Boronate inductive effect

Both sets of results may also be discussed in terms of inductive differences between hydrogen and deuterium (see Halevi, 1963). Brown et al. (1966) jDoint out that both the inductive and steric explanations qualitatively predict isotope effects in the same direction, but that an inductive effect would be expected to operate from the 3 and 4 positions nearly as effectively as from the 2 position . Furthermore, there is no observable isotope effect on the heat of reaction of 2,6-(dimethyl-de)-pyridine with the relatively small molecule diborane A AH = —20 18 cal mol ), but a significant effect is obtained with the larger molecule boron trifluoride AAH = 230 + 150 cal mol ). 

Concerning the inherent donor character of the ligand, this point simply considers the possibility that while the inductive effect might favor unsymmetrical cleavage, the base L could be a weak donor to boron compared to hydride. Thus formation of the symmetrical cleavage product could occur after initial unsymmetrical cleavage of the bridge system. 

It was correctly assumed and later determined unequivocally (64) that in each of these two reactions the BH vertex that was removed was always one of the two equivalent vertices found as the nearest neighbors of the two equivalent CH vertices. The fact that the carbon atoms in polyhedral surfaces of C2Bb 2H carboranes are electronic counterparts of boron atoms in the corresponding isoelectronic B H 2 polyhedral ions (62) requires these carbon atoms to resemble C+, a species present in carbonium ions. Consequently, the BH vertices which are nearest neighbors of two such carbon atoms will be activated for nucleophilic attack by base through the advent of a strong inductive effect. 

For the quantitative evaluation of the inductive effect of R3 and R4, the pKBH+ values of the conjugate acid of the enaminone, or the enthalpy of complex formation, AHBFi, of boron trifluoride with the carbonyl oxygen for different NR3R4 groups were proposed. The variation of <5H(2) was considered to be a measure of the steric interaction. The following correlations were found ... 

The increase of dipole moments in the order 1 4 < 3 < 2 gives evidence that in this series, the halogen atoms bound to the 12-boron vertices draw eletrons by their strong inductive effects from the positive center which is located on the main symmetry axis crossing the S(l) and B(12) vertices. A distinct increase of the dipole moments in the above series unambiguously confirms that the positive center must be the sulfur atom S(l),... 

The determination of the crystal structures of a series of silyl(amino)boranes shows that the B-Si bond is significantly shortened by the introduction of chlorine substituent at boron and is even Shorter in the presence of a Q atom at the silyl group. This definitely shows that the Cl atoms have a noticeable inductive effect on the length (and strength) of the B-Si single bond. 

Chlorine and bromine with inductive electron withdrawal ( —I) > mesomeric electron donation (-I- M) direct boron to the a position, the degree of selectivity varying with structure " . The direction of addition to allylic derivatives is influenced by steric and inductive effects of the substituent . Electron-withdrawing groups direct the boron... 

The second synthesis of fascaplysin (44), from labs at the CNRS, is also very high in overall yield (157). Palladium-catalyzed cross-coupling of the boronic ester 189 with the halogenated pyridine 190 lead to intermediate 191 (Scheme 20). Metalation of 191 with n-BuLi was regioselective, due in part to the inductive effects of nearby electronegative groups. The double cyclization of 192 to fascaplysin provides the natural product in 76% overall yield. 

The slower exchange of secondary alkyl groups compared to primary groups is due mostly to steric factors, although inductive effects may also play a role. As the number of atoms having free electron pairs increases, the tendency to form an electron-deficient bridge between two borons decreases. 

The electronic factor thus controls in the case of 9-BBN, which has the relative openness of the boron atom. The inductive effect of the methyl group increases the availability of electron of the double bond. The transition state involves the development of electron deficiency at C-2, with partial hydridic character at the >B-H moiety. 

Due to electronegativity differences (B = 2.05, C = 2.55) and notwithstanding the electronic deficiency of boron, which is mitigated by the two electron-donating oxygen atoms (vide supra), the inductive effect of a boronate group should be that of a weak electron-donor. The NM R alpha effect of a boronate group is very small [27]. 

Inductive effects. The formation of a co-ordination compound between, for example, boron and nitrogen as represented by the equation... 

Al-Ammar,A., Gupta, R. K., and Barnes, R. M. (1999). Elimination of boron memory effect in inductively coupled plasma-mass spectrometry by addition of ammonia. Spectrochim. 

Effective 1,4-asymmetric induction has been observed in reactions between 2-(alkoxyethyl)-2-propenylsilanes and aldehydes. The relative configuration of the product depends on the Lewis acid used. Titanium(IV) chloride, in the presence of diethyl ether, gave 1,4-ijn-products with excellent stereoselectivity with boron trifluoride-diethyl ether complex, the amt-isomer was the major product, but the stereoselectivity was less83. 

Dienones, such as 4-[4-(trimethylsilyl)-2-butenyl]-3-vinyl-2-cyclohexenone, are useful precursors for these particular transformations the allylsilane side chain is too short for effective 1,4-addition, but just right for 1,6-addition, resulting in six-ring annulation. Three different Lewis acids can be used titanium(IV) chloride, boron trifluoride diethyl ether complex, and ethylaluminum dichloride. The best chemical yields and complete asymmetric inductions were obtained with ethylaluminum dichloride. 

Additional acylation studies were also reported (24), (26). In the first case it is claimed that acylation of thiophene is achieved by means of HC104 and acetic anhydride affording a 65 % yield of 2-acetylthiophene. In the second paper Levine and coworkers reported that while 2,5-dimethylthiophene could be readily acetylated, 2,5-dichlorothiophene acetylated sluggishly. This is, however, readily explained, since the presence of chlorine atoms on the thiophene ring decreased its reactivity in electrophilic substitution reactions. In the case of methyl substitution, however, the 3 and 4 positions of the ring are activated toward electrophilic substitution by the inductive and hyperconjugative effects. Thus 2,5-dimethylthiophene was successfully acylated by the boron fluoride etherate method in high yield with three aliphatic anhydrides. 

Recently, the improved chiral ethyl ketone (5)-141, derived in three steps from (5)-mandelic acid, has been evaluated in the aldol process (115). Representative condensations of the derived (Z)-boron enolates (5)-142 with aldehydes are summarized in Table 34b, It is evident from the data that the nature of the boron ligand L plays a significant role in enolate diastereoface selection in this system. It is also noteworthy that the sense of asymmetric induction noted for the boron enolate (5)-142 is opposite to that observed for the lithium enolate (5)-139a and (5>139b derived from (S)-atrolactic acid (3) and the related lithium enolate 139. A detailed interpretation of these observations in terms of transition state steric effects (cf. Scheme 20) and chelation phenomena appears to be premature at this time. Further applications of (S )- 41 and (/ )-141 as chiral propionate enolate synthons for the aldol process have appeared in a 6-deoxyerythronolide B synthesis recently disclosed by Masamune (115b). 

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