丑亞杰, 程探宇, 林靜容

(上海師范大學 生命與環(huán)境科學學院,上海 200234)

手性稀土配合物在不對稱催化應用中的研究進展

丑亞杰, 程探宇, 林靜容*

(上海師范大學 生命與環(huán)境科學學院,上海 200234)

近年來隨著光學純化合物需求的不斷增大,不對稱催化逐漸成為科學研究的前沿領域之一.稀土元素則是由周期表中IIIB族Sc、Y和鑭系元素(La～Lu)的17種元素組成.相比其他過渡金屬與各類配體軌道間的作用,稀土金屬有其獨特之處,因此,近年來手性稀土配合物在不對稱催化領域的應用,也逐漸引起人們的重視.根據(jù)各類手性配體中所含配位原子的不同,對手性稀土配合物的類型及相關的不對稱催化反應作簡要綜述.

稀土元素; 鑭系金屬; 手性; 不對稱催化

0 引 言

近年來金屬不對稱催化已成為一個巨大的研究領域,其范圍隨著有機催化的發(fā)展也在迅速擴大.對于過渡金屬作路易斯酸進行不對稱催化的反應已有大量研究,但是對于使用諸如Sc、Y和La等稀土元素作為路易斯酸進行催化,發(fā)展較為緩慢.Bednarski等[1]在1983年報道了第一例手性鑭系元素絡合物作為路易斯酸催化手性hetero-Diels-Alder反應,獲得33%的對映選擇性(ee值).此后,稀土元素配合物催化劑在Shibasaki等[2]的研究下,取得了飛躍性的發(fā)展.手性鑭系元素絡合物能夠作為不對稱合成中的新催化劑與其獨特的化學和物理性質(zhì)有關.鑭系元素離子通常被認為是對氧原子具有高親和力的硬路易斯酸.4f軌道是鑭系元素的核心軌道,電子從La3+(4f0)到Lu3+(4f14)依次填充,但該軌道對配合物的鍵合作用沒有貢獻.因此,鑭系金屬-配體鍵合主要是靜電,從而得到一些不規(guī)則配位幾何形狀的復合物.Lu3+的離子半徑較大,其配位數(shù)比通常的d-過渡金屬更高,出現(xiàn)8,9或10的配位數(shù)也很常見.這些性質(zhì)對于稀土金屬結合各種手性配體是非常有利的,并且可以構建一些結構復雜的復合物,從而有效地控制反應的立體化學.本文作者便根據(jù)配體中配位原子的不同,對手性稀土配合物的類型及催化的相關反應作簡要綜述.

1 含O給體的手性稀土金屬配合物

1.1 β-二酮類配合物

β-二酮類鑭系配合物,早在20世紀初就開始被關注,并且被Whitesides等[3-6]用于核磁共振位移試劑的研究.最早應用于立體選擇性反應的斕系配合物,也是β-二酮類鑭系配合物[1].隨后優(yōu)化反應條件(-10 ℃,無溶劑),僅用摩爾分數(shù)為1%的催化劑,將相應的環(huán)加成產(chǎn)物ee值提高至58%[7].也有使用銪的絡合物[Eu(tfc)3]來催化不對稱Michael加成反應的例子[8],反應中催化劑用量較高,但所獲得的ee值并不是很理想.因此,之后該類配體主要被用于新型鑭系發(fā)光配合物中[9-10].

通過L-甲氧基取代的二酮配體L1與Gd制備的手性配合物,可以催化酮的不對稱硼氫化還原反應(圖1),得到了較好的ee值[11].

圖1 Gd[L1]3催化酮的硼氫化還原反應

1,3-雙(2-甲基二茂鐵基)丙烷-1,3-二酮(HBMPD)與Y(OiPr)3配制得到二酮絡合物,可以作為醛氰化反應的活性催化劑.研究初期其活性并不高,Abiko團隊[12]通過篩選條件,發(fā)現(xiàn)BMPD-Y絡合物對該反應具有很好的活性(圖2).催化劑減少至摩爾分數(shù)為0.2%時,也可以得到產(chǎn)率95%,ee值87%的產(chǎn)物[13].

