碳正离子重排反应合成Cyclocitrinol核心骨架

Cyclocitrinol是一类具有独特的桥环结构的天然产物,其桥环核心骨架可由高张力的降蒈烯酮开环制备得到.以商品化的19-羟基雄甾为原料,通过高烯丙基碳正离子重排反应,高效地合成了降蒈烯酮关键中间体,完成了Cyclocitrinols天然产物的核心骨架的合成.     

环式结构体系中的双碳正离子

在某些环式结构体系中,由于某些轨道的相互作用使正电荷离域,从而克服正电荷间的相互排斥形成长寿命的双碳正离子。本文介绍苯双碳正离子,双环[m.n.p]桥头双碳正离子、去氢金刚烷基双碳正离子及宝塔烷双碳正离子的生成、检测、性能及理论处理。     

五价碳原子

氯代降冰片烷(图a)在解离时生成正碳离子。对这种现象的解释,有两种相反的观点。诺贝尔奖金获得者美国普度大学的H.C.Brown是经典理论的支持者,认为氯代降冰片烷解离时产生两种正碳离子,这两种结构之间的相互转换,是通过碳-碳键像汽车挡风玻璃雨刷那样来回转动而实现的。而每一种结构又都符合碳原子价键的经典规则(图b)。     

TiCl_4共引发异丁烯正离子聚合合成反应活性聚异丁烯

由H2O/TiCl4/甲醇或乙醚体系引发异丁烯在二氯甲烷与己烷混合溶剂中进行正离子聚合,探讨甲醇用量、聚合时间等因素对正离子聚合以及产物分子量、分子量分布和末端基结构的影响,并在此基础上探讨TiCl4共引发混合C4馏分中异丁烯选择性正离子聚合以制备活性聚异丁烯的可行性.结果表明,含氧试剂对聚合反应起到明显的调节作用,可适当稳定碳正离子活性中心,降低链增长速率,降低聚合产物的分子量(Mn=1600~4600),使分子量分布明显变窄(Mw/Mn=1.35~2.05),并可调节大分子链末端基结构及其含量.降低聚合体系中微量单体浓度以及适当延长聚合反应时间,均有利于提高聚异丁烯大分子链末端α-双键结构含量.通过TiCl4共引发异丁烯正离子聚合制备出末端α-双键含量可以达到70%以上的低分子量高反应活性聚异丁烯.此外,该引发体系还可引发混合C4馏分原料中异丁烯进行高选择性正离子聚合,得到Mn=2000、Mw/Mn=2.59、端基α-双键含量为38.9%的聚异丁烯.     

亲核试剂在正离子聚合反应中的作用及其作用机理的研究

研究了亲核试剂对IB 1 ,3 二 (2 氯异丙基 )苯 (DiCumCl) TiCl4体系正离子聚合反应的规律 ,亲核试剂与增长的碳正离子作用的紫外光谱图 ;采用分子模拟 ,利用量子力学半经验方法计算了亲核试剂与质子反应的活化能 ,确定了不同亲核试剂与质子的作用能力 ;利用分子力学方法计算了在增长碳正离子末端的不同位置亲核试剂与之络合的稳定化能 ,根据能量最低原则确定了亲核试剂在碳正离子链端的作用位置 ;利用分子力学方法进行构象优化后 ,借助QEq方法得到了聚异丁烯碳正离子和聚苯乙烯碳正离子的电荷数值 .综合以上实验结果提出了亲核试剂在正离子聚合反应中的作用及其作用机理     

碳正离子相对稳定性的定量判断

利用某些碳正离子稳定性的活性指标,将碳正离子相对稳定性的定性解释提高到定量表述的水平.尤其对某些碳正离子用电性效应难以解释清楚的问题,采用文中介绍的几种活性指标都能得到较正确的结果.     

欧拉教授与碳正离子化学──1994年诺贝尔化学奖简介

本文简单地介绍了人们早期对碳正离子活性中间体的原始概念以及欧拉教授获奖的主要工作。这些工作包括在超强酸的介质中用ＮＭＲ等方法观察到长寿命碳正离子的存在和他对碳正离子的系统新概念。此外，本文还简单地介绍了欧拉教授的生平。     

离子基碳-氧键断裂能量的研究(Ⅱ)──负离子基的键能

合成了3个系列芳香类含碳一氧键的化合物,用循环伏安法测定了相关的氧化还原电势,通过热力学循环首次估测了负离子基的碳-氧键的断裂能量(ΔH_cleav[C-O]^·-).结果表明:得到1个电子形成负离子基可以活化碳-氧键。这与具有相同结构的正离子基的情况是一致的,但其活化程度略小。相关分析表明:ΔH_cleav[C-O]^·-vs(ΔE_red[C-O]+ΔpK_a[HA])和ΔH_cleav[C-O]^·+vs.(ΔBDE-ΔE_ox[C-O])均呈线性。讨论了负离子基和正离子基及其母体化合物中键的特征,并与文献中有关的键能数据进行了比较。     

