High-Energy Processes in Organometallic Chemistry - American

Chapter 16

Downloaded by UNIV OF QUEENSLAND on April 28, 2016 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch016

Activation of Carbon-Hydrogen Bonds by Metal Atoms 1

M. L. H. Green and Dermot O'Hare

Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, United Kingdom

Aspects of the apparatus for the synthesis using metal atoms are described. The reactions of the atoms of rhenium, tungsten, and osmium with hydrocarbons including alkanes are described. It is shown that metal atom reactions with alkanes can give isolable organometallic compounds including μ-alkylidene compounds.

The ability o f t h e s u r f a c e s o f many t r a n s i t i o n m e t a l s t o r e a c t w i t h a l k a n e s u n d e r m i l d c o n d i t i o n s has been l o n g known (1-3) . In t h e l a s t decade t h e r e has been i n c r e a s i n g i n t e r e s t i n t h e r e a c t i o n of discrete transition metal compounds with alkanes, where the t r a n s i t i o n metal i n s e r t s i n t o t h e carbon-hydrogen bonds o f t h e a l k a n e g i v i n g alkyl hydride derivatives (4 -6 ) . Very r e c e n t l y i t has been shown t o be p o s s i b l e f o r t r a n s i t i o n m e t a l compounds t o i n s e r t i n t o the c a r b o n - h y d r o g e n bonds o f methane, t h e most i n e r t of s a t u r a t e d hydrocarbons (7-9) . During a study designed t o understand the fundamental requirements for a transition metal center to interact with saturated carbon-hydrogen bonds we have investigated the reactions between t r a n s i t i o n m e t a l atoms and h y d r o c a r b o n s . 1

1

Current address: Ε. I. du Pont de Nemours & Co., Experimental Station, Wilmington, DE 19898 0097-6156/87/0333-0260$06.00/0 © 1987 American Chemical Society

Suslick; High-Energy Processes in Organometallic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

GREEN AND O'HARE

Activation of Carbon-Hydrogen Bonds

261

The general technique of the metal vapor experiments d e s c r i b e d below was t o co-condense the vapors of the transition metal with those of the chosen hydrocarbon o r hydrocarbon mixtures. In this paper we b r i e f l y o u t l i n e t h e t e c h n i q u e o f m e t a l atom synthesis and t h e n show how i t can be a p p l i e d t o a l k a n e activation reactions.


M E T A L VAPOR

SYNTHESIS

The g e n e r a l p r i n c i p l e s and p r a c t i c e o f m e t a l v a p o r s y n t h e s i s have been well reviewed (10-11) . The experiments d e s c r i b e d below were c a r r i e d out u s i n g a bell-jar design of apparatus shown i n Figure 1 incorporating a p o s i t i v e - h e a r t h electron-gun furnace (Figure 2) . T h i s d e s i g n was first constructed in Oxford. The e l e c t r o n gun furnace can achieve temperatures i n e x c e s s o f 4 0 0 0 ° C and c a n v a p o r i z e a l l the elements at a substantial rate. The positivehearth mode (12-13) r e d u c e s t h e damage t o b o t h t h e c o condensate a n d p r o d u c t s by r e d u c i n g t h e number o f r e f l e c t e d e l e c t r o n s e m i t t e d from the f u r n a c e . Typical rates of evaporation of a s e l e c t i o n of the t r a n s i t i o n metals from a 0.5ml c a p a c i t y e l e c t r o n gun f u r n a c e a r e given i n Table I.

Using the the l a r g e r s i z e hearth of c a p a c i t y 10ml, t y p i c a l l y , up t o 15g of t u n g s t e n o r 30g o f t i t a n i u m may be e v a p o r a t e d in a 4h e x p e r i m e n t producing multi-gram (5-35g) quantities of products, in favorable reactions. The success of t h i s reactor design owes much t o the close proximity of the electron-gun to the l a r g e 2000 litres.sec" cryopump ( F i g u r e 1) , to maintain the high vacuum required for i t s operation. The d e v e l o p m e n t o f t h i s reactor has e n a b l e d m e t a l atoms t o be c o n s i d e r e d as 'off the s h e l f reagents in synthesis. 1

