Preparation of Strained Axially Chiral (1,5)Naphthalenophanes by Photo-dehydro-Diels−Alder Reaction

The preparation of 10 (1,5)naphthalenophanes (10a−j) by photo-dehydro-Diels−Alder (PDDA) reaction is described. Owing to hindered rotation around the biaryl axis, compounds 10 are axially chiral and the separation of enantiomers by chiral HPLC was demonstrated in three cases (10a,b,e). The absolute configuration of the isolated enantiomers could be unambiguously determined by comparison of calculated and measured circular dichroism (CD) spectra. Furthermore, we analyzed ring strain phenomena of (1,5)naphthalenophanes 10. Depending on the length of the linker units, one can distinguish three classes of naphthalenophanes. Compounds 10a−c are highly strained (ESTR = 7−31 kcal/mol), and the strain is caused by small bond angles in the linker unit and deformation of the naphthalene moiety. Another type of strain is observed if the linker unit becomes relatively long (10g,h) originating from transannular interactions and is comparable with the well-known strain of medium sized rings. The naphthalenophanes 10d−f with a linker length of 10−14 atoms are only marginally strained. To clearly discriminate the different sources of strain, we defined two geometrical parameters (average central dihedral angle δC and naphthalene thickness DN) and demonstrated that they are well-suited to indicate naphthalene deformation of our naphthalenophanes 10 as well as of ten model naphthalenophanes (I−X) with different linker lengths and linking positions.

本研究报道了通过光脱氢-狄尔斯-阿尔德（photo-dehydro-Diels−Alder, PDDA）反应制备10种(1,5)萘环芳烷（(1,5)naphthalenophanes，10a−j）的方法。由于联芳轴周围存在旋转位阻，化合物10具有轴手性；针对其中3个化合物（10a、10b、10e），我们通过手性高效液相色谱（chiral HPLC）成功实现了其对映异构体的分离。分离得到的对映异构体的绝对构型，可通过比对计算与实测的圆二色性（circular dichroism, CD）光谱予以明确判定。此外，我们还对(1,5)萘环芳烷10的环张力现象进行了分析。依据连接单元的长度差异，可将此类萘环芳烷划分为三类：其一，化合物10a−c具有较高环张力（环张力能ESTR为7~31 kcal/mol），该张力源于连接单元内的小键角以及萘环片段的构象形变；其二，当连接单元长度相对较长时（对应化合物10g、10h），会出现另一类环张力，该张力由跨环相互作用引发，其强度与经典中环的环张力相当；其三，连接单元含10~14个原子的化合物10d−f仅具有极微弱的环张力。为清晰区分不同来源的环张力，我们定义了两个几何参数——平均中心二面角δ_C与萘环厚度D_N，并证实二者可有效表征本研究中的萘环芳烷10，以及另外10种具有不同连接长度与连接位点的模型萘环芳烷（I−X）的萘环形变情况。