Jiang Annie J, Simpson Jeffrey H, Müller Peter, Schrock Richard R
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Am Chem Soc. 2009 Jun 10;131(22):7770-80. doi: 10.1021/ja9012694.
Two diastereomers of the monoalkoxidepyrrolide (MAP) species, W(NAr)(CH(2))(Me(2)Pyr)(OR*) (1; Ar = 2,6-diisopropylphenyl, Me(2)Pyr = 2,5-dimethylpyrrolide, OR* = (R)-3,3'-dibromo-2'-(tert-butyldimethylsilyloxy)-5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphthyl-2-olate), were generated through addition of ROH to W(NAr)(CH(2))(Me(2)Pyr)(2). The unsubstituted tungstacyclobutane species, W(NAr)(C(3)H(6))(Me(2)Pyr)(OR) (2), was isolated by treating the mixture of diastereomers of 1 with ethylene. An X-ray study revealed 2 to have a trigonal bipyramidal structure in which the imido and phenoxide ligands are in axial positions. A variety of NMR experiments were carried out on 1 and 2. The major findings are the following: (i) the methylidene ligands in the two diastereomers of 1 rotate readily about the W horizontal lineC bond (k = 2-7 s(-1) at 22 degrees C); (ii) NMR studies are consistent with 2 breaking up to give an intermediate alkylidene/ethylene complex, (R)- and (S)-W(CH(2))(C(2)H(4)); and (iii) the ethylene in the (R)-W(CH(2))(C(2)H(4)) intermediate can rotate about the W-ethylene bond axis at approximately the same rate as 2 re-forms or ethylene is lost to give 1. Compound 1 reacts with trimethylphosphine to yield (R)-1(PMe(3)). Two intermediate PMe(3) adducts were observed and found to convert to (R)-1(PMe(3)) in an intramolecular fashion with an average rate constant at 5 degrees C of approximately 1.4 x 10(-4) s(-1). Both neophylidene (4) and methylidene (5) MAP species containing 2,3,5,6-tetraphenylphenoxide ligand also were prepared. Compound 5 can be heated to 80 degrees C, where methylidene rotation about the W=C bond is facile and observable in a variable-temperature (1)H NMR spectrum. A (1)H-(1)H EXSY spectrum of 5 in benzene-d(6) at 20 degrees C showed that the methylidene protons are exchanging with k = 90 s(-1). A structure of 5(THF) showed it to be a square pyramid with the methylidene ligand in the apical position and THF coordinated trans to the imido ligand. Exposure of 5 to ethylene generated the tungstacyclobutane complex, W(NAr)(C(3)H(6))(Me(2)Pyr)(OR) (6), whose structure is analogous to that of 2. Treatment of 5 with PMe(3) yielded yellow 5(PMe(3)), an X-ray study of which revealed it to be a square pyramid with the methylidene ligand in the apical position and the phosphine trans to the pyrrolide. These studies suggest that metallacyclobutane intermediates in metathesis reactions with MAP species are likely to contain axial imido and phenoxide ligands, that metallacycles are formed when an olefin approaches the metal in a MAP species trans to the pyrrolide, and that the configuration at the metal inverts as a consequence of each forward metathesis step.
通过将ROH添加到W(NAr)(CH₂)(Me₂Pyr)₂中，生成了单醇盐吡咯化物（MAP）物种W(NAr)(CH₂)(Me₂Pyr)(OR)（1；Ar = 2,6 - 二异丙基苯基，Me₂Pyr = 2,5 - 二甲基吡咯化物，OR* = (R)-3,3'-二溴 - 2'-(叔丁基二甲基硅氧基)-5,5',6,6',7,7',8,8'-八氢 - 1,1'-联萘 - 2 - 醇盐）的两种非对映异构体。通过用乙烯处理1的非对映异构体混合物，分离出未取代的钨环丁烷物种W(NAr)(C₃H₆)(Me₂Pyr)(OR*)（2）。X射线研究表明2具有三角双锥结构，其中亚氨基和酚盐配体处于轴向位置。对1和2进行了各种核磁共振实验。主要发现如下：（i）1的两种非对映异构体中的亚甲基配体很容易围绕W—C键旋转（在22℃时k = 2 - 7 s⁻¹）；（ii）核磁共振研究表明2分解生成中间体亚烷基/乙烯配合物(R)-和(S)-W(CH₂)(C₂H₄)；（iii）(R)-W(CH₂)(C₂H₄)中间体中的乙烯可以以与2重新形成或乙烯失去生成1大致相同的速率围绕W—乙烯键轴旋转。化合物1与三甲基膦反应生成(R)-1(PMe₃)。观察到两个中间体PMe₃加合物，并发现它们以分子内方式转化为(R)-1(PMe₃)，在5℃时的平均速率常数约为1.4×10⁻⁴ s⁻¹。还制备了含有2,3,5,6 - 四苯基酚盐配体的新亚基（4）和亚甲基（5）MAP物种。化合物5可以加热到80℃，在该温度下亚甲基围绕W = C键的旋转很容易，并且在变温¹H NMR光谱中可以观察到。在20℃下苯 - d₆中5的¹H - ¹H EXSY光谱表明亚甲基质子以k = 90 s⁻¹的速率交换。5(THF)的结构表明它是一个四方锥，亚甲基配体处于顶端位置，THF与亚氨基配体反位配位。将5暴露于乙烯中生成钨环丁烷配合物W(NAr)(C₃H₆)(Me₂Pyr)(OR)（6），其结构与2类似。用PMe₃处理5生成黄色的5(PMe₃)，对其进行的X射线研究表明它是一个四方锥，亚甲基配体处于顶端位置，膦与吡咯化物反位。这些研究表明，与MAP物种发生复分解反应中的金属环丁烷中间体可能含有轴向亚氨基和酚盐配体，当烯烃以与吡咯化物反位的方式接近MAP物种中的金属时会形成金属环，并且由于每个正向复分解步骤，金属处的构型会反转。