环己烯／HY体系中环烯碳正离子的形成及其转化

采用紫外漫反射光谱和红外光谱研究丁环已烯/HY体系中环烯碳正离子的形成及其进一步转化,两种方法结果相当吻合且互相充实。环己烯在室温吸附后,在UV-VIS、IR谱图中分别于350 nm、1530 cm~(-1)处产生与环烯碳正离子相关的吸收谱带。沸石酸性是出现这一结果的决定因素。在低温下(≤150℃)环烯碳正离子可转化成苯,从UV-VIS谱图尚看到链状多烯碳正离子存在,在温度高于150℃时,可观察到明显的催化剂结炭,所形成焦炭具有多支链多环芳烃结构性质。     

N—乙基吩噻嗪及其自由基正离子的结构和电子光谱的理论研究

用ＩＮＤＯ系列方法对Ｎ－乙基吩噻嗪及其自由基正离子进行了几何构型优化，得到中性分子为蝶状折叠形，自由基正离子为平面构型，以优化构型为基础计算其电荷密度，自旋密度键序和电子光谱。对光谱进行理论指认并讨论了从中性分子到离子谱带红移的原因，理论计算结果法与实验结果一致。     

Viability of Nonclassical Carbocations Proposed as Intermediates in the Biosynthesis of Atiserene,Beyerene, Kaurene,and Trachylobane Diterpenes

The results of quantum‐chemical calculations aimed at assessing the viability of nonclassical carbocations proposed to be involved in the biosynthesis of atiserne, beyerene, kaurene, and trachylobane diterpenes are presented. While the proposed edge‐protonated structure is much lower in energy than the proposed face‐protonated structure, neither is predicted to be a viable intermediate energetically.     

Nature of the 2-bicyclo[3.2.1]octanyl and 2-bicyclo[3.2.2]nonanyl cations.

Density functional and ab initio calculations have been employed to explore the nature of the cations formed in the solvolysis of 2-bicyclo[3.2.1]octanyl tosylate the 2-bicyclo[3.2.2]nonanyl tosylate. In contrast to recent conclusions in the literature, the various proposed classical carbocations postulated to explain the products of the 2-bicyclo[3.2.2]nonanyl tosylate solvolysis were found to have nonclassical structures.     

Multicenter bonding in carbocations with tetracoordinate protons

The nature of the bonding in a set of nonclassical carbocations with tetracoordinated protons "sandwiched" between two C=C pi bonds was scrutinized using the new computational methodology of generalized population analysis. The results of this theoretical analysis strongly corroborate the conclusions of a previous study in which the occurrence of delocalized 5-center 4-electron (5c-4e) bonding in the C-C-H-C-C fragments of such cations was anticipated.     

Ab initio/GIAO-CCSD(T) study of structures, energies, and 13C NMR chemical shifts of C4H7(+) and C5H9(+) ions: relative stability and dynamic aspects of the cyclopropylcarbinyl vs bicyclobutonium ions

The structures and energies of the carbocations C 4H 7 (+) and C 5H 9 (+) were calculated using the ab initio method. The (13)C NMR chemical shifts of the carbocations were calculated using the GIAO-CCSD(T) method. The pisigma-delocalized bisected cyclopropylcarbinyl cation, 1 and nonclassical bicyclobutonium ion, 2 were found to be the minima for C 4H 7 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level the structure 2 is 0.4 kcal/mol more stable than the structure 1. The (13)C NMR chemical shifts of 1 and 2 were calculated by the GIAO-CCSD(T) method. Based on relative energies and (13)C NMR chemical shift calculations, an equilibrium involving the 1 and 2 in superacid solutions is most likely responsible for the experimentally observed (13)C NMR chemical shifts, with the latter as the predominant equilibrating species. The alpha-methylcyclopropylcarbinyl cation, 4, and nonclassical bicyclobutonium ion, 5, were found to be the minima for C 5H 9 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level ion 5 is 5.9 kcal/mol more stable than the structure 4. The calculated (13)C NMR chemical shifts of 5 agree rather well with the experimental values of C 5H 9 (+).     

