Gámez José A, Guillemin Jean-Claude, Mó Otilia, Yáñez Manuel
Departamento de Química, C-9, Universidad Autónoma de Madrid, Cantoblanco, 28049-Madrid, Spain.
Chemistry. 2008;14(7):2201-8. doi: 10.1002/chem.200701254.
The effect that unsaturation has on the intrinsic acidity of boranes, alanes, and gallanes, was analyzed by B3 LYP and CCSD(T)/6-311+G(3df,2p) calculations on methyl-, ethyl-, vinyl-, and ethynylboranes, -alanes and -gallanes, and on the corresponding hydrides XH3. Quite unexpectedly, methylborane, which behaves as a carbon acid, is predicted to have an intrinsic acidity almost 200 kJ mol(-1) stronger than BH3, reflecting the large reinforcement of the C--B bond, which upon deprotonation becomes a double bond through the donation of the lone pair created on the carbon atom into the empty p orbital of the boron. Also unexpectedly, and for the same reason, the saturated and alpha,beta-unsaturated boranes are much stronger acids than the corresponding hydrocarbons, in spite of being carbon acids as well. The Al derivatives also behave as carbon acids, but in this case the most favorable deprotonation process occurs at C beta, leading to the formation of rather stable three-membered rings, again through the donation of the C beta lone pair into the empty p orbital of Al. For Ga-containing compounds the deprotonation of the GaH2 group is the most favorable process. Therefore only Ga derivatives behave similarly to the analogues of Groups 14, 15, and 16 of the periodic table, and the saturated derivatives exhibit a weaker acidity than the unsaturated ones. Within Group 13, boranes are stronger acids than alanes and gallanes. For ethyl and vinyl derivatives, alanes are stronger acids than gallanes. We have shown, for the first time, that acidity enhancement for primary heterocompounds is not only dictated by the position of the heteroatom in the periodic table and the nature of the substituent, but also by the bonding rearrangements triggered by the deprotonation of the neutral acid.
通过在甲基硼烷、乙基硼烷、乙烯基硼烷、乙炔基硼烷、铝烷和镓烷以及相应的氢化物XH₃上进行B3 LYP和CCSD(T)/6 - 311 + G(3df,2p)计算，分析了不饱和键对硼烷、铝烷和镓烷固有酸度的影响。非常出乎意料的是，表现为碳酸的甲基硼烷预计其固有酸度比BH₃强近200 kJ·mol⁻¹，这反映了C - B键的大幅增强，去质子化时，通过碳原子上产生的孤对电子向硼的空p轨道的供体作用，C - B键变成了双键。同样出乎意料的是，出于相同原因，尽管饱和和α,β - 不饱和硼烷也是碳酸，但它们比相应的烃类酸性强得多。铝衍生物也表现为碳酸，但在这种情况下，最有利的去质子化过程发生在Cβ位，同样通过Cβ孤对电子向铝的空p轨道的供体作用，导致形成相当稳定的三元环。对于含镓化合物，GaH₂基团的去质子化是最有利的过程。因此，只有镓衍生物的行为类似于元素周期表第14、15和16族的类似物，并且饱和衍生物的酸度比不饱和衍生物弱。在第13族中，硼烷比铝烷和镓烷酸性更强。对于乙基和乙烯基衍生物，铝烷比镓烷酸性更强。我们首次表明，一级杂化合物的酸度增强不仅取决于杂原子在元素周期表中的位置和取代基的性质，还取决于中性酸去质子化引发的键重排。