圖2 Y[BMPD]催化醛的不對稱氰化反應

1.2 醇鹽配合物

合成手性稀土醇鹽催化劑的一種方法是LnCl3( Ln為鑭系金屬)和堿金屬醇鹽之間的鹽交換反應.該方法得到了不飽和酮烯烴不對稱環(huán)氧化的有效催化劑:[Li2Cl] [Yb(L)2][14].

稀土醇鹽與TMSCN(氰基三甲基硅烷)進行交換反應可以形成稀土氰化絡合物,催化環(huán)氧乙烷的氰化,產(chǎn)率可達到99%[15].手性稀土醇鹽便開始被用作不對稱氰化反應的催化劑前體.Shibasaki團隊[16]隨后發(fā)現(xiàn)了一種氧化膦官能化二醇H2L3,其與Gd(OiPr)3反應形成Gd2[L3]3復合物,結構如圖3所示.衍生自Gd2[L3]3的催化劑也已經(jīng)用于酮[16-18]、亞胺[19-22]的對映選擇性氰化反應和氮丙啶[23-24]的開環(huán)反應,獲得了較高的活性和對映選擇性.

圖3 Gd2[L3]3復合物

Evans團隊[25]發(fā)現(xiàn)手性稀土醇鹽可以實現(xiàn)酮的不對稱還原反應(圖4),通過催化劑的篩選發(fā)現(xiàn)SmI3[L4]可以得到較高的ee值.另外,Sm的醇鹽可以催化Mukaiyama-aldol反應,但ee值較低,即使在2當量的催化劑時也只獲得30%的ee值,產(chǎn)率僅為16%.在使用0.2當量的催化劑時獲得了92%的產(chǎn)率,但此時,ee值卻降低到了2%[26].

圖4 SmI3[L4]催化酮的還原反應

1.3 聯(lián)萘酚配合物

1.3.1 單金屬聯(lián)萘酚配合物

稀土醇鹽Ln(OiPr)3和Ln(OtBu)3在商業(yè)上很容易獲得的,并且是理想的合成手性醇鹽的前體.早期,通過Ln(OiPr)3與1當量聯(lián)萘酚制備的催化劑,催化烯酮的對映選擇性環(huán)氧化獲得了不錯的ee值(94%)[27-29].隨后,Shibasaki等[30-33]開發(fā)了高效不對稱環(huán)氧化反應催化劑,如圖5所示.其中,中性配體(Ph3As=O或Ph3P=O)是必不可少的,其作用可能是為了防止催化劑低聚.該反應的結果(特別是產(chǎn)率)強烈依賴于鑭系金屬的選擇,通過金屬的篩選,元素Y復合物得到了最好的結果,而Sc復合物則基本無活性[34].通過La(OiPr)3與聯(lián)萘酚反應制備的催化劑也被用于催化酮的硼烷還原,得到61.8%的ee值[35].

圖5 Y(OiPr)3[L5]催化烯烴不對稱環(huán)氧化

Kobayashi[36-37]發(fā)現(xiàn)通過Yb(OTf)3與聯(lián)萘酚反應獲得的催化劑成功實現(xiàn)了Diels-Alder反應的不對稱催化,并獲得較好的ee值.Sm的聯(lián)二萘酚碘化物絡合物也被用于催化Diels-Alder反應[38].當使用催化劑Sm[L6]時,在-25 ℃下達到最高28%的ee值.在使用3和3′位置具有甲氧基苯基取代基的催化劑Sm[L7]時,在-28 ℃下得到81%的ee值(圖6).再將溫度降至-60 ℃時,觀察到ee值的反轉(zhuǎn),得到30%的ee值[39].和聯(lián)萘酚類似的聯(lián)萘胺衍生配體也可以用于不對稱D-A反應[40-41].

