said to be, in their system, disfavorable by the AZADO article I just reviewed) and subsequent oxidation to the triketone. From there, the triketone immediately enolizes and reacts again. Since there is a acid proton in the vicinity, it eliminates to generate the olefin of the ene-triketone. We need for excess salt, heating, and the polarity of the solvent all make sense with the mechanism (stabilizing enol form, 3 oxidations, steric repulsion). The mechanism is by far the best part of the article to me so here it is in all its glory:

    At that point we could have stopped and submitted the work. But we weren’t satisfied with just the oxidation, we wanted to see what sort of reactions we could do with the ene-triketones (being the only people to have access to them at that time). We decided that an excellent application would be as dienophiles in Diels-Alder reactions. That olefin is very electron-withdrawn and therefore would likely react in a shot. And, as predicted it did at room temperature! We found that most of our ene-triketones reacted quite well with fair to excellent endo : exo ratios. The sheer complexity of the molecules were generating was what really impressed me though. Well I really hope you go take a look at our article for more details and enjoy it! It certainly made my day yesterday! Ckellz…Signing Off…

    What the Heck is an AZADO?

    Hayashi, M.; Shibuya, M.; Iwabuchi, Y. Oxidative Conversion of Silyl Enol Ethers to α,β-Unsaturated Ketones Employing Oxoammonium Salts Org. Lett. ASAP December 19, 2011

    Happy holidays to all my readers!!! I hope that the holidays treated you all well. Thankfully, I have the week off this week which has allowed me to catch up on much of the writing I have put off. As I mentioned in my last article, our work with Dr. Fenteany’s group was accepted into Org. Lett. and (should) be up on the ASAPs this week. We received the proofs on Friday of last week and they were resubmitted on Christmas Day with very minimal corrections. I plan on doing a special post on it when it comes out so check back over the next few days. As for chemistry in the Leadbeater lab, nothing really new to report. The lab has the week off for the holidays allowing all of us to recoup and get ready for a very busy January. In the meantime, I have been working on prepping DiAndra’s work for submission to JOC. I’m still debating personally on whether it belongs there or Org. Lett. Either way it’s shaping up to be a phenomenal paper worthy (in my opinion) of both journals. In other news, one of our methods papers is just about complete and should be in by mid-January. I’m really excited about that one as it relates quite closely to the work DiAndra has been doing and gives a good feel for the sort of chemistry we’ve become interested in the Leadbeater lab. Our flow paper has (FINALLY) been submitted to Org. Lett. and, as far as I know, it is already out for review. So with all our hard work coming to fruition, 2012 is looking to be an excellent year in terms of publications. I hope to have at least 10 publications before I graduate from UConn (hopefully allowing me to graduate in less than 5 years). So far, that looks to be a very real possibility!
    Not that I enjoy politics, but I can’t help but comment on the UCLA incident and the current criminal charges being filed against Professor Patrick Harran. For those who do not know what I am talking about see the summary here and the latest news here. I think what’s happening to him is appalling and not justice but revenge for an act he didn’t have much responsibility for. Do I feel bad for Sheri Sangji, the research assistant who died from the burns she received? Absolutely, no one deserves the suffering she endured from the fire. However, that does not mean the PI is responsible. Much of the fault is in fact her own. She was wearing inappropriate lab attire, minimal eye protection, and working with a extraordinarily dangerous reagent that she had little familiarity with by herself. She was also using a syringe size that no experienced chemist would use for the particular reagent she was dealing with (t-BuLi). For those who haven’t worked with t-BuLi, it’s one of the few reagent that puts me a little on edge. It ignites in a beautiful but frightening purple flame on contact with air and is stored in pentane, adding to its volatile nature. So if a bottle were accidentally exposed to air, the bottle would explode into flames and that’s exactly what happened with Sangji. However, I don’t believe that Harran deserves to have his life destroyed (4.5 years in prison and basically he will have no chance of returning to chemistry) for an incident he is, at most, minimally responsible for. Some fines against the university are understandable but not jail time for the PI. What do you all think?
    Now on to some lighter, more academic news, a new review! I had another tough time again this week choosing an article. Here’s a few of the runner ups (which are they themselves very interesting and good chemistry!):
    1. CuCl/DABCO/4-HO-TEMPO-Catalyzed Aerobic Oxidative Synthesis of 2-Substituted Quinazolines and 4H-3,1-Benzoxazines Yu, W. et al JOC ASAP December 14, 2011
    2. A phosphine-free Pd catalyst for the selective double carbonylation of aryl iodides Castillón, S. et al Chem. Commun. Advanced Articles
    3.[1,4]Dithiepino[2,3-b]furans from Oxiranecarbaldimines and Lithiated 1,3-Dithiane: A Series of Rearrangement Reactions in One Pot Würthwein, E.-U. et al EJOC Early View
    4.Reaction of InCl3 with Various Reducing Agents: InCl3–NaBH4-Mediated Reduction of Aromatic and Aliphatic Nitriles to Primary Amines JOC ASAP

