Organometallics 1993,12, 2845-2848
2845
Titanium Ester Homoenolates: A Structural and Synthetic Study Pier Giorgio Cozzi, Tommaso Carofiglio, and Carlo Floriani' Section de Chimie, Universitb de Lausanne, Place du Chateau 3, CH-1005 Lausanne, Switzerland
Angiola Chiesi-Villa and Corrado Rizzoli Istituto di Strutturistica Chimica, Centro di Studio per la Strutturistica Diffrattometrica del CNR, Universitil di Parma, 1-43100 Parma, Italy Received February 12, 1993 Summary: The synthesis and structural characterization of the novel monomeric titanium homoenolates [(Cl)z(cp)Ti-CH2CH2COOEtl (21, and [(cp)2Ti-CH&H2COOEt]+ZnI3- (3;cp = $ - C a s ) are reported, along with astructural study on the well-knownhomoenolate fl(Cl),Ti-CHzCHzCOOEt)2(cl-C1)~( I ) , which is dimeric both i n solution and in the solid state. I n both classes of titanium homoenolates we observed the presence of a five-membered metallacycle containing a titaniumcarbon bond which is significantly longer in 3 than in 1. Crystallographic details: 1 is monoclinic, space group P21/c, with a = 8.889(2) A, b = ll.266(2) A, c = 11.040(2) A, CY = y = 90°, @ = 107.12(2)01Z = 2, and R = 0.077; 3 is monoclinic, space group P21/n, with a = 11.814(2) A, b = 12.084(3)& c=15.182(2)A1~ ~ = y = 9 0 ~ , @ = 9 7 . 9 5 ( 1 ) ~ , Z = 4, and R = 0.052. While homoenolates are extremely valuable in metalmediated organic synthesis,' some potential drawbacks to using them include (i) the occurrence of side reactions, such as the halogenation of monoaldol products, when titanium homoenolates are used, (ii)the low reactivity with substrates other than aldehydes,and (iii)the stereocontrol of the reactions. Tuning the reactivity and selectivity of the homoenolate functionality may be achieved by an appropriate use of a transition-metal fragment. We first focused on the structural characterization of c5 ' the well-known titanium homoenolate 1: both in the solid Figure 1. ORTEP view of complex 1 (30% probability state and in solution. It has two peculiar characteristics: ellipsoids). it contains a T i 4 u bond not supported by any conventional ancillary ligand, and it is made from a nonorgauncharacterized organic products. The dimeric nature of nometallic precursor. 1, as suggested by molecular weight determinations in solution,s was confirmed by an X-ray analysis. The Et0 OSiMe, centrosymmetric dimer is shown in Figure 1, while a +TIC14 CI,I-() + CISiMe3 (1) selection of bond distances and angles is listed in Table I. The titanium achieveshexacoordinationby sharing two 1 chlorine atoms at distances significantly longer (Table I) The stability of l3 is ascribed to the intramolecular than those with the terminal chlorines. A similar dimeric coordination of the ester group, leading to the production structure was found in some platinum @-ketoneC-bonded of a metalla~ycle.~ Strongly coordinating molecules such complexes? The metallacycle has the same structural as 2,2'-bipyridyl or THF led to the decomposition of 1 to characteristics found for the corresponding Sn(1V) degive [TiCldbpy)16 and [TiCb(THF)31, and a mixture of rivative? though the latter is not reactive with electrophiles. * To whom correspondenceshould be addressed. The solid-state structure of 1 is directly related to the (1) (a) Kuwajima, I.; Nakamura, E. In Comprehensive Organic lH NMR spectrum (CD2Cl2),which shows two triplets at Synthesis; Pergamon: Oxford, England, 1991; Vol. II, p 441. (b) Kuwajima, I.; Nakamura, E. In Top. Curr.Chem. 1990,155,3. (c)Hoppe, 6 3.43 and 2.78 ppm for the two nonequivalent methylene D. Angew. Chem., Znt. Ed. Engl. 1984,23,932. groups. The room-temperature 'H NMR spectrum in (2) Nakamura, E.; Kuwajima, I. J. Am. Chem. SOC.1983,105,651.
