3- methylpentanal, photolysis

Tile photolysis (A = 365 nm) of 42 alone resulted in a very slow conversion to the 1,4-dithiin 259, whereas the irradiation in the presence of nor-bornene quickly produced 260 in 95% yield (88CL657).Tlierefore, the conversion of 42 to 261 (or 262) and sulfur atom should be reversible. Tlie photolysis of 263 in 3-methylpentane at 77 K yielded a mixture of 261 and 262, as suggested by the electronic spectrum, which showed two absorption maxima at 580 and 370 nm (89TL2955). Similar results were also obtained for selenium species (88CL657 89TL2955). 

The photolysis of hexaarylcyclotrigermanes was used to synthesize the first stable digermenes. The photolyses are generally carried out at 254 nm in hydrocarbon solvents (e.g., cyclohexane or 3-methylpentane). Presumably, two equivalents of the cyclotrigermane form three equivalents of the digermene, the third equivalent being formed by dimerization of the diarylgermylene. 

Figure 8 Connection between primary C-H and C-C bond ruptures during radiolysis and photolysis. Alkanes (1) propane, (2) w-butane, (3) -pentane, (4) -hexane, (5) w-heptane, (6) n-octane, (7) w-decane, (8) isobutane, (9) neopentane, (10) 3-methylpentane, (11) 2,2-dimethylbutane, (12) isooctane, (13) cyclopentane, (14) cyclohexane, (15) cycloheptane, (16) cyclooctane, (17) cyclodecane, (18) methylcyclopentane, (19) methylcyclohexane, (20) ethylcyclohexane, (21) 1,1-dimethylcyclohexane, (22) cis-l,2-dimethylcyclohexane, (23) fraw5-l,2-dimethylcyclohexane, (24) cis-1,3-dimethylcyclohexane, (25) trarw-l,3-dimethylcyclohexane, (26) cw-l,4-dimethylcyclohexane, (27) trawi-l,4-dimethylcyclohexane. (From Refs. 18, 29, 91, 92, 99, 100, 108, 110, 111, 113, 114, and 160.)...

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al. detected a broad ultraviolet (UV) absorption band (3-niax = 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemicaUy or thermally, were examined (Scheme 14.6). ... 

Frey38 produced CH2 by CH2N2 photolysis in excess propane at 50°C. and found all the products (ethane, n-hexane, 2,3-dimethylbutane, 2-methylpentane, propene, and propane) which would be expected to arise from recombination and disproportionation reactions of the radicals CH3-, CH2CH2CH3, and CH3CHCH3. 

Alkoxy- and amino-substituted disilenes (17, 18 and 19) are produced by photolysis of the corresponding masked disilenes (20, 21 and 22)29. Thus, when a degassed 3-methylpentane (3-MP) matrix of 20 was irradiated at 77 K, the corresponding alkoxy-substituted disilene 17 was produced (equation 5) as indicated by its broad band at 373 nm. 

Photolytically generated 1-silabuta-l,3-dienes undergo a thermal reverse reaction to 2-silacyclobutenes. Thus 2-phenylsilacyclobut-2-ene 360 is easily opened to the 2-phenylsilabuta-1,3-diene 361 by irradiation in 3-methylpentane matrix at 77 K or by flash photolysis at ambient temperature (equation 97)183. The rate for the thermal reverse reaction was measured at room temperature and the activation energy for the 1 -siladiene ring closure was estimated to be 9.4 kcalmol-1183. 

Before that time, little was known about their UV spectra. Ando and coworkers extended our knowledge about this facet of organosilicon chemistry by photolysis of silyldia-zomethanes 4 in 3-methylpentane at 77 K yielding the expected silenes 519 (equation 2). The measured UV spectra together with previous results are summarized in Table 1. 

When di(t-butyl)silylene 321, generated in a 3-methylpentane glass at 77 K or in an argon matrix at 10 K by photolysis of a precursor bis-azide, was irradiated with 500-nm light, intramolecular C—H insertion occurred yielding the silacyclopropane 322 (equation 26)161. 

For comparative study, various silenes were isolated in 3-methylpentane matrix at 77 K by photolysis of the corresponding silyldiazo compounds and their ultraviolet spectra are measured. The results are summarized in Table 1. The introduction of trimethylsilyl group on carbon results in slight red shift compared with the parent silene (H2Si=CH2, 258 nm). As one might expect, considerable bathochromic shifts have been observed for conjugated silenes such as 6c, 6d and 2829,30. 

