Patemo mechanism

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

Oxetane Formation—The Patemo-Bnchi Reaction. A large number of carbonyl compounds, primarily aldehydes, ketones, and quinones, form oxetanes by photocycloadditions to olefins.61-63 In general, it is observed that (/) carbonyl compounds which have low-lying (77, ) triplet states and which are photoreduced in isopropyl alcohol form oxetanes most readily, and (2) oxetane formation takes place when energy transfer from the carbonyl compound to the olefin is unfavorable because of the relative location of their triplet levels.64,65 Hence, oxetanes are most readily formed from simple olefins and allenes63,66 but are seldom formed from dienes.67 An extensive review by Arnold63 covers the mechanism and scope of this reaction. 

The size and shape of molecules are as much a part of molecular structure as is the order in which the component atoms are bonded. Contrary to the impression you may get from structural formulas, complex molecules are not flat and formless, but have well-defined spatial arrangements that are determined by the lengths and directional character of their chemical bonds. It is not easy to visualize the possible arrangements of the bonds in space and it is very helpful to have some kind of mechanical model that reflects the molecular geometry, including at least an approximation to the relative lengths of the bonds. Ball-and-stick models such as the ones used by Patemo (Section 1-ID) fill this purpose admirably. 

The Generally Accepted Mechanism of the Patemo-Biichi Reaction... 

The 1,5-biradicaI BRi formed during the Patemo—Biichi reaction of excited benzo-quinone B with quadricyclane Q (for the formulas, see Chart 9.4) provides one of the extremely rare examples of a short-chain biradical that produces CIDNP of the radical pair type. Extracts of the CIDNP spectra are displayed in Fig. 9.7. The occurrence of both absorption and emission in the same product is clear evidence for this mechanism of polarization generation. 

The Patemo-Buchi reaction has been studied mechanistically ever since Patemo and Chieffi first reported that the photoreaction of ketones and alkenes produced, unexpectedly, the corresponding oxetane. Indeed, a recent surge of studies of the photoreaction mechanism has stimulated numerous applications in organic synthesis. 

The synthesis of oxetanes from alkenic precursors has been demonstrated and the range and scope of Patemo-Biichi photocycloadditions are broad. In general, both the reaction regioselectivity and stereoselectivity can often be predicted by considering the reaction mechanism (Section 2.4.2) for example, the directing effects of alkene substituents are readily imderstood. Synthetic applications are numerous owing to the rtq>id stereocontrolled assembly of multifimctional targets. 

Translation of these results into compound I leads to structure X. Unraveling of the strained zwitterion XI derived from this would yield keto aldehyde XII, a structure that plays a central role in the various possible reaction mechanisms that branch off from the starting material I. Furthermore, under photo-lytic conditions, some alkenes react with carbonyl compounds to form four-membered cyclic ethers, namely, oxetanes, by way of a [2-1-2] cycloaddition reaction known as the Patemo-Buchi process. Such a reaction would be all that is necessary to convert XII into the bicyclic cyclopropanone XIII required for the Favorskii-type rearrangement (see Scheme 42.3). Splitting by methanol attack would directly yield compound II. 

The Patemo-Buchi reaction is the photocycloaddition of an alkene with an aldehyde or ketone to form oxetanes. This transformation has been shown to proceed through a biradical intermediate, and up to three new stereocenters can be formed as a result of this reaction. A general mechanism for the reaction between an aldehyde and a chiral enol silyl ether is shown in Eq. (13.7) [18]. Allylic 1,3-strain is cited as the control element in reactions of this type, and diastereomeric ratios of >95 5 are reported for products 30 containing four contiguous stereocenters. Examples of photocyclizations of amino acid derivatives proceeding through biradical intermediates have been repotted [19]. 

By a detailed CIDNP investigation [117a] of the Patemo-Biichi reactions of anetholes 31 with quinones 30 in polar medium earlier mechanistic hypotheses were disproved. Stationary and time-resolved experiments showed the mechanism to have the following novel features (cf. Chart XIV) Spin-correlated radical ion pairs (i.e., 30 31,+) are key intermediates for cycloadduct formation free radical ions do not play a significant role. In the singlet state, these pairs undergo back electron transfer geminate reaction of triplet pairs leads to triplet biradicals, which are the precursors to the photoproducts. 

According to the Woodward-Hoflfmann rules, concerted thermal [2+2] cycloadditions are symmetry-forbidden, but should proceed via supra-antarafacial attack of the reactants. [2+2] cycloadditions of ketenes and related reactive intermediates generated in situ proceed by a stepwise mechanism. " Photochemical [2+2] cycloadditions are symmetry-allowed. Asymmetric [2+2] cycloadditions leading to 4-membered heterocycles, e.g. Staudinger reactions or Patemo-BUchi reactions, have been extensively studied in the past. 

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