Methine shielding

Table 13.1 collects chemical-shift infonnation for protons of various types. The beginning and major portion of the table concerns protons bonded to car bon. Within each type, methyl (CH3) protons are more shielded than methylene (CH2) protons, and methylene protons are more shielded than methine (CH) protons. These differences are small as the following two exanples illustrate. 

FIGURE 13.13 The magnetic moments (blue arrows) of the two possible spin states of the methine proton affect the chemical shift of the methyl protons in 1,1-dichloroethane. When the magnetic moment is parallel to the external field if.o (green arrow), it adds to the external field and a smaller 3 0 is needed for resonance. When it is antiparallel to the external field, it subtracts from it and shields the methyl protons. 

Spin of methine proton shields methyl protons from Mq. Methyl signal appears at higher field. 

Christl and Lang (390) noticed an upheld signal shift of about 10 ppm when they compared the methine carbon atoms in bicyclo[1.1.0]butane (287) (391) and octavalene (288). On the other hand, in benzvalene (289), the corresponding shielding is +48.3 (203), because in this molecule back-donation from the Walsh orbitals to the ir -orbital of the olefinic group (290) is conceivable. Such an interaction is not possible in 288, because the ir -orbital of the diene chromophore is of different symmetry (390). 

Ru < Os throughout. (The porphodimethene spectra are different and cannot yet be interpreted in the sense given here (55).) Concomitant with this hypsochromic shift, a bathochromic shift of the CO-stretching frequency and an increase in the shielding of the porphyrin methine protons indicate enhanced 7r-bonding, thus corroborating the w-bonding model (Sect. 4.2). 

Examination of the C-NMR spectra of roseadine (23) (Table XI) through comparison with vindoline (3) and leurosine (11) permitted the assignment of all carbons of the dihydroindole unit. The carbons of the indole nucleus were assigned by comparison with vinblastine (1), and the presence of three deshielded carbons, a methine carbon at 8 142.9 and two quaternary carbons at 8 133.2 and 169.2, were observed. The latter was assigned to the methoxycarbonyl carbon, which is shielded somewhat from its characteristic chemical shift of 8 174 1 ppm in the vinblastine series by attachment of an olefinic unit. The other two deshielded carbons at 8 133.2 and 142.9 could be assigned as C-18 and C-17, respec-... 

The first series involves cc-methyl-/Miydroxycarbonyl compounds where C-3 (carbinol) and the methyl carbon arc always shielded in the syn- compared to the aw/i-diastcrcomcr the chemical shift difference A d being 1.1 to 5 ppm. The results are less clearcut for C-2 (methine). The results can be explained after an inspection of the conformations for the diastereomers. In the syw-form the a-methyl group is partly in the axial position leading to a smaller chemical shift due to a higher number of gauche interactions. This is not the case for the w//-form because anti-1 (p 330) is the dominating conformation. 

The H-NMR spectrum of the perhydro-3a,6a,9a-triazaphenalene 353 (R = H) shows absorption for the methine proton at 5 2.31, indicative of the trans-trans-trans conformation 354.299,300 The shielding of the methine proton is shown by comparison300 with the absorption (5 3.67) of the methine proton in 355.30 The, 3C-NMR spectrum of 353 (R = Me) at ambient temperatures shows a chemical shift of the methyl carbon at 5 — 6.6, consistent with the predominance of the trans-trans-trans conformation. It has been estimated that the equilibrium contains 14% of the ds-cis-trans conformation 356.302... 

The 13C chemical shifts of the parent compound were assigned by comparison with the spectrum of cinnamic acid (see Section 4.11.2) and from the knowledge that for benzenoid methine carbons introduction of an acyloxy function into the benzene ring causes small shifts of the meta carbons, and shifts the signals of the para carbons to higher field, while the ortho carbons become even more shielded (Fig. 5.16) [635]. 

Alkylation Alkylation of the phenylindanone 31 with catalyst 3a by the Merck group demonstrates the reward that can accompany a careful and systematic study of a particular phase-transfer reaction (Scheme 10.3) [5d,5f,9,36], The numerous reaction variables were optimized and the kinetics and mechanism of the reaction were studied in detail. It has been proposed that the chiral induction step involves an ion-pair in which the enolate anion fits on top of the catalyst and is positioned by electrostatic and hydrogen-bonding effects as well as 71—71 stacking interactions between the aromatic rings in the catalyst and the enolate. The electrophile then preferentially approaches the ion-pair from the top (front) face, because the catalyst effectively shields the bottom-face approach. A crystal structure of the catalyst as well as calculations of the catalyst-enolate complex support this interpretation [9a,91]. Alkylations of related active methine compounds, such as 33 to 34 (Scheme 10.3), have also appeared [10,11]. 

You might wonder if such an approach can be extended to methine groups. Unfortunately, in such cases the three substituent groups tend to interfere significantly with each other so that their (de)shielding effects are not simply additive. 

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