Structural studies of nonclassical cyclobutylmethyl cations by the ab initio method

Ab initio calculations at the MP2/cc-pVTZ level show that the cyclobutylmethyl cation is a nonclassical sigma-delocalized species, which is distinct from the global minimum C2-symmetric cyclopentyl cation (Schleyer, P. v. R.; Carneiro, J. W. de M.; Koch, W.; Raghavachari, K. J. Am. Chem. Soc. 1989, 111, 5475). Relatively lower level DFT calculations, on the other hand, show that the primary cyclobutylmethyl cation spontaneously collapses into the cyclopentyl cation (Prakash, G. K. S.; Reddy, V. P.; Rasul, G.; Casanova, J.; Olah, G. A. J. Am. Chem. Soc. 1998, 120, 13362). The secondary 1-cyclobutylethyl cation is also a nonclassical carbocation, as shown by calculations at the MP2/cc-pVTZ level. Two structures having energy minima are identified for the latter cation on the potential energy surface. The conformer in which the methyl group is in the exo orientation is a global minimum and is favored over the corresponding endo conformer by 1.2 kcal/mol at the MP2/cc-pVTZ//MP2/cc-pVTZ +ZPE level of calculations. The tertiary 1-cyclobutyl-1-methylethyl cation, at this level of calculations, also involves substantial nonclassical sigma-delocalization, showing that the nonclassical stabilization is more important for cyclobutylmethyl cations relative to the cyclopropylmethyl cations. The 13C NMR chemical shifts obtained from GIAO-CCSD(T)/tzp/dz calculations further substantiate the nonclassical structures for these carbocations.     

Synthesis and Transformation of 5- and 2-Methyl- bicyclo[2.2.1]hept-2-enes and 2-Methylenebicyclo[2.2.1]heptane with Retrospective δ-Migration of Hydrogen over Oxide Catalyst

5-, 2-Methylbicyclo[2.2.1]hept-2-enes, 2-methylenebicyclo[2.2.1]heptane were synthesizedby a new procedure, and their transformations in a flow system over stationary layer of aluminosilicatecatalyst were studied at various temperatures. At 150-400°C these compounds undergo isomerization which is accompanied by -migration of hydrogen. The isomerization mechanism is discussed in terms of formation of classical carbocations, not invoking nonclassical carbenium ion.     

Cyclobutenes by platinum-catalyzed cycloisomerization reactions of enynes

1,6-Enynes bearing (electron-rich) aryl substituents on their alkyne moiety rearrange to cyclobutene derivatives in the presence of catalytic amounts of PtCl2 in toluene. The reaction is significantly accelerated when performed under an atmosphere of CO (1 atm), most likely by increasing the electrophilicity of the metal template by temporary coordination to this pi-acidic ligand. This transformation allows the build up of considerable strain in the products as witnessed by the productive formation of tricyclic skeletons, such as 7 or 9. Moreover, these products provide evidence for the mechanistic scenario of platinum-catalyzed cycloisomerization reactions previously proposed, which are thought to proceed via organo-platinum species that mimic the reactivity of metal-complexed "nonclassical" carbocations.     

Carbocations and Electrophilic Reactions

The general concept of carbocations encompasses all cations of carbon containing compounds which can be differentiated into two distinct classes: trivalent (“classical”) carbenium ions and pentacoordinated (“nonclassical”) carbonium ions. In this paper the preparation and structural study (by NMR, IR, Raman and ESCA spectroscopy) of stable carbenium ions and carbonium ions are discussed. As is well known, trivalent carbenium ions play an important role in electrophilic reactions of π- and n-donor systems. Similarly, pentacoordinated carbonium ions are the key to electrophilic reactions of σ-donor systems (single bonds). The ability of single bonds to act as σ-donor lies in their ability to form carbonium ions via triangular two electron, three-center bond formation. Consequently there seems to be no principle difference between the electrophilic reactions of π- and σ-bonds except that the former react more easily, even with weak electrophiles, whereas the latter necessitate more severe conditions. The role of carbocations in electrophilic reactions of π- and σ-donor systems is discussed.     

非经典碳正离子问题

本文论述了非经典碳正离子这一问题的有关争论,是根据近期评论译述的。争论的中心是,全去甲莰正离子是在C—C或C—H键电子的σ-参与下形成非经典的离子,还是如Brown所指出的那样,是一个定域的经典离子,但受到立体的张力效应,从而加速了它在溶剂解中的形成。     

Anab initio quantum-chemical study of proton addition to F-, Ch3-, and CF3-substituted ethylene derivatives

Protonated forms of the molecules of ethylene derivatives with the general formula C₂X₂Y₂ (X=Y=H) (1), F (2), CH₃ (3) CH₃ (4); X=F, Y=H:cis-(5)trans- (6)) were calculated by theab initio MP2/6-31 G^* method with full geometry optimization. The minima and saddle points located on the potential energy surface (PES) of the protonated ethylene molecule correspond to the stationary states and transition states of proton migration, respectively. The stationary states are characterized by a nonclassical geometry of carbocations similar to that of π-complexes, whereas the transition states have a classical structure. Unlike1, the carbocations of molecules2–6 have the classical structure. The saddle points on the PES of the ethylene derivatives correspond to the structures of the π-complex type, which are the transition states of proton migration between the C atoms of the ethylene bond. The barrier to rotation about, the C−C bond depends essentially on the substituent nature. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1333–1337, August, 2000.