1


262

HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY

ΕΤΓ-


-ft

F i g u r e 1 10KW m e t a l atom s y n t h e s i s a p p a r a t u s ; (a) V a r i a n VK10 cryopump; (b) G a t e v a l v e ; (c) C o p p e r matrix support; (d) G l a s s vacuum v e s s e l ; (e) I n l e t f o r washing solvent; ( f ) 10KW f u r n a c e ; (g) L i g a n d i n l e t r i n g ; (h) P i p e s f o r l i g u i d n i t o g e n c o o l a n t ; ( i ) Product exit pipe; (j) Argon i n l e t ; (k) R o u g h i n g valve; (1) F l e x i b l e p i p e t o r o t a r y pump; (m) Water cooling pipes.




GREEN AND O'HARE

Figure 2 3.5 KW e l e c t r o n beam f u r n a c e components: (a) Water-cooled furnace block; (b) C o p p e r hearth (c) C o o l i n g p i p e s ; (d) Top p l a t e of furnace (earth potential); (e) F i l a m e n t ; ( f ) F o c u s l i d ; (g) L i d : (h) High-tension supply; ( i ) Low-tension supply; (j) Ceramic i n s u l a t o r s ; (k) C o o l i n g p i p e s f o r top plate: (1) l o c a t i n g s t u d f o r h e a r t h a d j u s t m e n t ; and (m) Water c o n d u i t .


264


T a b l e I. Power i n p u t and r a t e s o f e v a p o r a t i o n m e t a l s from a 0.5ml h e a r t h o f t h e p o s i t i v e h e a r t h e l e c t r o n gun f u r n a c e

Power i n p u t ( K W )


Metal

a

of

Evaporation - 1

Rate(g.h ) Titanium Zirconium Hafnium Niobium Tantalum Molybdenum Tungsten Rhenium Osmium Uranium

1..3 0.,5 1..5 0..5 1..5 1..0 1..5 1..5 1,.5 0..5

1..0 2. .2 1,.8 2, .0 1,.8 1..2 1..8 1..8 1,.4 1,.4

b

a

Power input depends n o t o n l y on t h e temperature required f o r v a p o r i z a t i o n , but also on t h e f l u i d i t y of the molten sample and c o n s e q u e n t h e a t l o s s by v e r y e f f i c i e n t thermal c o n t a c t w i t h the water c o o l e d hearth. F o r example, t u n g s t e n m e t a l requires a vaporization temperature of ca. power input, whilst

3600°C corresponding to a 1. 8KW niobium which v a p o r i z e s at ca.

2 5 0 0 ° C r e q u i r e s ca. 2.0KW power i n p u t because at 2 5 0 0 ° C n i o b i u m i s a more f l u i d m e t a l t h a n tungsten at 3 600°C and so c a n dissipate t h e power more e f f i c e n t l y by t h e r m a l c o n d u c t i o n through the water c o o l e d h e a r t h . b

Uranium e v a p o r a t e d alloy.

from

Note:

Refs.

Data are

from

a p r e v i o u s l y prepared

14

and

40%

15.


U/Re

16.

GREEN AND O'HARE


265

ALKANE ACTIVATION BY METAL ATOMS


Introduction. In a typical experiment, the metal vapor i s co-condensed at a liquid nitrogen-cooled surface with the ligand substrate vapor. Upon warming, the r e a c t i o n mixture melts and i s washed from t h e r e a c t o r w a l l s w i t h a s u i t a b l e s o l v e n t . The p r o d u c t s a r e f i l t e r e d t o remove u n r e a c t e d b u l k m e t a l , and f i n a l l y t h e p r o d u c t s a r e s e p a r a t e d and p u r i f i e d by c o n v e n t i o n a l S c h l e n k v e s s e l o r vacuum l i n e t e c h n i q u e s . Kiabunde (JJL) was t h e f i r s t t o d e s c r i b e alkane activation by metal atoms. He showed t h a t c o condensation of nickel atoms and p e n t a n e yielded a solid m a t e r i a l which contained Ni,C and H. Hydrogenolysis of t h i s m a t e r i a l produced Cl~ 5 alkanes. Kiabunde suggested that t h e r e a c t i o n s was analogous to the well-documented heterogeneous cracking processes over n i c k e l catalysts, and that n i c k e l c r y s t a l l i t e s were r e s p o n s i b l e f o r t h e c h e m i s t r y observed. c

