[buRnINGbeND]

so i've run a particular kcarb deprotection of a TMS protected alkyne a number of times on a couple of related substrates and i always get a weird result.  my proton NMR looks fine, but both my APT and DEPT show what i would assume to be my terminal C-H to be quaternary.  additionally, HRMS data does not give me a match to what the compound is supposed to be.  on the other hand, in the next step, my n-BuLi deprotonation works like a charm. there are no other functional groups in the compound that should give me a problem.  i've done some poking in literature, but i can't find anything that would seem to give me a good explanation.  has anyone ever seen this and know what is going on?

[discodermolide]

Are you possibly forming the allene?

[buRnINGbeND]

no.  the proton NMR looks perfect and the problem with the APT is that the peak shows even instead of odd and the DEPT it shows quaternary (i.e. is absent).  the allene would be completely different and would probably tautomerize because i have a propargyl alcohol.

[Dan]

my proton NMR looks fine, but both my APT and DEPT show what i would assume to be my terminal C-H to be quaternary.

I suggest a C-H correlation experiment (e.g. HSQC) to see which Cs your Hs are attached to, if what you assume is a CH looks quaternary, it is likely that the assumption is bad. You might have two coincident peaks in the 13C, you will see this easily on an HSQC - this accounts for a missing CH but does not explain the extra quat C.

additionally, HRMS data does not give me a match to what the compound is supposed to be. 

What is the discrepancy? What is the technique?

has anyone ever seen this and know what is going on?

Structure and spectra would help enormously if you are in a position to post them.

[buRnINGbeND]

the pdfs are formatted on their sides but i'll get them up in a minute.

we can't do an HMQC or HETCOR on our instrument for some reason.  it's old, idk.

HRMS is CI, but we also send it to a different school, so i'm not sure exactly what's going on.  the analysis for every other compound in my sequence and all of my final products have been fine.

[buRnINGbeND]

PhCH₂CHOHCCH

[Dan]

Your NMR is identical to that reported in the Lit. Maybe the alkyne CH is very weak in the CH carbons APT due to some known effect - I'm by no means an NMR expert and do not know much about things like relaxation time etc. that give rise to such effects, but this might be why it seems to disappear.

I notice you've got a DEPTQ there, maybe a DEPT-135 or DEPT-90 (in which quaternaries do not appear) will make it clearer?

I can't think of a reasonable explanation for your ESI CI spectrum though. In the paper I linked to above they used EI successfully - so maybe try that method if you can - it is possible that this compound just does not fly ESI(+ve) CI.

[buRnINGbeND]

quaternaries don't appear in the DEPT, that's why i say that the peaks in the APT that don't show up in DEPT are quaternaries.

[buRnINGbeND]

i also said CI, not ESI.

[buRnINGbeND]

to be more clear on the DEPT spectrum, our scanner does a 45, 2 90s, and a 135.  quaternaries don't show up on any of them.  the spectrum posted is after a macro adds and subtracts to specifically tell us which peaks are which type of carbons.

[Dan]

Re: CI. Sorry, I was in a hurry and didn't read your post properly. I've never used this technique so I'm not sure what peaks would normally be expected. I would still suggest trying a different method in case CI is not compatible for some reason - from the lit it would appear that EI is a good choice.

Re: DEPT. A proton-carbon correlation experiment would be the best thing, but if that's not an option maybe check the DEPT spectrum before the Qs are added in. I think the alkyne CH is just weak rather than absent in the DEPT.

Weak peaks in the DEPT for terminal alkynes is a known phenomenon, I found this:

"Leakage" can occur in DEPT-90 spectra because 1JC-H varies as a function of environment, and the technique assumes that all 1JC-H are identical. This can result in small peaks for CH2 and CH3 signals, which should have zero intensity. For similar reasons the C-H of terminal acetylenes (C≡C-H) will show anomalous intensities in DEPT spectra (either nulled or very small in DEPT-90, or present in DEPT-135) because the C-H coupling is much larger (around 250 Hz) than the normal value of 125 Hz for which the DEPT experiment is usually parameterized. Of course, leakage can also result from an incorrectly calibrated pulse width for the spectrometer.

Source Link

[fledarmus]

Yes, as Dan reported, terminal alkynes tend to show much more like quaternary carbons than tertiary carbons in the carbon spectra. This has to do with anomalous spin-spin relaxations. This is why quaternary carbons appear so much smaller than carbons with hydrogens on them in a typical carbon spectra, unless you put enough delay to allow the simple to totally relax between pulses. Alkyne carbons, even terminal alkynes, will also appear very small unless fully relaxed.

This is also one of the things that makes it very difficult to use integration in carbon spectra - the differences in relaxation time, especially for quaternary and alkyne carbons, lead to rather large variabilities in their responses.