圖6 Sm[L6]與Sm[L7]催化Diels-Alder反應

在聯(lián)萘酚的3,3′位上連接手性噁唑啉基團形成新型的四齒配體,即BINOL-Box.該配體與稀土元素得到的催化劑對于圖7中所示的1,3-偶極加成反應可以得到87%的ee值[42-44].

圖7 Ln(OTf)3[L8]催化1,3-偶極加成反應

1.3.2 多金屬聯(lián)萘酚配合物

圖8 多金屬聯(lián)萘酚配合物M3 [RE(binol)3]

20世紀90年代初,Shibasaki團隊[45-46]發(fā)現(xiàn)了含稀土元素的多金屬催化劑 Ln-M3-三(1,1′-雙-2-萘酚)(M為第ⅠA族金屬,堿金屬),結構如圖8所示.Ln的高配位數(shù)允許構建由一個稀土元素、三個堿金屬和三個聯(lián)萘酚組成的多金屬配合物.RE是路易斯酸,二元配體則是布朗斯特堿,所以M3 [RE(binol)3]可以作為雙功能催化劑.并且該催化劑修飾的潛力是巨大的:Ln 可以是任何一種稀土元素,M可以是Li、Na或K.通過改變一系列RE和M 的組合,可以獲得具有不同O-金屬鍵長度,離子半徑和路易斯酸度的各種[RE(binol)3]絡合物.該類催化劑的第一個復合物是Li3[La(binol)3],其首先被應用于催化硝基甲烷的不對稱硝基Aldol反應并得到很好的ee值[47].隨后發(fā)現(xiàn)加入催化量的BuLi或其他堿會得到更高的轉(zhuǎn)化率[48-53].

催化不對稱Aldol縮合反應時(圖9),其中催化劑可以使供體烯醇化后,再對受體進行對映選擇性進攻[54-56].這種催化方法,為Aldol縮合反應碳-碳鍵形成過程提供了簡便的方法,并且避免了使用其他預烯醇化的試劑.實際上,這一發(fā)現(xiàn)引發(fā)了隨后對于金屬催化Aldol反應的廣泛研究[57].在所用的各種RE中,發(fā)現(xiàn)La在大多數(shù)反應中可以得到很好的反應結果.

圖9 Li3[La(R-binol)3(H2O)]催化Aldol縮合反應

丙二酸酯與環(huán)狀不飽和酮的邁克爾加成如圖10所示,該反應不僅對Ln有很大的依賴性,而且對堿金屬的性質(zhì)也有很大的依賴性,Li得到非常差的ee值,而Na卻可以獲得最好的ee值[58-60].雖然催化中間體的確切結構尚未確定,但有研究表明,脫質(zhì)子化的丙二酸酯與手性催化劑配位,才能實現(xiàn)高對映體選擇性[61].該催化劑同樣可以實現(xiàn)查爾酮的不對稱Michael加成[62],當Ln=Y或Dy時,產(chǎn)率和對映選擇性最好;Tm,Yb和Lu的反應效率較差,可能因為Ln金屬中心太小,無法有效地與查爾酮配位.

圖10 M3[La(R-binol)3]催化Michael加成反應

催化劑Na3[La(R-binol)3]成功實現(xiàn)烯酮的不對稱環(huán)氧化反應,得到83%的ee值,但該催化劑底物適應性較差[63-65].在非手性氧化膦添加劑的存在下,催化劑摩爾分數(shù)僅為1%～5%便可獲得很好地ee值(91%～97%),同時達到 88%～99%的產(chǎn)率.乙基酮和丙基酮則得到略低的ee值(67%～88%),這可能是空間位阻的原因[66].

[La(R-binol)3]和Li形成的催化劑在醛的氫化膦?；呋蝎@得較好的催化活性[67],而在亞胺的氫化膦?；^程中,K獲得了最佳結果[68](圖11).醛的不對稱氫化膦?；磻部梢杂蒐i2Binol與LaCl3的反應產(chǎn)生的絡合物[69]或3,3′-雙(甲氧基乙基)聯(lián)萘酚與NaOBut和LaCl3反應得到的絡合物催化.