Skell (17.) f o u n d t h a t c o - c o n d e n s a t i o n o f z i r c o n i u m atoms with neopentane g a v e a r e s i d u e w h i c h upon d e u t e r i o l y s i s gave Bu CH2D and p o l y d e u t e r a t e d C2-C4 hydrocarbons. He s u g g e s t e d t h a t t h e C 5 - p r o d u c t arose from a n e o p e n t y l z i r c o n i u m m o i e t y . t

The p h o t o e x c i t a t i o n o f m e t a l atoms i n a l k a n e matrices produces reactive excited-state species c a p a b l e o f a c t i v a t i n g a l k a n e s . In 1929 T a y l o r and H i l l , and s u b s e q u e n t l y S t e a c i e ( 18), showed t h a t a t o m i c Hg i n t h e P i s t a t e c a n a b s t r a c t an H atom f r o m an a l k a n e . 3

Billups and M a r g r a v e (JL2.) showed i n 1980 t h a t p h o t o e x c i t a t i o n o f Fe atoms i n a methane m a t r i x l e d t o an u n s t a b l e s p e c i e s which they suggested t o be HFeCH3. P h o t o e x c i t e d C u ( P ) has b e e n f o u n d a l s o to r e a c t w i t h methane, i n a m a t r i x a t 12K. The i n i t i a l photoproduct was proposed t o be HCUCH3, which s u b s e q u e n t l y decomposed p h o t o l y t i c a l l y t o CuH and CuMe ( 2 Û ) . 2

Activation

o f carbon-hydrogen

bonds by r h e n i u m atoms

Co-condensation o f t r a n s i t i o n m e t a l atoms w i t h a r e n e s s u c h a s b e n z e n e and t o l u e n e i s w e l l known t o y i e l d b i s - a r e n e - m e t a l compounds. However, i n many c a s e s t h e y i e l d s b a s e d on t h e m e t a l atoms a r e l e s s t h a n 40%. Evidence that competing r e a c t i o n s such as carbonhydrogen a c t i v a t i o n can o c c u r i s p r o v i d e d by t h e i s o l a t i o n of non-metal-containing products such as b i a r y l d e r i v a t i v e s (2JJ .



266


Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium^-arylidene compounds (2_2_) (Figure 3) . The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction i s in accord with s t a t i s t i c a l attack at a l l available s p C-H bonds. 3

Co-condensation of rhenium atoms with benzene alone gave no isolable products at ambient temperature. It was not expected, however, when rhenium atoms were co-condensed with a benzene :alkane mixture that μ-alkylidene complexes, analogous to the μ-arylidene complexes were formed. Thus, co-condensation of rhenium atoms with a ç_a_ 1:1 mixture of benzene and neopentane gave the dirhenium^-neopentylidene complex [ {Re (Tj-CgHg) } ^-CHBu* ) (μ-Η>2)] in moderate yield (Figure 4) (22.) . The single crystal Xray structure determination i s shown in Figure 5. The compound [ {Re (η-C gH g ) } (μ-ΟΗΒιι* ) ( μ-Η) 2 ) 1 represents the f i r s t isolated crystalline product arising from a metal atom-alkane experiment. 1

2

1

2

The a b i l i t y of the rhenium-benzene co-condensate to activate linear- and c y c l i c - alkanes i s quite general. We have co-condensed rhenium atoms with alkane: benzene mixtures using the alkanes; ethane, propane, butane, 2-methylpropane, neopentane, tetramethylsilane, cyclopentane, and cyclohexane, giving the μ-alklyidene compounds shown in Figures 4a and 4b (23.) . Co-condensation of rhenium atoms with 1:1 mixture of perdeuterobenzene and neopentane gave the compound [ {Re (TI-CgDg) } (μ-ΟΗΒν^ ) (μ-Η) ) ] and no H incorporation was detected in either the μalkylidene or μ-hydrido positions ( H n.m.r). 2

2

2

2


GREEN AND O'HARE


267


16.