K
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(3) b w a m i , R. J. Org. Chem. 1986,50, 6907. (4) Nakamura, E.; Shimada, J.; Kuwajima, I. Organometallics 1988, 4, 641. (6) Nakamura, E.; Oehino, H.; Kuwajima, I. J . Am. Chem. SOC.1986, 108,3745.
(6) Ikura, K.; Ryu, I.; Ogawa, A.; Sonoda, N.; Hnroda, 5.;Kaaai, N. Organometallics 1991, IO, 528. (7) Harrison, P. G.; King, T. J.; Healy, M. A. J. Organomet. Chem. 1979, 182, 17.
0276-7333/93/2312-2845$04.00/0 0 1993 American Chemical Society
2846 Organometallics, Vol. 12, No. 7, 1993 Table I. Relevant Bond Distances 1' Ti-CI( 1) Ti-CI( 1)* Ti-CI(2) Ti-Cl(3) Ti-O(l) Ti-C(l) 01-C(3) O( 1)-Ti-C( 1)
C1(3)-Ti-C( 1) C1(3)-Ti-O( 1) Cl(2)-Ti-C( 1) CI(2)-Ti-O( 1) C1(2)-Ti-C1(3) CI(l)*-Ti-C( 1) Cl( I)*-Ti-O( 1) a,
Notes
(A) and Angles (deg) for
Distances 2.455(2) 0(2)-~(4) 2.505(3) 0(2)-C(4)' 2.234(3) C( 1)-C(2) 2.214(4) C(2)-C(3) 2.072(6) C(3)-0(2) C(4)-C(5) 2.081(9) C(4)'-C(5)' 1.235(15)
1.496(20) 1.494( 18) 1.503(15) 1.503( 13) 1.283(14) 1.535(37) 1.518(24)
Angles 78.1(3) Cl( l)*-Ti-C1(3) 92.6(2) Cl(l)*-Ti-C1(2) 170.2(2) Cl(1)-Ti-C(l) 102.5(2) CI( 1)-Ti-O(l) 87.4(2) Cl(l)-Ti-C1(3) 97.4( 1) Cl( l)-Ti-CI(2) 83.8(3) Cl(1)-Ti-Cl(l)* 85.2(2)
91.1( 1) 169.1(1) 156.4(3) 85.5( 2) 102.6(1) 93.3(3) 78.1(1)
W c1,
Figure 2. ORTEP view of complex 3 (30% probability ellipsoids). Table 11. Relevant Bond Distances (A) and Angles (deg) for
3.
Primes denote a transformation of -x, -y, -2.
toluene gave a sharp singlet at 2.67 ppm for the two methylene groups; this split into two triplets on cooling the solution to 199 K. The mechanism by which the two methylene groups become equivalent at room temperature in toluene is still in question. A significant improvement in the nucleophilicity of the homoenolate functionality, along with a lowering of the chlorination side reaction, was attempted by replacing chlorine5in 1with alkoxo groups.6 Although this strategy was successful, these were one-pot reactions, without isolation of the species involved. In this context we planned the use of cyclopentadienylderivatives of titanium instead of TiCls, where chlorine is replaced by the more donating ligand cp. In addition, the cp ligand provides a better control on the coordination sphere of the metal and on the chemistry of the Ti-C functionality. The synthesis of cyclopentadienyl titanium homoenolates, however, cannot be performed uia reaction 1. We found that the Rieke methodology8 gave simple access to zinc homoenolates without using strong donor solvents or extreme reaction conditions9 which are incompatible with the synthesis of cyclopentadienyl derivatives.
cp( 1)-Ti( l)-cp(2)
Zn
I
Angles 134.2( 15) O( 1)-Ti( I)