圖11 M3[La(R-binol)3]催化醛/亞胺的氫化膦?；磻?/p>

2 含N給體的手性稀土金屬配合物

2.1 雙噁唑啉配合物(BOX)

雙噁唑啉配體可以與鑭系元素配位形成手性催化劑,Tian等[70]利用這種催化劑催化氨基烯烴的分子內(nèi)胺化反應(圖12),得到了中等ee值(67%),但是對于1,3偶極環(huán)加成[71]和Michael加成[72]均得到消旋產(chǎn)物.

圖12 Ln(BOX)[N(SiMe3)2]2催化氨基烯烴分子內(nèi)胺化反應

2.2 吡啶雙噁唑啉配合物(pybox)

1989年Nishiyama等[73]首次介紹了pybox配體,pybox是一種三齒N型配體,吡啶和亞胺的N原子都是強供電體,并且配體與金屬結合后會形成剛性平面結構,該系列配體結合各種稀土元素,可以得到多種催化劑,現(xiàn)已廣泛用于鑭系元素的催化應用.

Fukuzawa最初發(fā)現(xiàn)使用鑭系元素pybox復合物可以催化Diels-Alder反應[74],并且用[Sc(iPr-pybox)(OTf)3]獲得了82%的ee值.隨后使用(S)-TIPSOCH2-pybox Sc催化劑又得到了93%的ee值[75].(R,R)-Ph4-pybox Yb[76],和(R)-iPr-pybox Sc[77],也能得到84%的ee值.該反應中較大的鑭系元素(Sm,La和Yb)具有較低的對映體選擇性,并且產(chǎn)物都以內(nèi)型產(chǎn)物為主.值得注意的是,使用配體iPr-pybox時,分子篩的存在可以提高對映選擇性,而對于Ph-pybox配體,分子篩則降低對映體選擇性.

Schaus等[78]報道了由LnCl3·6H2O/pybox催化環(huán)氧化物的開環(huán)氰化反應.篩選了幾種水合的LnCl3,發(fā)現(xiàn)ee值隨著Ln3+半徑的減小而增加.還篩選了幾個pybox配體(tBu-,iPr-,Ph-,Bn-);(S)-Ph-pybox獲得最佳的ee(91%值).而LnCl3·6H2O-Ph2-pybox催化劑也可以應用于腙的氰化反應[79].在這個反應中,前面的元素Ln(La,Ce,Pr)活性非常差,Er獲得很好ee值后,從Er到Lu,ee值又開始快速下降.

Desimoni等[80-81]研究了pybox配體和Ln對不對稱Mukaiyama-Michael加成反應的對映選擇性影響.對于大多數(shù)反應,使用Sc或Yb得到最佳對映選擇性,但該反應中金屬La和Ce是最有效的催化劑,如圖13所示.這表明對于特定反應優(yōu)化Ln和配體是很重要的.

圖13 La/Ce(R,R-Ph4-Pybox)2催化Mukaiyama-Michael加成反應

Pybox衍生配體(R)-inda-pybox與Yb配合物高效地催化了β-酮酸的脫羧加成反應(圖14),獲得了一系列有生物價值的3-羥基吲哚,產(chǎn)率高達98%,最高ee值達了99%[82].

圖14 Yb(R-inda-Pybox)2催化β-酮酸的脫羧加成反應

3 含N/O混合給體的手性稀土金屬配合物

3.1 含O Schiff堿配合物

圖15 含O Schiff堿體L12

稀土金屬與Schiff堿配體的廣泛配位具有幾乎無限的修飾潛力.Fukuzawa等[83]篩選了幾種Schiff堿二醇基手性配體與Sc(OTf)形成配合物(圖15)催化Diels-Alder反應,配體L12獲得85%的ee值.