Figure 3 Summary of the reactions of rhenium atoms with alkylbenzenes. Coc o n d e n s a t i o n o f r h e n i u m atoms a t -196°C w i t h ; (i) Toluene; ( i i )p-Xylene; ( i i i )mesitylene; (iv) ethylbenzene.


268

HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY Ht


H

F i g u r e 4a Summary o f t h e r e a c t i o n s o f r h e n i u m atoms w i t h a c y c l i c s a t u r a t e d h y d r o c a r b o n s . Rhenium atoms were c o - c o n d e n s e d w i t h t h e i n d i c a t e d substrates at -196 ° C . ( i ) E t h a n e ; ( i i ) P r o p a n e ; ( i i i ) n-Butane; (iv) Neopentane; (v) 2 - M e t h y l p r o p a n e ; and ( v i ) Tetramethylsilane.



GREEN AND O'HARE

Activation of Carbon-Hydrogen

Bonds

269

F i g u r e 4b Summary of the r e a c t i o n s o f rhenium atoms with c y c l i c saturated hydrocarbons. Rhenium atoms were co-condensed with the indicated substrates a t -195°C; (i) Cyclopropane; ( i i ) cyclopentane; ( i i i ) cyclohexane.


270


Multi-step alkane dehydrogenation r e a c t i o n s have been observed f o r complexes of rhenium and iridium. For example, the dehydrogenation of cyclohexane by [IrH2 2 (Me2CO) 2] SbFg g i v i n g [Ir(T|CgHg) (PPh3) 2] SbFg (2J_) . We have observed that cocondensation of rhenium atoms with cyclohexane and PMe3 (20:1) gives the p r e v i o u s l y described compound

(PPÏ13)

[ReCn-CgHg) (PMe3)2H] (2Ji) . of

rhenium


gives

atoms with

Similarly,

cyclopentane

[Re(Ti-C H ) (PMe ) H ] 5

5

3

2

co-condensation

and

PMe

3

(10:1)

(2Â) .

2

A c t i v a t i o n of carbon-hydroaen

bonds bv tungsten atoms

Co-condensation of tungsten atoms with a mixture of cyclohexane and PMe gives a mixture a products shown i n F i g u r e 6 (2JL) . The identity o f the individual species has been determined by subsequent indepentant c o n v e n t i o n a l syntheses. Co-condensation of tungsten atoms with cyclohexene and PMe3 (10:1) gave (2) and (3) only (Figure 6) and there was no evidence f o r (1) and ( 4 ) . 3

When tungsten atoms were co-condensed with a mixture of cyclohexane and perdeuteriocyclohexane (3:1) and PMe3 the spectroscopic data indicated that there was no evidence f o r intermolecular hydrogen-deuterium scrambling and t h i s strongly suggests that (l)-(4) are formed by a process i n v o l v i n g i n t r a m o l e c u l a r carbon-hydrogen bond a c t i v a t i o n of the cyclohexane. In a more remarkable experiment i n which tungsten atoms are co-condensed with a mixture of cyclopentane and PMe3 the major component was identified as [W(T|C5H5) (PMe3) H ] (Figure 7) (21) . The other species p r e s e n t were i d e n t i f i e d as [W (TI-C5H5 ) (PMe3 ) 2H3 ] , [W(PMe )4H ] and [W(PMe3) H ] by comparison of the spectroscopic data with a u t h e n t i c samples. I t seems r e a s o n a b l e t o propose t h a t the f o r m a t i o n o f [W(T|C5H5) ( P M e 3 ) H 5 ] from cyclopentane proceeds v i a t h e migration of a l l f i v e hydrogens from one face of the C 5 ~ r i n g onto the tungsten i n a stepwise i n t r a m o l e c u l a r process. The p r o p o s a l i s supported by the almost complete absence of [W (T]-C5H5 ) (PMe ) H ] when cyclopentane i s r e p l a c e d by cyclopentene i n the above experiment. 5

3

4

5

2

3


5

271



16. GREEN AND O'HARE

Figure atoms.