    However, for the sake of a bit of foreshadowing (HINT!) of my own article and to stick with the theme of oxoammonium salts, I decided to go with an article detailing the oxidation of silyl-enol ethers to their corresponding α,β-unsaturated ketones. I had never heard of 2-azaadamantane N-oxyl (AZADO) before nor it’s the oxoammonium salt derived from it. In fact there are quite a few of these oxyl radicals derived ridged ring systems (9-azabicyclo-[3.3.1]nonane N-oxyl ABNO being another). According to this article, these bicyclic oxoammonium salts are far more effective oxidant than our friend TEMPO-BF4,a salt very similar to Bobbitt’s salt. However, if you dig a little bit, you soon realize that synthesizing AZADO is far more difficult than TEMPO-BF4 (9 steps to get to AZADO (of ten total) and only two (of three total) to get to TEMPO). I smiled a little bit when I saw the sheer number of references to Bobbitt on the first page alone. The first few paragraphs simply outline the advantages of oxoammonium salts and their use so I won’t bore you with that. The author’s goal of this article was to provide an alternative (possibly greener and safer) approach to performing α,β-dehydrogenation of ketones.

    Their first attempts to utilize AZADO for this transformation were met with mixed results. They tried using a cyclohexyl derivative to accomplish oxidation and got a mixture of the desired α,β-unsaturated ketone as well as the α-aminooxy ketone. As with most good reactions, their was the minor product and therefore the authors needed to find a way to turn the side reaction into the major reaction. By switching to lower temperatures (-78 oC) and using a tert-butyldimethyl silyl enol ether instead of the trimethylsilyl, they were able to obtain a 93:7 ratio of the dehydrogenated product to the α-aminooxy product. They compared salts too, finding that their original candidate, AZADO, was the reagent of choice for this oxidation.
    Next they did a comparison of counter-ions: BF4-, PF6-, ClO4-, SbF6-, Cl-, NO3-. They found that the less nucleophilic counter-ions (BF4-, PF6-, ClO4-, SbF6-) gave very similar ratios of dehydrogenation to addition of the α-aminooxy group with BF4- being the most selective. Cl- and NO3 gave far worse ratios. No explanation was given for this so I’d be very interested as to why there is a counter-ion effect.
    What I liked most about this article was the detail they went into in their experiments. Rather than immediately jumping to substrate screening, they focused on isolating a transient intermediate species in their oxidation. It turns out that an intermediate (before queching the reaction with saturated bicarbonate) is a mixed acetal. This gave the authors a unique insight into the reaction mechanism. They proposed the following:

    Also they used the same rational to explain why the TEMPO derived salt gave far more of the α-aminooxy product:

    After giving a plausible mechanism for their transformation, they then explored the scope. They found their reaction to be quite broad. Moreover they found their reaction proceeded stereoselectively in that acyclic species gave only the less sterically demanding E-isomer. Finally they noted that AZADOH (the reduction product of the spent oxidant) was recovered and re-oxidized quite easily, suggesting that their method was “recyclable” in oxidant.
    Overall this was a short but very practical article demonstrating the versatility of these salts for transformations other than simple carbinol oxidation. Hats off to Iwabuchi and co-workers for an excellent investigation and a useful method! Be on the lookout for another post within the next couple of days! Ckellz…Signing off…