At liquid nitrogen temperature, 77 K, matrix isolation in hydrocarbons is successful for many silylenes because they are singlet species and so do not abstract hydrogen from C—H bonds, as would be expected for triplet molecules. The usual hydrocarbon is 3-methylpentane (3-MP) which forms a rigid glass at 77 K, but sometimes mixtures of hydrocarbons are used which are softer at this temperature, to allow some mobility of the silylene in the matrix. In a few cases, silylenes have also been identified from transient spectra obtained in flash photolysis experiments. 

Laser flash photolysis at 347 nm generates a transient colored species (Amax 325 and 405 nm) with a lifetime of 435 xs in 3-methylpentane (3MP). Steady state UV irradiation at low temperature cleaves the S-C(2) bond and the merocya-nine so produced absorbs at 425 nm, shifting to 412 nm on prolonged irradiation, with additional bands at 340 and 256 nm. Both thermal and photo-bleaching occurs but the original spectrum is not restored. It is suggested that the first photoproduct is the cis isomer which is converted in part to the trans isomer (Equation 9) <2006JPH(177)34>. 

Methylpentanal. Early studies of propanal and 1-butanal in the gas phase have been reviewed by Cundall and Davies (61). Only a few other saturated aldehydes have been studied recently, and 3-methylpentanal is one of them. Rebbert and Ausloos (195) have compared the direct and the triplet-sensitized photolysis of 3-methylpentanal, since this molecule can undergo two kinds of intramolecular rearrangement process / Norrish type II process 18, a primary or a secondary a-hydrogen atom transfer to the carbonyl oxygen, giving 1-butene or trans/cis-2-butene, respectively. 

A recent review by Symons (176) dealt with ESR studies of radiation damage in inorganic solids. It is well known that trapped electrons or F centers may be produced by photolysis of inorganic compounds. Recently Froben and Willard (177) reported the photoionization of metal atoms, such as Li, K, Na, Mg, and Cd, and observed by ESR the electrons trapped in 3-methylpentane at 77°K. 

Extended photolysis of Mu2(CO)io in 3-methylpentane glass gives rise to Mn2(CO)s resulting from double CO-loss and an isomer of Mu2(CO)9 with a semibridging carbonyl. Dft calculations have been used to probe possible structures of the Mu2(CO)8 species. ... 

Carbene reagents also functionalize alkanes. Triplet tCHa adds unselectively to alkane C—bonds. The product mixture obtained from n-pentane was found to be 48% n-hexane, 35% 2-methylpentane and 17% 3-methylpentane, so that addition to a primary C—H bond appears to be favored. Monochloro-methylcarbene, CHCl, is less reactive and more electrophilic and so the normal tertiary > secondary > primary selectivity pattern was observed. Ethoxycarbonylcarbene, formed on photolysis of the corresponding diazo compound, inserts rather unselectively in to alkane C—H bonds to give the ethoxycarbo-nylmethyl derivatives in ca. 50% yield. Transition metals, such as copper(II) or rhodium(I), also usefully catalyze the insertion of caibenes into alkane C—bonds. 

Other types of sila systems also give di-n-methane products. In the case of (19), irradiation at 10 K in an argon matrix or at 77 K in a 3-methylpentane glass affords the di-n-methane product (20), which on further irradiation or upon heating reverts to the starting material (equation 21). Photolysis of (19) at room temperature gives instead retro Diels-Alder fragmentation to tetramethyldisilene, whose intermediacy was established by means of [4 + 2] cycloaddition with 2,3-dimethyl-1,3-butadiene. ... 

C3F7NO is similar . For 2-nitroso-2-methylpropane step (a) seems to be the major primary step although (b) cannot be excluded On the other hand, the photolysis of 2-nitroso-2,5-dimethylhexane seems to proceed mainly via step (b) although route (a) cannot definitely be excluded the same remarks apply to the photolysis of 4-nitroso-4-methylpentan-2-one . In the case of chloronitroso derivatives, a primary process analogous to (b) has been postulated , viz. 

Irradiation of the thiopyran derivative (292a) results in extrusion of HNCS and the formation of the pyridine (292b, 63.3%) as the major product. The photochemical reactivity of the pyranthione (293) is concentration dependent in 3-methylpentane as solvent. At low concentrations the thione reacts with the solvent but at higher concentrations the main reaction is the production of thiyl radicals. Laser flash photolysis has been used to identify that the triplet excited state of (294) is involved in addition reactions which occur to electron deficient alkenes such as acrylonitrile giving (295), for example. Other compounds related to thiourea are also photochemically reactive. Thus the photochemical cyclisation of (296) to afford (297) has been reported. ... 

Photolysis of (PhjSOjB or RB(SiPh3)2 (R = Me, mesityl) in 3-methylpentane at 196°C yields triphenylsilylboranediyl, which undergoes insertion on wanning, e.g. ° ... 

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