6

Activation

o f cyclohexane

by

tungsten


272


Me P


3

W atoms H

PMeo

Ο

W atoms

^7 \ l /

Me P 3

IV

H

PMeo

Me,P

Η

Me^P

ο

Me3P W(PMe3) H 5

W(PM*3) H 4

2

4

—

1

Η—-W. M*3P

Me

F i g u r e 7 R e a c t i o n o f t u n g s t e n atoms w i t h c y c l o p e n t a n e . ( i a n d i v ) C o - c o n d e n s a t i o n a t -196 ° C ; ( i i ) C y c l o p e n t a d i e n e i n b e n z e n e a t 60 ° C ; and ( i i i ) D i h y d r o g e n a t 50 atm.


16.

GREEN AND O'HARE


A c t i v a t i o n Qf carbon-hydrogen

273

bonds by osmium a t o m s

Co-condensation of osmium atoms with benzene gives moderate yields of [Os (T^-CgHg) (T^-CgHg) ] (23.) . The molecule i s f l u x i o n a l as demonstrated by variable temperature n.m.r experiments. H magnetization t r a n s f e r n.m.r experiments show that [Os (T^-CcHg) (TjG-CgHg) ] undergoes degenerate h a p t o t r o p i c r i n g exchange f o r which the rate constant (k) at 37°C i s 0.2~0.03s~l. Surprisingly, when osmium atoms are co-condensed with m e s i t y l e n e two p r o d u c t s can be isolated. The major yellow crystalline product was i d e n t i f i e d as [ {Os (Tl -C gH Me ) } 2 ^-CHC H Me2-3 5 ) ( η H)2]. Fractional c r y s t a l l i z a t i o n of the reaction mixture gave a second minor product as red c r y s t a l s which a c r y s t a l s t r u c t u r e determination showed t o be [{Os (Tl -C H Me ) } (T|-CHC6H Me2-3,5) ] as shown i n Figure 8 (2£) .


1

6

3

3

6

3

f

6

6

3

3

2

3

However, when osmium atoms are co-condensed with a mixture of benzene and 2-methylpropane only t r a c e amounts of [Os (T^-CgHg) (T^-CgHg) ] can be detected i n the r e a c t i o n mixture. More importantly, the t r i - n u c l e a r compound [ {Os (T1 -C H ) } ( μ - Η ) ( μ - (CH ) CH) ] can be i s o l a t e d (5) (Figure 8) (21). Co-condensation of osmium atoms with a mixture of perdueteriobenzene and 2methylpropane gives the compound [ {Os (Tj^-CgDg) } ( μ H) ( μ - ( C H ) CH) ] , showing that the three μ -Η groups a r i s e from the 2-methylpropane. The tri-osmium compound provides a s t r i k i n g model f o r a surface chemisorbed alkane (2Ώ.) . 6

2

6

6

3

3

3

2

3

2

3

2

3

3

2

3

CONCLUSION

The co-condensation reactions described above have l e d to the formation of i n t e r e s t i n g new compounds and sometimes very unexpected products. The nature of the products formed f o r example i n the osmium atom experiments indicate high degrees of specificity can be achieved. However, the d e t a i l e d mechanisms of the co-condensation reactions are not known. I t seems most l i k e l y that i n a l l cases the i n i t i a l products formed at the co-condensation temperature are simple l i g a n d - a d d i t i o n products and that the i n s e r t i o n of the metal i n t o the carbon-hydrogen bond occurs at some point during the warming up process. In support of t h i s h y p o t h e s i s we note the v i r t u a l absence of any




274

Figure 8 Summary of the r e a c t i o n s of osmium atoms with alkanes and alkylbenzenes. Osmium atoms were co-condensed with the indicated s u b s t r a t e s at -196°C. (i) Mesitylene; (ii) Benzene/2Methylpropane mixture (1:1, w/w).



16.

