Organometallics 1985, 4,1006-1014
1006
Titanacycles Derived from Reductive Coupling of Nitriles, Alkynes, Acetaldehyde, and Carbon Dioxide with Bis(pentamethylcyclopentadienyl)(ethylene)tltanium( I I)' Steven A. Cohen and John E. Bercaw" Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 1125 Received October 3, 1984
The reactions of bis(pentamethylcyclopentadieny1)(ethylene)titanium(II),( T ~ C ~ M ~ ~ ) ~ (I), T ~with (~-C~H~) alkynes, nitriles, carbon dioxide, and acetaldehyde have been examined and the products characterized spectroscopically. The most common reaction pathway observed is the addition of 1 equiv of substrate to 1 affording the respective monomeric metallacyclopentene ( T & ~ M ~ ~ ) ~ T ~ C ( R ) = C M ~ C (RH=, CHH ~ , and Me), metallacycloimine (7-C5Me5)2TiN=C(R)CHzCHz(R = CH3, CzH5, t-C4H9,and p-CH3C6H4), , metallalactone (rpC5Me5)2TiOC(0)CHzCH2, and oxymetallacyclopentane ( V - C ~ M ~ ~ ) ~ T ~ O C H ( C H ~ ) CH2CH2products. In each case, the preferred (often exclusive) product from the reaction has the sterically least-hindered substituent adjacent to the titanium atom. Depending upon the reaction conditions and the particular substrate chosen, insertion into a C-H bond or substitution of the C2H4 ligand in 1 by the substrate may be observed, as evidenced by the formation of (s-C5Me5)zTi(C=CR)(C2H5), where R = CH3 and t-C,H,, and (7-C,Me5),Ti(s-RC~CR), where R = CH3 and C6H5. Many of the Ti(1V) products are found to undergo further reactions under relatively mild conditions. For example, carbonylation of the , metallacyclopentene (7-C5Me5)2TiC(CH3)=C(CH3)CH2CH2 quantitatively yields ( T ~ C ~ M ~ ~ ) ~and T~(CO)~ the corresponding dimethylcyclopentenone while the metallacycloimine compounds (7, C5Me5)2TiN=C(R)CH2CH2undergo a 1,3 hydrogen shift to generate metallacycloenamine species (q, C5Me5)zTiNHC(R)=CHCH2,where R = CH3and C2H5 The latter rearrangement has been found to occur via an intramolecular pathway and is sensitive to the steric bulk of the R group. I
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1
i
I
I
I
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I
Introduction We recently reported the synthesis and structure of bis(pentamethylcyclopentadienyl)(ethylene)titanium(II), (q-C5Me&Ti(7-CzH4)(l),together with some preliminary studies of its reactivity.' Displacement of the ethylene from 1 by good r-acceptor ligands such as CO and CH3NC is observed, whereas treatment of 1 with H2or HC1 results in the liberation of ethane. Incorporation of ethylene in the organotitanium product was observed only in the case of the reversible reductive coupling of a second ethylene molecule, affording the titanacyclopentane (7r
1
C5Me5)2TiCH2(CH2)2CH2 (2) (eq 1). The instability of 2 (q-CsMe5 ) Z T I ( ? - C ~ H ~ t) 1
C2H4
(?-C5Me5 ) z T i 3 ( 1 1 2
has precluded further study of this titanacyclopentane species, however. Since substrates having more reactive carbon-carbon multiple bonds or those with carbon-nitrogen or carbon-oxygen multiple bonds were expected to provide greater stability for the titanacyclic products derived from reductive coupling with the ethylene ligand, we have examined the reactions of 1 with alkynes, nitriles, acetaldehyde, and carbon dioxide. The results of these studies are reported herein.
Results Reactions of (q-C5Mes)2Ti(q-C2H4) with Alkynes. Compound 1 reacts rapidly with 1 equiv of propyne at -50 "C, affording approximately equal amounts of the me, tallacyclopentene complex (p-C5Me5)2TiCH=C(CH3)CH2CHz (3) and the propynyl ethyl species (7-
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*Contribution No. 7103.
0276-7333/85/2304-106$01.50/0
C5Me5)zTi(C=CCH3)(CH2CH3) (4). Spectroscopic data
1
v
(IR 3, v(C=C) 1587,4, v(C
