Aldehydes protonation

Figure Bl.16.12. Experimental kineties of the CIDNP net eflfeet (A) for the aldehyde proton of the produets II and in of primary biradieal ( ) for the CH CHCOH) protons of the produets IV, V, and VI of seeondary...

Figure B2.4.3 shows an example of this in the aldehyde proton spectnim of N-labelled fonnamide. Some lines in the spectnim remain sharp, while others broaden and coalesce. There is no frmdamental difference between the lineshapes in figures B2.4.1 and figures B2.4.3—only a difference in the size of the matrices involved. First, the uncoupled case will be discussed, then the extension to coupled spin systems.

Figure B2.4.3. Proton NMR spectrum of the aldehyde proton in N-labelled fonnainide. This proton has couplings of 1.76 Hz and 13.55 Hz to the two amino protons, and a couplmg of 15.0 Hz to the nucleus. The outer lines in die spectrum remain sharp, since they represent the sum of the couplings, which is unaffected by the exchange. The iimer lines of the multiplet broaden and coalesce, as in figure B2.4.1. The other peaks in the 303 K spectrum are due to the NH2 protons, whose chemical shifts are even more temperature dependent than that of the aldehyde proton.

Carbonyl stretching frequency. Aldehyde proton, relative to TMS. Carbonyl carbon, relative to TMS. 

The alternative tran5-p-phenylcinnamic aldehyde F would display an additional coupling between the aldehyde proton and the vicinal alkene proton of the double bond whieh is not observed in speetmm 8 (but in speetmm 4 for eomparison). 

First the trans configuration of the C-2-C-3 double bond is derived from the large coupling constant Jhh = 15 Hz) of the protons at % = 5.10 and 7.11, whereby the middle CH proton (Sh = 5.10) appears as a doublet of doublets on account of the additional coupling ([Pg.189]

A cyanide anion as a nucleophile adds to an aldehyde molecule 1, leading to the anionic species 3. The acidity of the aldehydic proton is increased by the adjacent cyano group therefore the tautomeric carbanion species 4 can be formed and then add to another aldehyde molecule. In subsequent steps the product molecule becomes stabilized through loss of the cyanide ion, thus yielding the benzoin 2 ... 

The aldehyde proton signal at Cl (red) appears in the normal downfield position at 9.69 5 and is split into a doublet with / = 6 Hz by the adjacent proton at C2. 

Figure 19.18 1H NMR spectrum of acetaldehyde. The absorption of the aldehyde proton appears at 9.8 8 and is split into a quartet.

The ]H NMR spectrum shown is that of a compound isomeric with the one in Problem 19.65. This isomer has an IR absorption at 1730 cm-1. Propose a structure. [Note-. Aldehyde protons (CHO) often show low coupling constants to adjacent hydrogens, so the splitting of aldehyde signals is not always apparent.]... 

The vanillin ethers 36 and 39 exhibited the IR band of the lactone carbonyl group at 1710-1720 cm whereas the aldehydic carbonyl stretching was observed in the range of 1680-1690 cm In the NMR spectra all the protons resonated at expected fields. The aldehydic proton appeared downfield around 9-10 aromatic protons in the range of 7-8 and the C3 - H of coumarin around 6.5. The methylene, methoxy, and methyl protons resonated around 5, 3.8, and 2.2, S respectively. 

Compound 37a showed the absence of an aldehydic proton and the singlet around 8.15 ppm was assigned to the ethylenic proton located p with respect to the electron-withdrawing cyano and ester groups. The benzofuranyl coumarins 38 exhibited the carbonyl-stretching band around 1690 cm in the IR spectra (Table 6). PMR data for 13 compounds are given in Table 2. The El mass spectrum of 36a showed a molecular ion peak at m/z 324 (41%). 

So what about aromatic protons (<56.0-9.5) aldehyde protons (<59.5—9.6), or even protons oh double, nay triple bonds (<52.5-3.1) All these protons are attached to carbons with n bonds, double or triple bonds, or aromatic systems. The electrons in these n bonds generate their own little local magnetic field. This local field is not spherically symmetric — it can shield or deshield protons depending on where the protons are — it s anisotropic. In Fig. 137, the shielding regions have plusses on them, and deshielding regions have minuses. 

Diastereomerically pure iridium complexes of the formula [(ri -C5Me5)lr (/ )-Pro-phos (activated alkene)](SbF6)2 (activated alkene = enal, methacrylonitrile) are active, and selective catalysts for the DCR between one point binding activated alkenes and nitrones. Enals coordinate to the metal in a completely diastereoselec-tive way with a restricted geometry. From the point of view of the selectivity, a key point in enal coordination is the establishment of CH/n-attractive interactions between the CHO aldehyde proton and one (f )-Prophos phenyl group. This interaction fixes the methacrolein rotamer around the M-O bonds and renders the system enantioselective. 

Clausine R (34) was isolated from the acetone extract of the root bark of C. excavata (43). The UV spectrum (2max 241, 282, and 320 nm) resembled that of mukoeic acid (10) (see Scheme 2.4), which indicated a 1-oxygenated 3-carboxycarba-zole framework. The H-NMR spectrum is similar to that of clausine Q (19) (see Scheme 2.5), except for the presence of a carbomethoxy signal at 5 3.86 instead of the aldehyde proton at 10.01 as in clausine Q. The presence of a carbomethoxy group... 

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