Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, United States.
J Org Chem. 2011 Jan 21;76(2):380-90. doi: 10.1021/jo101369w. Epub 2010 Dec 17.
The induction of strain in carbocycles, thereby increasing the amount of s-character in the C-H bonds and the acidity of these protons, has been probed with regard to its effect on the rate constants for the enolization of cyclobutanone. The second-order rate constants for the general base-catalyzed enolization of cyclobutanone have been determined for a series of 3-substituted quinuclidine buffers in D(2)O at 25 °C, I = 1.0 M (KCl). The rate constants for enolization were determined by following the extent of deuterium incorporation (up to ∼30% of the first α-proton) into the α-position, as a function of time. The observed pseudo-first-order rate constants correlated to the [basic form] of the buffer and yielded the second-order rate constants for the general base-catalyzed enolization of cyclobutanone for four tertiary amine buffers. A Brønsted β-value of 0.59 was determined from the second-order rate constants determined. Comparison of the results for cyclobutanone to those previously reported for acetone and a 1-phenylacetone derivative, under similar conditions, indicated that the ring strain of the carbocycle appeared to have only a small effect on the general base-catalyzed rate constants for enolization. The similarity of the rate constants for the general base-catalyzed enolization of cyclobutanone to those determined for acetone allowed for an estimation of the limits of the rate constant for protonation of the enolate intermediate of cyclobutanone by the conjugate acid of 3-quinuclidinone (k(BH) = 5 × 10(8) - 2 × 10(9) M(-1) s(-1)). Combining the rate constants for deprotonation of cyclobutanone (k(B)) and protonation of the enolate of cyclobutanone (k(BH)) by 3-quinuclidinone and its conjugate acid, the pK(a) of the α-protons of cyclobutanone has been estimated to be pK(a) = 19.7-20.2.
已研究了在碳环中诱导应变，从而增加 C-H 键中的 s 特征和这些质子的酸度，以研究其对环丁酮烯醇化速率常数的影响。在 25°C、I = 1.0 M (KCl) 的 D(2)O 中，已确定了一系列 3-取代的奎宁啶缓冲剂对环丁酮的一般碱催化烯醇化的二级速率常数。通过跟踪氘掺入α-位的程度（最多约为第一个α-质子的 30%），作为时间的函数来确定烯醇化的表观一级速率常数。观察到的假一级速率常数与缓冲剂的[碱形式]相关，并为四个叔胺缓冲剂对环丁酮的一般碱催化烯醇化确定了二级速率常数。从确定的二级速率常数确定了 Brønsted β 值为 0.59。将环丁酮的结果与先前在类似条件下报道的丙酮和 1-苯乙酮衍生物的结果进行比较表明，碳环的环应变似乎对烯醇化的一般碱催化速率常数只有很小的影响。环丁酮的一般碱催化烯醇化的速率常数与丙酮的速率常数相似，允许估计环丁酮烯醇化物中间物被 3-奎宁啶的共轭酸质子化的速率常数的极限（k(BH) = 5×10(8) - 2×10(9) M(-1) s(-1)）。将环丁酮的脱质子化速率常数（k(B)）和环丁酮烯醇化物由 3-奎宁啶及其共轭酸质子化的速率常数（k(BH)）组合在一起，估计了环丁酮的α-质子的 pK(a)为 pK(a) = 19.7-20.2。
