Photolytically generated

In the bimolecular collision of the photolytically generated reagent, assumed to have a mass m. and laboratory speed Vp the centre-of-mass speed will be... 

Figure B2.3.16. Velocity diagram for die reaction of a photolytically generated reagent with an assumed stationary co-reagent. In this case, the relative velocity of the reagents is parallel to the velocity c of the centre of mass.

Photolytically generated carbene, as mentioned above, undergoes a variety of undiscriminated addition and insertion reactions and is therefore of limited synthetic utility. The discovery (3) of the generation of carbenes by the zinc-copper couple, however, makes carbene addition to double bonds synthetically useful. The iodo-methylzinc iodide complex is believed to function by electrophilic addition to the double bond in a three-center transition state giving essentially cis addition. Use of the... 

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

Figure 8, Photolytic generation of nitrogen dioxide and an aryl nitrite ester from a nitro compound, as formulated by Lippert and Kelm (27),...

Photolytic generation of silene Me3Si(Me)C=SiMe2 in the presence of an excess of D20 or a trace of H20 affords disiloxanes 25, presumably via condensation of the silanol 26 in the case of D20, or by reaction of the silanol 27 with further silene in the case of H20 (Scheme 7) (138, 139). 

Chiang, Y. Kresge, J. Zhu, Y. Flash photolytic generation of ortho-quinone methides in aqueous solution and study of its chemistry in that medium. J. Am. Chem. Soc. 2001,123, 8089-8094. 

EtMe4C5)Mn(CO)3 18 and reaction of the photolytically generated species [(jj5-EtMe4C5)Mn(CO)2 x THF] 19 with (f-Bu)2SiHCl 7 yields the p-biscarbyne complex 20, but also the carbonyl complex 22 with a manganese manganese triple bond (Scheme 3) [12],... 

As discussed above, the solution environment provides for a set of time scales different from the gas phase environment. In solution, there are typically 1013 collisions second"1 of a solute molecule with solvent molecules. Thus, if a photolytically generated species is expected to have a large cross section for reaction with solvent and it is desired to monitor that reaction, both generation and monitoring must be done on a picosecond (psecond) or even sub-psecond timescale. That monitoring this rapid is necessary has been confirmed in an experiment on Cr(CO)6 in cyclohexane solution where psecond photolysis and monitoring was not rapid enough to detect the naked Cr(CO)5 that existed before coordination with cyclohexane (55). 

However, there is another operative timescale in solution. This is that timescale for reaction with other photolytically generated species or with added reactants. This reaction cannot take place faster than the diffusion-limited reaction rate which is concentration dependent (59). Typical diffusion-controlled reaction rate constants are 109-1010 dm3 mol"1 second-1. By comparison, a typical gas-kinetic rate con-... 

Nevertheless, we were able to develop a transient absorption apparatus involving IR probe radiation that is suitable for gas phase studies, as have a number of other groups either coincident with or subsequent to our work [1]. In the remainder of this article we will discuss the apparatus and the results of our studies on three prototypical metal carbonyl species Fe(C0>5, Cr(C0>5 and Mn2(CO)] o The discussion in this article will center on the nature of the photolytically generated coordinatively unsaturated species, their kinetic behavior and photophysical information regarding these species. This latter information has enabled us to comment on the mechanism for photodissociation in these systems. Since most of the results that will be discussed have been presented elsewhere [3-10], we will concentrate on a presentation of data that illustrates the most important features that have come out of our research and directly related research regarding the kinetics, photophysics and photochemistry of coordinatively unsaturated metal carbonyls. 

Photolytic generation of carbene 19 from a precursor diazirine afforded cyclobutenes 27 (68%) and 28 (14%) via CH2 and CMe2 migrations, respectively cf. Eq. 15.45... 

When, however, carbenes are directly generated from diazoalkanes, RIES becomes significant.56 Photolytic generation of carbene 45 from diazoalkane precursors in the presence of >1.5 M pyridine gave values for the derived pyridinium ylides. 

RIES from diazoalkanes is also sensitive to the dihedral angle between the migrating a-H and the C-N bond of the diazo moiety.57 For example, the A values for the pyridine capture of the photolytically generated carbenes from 46 and 47 are in the ratio of 1.7 1. Similarly, the carbene from 46 is more efficiently generated and trapped in methanol, whereas the photolysis of 47 in methanol affords twice as much olefin (by 1,2-H RIES) compared to the photolysis of 46. These phenomena are attributed to conformational factors that favor RIES during the photolysis of 47, with the proximal excited state represented as a pyramidalized 1,3-C-N=N diradical.57... 

Accordingly, a re-examination of the benzylchlorocarbene system was performed, with close attention paid to the products formed at low temperature.71 Carbene 10a was photolytically generated from diazirine 9a in isooctane, methylcyclohexane, and tetrachloroethane at temperatures ranging from 30 to —75°C. At —70°C in isooctane, the products included 47% of P-chlorostyrenes 11a and 12a, 2.4% of a-chlorostyrene (49), 2% of dichloride 50, 5.5% of a C-H insertion product of 10a and isooctane, 4% of the dimers of 10a, and 30% of azine 48.71 The sum of the intermolecular products at —70°C was thus 41.5%, of which azine was the principal component. 

In the same vein is the observation that the lifetime of dipropylcarbene (59) in CH2C12 or cyclohexane is 0.3 ns,84 which, after statistical correction is 48 times less than the lifetime ( 21 ns) of Me2C in pentane.22 This reflects promotion by the propyl bystander groups of 59 of the 1,2-H shift to Z- and E-3-heptene.84 (Dipropylcarbene can be photolytically generated from either an oxadiazoline (diazoalkane)84 or diazirine85 precursor, but RIES lowers the efficiency of carbene production in either case.) Recently reported LFP lifetimes for Et2C and MeCEt in cyclohexane or benzene are 0.6-3 ns (cyclohexane) or 1-5 ns (benzene),14 in accord with the lifetimes of S822 and S9.84 The rate constants for carbene disappearance in cyclohexane ( 3 x 108 to 2 x 109 s 1) are presumably limited by 1,2-H shifts.14... 

In general, 1,2-C shifts do not compete effectively with the 1,2-H shifts of acyclic alkyl and alkylhalocarbenes. However, r-butylchlorocarbene (18) lacks the a-H needed for a 1,2-H shift, and so affords 1,3-CH insertion and 1,2-Me migration Eq. 14. Note that only for the thermally generated 18 is the 1,2-Me shift product (26) derived from the carbene. Photolytic generation of 18 from diazirine 24 gives only 1,3-CH insertion to dimethylchlorocyclopropane 25 in this case, the 1,2-Me shift product is formed by RIES of the diazirine.27 Based on the rate constant for the 1,3-CH insertion of t-BuCCl at 25°C (9.3 x 105 s 1), we can estimate A 105 s 1 for the 1,2-Me shift at 78°C. 

Herranz, M.A., et al., Spectroscopic characterization of photolytically generated radical ion pairs in single-wall carbon nanotubes bearing surface-immobilized tetrathiafulvalenes. Journal of the American Chemical Society, 2007.130(1) p. 66-73. 

Fig. 2.21. Photolytic generation and. synthetic applications of chromium ketene complexes.

Alkyl radicals have also been prepared by reaction of alkylbromides with photolytically generated Re(CO)5 (from Re2(CO)io) [17], photolysis of cobalt-alkyl complexes [20], photolysis of AIBN [17, 21, 22] or thermolysis of TEMPO adducts [23]. 

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