雙金屬催化的另一種方法是設計具有兩個不同結合位點的雙核配體.較小的結合位點具有N2O2的供電子體,其將有利于較小的金屬離子如d-過渡金屬,而O4供電子體將有利于Ln離子.這些雙金屬催化劑已經(jīng)證明了具有一些獨特的反應性:[(L13)Cu/Sm]是第一個催化順式選擇性硝基-Mannich反應的催化劑(圖16[84-87]).[(L13)Pd/Sm]催化硝基Aldol反應也得到了84%的ee值[88-89].

圖16 Cu/Sm(OAr)[L13]催化硝基-Mannich反應

稀土元素與二烷基胺(R2N-)配體的配位化學很普遍,二烷基胺是比相應的醇鹽更強的布朗斯特堿,并且它們在空間上要求更高,導致Ln(NR2)3通常具有更簡單的結構.RajanBabu團隊通過H2L13與Y[N(SiHMe2)2]3]的反應制備得到催化劑,在環(huán)氧化物的開環(huán)反應中顯示出很好的活性,催化劑負載量摩爾分數(shù)僅為0.01%(圖17)[90].RajanBabu團隊[91]后來又將Y[L13][N(SiHMe2)2]應用于更具挑戰(zhàn)性的氮丙啶開環(huán)的催化,并獲得88%的ee值.

圖17 Y[L13][N(SiHMe2)2]催化環(huán)氧化物的開環(huán)反應

3.2 N,N′-二氧配體

C2軸對稱的N,N′-二氧化物很容易從氨基酸合成,并且獲得各種的結構修飾類似物.N-氧化物官能團是稀土離子的良好供體,2003年,手性雙喹啉N,N′-二氧化物配體(L14)與稀土三氟甲磺酸鹽Sc(OTf)3結合使用催化不對稱Michael加成反應獲得良好的產(chǎn)率和ee值[92](圖18).Feng團隊[93-96]也已經(jīng)成功地應用[Ln(OTf)3(N,N′-二氧化物)]催化了各種不對稱反應.

圖18 Sc(OTf)3[L14]催化Michael加成反應

將Sc(OTf)3與配體L15結合,催化Aldol反應(圖19)獲得了高產(chǎn)率和高ee值[97].還發(fā)現(xiàn)該反應中加入3?分子篩會增加反應速率,但對對映體選擇性沒有影響.

圖19 Sc(OTf)3[L15]催化Aldol反應

在硫代乙醇酸酯與查爾酮的Michael加成中,當Ln=La(最大的稀土金屬)時獲得最高ee值,而較小的金屬Sc,Yb和Y獲得很低的ee值(分別為12%、4%和10%ee)[98].而在另一個Michael加成中(圖20),Sc(OTf)3[L16]獲得的對映選擇性與Y(OTf)3[L16]相反[99].并且,La、Sm、Gd、Dy、Er、Yb和Y均具有相同的高ee值.同時發(fā)現(xiàn)乙醇在Sc(OTf)3催化中對于實現(xiàn)高ee有重要作用.

圖20 Sc(OTf)3[L16]與Y(OTf)3[L16]催化Michael加成反應

3.3 大環(huán)N/O配合物

已經(jīng)有報道使用大環(huán)化合物與Ln(OTf)3的復合物催化Aldol反應并獲得較高的ee值(圖21)[100].該反應的對映體選擇性隨著鑭系元素半徑的減小而減小:用[Ce(OTf)3]獲得最佳結果.該大環(huán)配體L17的空腔尺寸應該與排在前面的鑭系金屬離子Ln3+半徑有很好匹配,這也可能是ee值隨半徑減少的原因,隨后的研究包括測定配體結合常數(shù)證實了這一假設[101].配體L18在環(huán)中具有N2O2供電子體,且N原子上連有手性基團.該配體對于Ln3+離子太小,親和力較低,所以不能共面結合.為了使Ln3+結合充分,需要2當量的配體L17[102-104].