GREEN AND O'H ARE


275

bis(arene)osmium compound, itself u n r e a c t i v e towards alkanes i n the co-condensation reaction of osmium atoms and a benzene :2-methylpropane mixture. A l t h o u g h many o f t h e p r o d u c t s a r e d i - o r t r i - n u c l e a r it seems unlikely that these arise from reactions involving metal dimers or trimers but that they are a consequence of intermolecular reactions of i n i t i a l l y formed mononuclear species. However, detailed studies of the product distribution as a function of temperature are n e e d e d b e f o r e we c a n h a v e any evidence of the nature of the intermediates. Given these assumptions i t seems that the reactions of t h e s e m e t a l atom co-condensates are s i m i l a r to that of thermal and photochemical reactions of discrete molecular systems i n s o f a r t h a t we b e l i e v e t h a t they proceed by initial formation of some unsaturated organometallic species which then subsequently inserts into the carbon-hydrogen bond. We n o t e that it is not p o s s i b l e under the conditions of the metal-atom experiments to distinguish between thepossibilities that they are simply thermal reactions or both thermal and/or photochemicallyinduced reactions. The conditions of the r e a c t i o n s are such that during the co-condensation the co-condensate r e c e i v e s r a d i a t i o n from the molten metal sample which i s c o m p a r a b l e t o a 1KW t u n g s t e n lamp. It is clear that alkanes should not be treated as 'inert solvents in reactions involving metal atoms, even when there is an excess of a more established ligand. 1

ACKNOWLEDGMENTS We t h a n k t h e D o n o r s o f t h e P e t r o l e u m R e s e a r c h Fund, administered by the American Chemical Society for partial support, and the Royal Commission f o r the E x h i b i t i o n o f 1851 f o r a R e s e a r c h F e l l o w s h i p t o D. 0 H. f

LITERATURE CITED 1. Kemball, C. In Catalysis Reviews; Heinemann, H., Ed.; Dekker, M.:New York, 1972; p33. 2. Muetterties, F.L.; Rhodin, T.N.; Band, E.;Brucker, C.F.; and Pretzer, W.R. Chem. Rev., 1979, 79, 91. 3. Somorjai, G. In Chemistry in Two-Dimensions: Cornell University Press: Ithaca, New York, 1981.


276


4. Shilov, A.E., Activation of Saturated Hydrocarbons by Transition Metal Complexes: D. Riedel Publishing Co: Dordrecht; 1984. 5. Green, M.L.H.; and O'Hare, D. Pure Appl. Chem., 1986, 51, 1, 897. 6. Crabtree, R.H.; Chem. Rev., 1985, 85, 245 and refs therein.


7. Watson, P.L. J. Am. Chem. Son., 1983,

6491.

8. Hoyano, J.K,; McMaster, A.D.; and Graham, W.A.G. J. Am. Chem.Soc., 1983, 105, 7190. 9. Fendrick, C.M.; and Marks, T.J. J. Am. Chem. Soc., 1984, 2214. 10. Blackborrow, J.R., and Young, D. In Metal Vapor Synthesis in Organometallic Chemistry: Springer Verlag: Berlin, 1979. 11. Klabunde, K.J. In Chemistry of Free Atoms and Particles:AcademicPress: New York, 1980. 12. Green, M.L.H. U.K. Patents 5953/74 and 53675/74, U.S. Patent 7,182,742,1980; 13. Green,M.L.H.; and Hammond, V.J. U.S. Patent 4,182,749, 1980. 14. Green, M.L.H. In Frontiers in chemistry; Laidler, K.J., Ed.;Pergamon Press: Oxford, 1982; p229. 15. Cloke, F.G.N.; and Green, M.L.H. J. Chem. Soc., Dalton Trans., 1981, 1938; 16. Davis,S.C.; and Klabunde, K.J. J. Am. Chem. Soc., 1978, 100, 5975. 17. Remick, R.J.; Asunta, T.A.; and Skell, P.J. J. Am. Chem. Soc., 1979, 101, 1320. 18. Bymater, S.; and Steacie,E.W.R. J. Chem. Phys., 1951, 19, 172. 19. Billups, W.E.; Konarski, M.M.; Hauge, R.H.; and Margrave, J.L. J. Am. Chem. Soc., 1980, 102, 7393. 20.Ozin, G.A.; Mcintosh, D.F.; and Mitchell, S.A. J. Am. Chem. Soc., 1981, l03, 1574. 21.Green, M.L.H. J. Organomet. Chem., 1980, 200, 119.


16. GREEN AND O'HARE Activation of Carbon-Hydrogen Bonds

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