DIASTEREOSELECTIVE HYDROFORMYLATION OF CHIRAL ALKENES IN ORGANIC SYNTHESIS

利用有机合成中烯基氢甲酰化反应的特点，考察了铑配合物催化的手性烯丙醇氢甲酰化反应。尽管在潜在的羟基引导的辅助下，产物中的氢化反应具有良好的非对映选择性，但氢甲酰化在主要的非对映异构体中表现出相反的选择性。因此，详细考察了以Rh(Acac)(CO)2为催化剂前体的非官能化底物4-叔丁基亚甲基环己烷(1)的氢甲酰化反应，考察了产物中反式(2)和顺式-4-叔丁基环己基乙醛(3)的非对映选择性与合成气体初始压力(H2-CO=1：1)的关系。结果表明，在较高的合成气压力(5-9.5兆帕)下，转化率是令人满意的，二向选择性2：3随初始压力的增加而均匀恶化，而在较低的合成气压力(2-0.5兆帕)下，非对映选择性2：3变得令人满意的高(高达92：8)，但以形成区域异构醛为代价。Cis-4-t-butyl-1-methylcyclohexane-carboxaldehyde(4)以及1的大量异构化为4-叔丁基-1-甲基环己烯(5)这些结果可以很好地解释为在较高的合成气体压力下底物1插入到[HRH(CO)_n]中的动力学控制的插入，从而导致较低的非对映选择性2：3，而在较低的压力下底物插入到[HRH(CO)_n]中的热力学控制步骤可以导致观察到的异构化为4或反向插入形成5，同时，[Rh(Acac)(CO)_2]-[Ru_3(CO)_2]-[Ru_3(CO)_1；作为助催化剂，可以抑制1的任何异构化，保持2：3的非对映选择性不变。因此，助催化剂系统彻底避免了上面观察到的两难境地。

Taking advantage of hydroformylation of alkenes in organic synthesis, the rhodium complex-catalyzed hydroformylation of chiral allylic alcohols was examined. Despite of related hydrogenation with excellent diastereoselectivity in the product based on a potential hydroxyl group-directed assistance, hydroformylation exhibited an inverse selectivity in the major diastereoisomer. Accordingly, hydroformylation of a non-functionalized substrate, 4-t-butylmethylenecyclohexane (1), using [Rh(acac)(CO)_2] as a catalyst precursor was examined in detail in terms of diastereoselectivity in the products, trans- (2) and cis-4-t-butyl-cyclohexylacetaldehyde (3), as a function of applied initial pressure of syn-gas (H_2-CO = 1 : 1). It was clearly demonstrated that the conversion to aldehydes was found to be satisfactory under a higher pressure of syn-gas (5 - 9.5 MPa), the distereoselectivity 2 : 3 being deteriorated uniformly as the initial pressure increased, while under a lower pressure of syn-gas (2 - 0.5 MPa), diastereoselecivity 2 : 3 became satisfactorily high (up to 92 : 8) at the expense of the formation of a regioisomeric aldehyde, cis-4-t-butyl-1-methylcyclohexane-carboxaldehyde (4) as well as much isomerization of 1 into 4-t-butyl-1-methylcyclohexene (5) These results may well be interpreted in terms of a kinetic controlled insertion of the substrate 1 into [HRh(CO)_n], under a higher pressure of syn-gas, resulting in lower diastereoselectivity of 2 : 3, whereas a thermodynamic controlled step of substrate insertion into [HRh(CO)_n] under a lower pressure may give rise to the observed isomerization to 4 or inverse insertion to form 5, at the same time, affording the highest diastereoselectivity 2 : 3 = 92.8.Use of [Rh(acac)(CO)_2]-[Ru_3(CO)_<12>] as a co-catalyst was found to depress any isomerization of 1, keeping the diastereoselectivity 2 : 3 intact. Thus, the co-catalyst system has averted cleanly a dilemma observed above.