圖21 Ce(OTf)3[L17]與Nd(OTf)3[L18]催化Aldol反應

4 含P給體的手性稀土金屬配合物

衍生自聯(lián)萘酚的手性膦酸酯配體與稀土離子形成穩(wěn)定的絡合物.這些配合物即使在室溫下也顯示出良好的活性和對映選擇性.如圖22所示,在催化hetero-Diels-Alder反應時添加2,6-二甲基吡啶后得到89%ee值[105].該類催化劑也可催化查爾酮Michael加成,在該反應中,Sc獲得最好的ee值69%,并且明顯優(yōu)于Yb[106].

圖22 Yb[L19]催化hetero-Diels-Alder反應

5 環(huán)戊二烯基配合物

由于Ln3+的大尺寸以及鑭系雙(環(huán)戊二烯基)復合物的非剛性結構,合成具有明確手性結合位點的此類復合物是一個挑戰(zhàn).1992年,Marks團隊[107]首先使用烷基絡合物L20催化烯烴氫化(圖23).催化過程中的對映選擇性步驟推測是烯烴不可逆地嵌入到了Ln-H鍵中.

圖23 Sm-N(SiMe3)2[L20]催化烯烴氫化反應

CH(SiMe3)2或N(SiMe3)2的絡合物可用作對映選擇性氫胺化/環(huán)化反應的催化中[108].CH(SiMe3)2或N(SiMe3)2可以被底物的氨基通過質(zhì)子分解快速置換,該過程會加速整個反應.然而產(chǎn)物的絕對構型對催化劑的手性結構幾乎不敏感,而僅僅取決于手性取代基R 的手性結構[109].該類催化劑也可以催化烯烴硅烷化反應,并且得到了68%的ee值[110].

6 展 望

本文作者簡要綜述了各類手性稀土配合物在不對稱催化中的應用.已經(jīng)認識到Ln鑭系金屬的半徑較大,由于配位層的流動性,從而想要得到足夠剛性的結構產(chǎn)生高對映選擇性,是一個很大的挑戰(zhàn).可以看到聯(lián)萘酚、pybox尤其是Shibasaki的多金屬M3[RE(binol)3]催化劑已經(jīng)取得了很大的成功.中國是稀土資源大國,可以預見在今后的發(fā)展中,稀土金屬在不對稱催化中的應用還會不斷深入.

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Researchprogressofchiralrareearthcomplexesinasymmetriccatalysis

ChouYajie,ChengTanyu,LinJingrong*

(College of Life and Environmental Sciences,Shanghai Normal University,Shanghai 200234,China)

With the increasing demand of the optical compounds,asymmetric catalysis becomes a hot research area.The rare earth elements are defined as the elements of lanthanide from La to Lu,together with Sc and Y.Compared to the orbital interaction between the other transition metals and ligands,the one between rare earth metals and ligands is unique.Therefore,the application of chiral rare earth complexes in asymmetric catalysis has also attracted much attention.This paper reviewed research progress of chiral rare earth complexes in asymmetric catalysis.

rare earth elements; lanthanides; chiral; asymmetric catalysis

10.3969/J.ISSN.1000-5137.2017.06.012

2017-09-18

丑亞杰(1991-),男,碩士研究生,主要從事不對稱催化方面的研究.E-mail:choumars@163.com

*通信作者: 林靜容(1968-),女,博士,副教授,主要從事天然產(chǎn)物合成等方面的研究.E-mail:jrlin@shnu.edu.cn

丑亞杰,程探宇,林靜容.手性稀土配合物在不對稱催化應用中的研究進展 [J].上海師范大學學報(自然科學版),2017,46(6):865-879.

formatChou Y J,Cheng T Y,Lin J R.Research progress of chiral rare earth complexes in asymmetric catalysis [J].Journal of Shanghai Normal University(Natural Sciences),2017,46(6):865-879.

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1000-5137(2017)06-0865-15

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